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  Subjects -> EARTH SCIENCES (Total: 623 journals)
    - EARTH SCIENCES (460 journals)
    - GEOLOGY (67 journals)
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EARTH SCIENCES (460 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 9)
Aeolian Research     Hybrid Journal   (Followers: 3)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
AMBIO     Hybrid Journal   (Followers: 12)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 6)
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 Review     Hybrid Journal   (Followers: 3)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 7)
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: 21)
Arctic Science     Open Access  
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 7)
Atmospheric and Climate Sciences     Open Access   (Followers: 17)
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: 12)
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: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 4)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 6)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 12)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 4)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 11)
Coastal Management     Hybrid Journal   (Followers: 19)
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: 18)
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: 4)
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: 4)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 224)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 8)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 14)
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: 2)
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: 11)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 11)
Environmental Geology     Hybrid Journal   (Followers: 12)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)

        1 2 3 4 5 | Last

Journal Cover   Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [14 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2586 journals]
  • Extraction of α-cellulose from mummified wood for stable isotopic
           analysis
    • Abstract: Publication date: Available online 15 April 2015
      Source:Chemical Geology
      Author(s): Benjamin A. Hook , Jochen Halfar , Jörg Bollmann , Ze’ev Gedalof , M. Azizur Rahman , Julito Reyes , Daniel J. Schulze
      Stable isotope ratios of carbon (δ13C) and oxygen (δ18O) from tree ring cellulose can provide valuable paleoclimatic information at annual and subannual resolution, from time periods long before instrumental climate records. Recently, mummified (non-permineralized) wood was discovered within Canadian Subarctic kimberlites, inviting paleoclimatic investigations of the time in which the trees grew. In the mummified wood, polysaccharides (hemicellulose, α-cellulose) are frequently preferentially degraded, in the low-silica anaerobic burial environment of the kimberlites, resulting in a lignin-rich material. However, some samples from the Ekati Panda kimberlite pipe (ca. 53.3 Ma) contain remnant cellulose, demonstrating the extraordinary preservation potential of kimberlites. Preservation of α-cellulose is important because it allows for the construction of stable isotopic proxy records of deep-time paleoclimates at annual to subannual resolution. Established α-cellulose extraction methods [i.e., using a 17.5 % sodium hydroxide (NaOH) solution] were unsuitable for this material because all holocellulose was dissolved. Therefore, we tested variants of two cellulose extraction methods [i.e., Brendel et al. (2000) and Jayme-Wise (Leavitt and Danzer 1993, Loader et al. 1997) to optimize a procedure for extraction of a consistent yield of mummified wood cellulose for stable isotopic analysis. Stable carbon (δ13C) and oxygen (δ18O) isotopes were measured from cellulose from each extraction method, as well as extractive-free wood and unextracted mummified wood. Vitrinite Reflectance, used to assess thermal alteration of the unextracted material to estimate post-burial temperatures within the kimberlite, suggested low post-burial peak temperatures (Tpeak = 60 °C). We detected no mineral contaminants (i.e., iron oxides) in the material using Energy-Dispersive X-ray Spectroscopy (EDX). Despite low cellulose yield (<5 %), the Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectra of mummified cellulose samples strongly resembled modern α-cellulose. All cellulose treatments were similar in stable isotope values and ATR-FTIR peaks, but significantly different from unextracted wood and extractive-free wood. As, reported by other sources (Anchukaitis et al. 2008, Brookman and Whittaker 2012), use of the Brendel method may cause cellulose acetylation, introducing an FTIR peak near 1720 cm-1, thus complicating hemicellulose detection at a peak near 1725 cm-1. For this reason, the Jayme-Wise method is recommended for detection of hemicelluloses in mummified wood. If hemicelluloses are not present in holocellulose, due to groundwater hydrolysis, the NaOH step may be omitted or reduced in concentration and still produce α-cellulose.


      PubDate: 2015-04-16T06:47:35Z
       
  • The importance of organic-rich shales to the geochemical cycles of rhenium
           and osmium
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Andrea Dubin , Bernhard Peucker-Ehrenbrink
      The role that marine organic-rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high Re/Os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust. The high Re/Os leads to rapidly increasing 187Os/188Os ratios. We determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187Re/188Os and 187Os/188Os ratios of organic-rich sediments. We then utilize estimates of the abundance of organic-rich sediments in the geologic record of different ages to estimate an average 187Os/188Os for these sediments. We find that the calculated 187Os/188Os value of organic-rich sediments varies between 2.22 and 2.27 depending on the volume–age model used, with error estimates of 2.07 to 2.42 and 2.12 to 2.48, respectively. Despite the fact that these sediments constitute a relatively small proportion of the upper continental crust, they contribute a disproportionately large amount (>50%) to the upper continental crustal rhenium and, to a lesser extent (5–10%), osmium inventories owing to their high concentrations of these elements.


      PubDate: 2015-04-16T06:47:35Z
       
  • Novel 1H-Pyrrole-2,5-dione (maleimide) proxies for the assessment of
           photic zone euxinia
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Sebastian Naeher , Kliti Grice
      1H-Pyrrole-2,5-diones (maleimides) and 1-alkyl-2,3,6-trimethylbenzenes (aryl isoprenoids), degradation products of tetrapyrrole pigments and carotenoids respectively, were analysed and compared with pristane/phytane (Pr/Ph) ratios to reconstruct past redox conditions in three geologic sections. One section comes from the Middle–Late Devonian and was deposited before the Frasnian–Famennian boundary mass extinction. The two other sections span the Late Permian to the Early Triassic as well as the Late Triassic to the Early Jurassic, and recorded the Permian–Triassic (P/T) and Triassic–Jurassic (T/J) extinction events respectively. The 2-methyl-3-iso-butyl-maleimide (Me,i-Bu maleimide) to 2-methyl-3-ethyl-maleimide (Me,Et maleimide) and 2-methyl-3-n-propyl-maleimide (Me,n-Pr maleimide) to Me,Et maleimide ratios (Me,i-Bu/Me,Et and Me,n-Pr/Me,Et ratios) in the studied sections revealed a moderate to strong negative correlation to the aryl isoprenoid ratio (AIR), defined as (C13–C17 1-alkyl-2,3,6-trimethylbenzenes)/(C18–C22 1-alkyl-2,3,6-trimethylbenzenes), indicating that these maleimide ratios can be used as robust, specific indicators of photic zone euxinia (PZE). These results agreed with Pr/Ph ratios, which were used as diagnostic indicators to differentiate between oxic and anoxic conditions. In agreement with previous studies, the novel maleimide proxies suggest that all three mass extinctions were largely characterised by PZE depositional conditions.


      PubDate: 2015-04-16T06:47:35Z
       
  • Using a mathematical model of a weathering clast to explore the effects of
           curvature on weathering
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): M.I. Lebedeva , P.B. Sak , L. Ma , S.L. Brantley
      For the first time, we show that rind thicknesses developed on surfaces of a clast with different values of curvature can be used to estimate the duration of clast weathering. To obtain an analytical expression for the velocity of the curvilinear weathering front on a clast of arbitrary shape, we approximate our previous multi-mineral reactive-diffusion model and explore a simplified 2-D model numerically and analytically. Our analysis documents that with increasing curvature of the weathering front, the mathematical description of weathering advance is equivalent to that derived for advection as the dominant solute transport mechanism, even for the case where transport is occurring by diffusion only. Specifically, for a curvilinear weathering front with constant curvature K <0, diffusivity (D), and porosity (ϕ), the normal component of the weathering advance rate can be calculated using an advection-like term where the advection velocity v can be expressed as v = Dϕ K . Therefore, at points along the rind–core interface with K <0, rind thickness is directly proportional to the absolute value of the curvature of the core–rind interface. The reaction front thickness also increases with K. These inferences are in agreement with field observations. This quantitative analysis allows an assessment of the duration of weathering if certain parameters are known. For example, using the difference in curvature observed at two positions for a clast that weathered in Guadeloupe (0.12mm−1 and 0.018mm−1) and the corresponding rind thickness difference (35.8mm and 20.6mm), we estimated the duration of weathering to be about 118ky, which is consistent with the weathering ages previously determined by U-series isotope disequilibrium.


      PubDate: 2015-04-16T06:47:35Z
       
  • Geochemistry of vein and wallrock carbonates from the Ediacaran system in
           South China: Insights into the origins of depositional and
           post-depositional fluids
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Yan-Yan Zhao , Yong-Fei Zheng
      A combined study of carbon–oxygen isotopes and major-trace elements was carried out for vein and wallrock carbonates from the Lantian Formation of the Ediacaran system in South China. The results provide geochemical constraints on the origins of depositional and post-depositional fluids with respect to recognition of the post-depositional alteration. The Lantian Formation was subdivided into the Upper and Lower carbonate units. For the Upper Unit (UU), petrographic observations indicate that wallrock carbonates only experienced limited post-depositional alteration. The wallrock exhibits δ18O values of −19.2 to −13.1‰ and δ13C values of −9.6 to −8. 9‰ (both relative to VPDB). The vein carbonates are divided into Groups A and B based on their δ18O values and REE+Y patterns. Group A veins show nearly similar δ18O and δ13C ranges to, but different REE+Y patterns from, the wallrock. These data indicate that the origin of fluids for Group A veins may be similar to that of wallrock, which was probably the mixture of continental and marine glacial meltwaters. Group B veins show both different δ18O values and REE+Y patterns from, but similar δ13C values to, the wallrock. This indicates that post-depositional fluids are different from the depositional fluids and thus of external origin. Low δ18O values −23.7 to −18.1‰ for Group B veins indicate that the post-depositional fluids were derived from continental glacial meltwater. There is no correlation between δ18O and δ13C values for these vein and wallrock carbonates, suggesting that the wallrock was not extensively altered by the post-depositional fluids. For the lower unit (LU), veins exhibit different C–O isotope compositions and REE+Y patterns from wallrock carbonate, indicating that post-depositional fluids have different geochemical compositions from depositional fluids and thus are of external origin. The simultaneous changes in δ18O and δ13C values with different laminations in the wallrock carbonates suggest the preservation of the primary geochemical compositions. The δ18O values and REE+Y patterns for the LU veins indicate that the post-depositional fluids were probably the mixtures of continental and marine glacial meltwaters. Therefore, the Lantian carbonates record the incorporation of continental glacial meltwater into the marginal sea of Ediacaran age. Nevertheless, the geochemical screening can provide constraints on the preservation of primary signatures and thus on the origins of depositional and post-depositional fluids in the Neoproterozoic marine carbonates.


      PubDate: 2015-04-16T06:47:35Z
       
  • Isotope Dilution-AMS technique for 36Cl and Cl determination in low
           chlorine content waters
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Camille Bouchez , Julie Pupier , Lucilla Benedetti , Pierre Deschamps , Valéry Guillou , Karim Keddadouche , Georges Aumaître , Maurice Arnold , Didier Bourlès
      The cosmonuclide 36Cl is relevant for hydrological applications due to its conservative chloride form. 36Cl measurements are commonly performed by Accelerator Mass Spectrometry (AMS) while stable chlorine concentration measurements are usually performed by ion chromatography (IC). They can also be achieved by Isotope Dilution-Accelerator Mass Spectrometry (ID-AMS) which offers the advantage of the simultaneous determination of both Cl and 36Cl contents. The range of applicability of this method has been assessed on rock samples but, to our knowledge, no experimental tests were conducted on water samples characterized by low chloride contents. This study aims at investigating the accuracy and precision of the ID-AMS method for such low chlorine water samples for which chloride and 36Cl determinations remain highly challenging. First, internal and external errors were evaluated on the measured ratios and uncertainties on Cl and 36Cl concentrations were assessed at ±5% and ±7% respectively using standard error propagation calculations. Focused on the 0.1 to 10mg·L−1 concentration range, the Cl concentrations of thirty-five gravimetric standards were determined by ID-AMS leading to a 1:1 line (R2 =0.99) and a 1-sigma uncertainty on [Cl−] of ±5%, consistently with the propagated uncertainty. Finally, the optimal sample volume to reach accurate 36Cl and Cl measurements with ID-AMS for water sample concentrations within the 0.1 to 10mg·L−1 range has been theoretically estimated.


      PubDate: 2015-04-11T03:26:04Z
       
  • The continuous re-equilibration of carbon isotope compositions of hydrous
           Mg carbonates in the presence of cyanobacteria
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Vasileios Mavromatis , Irina A. Bundeleva , Liudmila S. Shirokova , Christian Millo , Oleg S. Pokrovsky , Pascale Bénézeth , Magali Ader , Eric H. Oelkers
      The hydrous magnesium carbonate minerals dypingite (Mg5(CO3)4(OH)2·5H2O) and nesquehonite (MgCO3·3H2O) were precipitated in the presence and absence of Gloeocapsa sp. and Synechococcus sp. cyanobacteria in batch reactors. The Δ13Cmineral–DIC is similar in both biotic and abiotic systems. Stable carbon isotope analyses of the precipitated minerals and co-existing fluids indicated that the δ13C values of the precipitated carbonates co-vary over time with the δ13CDIC of the fluid phase, even after the precipitation of the carbonate is complete. This observation indicates the continuous isotopic exchange between the carbonates and the fluid, a process that efficiently resets the original δ13C values of the solids. Therefore although cyanobacterial-induced Mg carbonates are 10±5‰ enriched in 13C compared to inorganic carbonates, the δ13C values of natural hydrous Mg carbonates may reflect post precipitation processes and may not be reliable for paleo-environmental reconstructions.


      PubDate: 2015-04-11T03:26:04Z
       
  • The densities and dimensions of recoil-track etch pits in mica
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Konstanze Stübner , Raymond Jonckheere , Lothar Ratschbacher
      Measurements of alpha-recoil-track densities in mica as a basis for geological dating depend on an etch model relating the number of tracks per unit volume (N RT) to the counted number of etched tracks per unit area (ρ RT). The model of Gögen and Wagner (2000) implies that ρ RT increases linearly with etch time (t E), so that N RT can be calculated from the slopes or intercepts of step-etch functions ρ RT(t E). This model rests on the assumption that the etch rate of the mica surface (v V) and the horizontal etch pit growth rate (v H) are both constant, in contradiction with experimental results and computer simulations of mineral dissolution. We present results of four different experiments aimed at relating etch pit densities to volumetric track densities. Step-etch data are vulnerable to observation-related artefacts at increasing t E and lack the resolution to confirm or refute the supposed linear increase of ρ RT with t E. The intercepts of regression lines fitted to step-etch data are imprecise and perhaps inaccurate. Intercept estimates based on mirror-image counts and (etch)–anneal–etch experiments indicate that v V increases during the initial etching stages. The (etch)–anneal–etch results show that no pre-etch is in fact required, implying that surface tracks are more resistant to annealing than tracks in the bulk of the mineral. Track-size measurements confirm that v H is also not constant but decreases with increasing etch-pit size. The results show that no recoil-track etch pit in phlogopite etched in 40% HF grows larger than ca. 7μm and that the track-size distribution becomes quasi-invariant at >6min etching. This signifies that there exist accessible etching conditions at which the etch-pit size distribution becomes a fixed, distorted reflection of the size distribution of latent recoil tracks. It is not improbable that track addition and loss also cancel each other out in this equilibrium state, in which case both the etch-pit size distributions and densities ρ RT become independent of etch time.


      PubDate: 2015-04-11T03:26:04Z
       
  • Can Mn–S redox cycling drive sedimentary dolomite formation? A
           hypothesis
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Daniel A. Petrash , Stefan V. Lalonde , Gabriela González-Arismendi , Robert A. Gordon , José A. Méndez , Murray K. Gingras , Kurt O. Konhauser
      The formation of dolomite in modern peritidal environments is linked to the degradation of buried microbial mats, with complexation of Ca and Mg by extracellular polymeric substances (EPSs) and alkalinity generation through organic carbon respiration facilitating the nucleation of dolomite precursors. In the past two decades, microbial sulfate reduction, methanogenesis, and methanotrophy have all been considered as potential drivers of the nucleation process, but it remains unclear why dolomite formation could not also occur in suboxic sediments where abundant alkalinity is produced by processes linked to Mn(IV) and/or Fe(III) reduction coupled with the diffusion and reoxidation of reduced sulfur species. Here we report the interstitial occurrence of spheroidal aggregates of nanometer-scale Ca-rich dolomite rhombohedra within suboxic sediments associated with remnant microbial mats that developed in the peritidal zone of the Archipelago Los Roques, Venezuela. Multiple analytical tools, including EPMA, ICP-MS, synchrotron-based XRF and XRD, and spatially resolved XANES microanalyses, show that the dolomite-cemented interval exhibits depleted bulk iron concentrations, but is interstitially enriched in Mn and elemental sulfur (S0). Manganese occurs in several oxidation states, indicating that the dolomite-cemented interval was the locus of complex biological redox transformations characterized by coupled Mn and S cycling. The tight correspondence between sedimentary Mn and MgCO3 concentrations further hints at a direct role for Mn during dolomitization. While additional studies are required to confirm its relevance in natural settings, we propose a model by which coupled Mn–S redox cycling may promote alkalinity generation and thus dolomite formation in manner similar to, or even more efficiently, than bacterial sulfate reduction alone.


      PubDate: 2015-04-11T03:26:04Z
       
  • Unusually low TEX86 values in the transitional zone between Pearl River
           estuary and coastal South China Sea: Impact of changing archaeal community
           composition
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Jin-Xiang Wang , Yuli Wei , Peng Wang , Yiguo Hong , Chuanlun L. Zhang
      TEX86, calculated based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), is used worldwide for paleotemperature reconstruction in marine and lacustrine environments. Recently, however, increasing evidence showed that TEX86 could be affected by multiple environmental variables. In this study, TEX86 was calculated for core and polar GDGTs obtained from water column and surface sediments in the lower Pearl River and its estuary. Together with previously published core GDGT data from the coastal and open South China Sea (SCS), a comparison was made between TEX86-derived and satellite-based surface water temperature, which showed a TEX86-temperature minimum that is considerably lower than winter satellite temperature in the transitional zone between the Pearl River estuary and the coastal SCS. TEX86 showed significantly positive correlation with GDGT-2 and GDGT-3 in this transitional zone, indicating that they are the primary compounds for the TEX86 bias toward cooler temperature. Lipid and molecular DNA data both indicate that the variation in archaeal community composition rather than water depth or seasonality is likely the crucial factor causing the deviation of TEX86 in the transitional area. Our study has implications for studies in ancient continental margins where unusually low TEX86 temperatures may be observed in the geological record.


      PubDate: 2015-04-06T07:22:26Z
       
  • Geochemical evolution and residence time of porewater in low-permeability
           rocks of the Michigan Basin, Southwest Ontario
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Tom A. Al , Ian D. Clark , Laura Kennell , Mark Jensen , Ken G. Raven
      Drill cores collected from an 860-m-thick Paleozoic (Cambrian to Devonian) sedimentary sequence of the Michigan Basin in southwest Ontario, Canada, were used to develop vertical profiles of natural tracers in porewater from the sedimentary sequence and build a conceptual model regarding processes and timing of solute migration. In this paper, the conceptual model is quantitatively assessed with numerical simulations of diffusion-dominated solute transport (Cl− and 18O). The conceptual model suggests that the tracer profiles result from mixing between a deep shield brine with isotopic characteristics consistent with water–rock interaction over a long period of geologic time, Ordovician seawater at intermediate depth, and an overlying hypersaline brine formed by evaporation of seawater in a restricted basin during the Silurian. Between the peak of basin consolidation at 300MaBP and the Pleistocene (2.6MaBP), salinity and isotopic concentration gradients are thought to have driven mixing, dominantly by diffusion. Water isotope data indicate that in the Pleistocene, melt water infiltrated to a maximum depth of 330m in the Silurian Salina Formation, likely in response to transient glacial loading and elevated sub-glacial hydraulic pressures. Also in the Pleistocene, over-pressured conditions in deep regions of the basin drove high-salinity brine eastward in the basal Cambrian aquifer, interrupting pre-existing tracer-diffusion profiles. A base case set of boundary and initial conditions provides a good fit between simulated and measured tracer profiles, but requires effective diffusion coefficients (De ) in the Ordovician limestones that are lower than the average values obtained in laboratory measurements by as much as a factor of 10. The base-case values are still within the overall range of measured values. At the formation scale, relatively low in-situ De values are attributed to the intermittent occurrence of gas and liquid hydrocarbons in the Ordovician carbonate units and the effect of lithostatic confining pressure; neither of which are accounted for in the laboratory measurements but both would have the effect of decreasing De for aqueous solutes. The numerical model is most sensitive to the formation-specific De values and geologically reasonable variations in initial and boundary conditions have a relatively small influence on the results. Additional simulations indicate that hydraulic disturbance and ingress of meltwater during the Pleistocene can explain second-order features observed in the measured tracer profiles. This work provides supporting evidence for previous research indicating that the porewater residence time in the Ordovician sequence approaches the age of the rocks.


      PubDate: 2015-04-06T07:22:26Z
       
  • Biogenic precipitation of manganese oxides and enrichment of heavy metals
           at acidic soil pH
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Sathish Mayanna , Caroline L. Peacock , Franziska Schäffner , Anja Grawunder , Dirk Merten , Erika Kothe , Georg Büchel
      Natural Mn oxides are largely biogenic in origin, formed via the microbial oxidation of Mn(II). These minerals are extremely efficient scavengers of heavy metals, yet to date microbial Mn oxide precipitation and subsequent heavy metal sorption have received little attention in mining-impacted environments, where heavy metal concentrations are elevated but (bio)geochemical conditions are typically unfavourable for both abiotic and biogenic Mn oxide precipitation, featuring acidic pH and low organic carbon contents. Here we investigate the formation of Mn oxide (bio)geochemical barrier layers, and the immobilisation of heavy metals in these layers, in soil profiles from a former uranium mining site in Ronneburg, Germany. Detailed soil profiling shows the site has an acidic soil pH that varies from 4.7 to 5.1 and Eh values from 640 to 660mV. Using synchrotron X-ray diffraction and X-ray absorption spectroscopy, together with scanning electron microscopy and electron microprobe analysis, we find that the dominant Mn oxide present in the Mn oxide layers is a poorly crystalline hexagonal birnessite, akin to synthetic δ-MnO2, covering and cementing quartz grains. Using phylogenetic analysis based on 16S rDNA, we identify and characterise six strains of manganese oxidising bacteria (MOB) from the acidic Mn oxide layers which we subsequently culture to produce poorly crystalline hexagonal birnessite akin to that found at the study site. Specifically, we identify three Gram-positive spore-forming firmicutes affiliated to Bacillus safensis, Bacillus altitudinis and Brevibacillus reuszeri, which are able to oxidise Mn after initiating spore formation, two Gram-positive actinobacteria belonging to the genera Arthrobacter and Frondihabitans, and one Gram-negative proteobacteria belonging to the genus Sphingomonas. Geochemical thermodynamic speciation modelling indicates that the abiotic precipitation of Mn oxides in the Mn oxide layers is unfavourable and we suggest that the Mn oxides in the (bio)geochemical barriers at our study site are biogenically precipitated in an acidic soil environment. To our knowledge, this is the first report to identify the above six bacterial strains, and specifically identify spore-forming bacteria, as MOB in an acidic soil environment. We find that the poorly crystalline hexagonal birnessite precipitated in the Mn oxide layers efficiently immobilises Ba, Ni, Co, Cd, Zn and Ce, and as such we find that MOB and biogenically precipitated Mn oxides can exert a strong control on the fate and mobility of metals in mining-impacted environments.


      PubDate: 2015-04-06T07:22:26Z
       
  • Oxygen isotopes in authigenic quartz from massive salt deposits
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): S. Halas , Ana-Voica Bojar , T.M. Peryt
      We describe a new geochemical tool that could provide temperatures of ancient basins in which massive evaporites were deposited: the oxygen isotope composition of fine crystalline quartz found in large halite bodies. Such quartz is well preserved from post-depositional alterations and it can be relatively easily separated. For the purpose of this study, five halite samples were selected from four various evaporite basins, spanning in age from the Early Cambrian to the Late Jurassic. The obtained isotope temperatures refer to locations where evaporites were deposited, i.e. in subtropical zones of the Earth, as it may be estimated from continental distribution during Phanerozoic times. Reasonable temperatures are obtained, with an assumption for the δ18O of brines ranging from −1‰ to 0‰ during halite deposition. The assumption of higher δ18O values leads to unrealistic temperatures, higher than the homogenization temperatures of fluid inclusions in halite.


      PubDate: 2015-04-06T07:22:26Z
       
  • U–Th–Pb zircon geochronology by ID-TIMS, SIMS, and laser
           ablation ICP-MS: Recipes, interpretations, and opportunities
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): U. Schaltegger , A.K. Schmitt , M.S.A. Horstwood
      The chronologic record encoded in accessory minerals, based on the radioactive decay of U and Th, is indispensable to extract quantitative process rates over timescales encompassing Earth's evolution from the Hadean to the Holocene, and extending from terrestrial to extra-terrestrial realms. We have essentially three different U–Pb dating tools at hand, a high-precision, whole-grain bulk technique (isotope-dilution thermal ionization mass spectrometry, ID-TIMS), and two high-spatial resolution but less precise in-situ techniques (secondary ion mass spectrometry, SIMS, and laser ablation inductively-coupled plasma mass spectrometry, LA-ICP-MS), all of which are predominantly applied to the mineral zircon. All three have reached a technological and methodological maturity in data quality and quantity, but interpretational differences, which are often common (albeit at different temporal and spatial scales) to all isotopic dating techniques, remain largely unresolved. The choice to use one of these techniques should be governed by the scientific question posed, such as (1) the duration of the geological process to be resolved; (2) the size and abundance of the material to be analyzed; (3) the complexity of the sample material and of the geological history to be resolved; and (4) the number of dates needed to address the question. Our compilation demonstrates that, ultimately, the highest confidence geochronological data will not only result from the optimal choice of appropriate analysis technique and the accurate treatment of analytical and interpretational complexities, but also require comprehensive sample characterization that employs the full gamut of textural (e.g., cathodoluminescence, charge contrast imaging, electron backscatter diffraction) and compositional (e.g., trace element, stable and radiogenic isotope) analysis.


      PubDate: 2015-04-06T07:22:26Z
       
  • 40Ar/39Ar ages and residual volatile contents in degassed subaerial and
           subglacial glassy volcanic rocks from Iceland
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): P.L. Clay , H. Busemann , S.C. Sherlock , T.L. Barry , S.P. Kelley , D.W. McGarvie
      Major volatile contents (H2O, CO2, F, Cl, and S) and 40Ar/39Ar ages have been determined in variably degassed rhyolite obsidians from Pleistocene–Holocene aged subaerial and subglacial eruption environments from the Torfajökull volcanic center and the monogenetic volcano at Prestahnúkur (Iceland). Icelandic subglacial rhyolites preserve residual H2O contents of 0.08–0.69wt.%, undetectable CO2, 840–1780ppm F, 430–2000ppm Cl and 6–45ppm S. Most subglacial obsidians have degassed volatile signatures at the time of their eruption under ice. One eruption (Bláhnúkur, Torfajökull) showed H2O contents which exceed those expected for quenching at atmospheric pressures (up to 0.69wt.% H2O) and are consistent with eruption at ~40kbar of pressure or equivalent to under ~450m of ice. Altered and microcrystalline groundmass in some subglacial rhyolites yield variable volatile contents that are likely the result of micro-scale variability and the presence of alteration products. Two subaerial rhyolite obsidians gave low volatile contents consistent with quenching at the surface. New 40Ar/39Ar ages were determined on the subglacially erupted rhyolites and provide broad constraints on the timing and frequency of glacio-volcanism during the Pleistocene. Three subglacial eruptions at Torfajökull yielded ages of 108±22ka (Bláhnúkur), 255±20ka (Hábarmur) and 236±7ka (Háskerðingur) which fit into a range of previously determined ages at Torfajökull of 67–384ka. A new age of 132±19ka was determined for the monogenetic subglacial eruption at Prestahnúkur. Holocene-aged subaerial volcanic glass produced no ages due to large amounts of air-derived 40Ar (84–93%). Preliminary apparent 40Ar/39Ar ages from the limited feldspar phenocrysts available in these crystal-poor samples yield erroneous apparent plateau ages far older than eruption ages suggesting a homogenously distributed excess Ar component. Disturbances to the 40Ar/39Ar geochronometer are correlated with a heterogeneous distribution of volatiles in glass and linked to hydration (e.g., Kaldaklofsfjöll), mild alteration (e.g., Hábarmur) and alteration and/or recrystallization of glass (e.g., Bláhnúkur). Our observations indicate however, that unaltered, subglacially erupted glasses that are partially to wholly degassed are promising materials for the application of the 40Ar/39Ar geochronometer. The precision to which we can date these young glassy samples currently limits the use of 40Ar/39Ar geochronometer as a tool for understanding the exact timing and duration of glaciovolcanism in the recent past.


      PubDate: 2015-04-06T07:22:26Z
       
  • Changes in porosity, permeability, water retention curve and reactive
           surface area during carbonate rock dissolution
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Tobias S. Rötting , Linda Luquot , Jesus Carrera , Diosenia J. Casalinuovo
      Chemical reactions between fluids and the solid phase change porous media by dissolution and precipitation processes that modify the pore space geometry and associated hydraulic and chemical properties (saturated and relative permeability, porosity, capillary pressure curve, pore size distribution, reactive surface area). Knowledge of these properties and how they evolve with time is important for understanding and modelling multi-phase flow or reactive transport. Most published studies for reservoir rocks concentrate on the changes of porosity and saturated liquid permeability, but few data are available on the other parameters, and even less on their evolution during interaction with acidic solutions. We investigate the joint evolution of transport parameters of sedimentary carbonate rocks upon reaction with dilute acid (HCl). We first characterize the initial permeability, porosity and pore size distribution. We then attack the rock samples by permeation with HCl (pH3.5 or 4.1) and characterize them again after partial dissolution. Several dissolution-characterization cycles were performed on each sample in order to study the evolution and interaction of the different parameters. In two coarse calcite cores C1 (dissolved at pH3.5) and C2 (pH4.1) with high initial permeability, significant dissolution occurred only in pores with diameters greater than 0.022mm. Permeability increased very little even though porosities increased by 2.6–5.8%. In fine-grained dolomite cores, dissolution affected a broader range of pore sizes, down to 0.001mm in D1 (pH3.5), and down to 0.0066mm in D2 (pH4.1). A wormhole broke through in D1 after percolation of only 5000 pore volumes of acid. Effective reactive surface areas decreased by a factor of 240–250 in the cores dissolved at pH3.5, but only by a factor of 2–25 in the cores dissolved at pH4.1. A simple 1-D model was unable to simulate the evolution of the reactive surface during wormhole formation. Effective reactive surface area correlated negatively with permeability, and was consistent with the geometric area of the pores that carried most flux, which was calculated from the retention curves.
      Graphical abstract image

      PubDate: 2015-04-06T07:22:26Z
       
  • Kinetics of calcite dissolution in CO2-saturated water at temperatures
           between (323 and 373)K and pressures up to 13.8MPa
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Cheng Peng , John P. Crawshaw , Geoffrey C. Maitland , J.P. Martin Trusler
      We report measurements of the calcite dissolution rate in CO2-saturated water at pressures ranging from (6.0 to 13.8)MPa and temperatures from (323 to 373)K. The rate of calcite dissolution in HCl(aq) at temperatures from (298 to 353)K was also measured at ambient pressure with pH between 2.0 and 3.3. A specially-designed batch reactor system, implementing a rotating disc technique, was used to obtain the dissolution rate at the solid/liquid interface of a single crystal, free of mass transfer effects. We used vertical scanning interferometry to examine the texture of the calcite surface produced by the experiment and the results suggested that at far-from-equilibrium conditions, the measured calcite dissolution rate was independent of the initial defect density due to the development of a dynamic dissolution pattern which became steady-state shortly after the onset of dissolution. The results of this study indicate that the calcite dissolution rate under surface-reaction-controlled conditions increases with the increase of temperature from (323 to 373)K and CO2 partial pressure from (6.0 to 13.8)MPa. Fitting the conventional first order transition state kinetic model to the observed rate suggested that, although sufficient to describe calcite dissolution in CO2-free HCl(aq), this model clearly underestimate the calcite dissolution rate in the (CO2 +H2O) system over the range of conditions studied. A kinetic model incorporating both pH and the activity of CO2(aq) has been developed to represent the dissolution rates found in this study. We report correlations for the corresponding reaction rate coefficients based on the Arrhenius equation and compare the apparent activation energies with values from the literature. The results of this study should facilitate more rigorous modelling of mineral dissolution in deep saline aquifers used for CO2 storage.


      PubDate: 2015-04-06T07:22:26Z
       
  • In-situ iron isotope analyses of pyrites from 3.5 to 3.2Ga sedimentary
           rocks of the Barberton Greenstone Belt, Kaapvaal Craton
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Kazumi Yoshiya , Yusuke Sawaki , Takazo Shibuya , Shinji Yamamoto , Tsuyoshi Komiya , Takafumi Hirata , Shigenori Maruyama
      The Barberton Greenstone Belt (BGB), South Africa, consists of volcano-sedimentary and sedimentary successions deposited between 3.5 and 3.2Ga, which are subdivided into three groups: the Onverwacht, Fig Tree, and Moodies. After deposition, the Barberton Greenstone Belt underwent relatively low-grade tectonothermal events, enabling estimation of surface environmental events and biological evolution in the Middle Archean. Isotopic studies suggest the activity of methanogen, sulfate-reducing bacteria, and photosynthetic bacteria at 3.4Ga. Microbial dissimilatory iron reduction (DIR) is also considered one of the earliest expressions of metabolism on Earth, but the isotopic signal of microbial DIR is still uncertain in the Archean. We performed in-situ iron isotope analyses of individual pyrites in sedimentary rocks from the BGB, using a femtosecond laser ablation multi-collector ICP-MS technique (fs-LA-MC-ICP-MS) to find isotopic evidence for the microbial activity. We obtained a large variation in δ56Fe values, ranging from −1.84 to +3.79‰. Most of the δ56Fe values of pyrites from the Hooggenoeg Complex show positive values, whereas those from the Noisy Complex and subsequent sedimentary sequences show a wide variation from negative to positive δ56Fe values. One of the main differences between these complexes is their depositional depth. The Hooggenoeg Complex was probably deposited in a deep-ocean environment, whereas the Noisy Complex was in shallow water. The negative δ56Fe values of pyrite grains in the Noisy Complex, Onverwacht Group indicate the occurrence of partial reduction caused by microbial DIR in a Middle Archean shallow sea.


      PubDate: 2015-04-06T07:22:26Z
       
  • A new analytical approach to determining Mo and Re speciation in sulfidic
           waters
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Trent P. Vorlicek , Anthony Chappaz , Laura M. Groskreutz , Natalie Young , Timothy W. Lyons
      The distinct reactivities of Mo and Re in oxic versus anoxic waters make these elements ideally suited for use as redox proxies. However, their full exploitation as geochemical tracers requires that their chemical transformations in sulfidic water be well understood. While thermodynamic data have been used to predict Mo and Re speciation within sulfidic waters, these predictions remain unsubstantiated because effective methodologies for separating the thiomolybdate (MoVIOxS4−x 2−) and thioperrhenate (ReVIIOxS4−x −) anions within natural settings have yet to be developed. Anion exchange chromatography (AEC) is often employed to provide separation of environmentally important anions. For example, thiometalates of As and Sb have been quantified in sulfidic geothermal waters using AEC coupled with inductively coupled plasma mass spectrometry (ICP-MS). Unfortunately, AEC methods are incapable of separating the thiomolybdate or thioperrhenate anions due to the extreme retention these thiometalates experience on AEC columns. Because reverse phase ion pair chromatography (RP-IPC) offers greatly diminished retention times, we have developed RP-IPC methods that are capable of separating all stable thiomolybdates and thioperrhenates within 40min. An isocratic method provides effective separation of the thiomolybdates. Thioperrhenates and mixtures of the thiomolybdates and thioperrhenates require gradient methods. Addition of p-cyanophenol (p-CP) to the eluent markedly reduces thiometalate retention and facilitates their separation. Our efforts could lead in the near future to coupling RP-IPC with ICP-MS or multi-collector ICP-MS for characterizing Mo and Re speciation in natural sulfidic waters as well as potential fractionation among Mo and Re isotopes during speciation changes.
      Graphical abstract image

      PubDate: 2015-04-06T07:22:26Z
       
  • High-resolution line-scan analysis of resin-embedded sediments using laser
           ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Rick Hennekam , Tom Jilbert , Paul R.D. Mason , Gert J. de Lange , Gert-Jan Reichart
      Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) line-scanning is a promising technique for producing high-resolution (μm-scale) geochemical records on resin-embedded sediments. However, this approach has not yet been thoroughly tested on sediment samples of known elemental compositions. Here, we address this through the analysis of resin-embedded quartz, calcite, and clay (montmorillonite) sediments spiked with Al, V, Mo, and Ba across a range of concentrations. LA-ICP-MS spot analyses on these samples were compared to data independently obtained by conventional techniques: solution nebulization-ICP-optical emission spectroscopy (SN-ICP-OES), SN-ICP-MS, and X-Ray Fluorescence on fused glass beads. Data were reported as (log-)ratios of the analyte elements normalized to Ca, Si, and Al for calcite, quartz and clay respectively to correct for varying ablation yields. Our LA-ICP-MS data demonstrate close agreement to within 6% of the reference values determined by the conventional techniques, with high correlation coefficients (R2 ≈1.0) across the full range of concentrations, and high precision (<3% relative standard deviations after three repeated analyses). Barium was hosted in both aluminosilicates and carbonates in the clay matrix, giving different yields (elemental fractionation), and leading to variable accuracy (mean deviation of 15%). A selection of the spiked sediments was used to produce artificial laminated sediments to further test the effects of line scanning on LA-ICP-MS accuracy and precision in resin-embedded samples. It appears that LA-ICP-MS line-scan analyses have good accuracy (deviations from the reference values generally <5%) and high precision (relative standard deviations of repeated analyses on the same sample generally <5%) for our target elements. Moreover, the LA-ICP-MS line-scanning closely records the alternating geochemical profiles of our artificial laminations. However, calcium showed a clear tailing signal at the transition from a calcite to a quartz layer, indicating that geochemical signals can be smeared at transitions to sediment layers devoid of analyte element(s). We also analyzed resin-embedded natural sediments for comparison of LA-ICP-MS line-scan data to parallel sub-samples measured by SN-ICP-OES and SN-ICP-MS. In general, the results from LA-ICP-MS line-scans of natural sediments correspond well with reference values and with high reproducibility, corroborating the results of the artificial laminations. The results for Ca, V, Mn, Sr, Mo, Ba, and U (in log-ratios to Al) suggest that these elements can be analyzed semi-quantitatively, showing correlation coefficients (R2) ranging from 0.59 to 0.85 compared to conventional analytical techniques. However, we show that using non-matrix matched calibration standards such as NIST SRM 610 may induce deviations from the LA-ICP-MS values to the reference values. Consequently, we recommend a simple protocol for further data correction in which binned-mean LA-ICP-MS values are calibrated to a parallel series of discrete samples analyzed by these conventional techniques.


      PubDate: 2015-04-06T07:22:26Z
       
  • Microbe–mineral interactions: The impact of surface attachment on
           mineral weathering and element selectivity by microorganisms
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Engy Ahmed , Sara J.M. Holmström
      One of the major gaps within the field of biogeochemistry is the lack of a detailed and deep understanding of the mechanism behind the microbial inducement of mineral dissolution. The association of microorganisms with the mineral surfaces is an important issue for understanding processes like mineral weathering, biomineralization, bioremediation and biofouling. The present study aims to investigate the performance of attached and unattached soil fungal and bacterial species in biotite weathering and in the selectivity of elements from biotite. Sterilized microplate devices were filled with biotite (>2mm) followed by an iron limited liquid growth medium and were inoculated separately with six different microbial species isolated from podzol soil: Erwinia amylovora, Pseudomonas stutzeri, Pseudomonas mendocina, Streptomyces pilosus, Neurospora crassa and Penicillium melinii. The experiment was designed in two set-ups: 1) attached form, in which the microorganisms were inoculated directly to the biotite surface, and 2) unattached form, in which 0.4μm PET track etched devices were used to separate the microbial cells from the biotite surface. Our findings indicate that the surface attached microorganisms led to a greater dissolution of elements from biotite than the unattached microorganisms that was evidenced by 1) higher dissolution of Fe, Al and Si, 2) greater decrease in pH of the liquid growth medium, and 3) relatively higher production of siderophores. Furthermore, there was no significant difference in the capability of element selectivity between the attached and unattached microbial forms. The biotite dissolution was promoted initially by complexation processes and later by acidification processes for most of the attached and unattached microorganisms. Thus, we conclude that despite the mineral dissolution induced by microbial attachment on the mineral surface, the element composition of the biotite and nutritional need of the microorganisms were the main factors affecting the element selectivity.


      PubDate: 2015-04-06T07:22:26Z
       
  • Numerical simulation of porosity and permeability evolution of Mount Simon
           sandstone under geological carbon sequestration conditions
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Liwei Zhang , Yee Soong , Robert Dilmore , Christina Lopano
      A numerical model was developed with the use of reactive transport code CrunchFlow to estimate porosity, permeability and mineral composition changes of Mount Simon sandstone under typical geological carbon sequestration conditions (P =23.8MPa and T =85°C). The model predicted a permeability decrease from 1.60mD to 1.02mD for the Mount Simon sandstone sample in a static batch reactor after 180days of exposure to CO2-saturated brine, which is consistent with measured permeability results. Model-predicted solution chemistry results were also consistent with laboratory-measured solution chemistry data. SiO2 (am) was the primary mineral that causes permeability decrease, followed by kaolinite. Both SiO2 (am) formation and kaolinite formation were attributed to the dissolution of quartz and feldspar. This study shows that the formation of SiO2 (am) and kaolinite in the pore space of host rock is possible under typical CO2 sequestration conditions. SiO2 (am) and kaolinite precipitation at the CO2 plume extent could reduce the permeability of host rock and improve lateral containment of free-phase CO2, contributing to overall security of CO2 storage.


      PubDate: 2015-04-06T07:22:26Z
       
  • Bromine speciation in hydrous silicate melts at high pressure
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): B. Cochain , C. Sanloup , C. de Grouchy , C. Crépisson , H. Bureau , C. Leroy , I. Kantor , T. Irifune
      Br speciation in hydrous silicate melts at high pressure has been investigated up to 7.6GPa using X-ray absorption spectroscopy (XAS) at the Br K-edge in a Paris–Edinburgh press. Br in silicate melts is surrounded by an average of 6 Na cations, a number slightly increasing with pressure (5.8 to 6.6), with a Br–Na distance increasing from 3.49 to 3.72Å. Two oxygens, either from a water, an –OH molecule or from the tetrahedral silicate network, with an average Br–O distance of 1.80Å, form the closest coordination shell around Br ions. The persistence of an alkali shell around Br, in a structure similar to crystalline NaBr, throughout the pressure range investigated shows that Br can be retained in the melt structure at relatively high pressure and supports the idea of its deep recycling. Finally, our results confirm that Br could be efficiently degassed with water at low pressures and that Br may also have been efficiently degassed along with water during the early stages of an oxidized magma ocean.


      PubDate: 2015-04-06T07:22:26Z
       
  • Limited Zn and Ni mobility during simulated iron formation diagenesis
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Leslie J. Robbins , Elizabeth D. Swanner , Stefan V. Lalonde , Merle Eickhoff , Megan L. Paranich , Christopher T. Reinhard , Caroline L. Peacock , Andreas Kappler , Kurt O. Konhauser
      Iron formations (IF) are iron- and silica-rich chemical precipitates that were deposited during the Precambrian. Several recent studies have demonstrated how the trace metal abundances in IF can be used as proxies for the bioavailability of trace metals in ancient seawater; with the ultimate goal being to understand first-order controls on the composition of the ancient biosphere. However, the utility of IF as proxies depends on the immobilization of trace metals during diagenesis. Here, we assess the mobility of Zn and Ni from ferric oxyhydroxides (ferrihydrite) in the absence and presence of organic matter (glucose) during simulated diagenesis (170°C, 1.2kbar); similar to what some Precambrian IF experienced. Quantitative concentration data, coupled with X-ray diffraction analysis and electron microprobe element mapping, demonstrate that both metals are relatively immobile during simulated diagenesis. Additionally, the mechanism for initial Ni sorption is examined using X-ray adsorption spectroscopy. For the initial sorption of trace elements in abiotic ferrihydrite experiments, 93.38% Zn and 65.95% Ni were initially sorbed. In experiments utilizing biogenic ferrihydrite, 97.03% of Zn and 93.38% of Ni were initially sorbed. Following the diagenetic capsule treatments, more than 99% of Zn and more than 91.9% of Ni were retained under the varied conditions considered here. Capsule experiments suggest the strong retention of Zn and Ni following the diagenesis of either abiotic or biogenic ferrihydrite. Overall, our results indicate that paleomarine Zn and Ni concentrations are likely to be faithfully recorded in well-preserved IF deposits.


      PubDate: 2015-04-06T07:22:26Z
       
  • Experimental constraints on kinetic and equilibrium silicon isotope
           fractionation during the formation of non-biogenic chert deposits
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Desiree L. Roerdink , Sander H.J.M. van den Boorn , Sonja Geilert , Pieter Z. Vroon , Manfred J. van Bergen
      Silicon isotopic compositions (δ30Si) of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments, but interpretations are hampered by the lack of experimentally validated fractionation factors. Here, we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch-reactors at low temperature (10–35°C) and near-neutral pH (7.5–8.5), as analogue for non-biogenic chert formation. Instantaneous fractionation factors, derived from δ30Si-values of the total dissolved (SiTD) silica and mass balance computations with αinst =(δ30Sippt +1000)/(δ30SiTD +1000), decrease with progressive precipitation and reduced reaction rates. This suggests that silica deposition in the batch-reactors is kinetically-dominated at the start of the experiments but approaches a metastable equilibrium after ca. 400hours. Modelled kinetic fractionation factors range from 0.9965 at 10°C, to 0.9976 at 20°C and 0.9993 at 35°C and pH8.5, whereas equilibrium isotope effects are smaller and range from 0.9995 at 10°C, to 1.000 at 20°C and 1.0005 at 35°C. Our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated, implying that the kinetic conditions of non-biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks, with particular relevance for those preserved in the Archean chert record.


      PubDate: 2015-04-06T07:22:26Z
       
  • The effect of particle density on the sources, distribution, and
           degradation of sedimentary organic carbon in the Changjiang Estuary and
           adjacent shelf
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Jinpeng Wang , Peng Yao , Thomas S. Bianchi , Dong Li , Bin Zhao , Xingqian Cui , Huihui Pan , Tingting Zhang , Zhigang Yu
      Hydrodynamic sorting processes have a significant influence on the dispersal of sedimentary organic carbon (OC) in large-river delta-front estuaries (LDEs). Density fractionation was conducted on 6 sediments from the Changjiang LDE to investigate the sources, distribution, and degradation of OC among density fractions. Mass, elemental composition (C and N) and stable carbon isotope ratios, lignin, and specific surface area (SSA) of density fractions and bulk sediments were determined. Mass was concentrated in high density fractions (2.0–2.5gcm−3 and >2.5gcm−3) (up to 67.8%) while OC was dominated in 2.0–2.5gcm−3 fraction (up to 77.3%) due to extremely low OC in the highest density fraction (%OC<0.10%). The less dense fractions (<1.6gcm−3 and 1.6–2.0gcm−3) contained the highest OC (up to 21.4%) and lignin concentrations (up to 3.17mgg−1 or 2.85mg/100mg OC), and thus were most depleted δ13C (down to −25.2‰). A three end-member mixing model using δ13C and lignin content (Λ8) as source markers and Monte-Carlo simulation, to distinguish the relative importance of terrestrial vascular plant, soil, and marine plankton OC sources, showed that less dense fractions were dominated by terrestrial OC (the sum of soil- and vascular plant-derived OC), whereas marine OC was the predominant OC source in high density fractions. Distributions of OC from different sources in density fractions and bulk sediments both indicated a differential transport and deposition of OC along the coast, showing a gradual decreasing of terrestrial OC and addition of marine OC. Ratios of vanillic acid to vanillin (Ad/Al)v, 3,5-Bd to total V phenols (3,5-Bd/V) and total p-hydroxy phenols to sum of vanillyl and syringyl (P/(S+V)) among lignin oxidation products increased with increasing particle density suggesting selective degradation of lignin among the different density fractions. Distinctively low OC/SSA loadings (<0.40mgm−2) were observed in the highest density fraction and the bulk sediments, indicating low preservation of OC in these sediments, consistent with the results of lignin decay parameters.


      PubDate: 2015-04-06T07:22:26Z
       
  • Nickel isotope fractionation during tropical weathering of ultramafic
           rocks
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): G. Ratié , D. Jouvin , J. Garnier , O. Rouxel , S. Miska , E. Guimarães , L. Cruz Vieira , Y. Sivry , I. Zelano , E. Montarges‐Pelletier , F. Thil , C. Quantin
      Although Ni isotopes have been shown to be significantly fractionated in terrestrial samples, their use in continental environmental studies has not yet been evaluated. The present study focuses on an ultramafic (UM) massif (Barro Alto, Goiás, Brazil) because such areas are naturally rich in Ni. We present developed lateritic weathering profiles. The goal of the study is to evaluate the potential of using Ni isotopes in environmental continental studies by combining its isotopic signature with mineralogy, in order to better understand the geochemical cycling of Ni in UM settings during weathering. As such, Ni isotope values were measured in samples from the Barro Alto UM complex in the main stages of the lateritic weathering profile of UM rocks, including bedrock, ores (saprolitic and lateritic samples) and soil. The mineralogical composition of the samples, with a focus on the different Ni-bearing minerals, was also determined to decipher the potential links between isotopic fractionation and weathering dynamics. Isotopic signatures (δ60Ni) from the natural Ni geochemical cycle include: bedrock samples (δ60Ni=0.28±0.08‰), ore samples (saprolitic and lateritic, δ60Ni from −0.60 to 0.30‰) and soil samples (δ60Ni from −0.19 to −0.02‰). An overall trend of heavier isotope depletion was observed in the solid phase during weathering (Δ60NiSoil–Bedrock =−0.47‰). The mineralogical results were consistent with the literature and showed that the mineralogy of the lateritic part and soil was dominated by Fe-oxides, whereas clay minerals were the primary Ni phase scavengers in the saprolitic part of the profile. Thus, the formation of Ni-bearing clay minerals and Fe-oxides appeared to lead to depletion in heavier isotopes, which indicates preferential export of heavy isotopes in the dissolved phase. This result is consistent with isotopic signatures measured in the exchangeable pool of the solid phase (Δ60Niexch-total up to 0.47‰), and Ni isotopes appear to be a promising tracer to better understand the biogeochemical Ni cycling on the Earth's surface.


      PubDate: 2015-04-06T07:22:26Z
       
  • Redox evolution of silicic magmas: Insights from XANES measurements of Ce
           valence in Bishop Tuff zircons
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Dustin Trail , Nicholas D. Tailby , Antonio Lanzirotti , Matthew Newville , Jay B. Thomas , E. Bruce Watson
      The potential for zircon to record continuous and evolving magmatic redox conditions is investigated by quantifying Ce valence in natural and synthetic crystals by X-ray Absorption Near Edge Structure (XANES). Valence was determined at high spatial resolution (2×4μm) by analysis of the Ce L 3 edge for synthetic zircons and crystals from the Bishop Tuff Ig2E sequence; analyses included both core-to-rim and cross-sector measurements. Core-to-rim zonation among natural grains reveals a systematic increase in Ce4+/ΣCe, with core regions that range from ~0.4 to 0.6 Ce4+/ΣCe (i.e., ~40–60% Ce4+), while zircon rims range from ~0.7 to 1.0 Ce4+/ΣCe (i.e., ~70–100% Ce4+). Repeat analysis on an individual point suggests, on average, a Ce4+/ΣCe reproducibility at the 5% level or less. Changes in spectral features with grain orientation were also investigated by rotating and analyzing synthetic zircons every 45o. This resulted in changes to the calculated Ce valence of <5%, which is much smaller than the range observed in natural samples. The core-to-rim increase in Ce4+/ΣCe of Bishop Tuff samples may indicate a continuous crystal-melt evolution to more oxidizing conditions prior to eruption, but this cannot be uniquely decoupled from other effects that may influence Ce valence in zircon, which potentially include temperature changes or kinetic processes related to the mineral growth surface. Cathodoluminescence imaging couples with XANES spectra for Bishop Tuff samples reveal that different sectors yield notably different Ce4+/ΣCe, implying anisotropic decoupling of Ce3+ and Ce4+ in the zircon near-surface during crystallization. Broadly correlative (albeit with lower spatial resolution) Ti-thermometry and light rare earth element LA-ICP-MS data are also reported for zircon grains; there is no correlation between measured Ce anomalies and Ce4+/ΣCe. Cerium valence measurements of zircon may be able to constrain magma redox evolution with time, without relying on the nearly ubiquitous low concentrations of La and Pr, which are classically used to calculate Ce anomalies.


      PubDate: 2015-04-06T07:22:26Z
       
  • Cave ventilation and rainfall signals in dripwater in a monsoonal setting
           – a monitoring study from NE India
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Sebastian F.M. Breitenbach , Franziska A. Lechleitner , Hanno Meyer , Gregory Diengdoh , David Mattey , Norbert Marwan
      Detailed monitoring of subterranean microclimatic and hydrological conditions can delineate factors influencing speleothem-based climate proxy data and helps in their interpretation. Multi-annual monitoring of water stable isotopes, air temperature, relative humidity, drip rates and P CO2 in surface, soil and cave air gives detailed insight into dripwater isotopes, temperature and ventilation dynamics in Mawmluh Cave, NE India. Water isotopes vary seasonally in response to monsoonal rainfall. Most negative values are observed during late Indian Summer Monsoon (ISM), with a less than one-month lag between ISM rainfall and drip response. Two dry season and two less-well distinguishable wet season dynamic ventilation regimes are identified in Mawmluh Cave. Cave air temperatures higher than surface air result in chimney ventilation during dry season nights. Dry season days show reduced ventilation due to cool cave air relative to surface air and cold-air lake development. Both, high water flow and cooler-than-surface cave air temperatures result in air inflow during wet season nights. Wet season daytime ventilation is governed by river flow, but is prone to stagnation and development of cold air lakes. CO2 monitoring indicates that P CO2 levels vary at diurnal to annual scale. Mawmluh Cave seems to act as CO2 sink during part of the dry season. While very likely, additional data is needed to establish whether wet season cave air CO2 levels rise above atmospheric values. Drip behavior is highly nonlinear, related to effective recharge dynamics, and further complicated by human influence on the epikarst aquifer.


      PubDate: 2015-04-06T07:22:26Z
       
  • Trace element composition of freshwater pearl mussels Margaritifera spp.
           across Eurasia: Testing the effect of species and geographic location
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): I.N. Bolotov , O.S. Pokrovsky , Y. Auda , J.V. Bespalaya , I.V. Vikhrev , M.Y. Gofarov , A.A. Lyubas , J. Viers , C. Zouiten
      To reveal the geographical and inter-species variability of the major and trace element (TE) compositions of freshwater mussel shells across Eurasia, we used ICP MS after acid digestion to analyze 50 samples of shells from five species of the genus Margaritifera spp. (Bivalvia: Margaritiferidae) collected in 20 minor rivers located in NW Russia, Sakhalin, Amur basin, Kuril Islands, Kamchatka and Laos. The variations between replicates from the same site were smaller than the variations between samples from different localities or between different mollusk species. Using normed PCA, we observed significant biological and geographical controls of the trace element composition in freshwater shells, with five main species exhibiting distinct features of TE concentration corresponding to four major geographical locations. Four PCA factors explain 81% of the TE variability, which is closely linked to specific geographical location and weakly linked to the identity of the species. The first two factors (F1×F2) are the element concentration and its biological affinity. The other two factors (F3×F4) likely are the nutrient status of the river, corresponding to its proximity to wetlands or mountains, and the degree of the influence of volcanic/hydrothermal activity. The analysis of water samples collected during the active growth period in summer baseflow was used to quantify the distribution coefficients (Kd ) of trace elements between the aragonite shells and the river water. All species are significantly enriched in Mn relative to the river water, with distribution coefficients of up to 5. The other elements exhibit Kd values that were similar for the five species. This suggests that the obtained distribution coefficients may be universal constants reflecting uptake of each element by the organism from the river water and its intracellular transport and biocalcification processes. Taken together, the chemical composition data for pearl mussel shells may not only reflect the geographical locality and species identity but also provide insights into biochemical processes of element uptake in the form of biominerals.
      Graphical abstract image

      PubDate: 2015-04-06T07:22:26Z
       
  • Portrait of a reference material: Zircon production in the Middledale
           
    • Abstract: Publication date: 8 May 2015
      Source:Chemical Geology, Volume 402
      Author(s): Kieran A. Iles , Janet M. Hergt , Keith N. Sircombe , Jon D. Woodhead , Simon Bodorkos , Ian S. Williams
      The Middledale Gabbroic Diorite (MGD) in New South Wales, Australia, is host to the internationally distributed TEMORA 2 zircon reference material and its prototype, TEMORA 1. The original characterisation of the source of the reference zircons revealed that the more altered TEMORA 2 host rock contains an order of magnitude more zircon than the TEMORA 1 host, despite similar bulk-rock Zr concentrations. Although TEMORA 1 and 2 preserve the same U–Pb age, they are variable in oxygen isotope composition. In this study, petrographic observations in concert with bulk-rock and mineral geochemistry and zircon U–Pb geochronology have been applied to new samples of the MGD to investigate the link between alteration and zircon abundance. Trace element maps reveal that the products of late-stage, deuteric alteration (particularly actinolite after hornblende) are depleted in Zr, and many other trace elements, relative to the unaltered mafic, magmatic phases. It is posited that the conversion of hornblende to secondary amphiboles in the latter stages of magma solidification liberated Zr, permitting the crystallisation of additional zircon. New high resolution SIMS U–Pb determinations on four samples confirm the age homogeneity of the zircon across the pluton and reaffirm the value of TEMORA 2 as a valuable geochronological reference material. Zircon oxygen isotope data have been acquired for these same samples and the mean δ18O values encompass the accepted values for TEMORA 1 and 2. Likewise, the Hf-isotope determinations are similar to the accepted TEMORA 2 composition. Together with petrographic observations, these data reveal the TEMORA 2 zircon and its host to be broadly reflective of the relatively coarse-grained portions of the MGD, and that the isotopically less evolved compositions (i.e. the lower δ18O of TEMORA 1 and low δ18O, high 176Hf/177Hf of one sample from this study) are associated with a relatively fine-grained, marginal lithology. Given δ18O values greater than typical mantle-derived zircon and the broad correlation between O- and Hf-isotopic compositions, the data imply the pluton evolved by crustal contamination of a primitive magma.


      PubDate: 2015-04-06T07:22:26Z
       
  • Editorial Board
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401




      PubDate: 2015-04-06T07:22:26Z
       
  • The TICE event: Perturbation of carbon–nitrogen cycles during the
           mid-Tournaisian (Early Carboniferous) greenhouse–icehouse transition
           
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Le Yao , Wenkun Qie , Genming Luo , Jiangsi Liu , Thomas J. Algeo , Xiao Bai , Bo Yang , Xiangdong Wang
      Carbonate carbon (δ13Ccarb) and bulk nitrogen (δ15N) isotopic variation during the mid-Tournaisian was analyzed in the Malanbian and Long'an sections of South China. Their C-isotope profiles document a large positive excursion, herein termed the ‘mid-Tournaisian carbon isotope excursion’ (TICE), during the Siphonodella isosticha conodont Zone, although its magnitude differed between the two sections (>6‰ at Malanbian versus ~3‰ at Long'an). The TICE event coincided with sedimentologic and oxygen-isotopic evidence of climatic cooling and glaciation during the mid-Tournaisian. It was probably triggered by an increase in organic carbon burial rates linked to changes in global-ocean circulation. The study sections also document a large positive shift in δ15N, from 1.7‰ to 4.2‰ at Malanbian and from 1.5‰ to 3.8‰ at Long'an. The N-isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15N-enriched marine nitrate that lasted for the duration of the Late Paleozoic Ice Age. Its initiation coincided with TICE and thus may have been linked to ocean-circulation changes that resulted in intensified upwelling and an increase in water-column denitrification. The continuation of the N-isotope shift over millions of years may have been linked to glacio-eustatic fall and a long-term shift in the locus of denitrification from continental-shelf sediments to continent-margin oxygen-minimum zones. The TICE event thus marks the onset of sustained continental glaciation during the Late Paleozoic Ice Age.


      PubDate: 2015-04-06T07:22:26Z
       
  • Integrated O–Sr–Nd isotope constraints on the evolution of
           four important Fe–Ti oxide ore-bearing mafic–ultramafic
           intrusions in the Emeishan large igneous province, SW China
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Song-Yue Yu , Xie-Yan Song , Edward M. Ripley , Chusi Li , Lie-Meng Chen , Yu-Wei She , Yan Luan
      In the Emeishan large igneous province, SW China, there are many layered mafic–ultramafic intrusions such as the Baima, Hongge, Panzhihua and Taihe intrusions that host world-class Fe–Ti oxide ore deposits. Despite numerous studies, the origin of these deposits still remains elusive. This includes the role of crustal contamination, especially addition of external CO2 from carbonate country rocks during contact metamorphism, in triggering Fe–Ti oxide crystallization from high-Mg basaltic magma. To address this important issue, we have carried out an integrated O–Sr–Nd isotope study of these ore-bearing intrusions and the country rocks. Our results show that in these intrusions clinopyroxene is much less susceptible to fluid–mineral oxygen isotope exchange than coexisting plagioclase and Fe–Ti oxides, which is similar to other intrusions worldwide (e.g., Taylor, 1967; Gregory and Taylor, 1981). Our calculations based on the least exchanged clinopyroxene oxygen isotope data show that the mean δ18O values for the parental magmas of these intrusions are Baima=5.7‰, Panzhihua=6.1‰, Taihe=5.9‰. The estimated mean δ18O value for the parental magma of the Upper and Middle Zones of the Hongge intrusion is 6.2‰, which is similar to those for the parental magmas of the other intrusions (Baima, Panzhihua and Taihe). By contrast, the estimated mean δ18O value for the parental magma of the Lower Zone of the Hongge intrusion is higher (6.9‰). This difference, together with higher initial 87Sr/86Sr ratios (0.7057 to 0.7076) and lower εNdt values (−2.82 to −0.07) for this zone, can be attributed to higher degrees of contamination with siliceous crustal materials in this zone than elsewhere in this intrusion. Comparison of O–Sr–Nd isotope compositions between the intrusions and country rocks reveals that bulk assimilation of carbonate country rocks is negligible in all of these intrusions. Mixing calculations using the O–Sr–Nd isotope data are consistent with variable degrees of contamination with siliceous crustal materials in the intrusions: Panzhihua, <5%, Baima and Taihe, <10%, the Middle and Upper Zone of the Hongge intrusion, <10%, the Lower Zone of the Hongge intrusion, <15%. These percentages are maximum values and may be reduced if contamination was selective in nature, involving Sr- and Nd-bearing fluids or partial melts. Based on the oxygen isotope results, an iterative calculation with a CO2/magma mass ratio=1/1000 for each increment reveals that the Panzhihua magma reacted with <1wt.% of CO2 released from the footwall during contact metamorphism. This amount is not sufficient to increase the oxidation state of the magma to the level that Fe–Ti oxides would crystallize alone from the magma. Therefore, we conclude that external CO2 did not play a major role in the formation of the Fe–Ti oxide ores in the Panzhihua intrusion.


      PubDate: 2015-04-06T07:22:26Z
       
  • Formation of the Jurassic South China Large Granitic Province: Insights
           from the genesis of the Jiufeng pluton
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Hui-Qing Huang , Xian-Hua Li , Zheng-Xiang Li , Wu-Xian Li
      The magma source and thermal regime for the formation of Large Granitic Provinces (LGPs) have been the subjects of much debate. As mineralogical, geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution, the Jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the South China LGP. SIMS zircon U-Pb analyses of a biotite granite (158.9±1.3Ma), three two-mica granites (stage I: 159.7±1.2Ma, stage II: 158.6±1.2Ma and 156.8±1.2Ma) and a muscovite granodiorite (159.0±4.4Ma) of the Jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5.3m.y. Limited variations in whole-rock Nd isotope (εNd(t)=−12.6 to −9.5) and zircon Hf-O isotopes (εHf(t)=−12.0 to −6.3, δ18O=6.8‰−9.4‰) suggest that the Jiufeng pluton was mainly derived from melting of a common metasedimentary source, possibly with a minor basaltic contribution. We consider the geochemical variations of the Jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step-like transition from fluid-saturated to fluid-absent melting of the source. The muscovite granodiorite, with high Na2O (>3.80wt.%; K2O/Na2O=~1), is interpreted to have been produced by fluid-saturated melting at low temperature (~650°C). High P2O5 (0.09–0.17wt.%), zircon saturation temperature (TZr =769–816°C) and La/Yb ratios (8.4–55.8) of the stage II two-mica granite support its formation from high-temperature (>800°C) biotite-dehydration melting, whereas lower P2O5 (<0.02wt.%), TZr (685–742°C) and La/Yb (<3) of the stage I two-mica granite suggest its generation at lower temperature, likely by muscovite-dehydration melting. We propose that extensive emplacement of basaltic melts in the lower crust most likely drove the rapid increase of mid-crustal (~20km) temperature (~50°C/m.y.) and widespread crustal melting for the formation of the Jurassic South China LGP. Therefore, formation of the LGP signifies prominent crustal growth as well as crustal reworking in an intraplate setting and was likely a response to flat-slab delamination and foundering.


      PubDate: 2015-04-06T07:22:26Z
       
  • Iron availability as a dominant control on the primary composition and
           diagenetic overprint of organic-matter-rich rocks
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Nicolas Tribovillard , Ebraheem Hatem , Olivier Averbuch , Florent Barbecot , Viviane Bout-Roumazeilles , Alain Trentesaux
      Iron is known to stimulate surface ocean productivity, as well as intervene with bacterially-mediated processes of organic matter remineralization, during early diagenesis. In this paper, we examine the influence of iron supply on the geochemistry (trace metals, δ34S, organic matter) of sedimentary rocks deposited in a clastic-dominated marine ramp environment. To this end, we studied two Late Jurassic formations of the Boulonnais area (North-France). Both formations were deposited under quite similar conditions, but they differ in the reactive-iron supply they received. Only one of the two formations was affected by the particulate iron shuttle process. Our results indicate that 1)the iron shuttle may be recorded through concomitant enrichments in P, Mo, As and Sb; 2)a limited reactive-iron supply will allow the sulfurization of organic matter, even in a context of moderate productivity. Thus sulfurization can be a factor favoring a noticeable accumulation of organic matter: iron may thus be an important agent in the C cycle.


      PubDate: 2015-04-06T07:22:26Z
       
  • The solubility of xenotime-(Y) and other HREE phosphates (DyPO4, ErPO4 and
           YbPO4) in aqueous solutions from 100 to 250°C and psat
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Alexander P. Gysi , Anthony E. Williams-Jones , Daniel Harlov
      Xenotime (YPO4) is a heavy rare earth element (HREE) phosphate, the principal host to economic HREE mineralization and a common accessory mineral in igneous, metamorphic and sedimentary rocks. Many occurrences of xenotime-(Y) are of hydrothermal origin, and therefore, determination of its solubility in aqueous fluids may help to evaluate the mobility of HREE in crustal fluids. We have measured the solubility of the HREE phosphate end-members YPO4, ErPO4, DyPO4 and YbPO4 in aqueous HClO4–H3PO4 solutions at temperatures from 100 to 250°C and saturated water vapor pressure (p sat) corresponding to the reaction: HREEPO4 =REE3+ +PO4 3− (Ks0). The logarithm of the solubility products (logKs0) determined for these end-members, can be evaluated as a function of temperature (in K) using the equation logKs0 =A+BT+C/T+Dlog(T). Reliable fits to our data yields the following coefficients: YPO4, A=−876.93, B=−0.25127, C=2.327e+4, D=342.82; DyPO4, A=−1150.2, B=−0.31778, C=3.132e+4, D=450.32; ErPO4, A=−555.90, B=−0.17933, C=1.373e+4, D=217.42; and YbPO4, A=−419.76, B=−0.15215, C=8.941e+3, D=165.27. A comparison of our solubility products to those calculated with the available enthalpy, entropy and heat capacity values from calorimetric determinations, and the corresponding properties of the aqueous species (Y3+, Dy3+, Er3+, Yb3+, PO4 3−), shows that our solubility products for YPO4 and ErPO4 are orders of magnitude different from the calculated solubility products. By contrast, the calculated solubility products for YbPO4 are in good agreement with the measured values for the range of temperature considered in our experiments. There are no data available for the heat capacity of DyPO4, and consequently our experimental results provide the first data that permit the solubility of this HREE phosphate to be calculated for elevated temperature. We have developed a method that allows our data to be extrapolated to 25°C and have reconciled the measured calorimetric and solubility data. The results are used to recommend standard thermodynamic properties for the HREE phosphates at 25°C and 1bar.


      PubDate: 2015-04-06T07:22:26Z
       
  • Effects of melt percolation on iron isotopic variation in peridotites from
           Yangyuan, North China Craton
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Xin-Miao Zhao , Hong-Fu Zhang , Xiang-Kun Zhu , Bin Zhu , Hui-hui Cao
      The petrology, geochemistry and Fe isotopic composition of two suites of mantle xenoliths (magnesium-lherzolites and Fe-rich lherzolites) and their associated mineral separates from Yangyuan, in the Central Zone of the North China Craton, were measured in detail to investigate the effects of metasomatism on Fe isotope fractionation and mantle Fe isotopic heterogeneity. The magnesium-lherzolites (Mg#=89.8 to 90.9; TFeO=8.3 to 8.9wt.%) from Yangyuan are residual mantle lherzolites with no or only moderate metasomatism. They have a relatively limited Fe isotopic variation (δ57Fe=0.007 to 0.169‰) with an average of 0.083±0.044‰ (2SD/√n, n=8), similar to the range reported previously for typical mantle lherzolites, and they show no significant inter-mineral disequilibria. In contrast, the Fe-rich lherzolites examined in detail in this study are strongly metasomatized and are enriched in iron (Mg#=89.8 to 90.9; TFeO=11.2 to 16.8wt.%). These Fe-rich lherzolites exhibit extremely light Fe isotopic compositions, with δ57Fe ranging from −0.615 to −0.311‰, and disequilibrium Fe isotope fractionation occurs between most of their coexisting minerals (Δ57FeOpx–Ol =−0.616 to 0.522‰; Δ57FeCpx–Ol =−0.003 to 0.230‰; Δ57FeSp–Ol =0.191 to 0.522‰; Δ57FeSp–Opx =0.206 to 0.510‰; Δ57FeSp–Cpx =0.112 to 0.297‰; Δ57FePhl–Ol =0.097 to 0.450‰; and Δ57FeAmp–Ol =0.048 to 0.147‰). Overall, the large disequilibrium isotope fractionations in Fe-rich lherzolites likely results from the kinetic nature of the isotopic exchange between the wall-rock mantle and the percolating melt. This process results in the heterogeneous Fe isotopic composition of the lithospheric mantle beneath the Central Zone of the North China Craton.


      PubDate: 2015-04-06T07:22:26Z
       
  • Chemical and Pb isotope composition of olivine-hosted melt inclusions from
           the Hannuoba basalts, North China Craton: Implications for petrogenesis
           and mantle source
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Sheng-Ping Qian , Zhong-Yuan Ren , Le Zhang , Lu-Bing Hong , Jian-Qiang Liu
      Melt inclusions and their host olivine provide unique information about the nature, distribution and scale of mantle source heterogeneity. We present the first analyses of the chemical and Pb isotope compositions of melt inclusions and their host olivine from the Cenozoic Hannuoba basalts, North China Craton, which contains coexisting suites of alkali and tholeiitic basalts. There is limited variation in major and trace element composition, but significant Pb isotopic variation, in the tholeiitic samples. This contrasts with the substantial variation in major and trace elements, but limited Pb isotopic variation, in the alkali basalt samples. Based on the results of the major- and trace-element modeling, the compositional variation of the alkali basalts may be primarily attributed to garnet pyroxenite melting, with only a small input from garnet peridotite melts. The garnet–pyroxenite component involved in the genesis of the alkali basalts, of metasomatic origin, may have formed in the lithosphere. The wide range of isotopic composition of the tholeiitic basalts and remarkably limited variability of major and trace element composition is argued to be characteristic of the source region, which is a mixture of peridotite with small amounts of pyroxenite transformed from recycled ancient oceanic crust and sediment. The presence of recycled oceanic crust in the mantle source and the low–velocity anomaly observed beneath the Taihang Mountains, located 50km to the southeast of the Hannuoba region, supports a plume model for their origin.


      PubDate: 2015-04-06T07:22:26Z
       
  • In-situ iron isotope analysis of pyrites in ~3.7Ga sedimentary protoliths
           from the Isua supracrustal belt, southern West Greenland
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Kazumi Yoshiya , Yusuke Sawaki , Takafumi Hirata , Shigenori Maruyama , Tsuyoshi Komiya
      The timing of the emergence of life remains one of the principal unresolved questions in the Earth sciences. Putative relicts of microorganisms in the Eoarchean (ca. 3.6–3.85Ga) high-grade metamorphic terranes do not preserve morphological evidence for early life, but some relicts can be identified by their geochemical signatures created by metabolic processes. Among the oldest rocks of sedimentary origin (ca. 3.8Ga) occur in the Isua supracrustal belt (ISB), southern West Greenland; these have undergone metamorphism up to the amphibolite facies. Despite intense metamorphism, the carbon isotope compositions of graphite clots from the Isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process. Microbial dissimilatory iron reduction (DIR) is considered to be an early form of metabolism. The microbial DIR produced Fe2+ aq with a lower δ56Fe value from a precursor Fe3+-bearing iron mineral. However, δ56Fe values lower than −1‰ are not found in sedimentary rocks prior to about 2.9Ga. Here, we report in-situ iron isotope analysis of pyrites in sedimentary rocks from the ISB, using a near infrared-femtosecond-laser ablation-multicollector-ICP-MS (NIR-fs-LA-MC-ICP-MS). A large variation of δ56Fe values from −2.41 to +2.35‰, was documented from 190 points within pyrite grains from 11 rock specimens, including those interpreted to be banded iron-formations (BIFs), chert, amphibole-rich chert, quartz-rich clastic sedimentary rocks, mafic clastic sedimentary rocks, carbonate rocks and conglomerates. We found that the distribution of δ56Fe values depends on the lithology, whereas there is no correlation between their δ56Fe values and the metamorphic grade. The δ56Fe values of pyrites in BIFs range from +0.25 to +2.35‰, indicating partial oxidation in the deep ocean. Especially, the high δ56Fe values, up to +2.35‰, suggest that the BIF was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions. Pyrite grains in a conglomerate, carbonate rocks, mafic clastic sedimentary rocks, and amphibole-rich cherts show negative δ56Fe values around −1.5‰, down to −2.41‰, pointing to microbial DIR in the Eoarchean shallow sea. In addition, the relatively low δ56Fe values of pyrites in the shallow water sediments suggest anoxic, anoxygenic photoautotrophic iron oxidation in the photic zone.


      PubDate: 2015-04-06T07:22:26Z
       
  • Lead concentrations and isotope ratios in speleothems as proxies for
           atmospheric metal pollution since the industrial revolution
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Mohammed Allan , Nathalie Fagel , Maité Van Rampelbergh , James Baldini , Jean Riotte , Hai Cheng , R. Lawrence Edwards , David Gillikin , Yves Quinif , Sophie Verheyden
      Lead concentrations and isotope ratios from two speleothems from the Han-sur-Lesse cave in southern Belgium were measured in order to study the ability of speleothems to act as archives of atmospheric pollution. To address this aim we analyzed trace elemental Al and Pb compositions by LA-ICP-MS and ICP-MS as well as Pb isotopes by MC-ICP-MS. The results help to identify three intervals characterized by particularly high enrichment of Pb: from 1880 to 1905AD, from 1945 to 1965AD, and from 1975 to 1990AD. The speleothem record shows similar changes as the known historical atmospheric pollution level in Belgium. Lead isotope ratios discriminate between Pb sources and confirm that coal and gasoline combustion, combined with regional metallurgical activities, were the predominant Pb pollution sources in the stalagmites during the last 250years. This study opens possibilities to determine anthropogenic versus natural metal sources in well-dated speleothem archives.


      PubDate: 2015-04-06T07:22:26Z
       
  • Evaluating chemical equilibrium in metamorphic rocks using major element
           and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA
           
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Matthew P. Gatewood , Besim Dragovic , Harold H. Stowell , Ethan F. Baxter , David M. Hirsch , Rose Bloom
      A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume. Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.


      PubDate: 2015-04-06T07:22:26Z
       
  • The mineral and aqueous phase evolution of sulfur and nickel with
           weathering of pyrrhotite in a low sulfide, granitic waste rock
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Jeff B. Langman , David W. Blowes , Harish Veeramani , David Wilson , Leslie Smith , David C. Sego , Dogan Paktunc
      The release of S and metals from the weathering of waste rock pyrrhotite [Fe(1−x)S] is affected by oxidation pathways, the presence and concentration of Fe-substitution metals (e.g., Ni), and Fe (oxyhydr)oxide products. A 4C (monoclinic) pyrrhotite [Fe7S8, Ni:Fe substitution of about 1:200] in a low sulfide waste rock was weathered in the laboratory and at a field site to identify the evolution of S oxidation, distribution and coordination of Ni, and possible adsorption of Ni on secondary Fe minerals. Mineral phase S was present in the −2, −1, 0, +4, and +6 oxidation states prior to its release as SO4 2− into solution. The thiosulfate [S2O3 2−] pathway of sulfide oxidation was indicated by the presence of polythionates and S2O3 2−. Sulfur as S2O3 2− was more persistent than sulfite [SO3 2−] prior to oxidation to SO4 2−. The release of Ni during the oxidative dissolution of pyrrhotite was influenced by its distribution in the grain: diffuse throughout the grain (solid solution mineral) or concentrated in exsolved pentlandite [(Fe,Ni)9S8] lamellae within the pyrrhotite. With the oxidation of S, Ni coordination changed from bonding with reduced S species to bonding with oxygen prior to its release with SO4 2−. Sulfur as SO4 2− typically stayed in solution with release from the pyrrhotite, but the solubility of released Ni was dependent on leachate pH. A near-neutral pH induced the adsorption of Ni onto Fe (oxyhydr)oxides, which formed during oxidation of the pyrrhotite. In some grains, Ni was found concentrated in Fe distal precipitates as opposed to the closer alteration layers/zones of the sulfide grain. Subsequent dissolution of the Fe (oxyhydr)oxides or desorption of Ni with a change in pH provides a delayed release of Ni into solution. This improved understanding of the oxidation pathway of S, coordination and release of Ni, and influence of Fe (oxyhydr)oxides in the weathering of pyrrhotite will assist in determining appropriate reaction-rate factors and element release rates for prediction of acid rock drainage.


      PubDate: 2015-04-06T07:22:26Z
       
  • Editorial Board
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400




      PubDate: 2015-04-06T07:22:26Z
       
  • Modeling plutonium sorption to kaolinite: Accounting for redox equilibria
           and the stability of surface species
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Rémi Marsac , Nidhu L. Banik , Johannes Lützenkirchen , Razvan A. Buda , Jens V. Kratz , Christian M. Marquardt
      Plutonium with its particularly complex redox chemistry may be thermodynamically stable in the states +III to +VI depending on the redox conditions in the environment. Mineral surfaces can also affect Pu redox speciation. Therefore, the interpretation of Pu sorption data becomes particularly challenging, even for simplified laboratory experiments. The present study focuses on Pu sorption to kaolinite. Am(III), Th(IV), Np(V) and U(VI) literature sorption data are used as analogues for the corresponding Pu redox states to calibrate a simple surface complexation model, and the Nernst formalism is applied. Two independent pH–pe diagrams, one for the kaolinite surface and another for the aqueous phase, are constructed and superimposed. This allows visualization of the prevalent Pu redox state in both phases. The model suggests that the stability field of the most strongly adsorbing redox state is larger at the surface than in solution. Because Pu(V) weakly sorbs to kaolinite, it never prevails at the surface. Within the stability field of Pu(V) in 0.1M NaClO4 solution, Pu(VI) and Pu(IV) prevail at the kaolinite surface under oxidizing and slightly reducing conditions, respectively. By contrast, the Pu(IV)/Pu(III) boundary is hardly affected because both redox states strongly sorb to kaolinite, especially for pH>6. The present method is applied to literature data for Pu sorption to kaolinite. By estimating the pe from a Pu redox state analysis in solution, overall Pu uptake could be predicted. Generic equations are derived that are applicable to minerals and actinides other than kaolinite and Pu. The present study provides important progress in understanding Pu geochemistry, especially in the context of nuclear waste disposal where thermodynamic models are particularly necessary to predict Pu mobility.


      PubDate: 2015-04-06T07:22:26Z
       
  • Accurate determination of elements in silicate glass by nanosecond and
           femtosecond laser ablation ICP-MS at high spatial resolution
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Zhen Li , Zhaochu Hu , Yongsheng Liu , Shan Gao , Ming Li , Keqing Zong , Haihong Chen , Shenghong Hu
      Despite the large number of successful applications of LA-ICP-MS, elemental fractionation remains the main limitation for many of its applications in the Earth sciences. This limitation is particularly notable for high spatial resolution analysis. Elemental fractionation and mass-load effect in silicate glasses NIST SRM 610 and GSE-1G were investigated by using 193nm ArF excimer nanosecond (ns) laser and 257nm femtosecond (fs) laser ablation systems coupled to inductively coupled plasma mass spectrometry. Contrary to those observed in ns-LA-ICP-MS, the most elemental fractionation at the small spot sizes of 16–24μm are lower than that at the large spot sizes of 44–60μm in fs-LA-ICP-MS. The significantly different fractionation behaviors of Li, Na, Si, K, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Cs and U between silicate glass materials NIST SRM 610 and GSE-1G observed in 193nm excimer LA-ICP-MS are eliminated by using 257nmfs-LA-ICP-MS at high spatial resolution. In addition, the mass load effect and matrix dependent mass load effect are also found to be reduced by using fs-LA-ICP-MS in comparison with ns-LA-ICP-MS. Except for Sb, Pb and Bi, the elemental fractionation is independent on the laser fluence chosen, which is irrespective of ns- or fs-LA-ICP-MS. In this study, a spot size of 24μm was used to test the capabilities of LA-ICP-MS analysis at high spatial resolution. The agreement between our data and the reference values is better than 10% for most of the elements in MPI-DING, USGS, and NIST glasses by using fs-LA-ICP-MS. For ns laser ablation analysis, the accuracy is highly dependent on the calibration strategies used (conventional external calibration method or 100% oxide normalization method) and the selected external reference materials (NIST SRM 610 or GSE-1G). The much less laser-induced elemental fractionation and matrix effect in fs-LA-ICP-MS in comparison with 193nm excimer LA-ICP-MS make it more suitable for the analysis of silicate materials at high spatial resolution.
      Graphical abstract image

      PubDate: 2015-04-06T07:22:26Z
       
  • Iron isotope fractionation during Fe(II) and Fe(III) adsorption on
           cyanobacteria
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Daniel S. Mulholland , Franck Poitrasson , Liudmila S. Shirokova , Aridane G. González , Oleg S. Pokrovsky , Geraldo R. Boaventura , Lucieth C. Vieira
      The present study aimed at testing the hypothesis that Fe adsorption and precipitation on bacterial phytoplankton likely cause significant isotopic fractionation with preferential adsorption of heavy isotopes on the cell surface. We measured the iron isotopic fractionation during the interaction of aqueous Fe with planktonic cyanobacteria (Gloeocapsa sp., Synehococcus sp., and Planthothrix sp.) in six independent experiments using two distinct Fe oxidation states (Fe(III) and Fe(II)) at pH3 and 6. Isotopic analyses demonstrated that the Fe adsorption on bacterial planktonic biomass yields a clear enrichment of heavy isotopes on the cell surfaces, producing isotopically light δ57Fe values in solutions. The adsorption experiments with Fe(II) in the initial solution yielded a Δ57Fecell-solution =2.66±0.14‰, whereas the adsorption experiments with Fe(III) in the initial solution yielded Δ57Fecell-solution =0.97±0.19‰. Because these data approached closed system equilibrium isotopic fractionation lines rather than Rayleigh curves, the most likely mechanism is a steady state isotopic fractionation, linked with short-term reversible Fe adsorption on cells. In agreement with X-ray Absorption Spectroscopy structural data obtained on the same adsorption experiment samples (González et al., 2014), the preferential enrichment of heavy Fe isotopes on the cell surfaces is attributed to the stronger covalent metal-ligand bonding (FeOC/P) of octahedrally coordinated Fe with phosphoryl or carboxyl groups on the cell walls when compared with the Fe aquacomplexes (OFeO) in solution. The larger isotopic fractionation factor in the experiments starting with Fe(II) results from the iron oxidation to Fe(III) which is itself accompanied by a strong isotopic effect. A natural case study of warm hydrothermal spring depositing Fe oxy(hydr)oxide with and without cyanobacterial biomass yielded an isotopic fractionation between the solid phase and aqueous solution (Δ57Fesolid-solution) of 0.62±0.16‰ (biotic) and 0.80±0.06‰ (abiotic). This natural case study therefore confirms experimental results, albeit of smaller magnitude. These findings suggest that Fe adsorption on cyanobacteria cell surfaces in nature should produce cell enrichments in heavier isotopes relative to the coexisting aqueous solution.


      PubDate: 2015-04-06T07:22:26Z
       
  • Petrogenesis of mafic collision zone magmatism: The Armenian sector of the
           Turkish–Iranian Plateau
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Iain Neill , Khachatur Meliksetian , Mark B. Allen , Gevorg Navasardyan , Klaudia Kuiper
      The Turkish–Iranian Plateau grew after the Middle Miocene following the initial Paleogene Arabia–Eurasia collision.
      Authors attribute uplift to break-off of the southern Neo-Tethys slab beneath the Bitlis–Zagros Suture at ~15–10Ma, coupled with continued plate convergence and regional crustal shortening. Since this time there has been an upsurge in mantle-derived collision magmatism over large parts of NW Iran, Eastern Anatolia and the Lesser Caucasus, potentially hundreds of kilometres from the site of southern Neo-Tethys slab break-off, >10Myr after the proposed break-off event. Whole rock elemental and Sr–Nd–Pb–Hf isotope data are presented for <3Ma trachy-basalt to trachy-basaltic andesite lavas erupted in Armenia in the South Caucasus. Samples formed by <5% melting of fertile subduction-modified spinel-facies lithospheric mantle, and few display elemental or isotopic evidence for contamination by the 45-km thick Mesozoic–Paleogene arc crust or South Armenian Block continental crust. Recent magmatic activity in Armenia may not be a direct consequence of southern Neo-Tethys slab break-off 300–450km away, beneath the Bitlis Suture. Late Miocene break-off of a second (northern Neo-Tethys) slab beneath the Pontide Arc may have allowed asthenospheric upwelling over a wider area than was affected by southern Neo-Tethyan break-off. However, whole-scale delamination of mantle lithosphere is ruled out due to the modest degrees of partial melting, a lack of asthenospheric components and limited crustal involvement in magmatism. Small-scale sub-lithospheric convection may be complementary to break-off, causing localised removal of lithospheric mantle and aiding the occurrence of melting for a significant time interval after the break-off event(s). Collision magmas such as those in Armenia represent mantle-derived additions to continental crust, enriched in incompatible elements but with Th/La ratios≤0.2, much lower than those calculated for continental crust (0.25–0.3). Collision magmatism in Turkic-style orogens must be balanced by infra-crustal recycling and delamination to produce bulk continental crust.


      PubDate: 2015-04-01T15:42:18Z
       
  • A remarkable relationship of the stable carbon isotopic compositions of
           wood and cellulose in tree-rings of the tropical species Cariniana
           micrantha (Ducke) from Brazil
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): G.H. Schleser , D. Anhuf , G. Helle , H. Vos
      The stable isotopes of carbon were analysed in total wood and cellulose from the tree-rings of the tropical wood species Cariniana micrantha (Ducke). The aim was to examine the isotopic relationship between total wood and its cellulose over the last two and a half centuries. Although the correlation for the whole time period is very high (r=0.96) it is remarkable that different sub-periods deviate strongly from this close relationship. Consequently, a good correlation from the subset of a longer isotopic record cannot necessarily prove its validity for the whole record. The study indicates that changes of the carbon isotopes of cellulose and of total wood show sometimes during short sub-periods different isotope patterns presumably caused by different environmental effects. Thorough calculations indicate that strong variations within the isotopic record especially changes of the isotopic level along a chronology lead to high correlations between δ13Cwood and δ13Ccel. Contrary thereto subsections with low isotopic variability lead to low correlations. The results imply that long term trends provide similar patterns. Therefore, if long term trends are of interest such as e.g. in climate reconstruction then total wood can be analysed in favour of cellulose, thus saving a tremendous amount of work. However, if short term aspects from a longer record are of interest, cellulose and total wood may sometimes provide different information. In addition it is hypothesized that during intervals of low isotopic variability the proportions of the various wood components may change relative to each other, leading for certain time intervals to different isotope patterns.


      PubDate: 2015-03-08T16:21:17Z
       
  • Chemical fluxes from time series sampling of the Irrawaddy and Salween
           Rivers, Myanmar
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Hazel Chapman , Mike Bickle , San Hla Thaw , Hrin Nei Thiam
      The Irrawaddy and Salween rivers in Myanmar deliver water fluxes to the ocean equal to ~70% of the Ganges–Brahmaputra river system. Together these systems are thought to deliver about half the dissolved load from the tectonically active Himalayan–Tibetan orogen. Previously very little data was available on the dissolved load and isotopic compositions of these major rivers. Here we present time series data of 171 samples collected fortnightly at intervals throughout 2004 to 2007 from the Irrawaddy and Salween at locations near both the river mouths, the up-stream Irrawaddy at Myitkyina, the Chindwin, a major tributary of the Irrawaddy and a set of 28 small tributaries which rise in the flood plain of the Irrawaddy between Yangon and Mandalay. The samples have been analysed for major cation, anion and 87Sr/86Sr ratios. The new data indicates that the Irrawaddy has an annual average Na concentration only a third of the widely quoted single previously published analysis. The Irrawaddy and Salween drain about 0.5% of global continental area and deliver about 3.3% of the global silicate-derived dissolved Ca+Mg fluxes and 2.6% of the global Sr riverine fluxes to the oceans. This compares with Ganges and Brahmaputra which deliver about 3.4% of the global silicate-derived dissolved Ca+Mg fluxes and 3.2% of the global Sr riverine fluxes to the oceans from about 1.1% of global continental area. The discharge-weighted mean 87Sr/86Sr ratio of the Irrawaddy is 0.71024 and the Salween 0.71466. The chemistry of the Salween and the Irrawaddy waters reflects their different bedrock geology. The catchment of the Salween extends across the Shan Plateau in Myanmar through the Eastern syntaxis of the Himalayas and into Tibet. The Irrawaddy flows over the Cretaceous and Tertiary magmatic and metamorphic rocks exposed along the western margin of the Shan Plateau and the Cretaceous to Neogene Indo-Burma ranges. The 87Sr/86Sr compositions of the Salween and Upper Irrawaddy (between 0.7128 and 0.7176) are significantly higher than the downstream Irrawaddy (0.7095 to 0.7108) and the Chindwin (0.7082 to 0.7095). The Irrawaddy and the Chindwin exhibit lower 87Sr/86Sr and Na/Ca ratios during and immediately post-monsoon, interpreted to reflect higher weathering of carbonate at high flow. The Salween exhibits higher 87Sr/86Sr ratios but lower Na/Ca ratios during the monsoon, interpreted to reflect higher inputs from the upper parts of the catchment in the Himalayas.


      PubDate: 2015-03-08T16:21:17Z
       
 
 
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