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  Subjects -> EARTH SCIENCES (Total: 598 journals)
    - EARTH SCIENCES (443 journals)
    - GEOLOGY (67 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (44 journals)

EARTH SCIENCES (443 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 2)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 4)
AMBIO     Hybrid Journal   (Followers: 12)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 15)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal   (Followers: 1)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 7)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 9)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 4)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 17)
Bulletin of Volcanology     Hybrid Journal   (Followers: 13)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 10)
Coastal Management     Hybrid Journal   (Followers: 19)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 11)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 179)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 12)
Ecohydrology     Hybrid Journal   (Followers: 11)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2571 journals]   [SJR: 1.714]   [H-I: 111]
  • An improved protocol for 87Sr/86Sr by laser ablation multi-collector
           inductively coupled plasma mass spectrometry using oxide reduction and a
           customised plasma interface
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): J. Lewis , C.D. Coath , A.W.G. Pike
      Strontium isotope analysis has become one of the most effective means to characterise mobility and migration of humans and animals in the past. Analysis of strontium isotopes in bulk dental enamel by dissolution and chemical separation is precise, reliable and accurate. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offer high spatial, and therefore temporal, resolution sampling. However, the application of LA-MC-ICP-MS to strontium isotopes in bioapatites has been limited due to the presence of isobaric interferences. In particular, 40Ca31P16O+ (CaPO+) on 87Sr+ has been identified as causing a consistent positive offset in 87Sr/86Sr by LA-MC-ICP-MS, relative to dissolution methods, of approximately 500 to 1500ppm. It has been demonstrated that tuning the mass spectrometer for reduced oxide production can reduce the size of this effect but not eliminate it completely. Here we present a revised protocol using a customised plasma interface through which helium gas is introduced. We find that using the oxide reduction and the customised plasma interface protocols can further reduce the offset such that we can achieve accuracy to within typical analytical precisions (±50ppm, 2σ) without any need for further mathematical corrections. We test the methodology by ‘blind’ comparisons between LA-MC-ICP-MS and micro-drill/dissolution thermal ionisation mass spectrometry on two archaeological teeth. Both techniques are in excellent agreement.


      PubDate: 2014-11-19T04:57:32Z
       
  • Effects of CeO2 nanoparticles on microbial metabolism
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Shota Masaki , Hiroyuki Shiotsu , Toshihiko Ohnuki , Fuminori Sakamoto , Satoshi Utsunomiya
      To understand the effects of nanoparticles on microorganisms, we experimentally investigated the effects of CeO2 nanoparticles (CeNPs) on yeast (S. cerevisiae) focusing on microbial metabolites and intracellular proteins. The yeast were harvested from a yeast extract peptone dextrose medium containing 0, 10, 100, and 250ppm of CeNPs and incubated for 120h in 1mM NaCl solution at three different pH values: 3, 5, and 7. The yeast released organic matter, P, K, and Mg into the NaCl solution at all pH values, even without CeNPs. Distinct differences were detected in the released organic species and intracellular proteins after exposure to CeNPs. High-performance liquid chromatography revealed that various organic species released from the yeast were expressed or suppressed after exposure to CeNPs. Although cytotoxicity was not caused by CeNPs, the results of the peptide mass fingerprint analysis of the intracellular protein revealed that Eno2p, a glycolysis enzyme, was expressed after exposure to CeNPs. These results suggest that nanoparticles have the potential to alter microbial metabolism, leading to changes in the compositions of the released substances in the surrounding environment.


      PubDate: 2014-11-19T04:57:32Z
       
  • Rare earth elements as reactive tracers of biogeochemical weathering in
           forested rhyolitic terrain
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Angélica Vázquez-Ortega , Julia Perdrial , Adrian Harpold , Xavier Zapata-Ríos , Craig Rasmussen , Jennifer McIntosh , Marcel Schaap , Jon D. Pelletier , Paul D. Brooks , Mary Kay Amistadi , Jon Chorover
      Rare earth elements (REEs) were evaluated as potential tracers of biogeochemical weathering at pedon, hillslope, and catchment scales in the Jemez River Basin Critical Zone Observatory (JRB-CZO), Valles Caldera National Preserve, NM, USA. We investigated time series of REE patterns in precipitation, soil pore water, groundwater, and stream water, and related these data to REE composition of soil, rock and atmospheric dust. REE signatures in stream waters are dynamic, reflecting processes that occur along hydrologic flowpaths during transport to the stream, including organic matter complexation, primary and secondary mineral weathering, water/soil/bedrock interaction, and atmospheric deposition. Strong compositional similarities for the REE between soil waters and stream waters during the initial snowmelt are consistent with shallow subsurface flows to streams. Most (bio)chemical denudation of REE occurred during the snowmelt-derived dissolved organic carbon (DOC) pulse, during which time apparent colloidal mobilization of REE occurred in association with Fe and Al (oxy)hydroxides. The REE and DOC concentrations in stream waters were positively correlated (R2 =0.80, p<0.0001) during snowmelt, suggesting REE complexation and mobilization in association with organic ligands during the period of shallow subsurface flow. Positive Eu-anomalies occur in the soil matrix ([Eu/Eu⁎]RT range from 1.79 to 2.52), soil solutions ([Eu/Eu⁎]RT range from 1.89 to 5.98), and stream waters ([Eu/Eu⁎]RT range from 1.01 to 2.27) with respect to the host lithologies—effects attributable to both eolian deposition and preferential feldspar dissolution. Cerium anomalies in soil solids and porewaters indicate seasonally dynamic translocation and oxidative accumulation in subsurface soil horizons (surface horizons: [Ce/Ce⁎]RT range from 0.70 to 1.1; subsurface horizons: [Ce/Ce⁎]RT range from 0.95 to 1.29), consistent with prior research reporting Ce(IV) co-precipitation with Fe- and Mn-oxide minerals.


      PubDate: 2014-11-19T04:57:32Z
       
  • Determining in situ pH values of pressurised fluids using stable carbon
           isotope techniques
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): A. Myrttinen , V. Becker , B. Mayer , R. van Geldern , J.A.C. Barth
      Accurate determination of pH values is fundamentally important for understanding and predicting geochemical processes in surface and subsurface environments. However, conducting in situ pH measurements of fluids from geological formations exposed to high CO2 pressures and elevated temperatures is technically and logistically challenging. Here we introduce a proxy-method for determining in situ pH values based on stable carbon equilibrium isotope fractionation (103lnα13C) between dissolved inorganic carbon (DIC) and headspace CO2, which is valid for a DIC system composed of a mixture of H2CO3 ⁎ (CO2(aq) +H2CO3) and HCO3 −. The method is based on determination of DIC speciation and δ13C measurements of DIC and headspace CO2 on de-pressurised fluid samples in the laboratory. Since 103lnα13CDIC–CO2 values vary predictably dependent upon temperature and pH, DIC speciation and carbon isotope ratio measurements on two carbon species (CO2 and DIC) enable the determination of in-situ pH values if temperatures are known. This approach was verified in a series of laboratory experiments with water samples equilibrated with gaseous CO2 at 55bars for various temperatures and time periods. Concentration and carbon isotope measurements upon opening of the reaction vessels yielded 103lnα13CDIC–CO2(g) values between −1.3±0.1 and +1.4±0.2‰ corresponding to reconstructed in situ pH values ranging from 5.5 to 7.4 at temperatures between 20 and 120°C. Measurements with a pH-metre immediately after opening the reaction vessels were on average 1.4pH units higher than those determined by the proxy method due to CO2 outgassing that increased the measured pH. This CO2 degassing effect does not impact the here presented carbon isotope-based pH reconstruction method, which constitutes a viable alternative to direct pH measurements.


      PubDate: 2014-11-19T04:57:32Z
       
  • Sedimentary iron-phosphorus cycling under contrasting redox conditions in
           a eutrophic estuary
    • Abstract: Publication date: Available online 15 November 2014
      Source:Chemical Geology
      Author(s): Peter Kraal , Edward D. Burton , Andrew L. Rose , Benjamin D. Kocar , Robert S. Lockhart , Kliti Grice , Richard T. Bush , Eileen Tan , Samuel M. Webb
      Phosphorus (P) is often a limiting nutrient within freshwater and estuarine systems, thus excess inputs of P from anthropogenic activities (dominantly agriculture) are known to induce eutrophication in receiving water bodies. The sequestration of P within estuarine sediments is controlled by sorption and precipitation processes, which are impacted by local redox conditions and burial environment. Despite the global spread of oxygen depletion in coastal marine systems, P burial under reducing conditions remains poorly understood. We investigated P cycling in relation to iron (Fe) redox chemistry in sediments from the eutrophic Peel-Harvey Estuary in Western Australia, using a combination of porewater analysis, sequential chemical P extractions, synchrotron-based micro-scale X-ray fluorescence mapping and Fe K-edge X-ray absorption spectroscopy, and PO4 3- sorption experiments. The sediments represented redox regimes varying from strongly reducing, organic-rich sediments with little or no reactive Fe(III) (oxyhydr)oxides to oxygenated sediments that were enriched in reactive Fe(III) phases. Organic P and Fe-associated P were the main P burial phases, and the latter was quantitatively important even under strongly reducing conditions. We attribute this to adsorption of P onto micro-scale Fe(III) oxyhydroxide enrichments and/or Fe-bearing clay minerals. The organic-rich sediments showed a strong decline in P contents with depth; P was released from organic matter and Fe phases but apatite precipitation was apparently inhibited in these sediments. Despite greater and stronger PO4 3- sorption capacity, the oxic sediments contained relatively little P due to a lack of the primary P source in marine sediments: organic matter. Our results provide detailed insight into P burial in dynamic estuarine sediments and show that micro-scale spectroscopic analyses greatly advance our understanding of P sequestration processes.


      PubDate: 2014-11-19T04:57:32Z
       
  • The thermodynamic properties of bastnäsite-(Ce) and parisite-(Ce)
    • Abstract: Publication date: Available online 15 November 2014
      Source:Chemical Geology
      Author(s): Alexander P. Gysi , Anthony E. Williams-Jones
      The rare earth elements (REE) are critical metals that play a major role in emerging high technology and green industries. The light (L)REE occur dominantly in fluorocarbonate minerals and consequently information on the stability of these minerals is essential for a better understanding of the genesis of REE deposits and for the efficient processing of their ores. We have investigated the thermochemical properties of natural bastnäsite-(Ce) (Ce0.50La0.25Nd0.20Pr0.05CO3F) and parisite-(Ce) (CaCe0.95La0.60Nd0.35Pr0.10(CO3)3 F2) using differential scanning calorimetry at temperatures from 323K to 1022K and a pressure of 1bar using heat ramping and isothermal methods. Crystal lattice parameters of the REE fluorocarbonate minerals and reaction products from the experiments were determined using X-ray diffraction. The measured isobaric heat capacity (Cp°) for bastnäsite-(Ce) can be described by the relationship 134.3-2.032×106 T-2 between 343.15 and 528.15K and for parisite-(Ce) by the relationship 398.8-1048T-0.5-4.202×106 T-2 between 343.15 and 643.15K, where T is temperature in K. Bastnäsite-(Ce) decomposed irreversibly at >612K to form REE oxyfluorides and CO2. An endothermic peak at 824.2K yielded a heat of reaction of 245.2±2.5kJ/mol. Parisite-(Ce) decomposed irreversibly at >664K to form REE oxyfluorides, CaCO3 and CO2. An endothermic peak at 842.7K yielded a heat of reaction of 522.6±5.2kJ/mol. The enthalpy of formation at 298K and 1bar was retrieved from the decomposition enthalpies, yielding -1808.4±12.0kJ/mol and -4848.0±23.8kJ/mol for bastnäsite-(Ce) and parisite-(Ce), respectively. The measured molar volumes for bastnäsite-(Ce) and parisite-(Ce) are 42.91cm3/mol and 122.71cm3/mol, respectively. An estimation method, based on the dependence of entropy on volume, was used to retrieve the third law entropy (S°) at 298.15K, and together with the measured thermodynamic properties, permitted us to construct the first quantitative mineral-fluid stability diagrams involving bastnäsite-(Ce), parisite-(Ce), fluocerite-(Ce), calcite and fluorite at P-T-x conditions relevant for the study of natural Ca-REE-C-O-H-F systems. Further studies of the thermodynamic properties of REE-bearing minerals are urgently needed to better understand the genesis of REE ore deposits.


      PubDate: 2014-11-19T04:57:32Z
       
  • The intensity of chemical weathering: Geochemical constraints from marine
           detrital sediments of Triassic age in South China
    • Abstract: Publication date: Available online 15 November 2014
      Source:Chemical Geology
      Author(s): Ming-Yu Zhao , Yong-Fei Zheng
      A geochemical study of major-trace elements in detrital sediment and carbon-oxygen isotopes in carbonate was carried out for a marine stratigraphic profile of Early Triassic that is composed of argillaceous limestone and calcareous mudstone in the Lower Yangtze basin, South China. The results place constraints on the geochemical behaviors of various elements in the detrital sediment that was deposited in the residual Paleotethyan seawater. This leads to establishment of new geochemical proxies for chemical weathering of continental crust. In terms of the correlations between element concentrations and their variations in the profile, the elements are categorized into four groups with respect to the difference in their geochemical behaviors. The first group is composed of Al, Th, Sc, Be, In, Ga, K, Rb and Cs that are tightly correlated due to their immobility during chemical weathering. The second group is composed of Ca and Na that show opposite variation trends with Th and Sc, on account of their mobile behavior in the weathering profile. The third group is composed of high field strength elements such as Ti, Nb, Ta, Zr and Hf that are closely correlated with each other because they were primarily taken up by heavy minerals from sedimentary provenance. The fourth group is composed of redox sensitive elements such as Co, Cu, Fe, Mn and Ni that are correlated with S and thus mainly hosted by sulfides. Th, Sc, Ca and Na were not amenable to changes in sedimentary provenance, and thus are selected to establish the new proxies for chemical weathering. These are composed of logarithmic parameters such as log(Th/Ca), log(Sc/Ca), log[Th/(Na/5+Ca)] and log[Sc/(Na/5+Ca)]. They exhibit synchronous increases at the Permian-Triassic boundary, the middle Griesbachian and the early Smithian, indicating the enhancements of chemical weathering. High proxy values approaching the values for the extremely weathered product of granodiorite occurred in the middle to late Griesbachian and early Smithian, demonstrating the occurrences of extreme chemical weathering and very warm paleoclimate in those periods. These paleoclimatic changes are concordant with results from geochemical studies elsewhere in the world. Therefore, the intensity of chemical weathering can be indicated by the new geochemical proxies for the different properties of elements in marine detrital sediments.


      PubDate: 2014-11-19T04:57:32Z
       
  • Silica hydrate preserved with δ18O-rich quartz in high-temperature
           hydrothermal quartz in the high sulfidation copper-gold deposit at El
           Indio, Chile
    • Abstract: Publication date: Available online 16 November 2014
      Source:Chemical Geology
      Author(s): Dominique Tanner , Richard W. Henley , John A. Mavrogenes , Peter Holden , Terrence P. Mernagh
      Quartz microcrystals from the El Indio Au-Ag-Cu deposit (Chile) preserve a rare glimpse into the high-temperature evolution of silica. Here, we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates ‘opal’ and moganite. We propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase. Evidence for sequential dehydration to from silica hydrate to quartz (silica hydrate⌫opal⌫moganite⌫quartz) is provided by SHRIMP 18O microanalytical data that show oscillatory isotopic zoning from 3.6 to 16.2‰ δ18O (±0.5‰) coupled with K and Al variations. We estimate that the precursor silica hydrate deposited between ~480-680°C and contained 32-63wt% H2O. Silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation, a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the El Indio copper-gold deposit. Modern understanding of ore-forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions. The discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids. Quartz matured from silica hydrate would record ‘pseudo-primary’ fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while δ18O data may bias fluid provenance interpretation within sub-volcanic systems.
      Graphical abstract image

      PubDate: 2014-11-19T04:57:32Z
       
  • Editorial Board
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388




      PubDate: 2014-11-19T04:57:32Z
       
  • Strontium isotope variation and carbonate versus silicate weathering in
           rivers from across Alaska: Implications for provenance studies
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Sean R. Brennan , Diego P. Fernandez , Glen Mackey , Thure E. Cerling , Clement P. Bataille , Gabriel J. Bowen , Matthew J. Wooller
      The characterization of strontium (Sr) isotopic (87Sr/86Sr) variation in surface waters of Alaska (AK) has significant implications for provenance studies aiming to track movement patterns of animals. This study presents 87Sr/86Sr ratios, concentrations of Sr, Ca, Mg, Na and K, and selected molar ratios (Sr/Na, Ca/Na, Mg/Na and Ca/Sr) from 61 rivers from across AK to characterize regional patterns in i) 87Sr/86Sr variation and ii) carbonate versus silicate weathering influencing 87Sr/86Sr ratios. 87Sr/86Sr ratios range from 0.70422–0.74041±0.00009. Rivers north of the Denali Fault exhibit more radiogenic and variable 87Sr/86Sr ratios (0.70763 to 0.74041) than rivers to the south (0.70422 to 0.70895), reflecting the tectonic growth of southern AK via accretion of relatively young mafic-rich exotic island arcs along North America's continental margin during the Mesozoic. Within interior AK an east–west gradient exists. Radiogenic 87Sr/86Sr ratios (>0.725) in east-central AK and relatively low ratios (<0.709) in western AK reflect AK's progressive western growth along the ancestral North American miogeocline from the Precambrian to Mesozoic. High relief rivers draining north of the Brooks Range are more radiogenic (>0.711) than lowland rivers (<0.709). Elemental ratios indicate carbonate weathering has the strongest influence on 87Sr/86Sr ratios in high relief watersheds north of the Denali Fault and in watersheds of south-central AK. Elemental and 87Sr/86Sr ratios indicate silicate weathering is important across AK, but is most influential in lowland basins north of the Denali Fault and in southwestern AK. This study illustrates the large Sr isotopic heterogeneity across AK on multiple spatial scales and provides necessary baseline information for provenance studies.


      PubDate: 2014-11-19T04:57:32Z
       
  • Editorial Board
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389




      PubDate: 2014-11-19T04:57:32Z
       
  • The influence of pH on barite nucleation and growth
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Cristina Ruiz-Agudo , Christine V. Putnis , Encarnación Ruiz-Agudo , Andrew Putnis
      Nanoscale Atomic Force Microscopy (AFM) experiments show that barite (BaSO4) growth is influenced by the pH of the growth solution. AFM observations provide evidence that growth and nucleation rates measured along the [100] crystallographic direction on the initial layer grown on barite (001) natural surfaces increase at both high and low pH of the growth solutions. At alkaline pH, growth is arrested in the second and successive layers, possibly as a result of the structure distortion resulting from incorporation of foreign ions (OH− and/or CO3 2−). Macroscopic nucleation experiments also show that with increasing pH, the induction times, the precipitation rate and the interfacial tension are all reduced, consistent with nanoscale observations. Smaller particle size at high pH provides further evidence for enhanced barium sulfate nucleation in alkali solutions. This enhancement in growth as well as in nucleation of barite at high pH could be explained by taking into account the effect of hydroxyl ions on hydration shells of aqueous Ba2+ and SO4 2− in solution and on the barite surface. The energetic interaction between water molecules and the barite building units is affected by the presence of OH− ions in solution. The frequency of water exchange around Ba2+ and SO4 2− could increase due to the effect of OH− ions on the structure of water and consequently promote nucleation and growth. Increased growth at low pH can be attributed to increased Ba2+ activity with respect to SO4 2−.


      PubDate: 2014-11-19T04:57:32Z
       
  • 238U–230Th dating of chevkinite in high-silica rhyolites from La
           Primavera and Yellowstone calderas
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Jorge A. Vazquez , Noel O. Velasco , Axel K. Schmitt , Heather A. Bleick , Mark E. Stelten
      Application of 238U–230Th disequilibrium dating of accessory minerals with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical evolution of magma within the framework of absolute time. Chevkinite, a Th-rich accessory mineral that occurs in peralkaline and metaluminous rhyolites, may be particularly useful as a chronometer of crystallization and differentiation because its composition may reflect the chemical changes of its host melt. Ion microprobe 238U–230Th dating of single chevkinite microphenocrysts from pre- and post-caldera La Primavera, Mexico, rhyolites yields model crystallization ages that are within 10's of k.y. of their corresponding K–Ar ages of ca. 125ka to 85ka, while chevkinite microphenocrysts from a post-caldera Yellowstone, USA, rhyolite yield a range of ages from ca. 110ka to 250ka, which is indistinguishable from the age distribution of coexisting zircon. Internal chevkinite–zircon isochrons from La Primavera yield Pleistocene ages with ~5% precision due to the nearly two order difference in Th/U between both minerals. Coupling chevkinite 238U–230Th ages and compositional analyses reveals a secular trend of Th/U and rare earth elements recorded in Yellowstone rhyolite, likely reflecting progressive compositional evolution of host magma. The relatively short timescale between chevkinite–zircon crystallization and eruption suggests that crystal-poor rhyolites at La Primavera were erupted shortly after differentiation and/or reheating. These results indicate that 238U–230Th dating of chevkinite via ion microprobe analysis may be used to date crystallization and chemical evolution of silicic magmas.


      PubDate: 2014-11-15T04:38:19Z
       
  • Sensitivity of carbonate weathering to soil CO2 production by biological
           activity along a temperate climate transect
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Damien Calmels , Jérôme Gaillardet , Louis François
      We investigated the controls on carbonate weathering in a well-drained pure carbonate area subject to strong environmental gradients, the Jura Mountains, Western Europe. The water chemistry of sampled springs and resurgences is dominated by Ca2+ (87 to 96Eq% of the cationic charge) and HCO3 − (90 to 97Eq% of the anionic charge), reflecting the overwhelming imprint of calcium carbonate dissolution by atmospheric/biogenic CO2. Ca2+ concentration, which directly gives access to the amount of calcium carbonate dissolved per unit of water runoff, shows a gradual two-fold decrease (from 3000 to 1400μmol/L) along the elevation gradient (from 300 to 1200m). After discussing the possible influence of each environmental parameter on the observed water chemistry gradient, a decreasing soil pCO2 (the main source of acidity) with increasing altitude appears as the most likely explanation. As no spatial and temporal record of soil pCO2 are available for the Jura Mountains, we performed soil pCO2 modeling using the ecological and hydrological ASPECTS model that allows reconstructing carbon and water exchange fluxes between the vegetation, soil and atmosphere. Modeling results suggest that soil pCO2 decreases with altitude in response to both the change in vegetation species from deciduous-dominated forest in the lowlands to evergreen-dominated forest above 800m (responsible for 65% of the variation) and the change in climate and soil properties (responsible for 35% of the variation). Carbonate weathering would thus be strongly sensitive to the type of vegetation, which drives both temporal and spatial variations of soil carbon and water budgets. Based on field observations, we show that carbonate weathering rates are 20–30% higher under deciduous vegetation cover than under conifers (at a given water runoff value), in agreement with modeling results. Chemical denudation rates of carbonate in the Jura Mountains vary from 152 to 375t/km2/yr, corresponding to 60–150mm/ka of carbonate being removed. Carbonate weathering within the 10,000km2 of the study area accounts for an atmospheric CO2 consumption of 0.3 TgC/yr, showing that carbonate rocks have an enhanced capacity of atmospheric CO2 neutralization at least transiently. This study demonstrates that carbonate weathering is sensitive to the ecosystem dynamics, a conclusion that might be much more general, and suggests that carbonate weathering and associated CO2 consumption fluxes quickly react to any global change or land use modification.


      PubDate: 2014-11-15T04:38:19Z
       
  • Dissolution rates of actinolite and chlorite from a whole-rock
           experimental study of metabasalt dissolution from 2≤pH≤12 at
           25°C
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): T. Critelli , L. Marini , J. Schott , V. Mavromatis , C. Apollaro , T. Rinder , R. De Rosa , E.H. Oelkers
      The dissolution rates of the minerals actinolite and chlorite were determined from metabasalt element release rates measured at 25°C and 2<pH<12 in mixed flow reactors. At pH 2.0 and 3.2, chlorite rates are 3 and 5 times faster, respectively, than corresponding actinolite rates, whereas the Si release rates from metabasalt are intermediate between chlorite and actinolite rates. In contrast, at pH 7.2 and 12.0, chlorite, actinolite and the metabasalt release Si at the same rates within analytical uncertainties. At pH 6.3, it was only possible to obtain the chlorite dissolution rate; at this pH the measured chlorite dissolution rate is 10−11.86 mol/m2/s. Mineral dissolution rates obtained in this study are within the range of corresponding values reported in the literature. This observation suggests that the dissolution rates of major-constituent minerals in a multi-phase rock are not affected by the presence of the other minerals. This conclusion validates the common assumption that the dissolution rate of an individual mineral is equal to that of the same mineral in a dissolving multi-mineralogic rock, at least for major constituents.


      PubDate: 2014-11-15T04:38:19Z
       
  • Enhanced heterotrophic denitrification in clay media: The role of mineral
           electron donors
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Marc Parmentier , Patrick Ollivier , Catherine Joulian , Achim Albrecht , Jebril Hadi , Jean-Marc Greneche , Hélène Pauwels
      Denitrification is expected to occur at and near the interface with clayey formations dedicated to radioactive waste disposal, due to the nitrate content in some waste canisters and the high probability of introducing denitrifiers during the operational phase. Nitrate reduction to nitrous-oxide gas by pure-strain Pseudomonas mandelii, in the presence or absence of sterilized Callovian–Oxfordian (COx) clay rock, was studied over a period of ~41.5months (1267days) by means of batch experiments. A culture medium with a similar porewater chemistry to that of COx rocks was used, supplemented with acetate and nitrate. Bacterial growth was monitored by genomic-DNA and narG-gene quantification. Nitrite accumulated in solution concomitantly with a decrease in nitrate content and the weak generation of nitrous oxide, but denitrification rates drastically decreased over the study period. Acetate was both oxidized to inorganic carbon and incorporated into biomass. The presence of solid COx particles significantly affected the geochemical reactions and particularly caused an enhanced nitrate reduction, a higher bacterial growth and the precipitation of calcium carbonate. Moreover, in the presence of COx a delay of several weeks was observed before the accumulation of nitrite in solution, leading to an imbalance between nitrate consumption and the production of nitrite, nitrous oxide and ammonium. Chemical oxidation of clay along with nitrite reduction to dinitrogen is expected to occur, explaining both the delay in nitrite accumulation and the apparent imbalance in nitrogen species. Although the electron donor of the COx oxidation was not identified, several hypotheses may be advanced, and we provide new insight into the biogeochemical and geochemical processes that may occur concomitantly at the excavation damaged interface of the clayey host rock after closure, resaturation and release of waste components in a nuclear waste repository. Because of denitrification, the oxidative impact of nitrate released from waste will diminish. However, nitrites produced through denitrification and diffusing into the rock will likely play in redox reaction via their abiotic reactivity which may occur within the compact clay formation despite a lack of space for bacterial activity.


      PubDate: 2014-11-15T04:38:19Z
       
  • Selenium isotopes trace anoxic and ferruginous seawater conditions in the
           Early Cambrian
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Hanjie Wen , Jean Carignan , Xuelei Chu , Haifeng Fan , Christophe Cloquet , Jing Huang , Yuxu Zhang , Huajin Chang
      Selenium (Se) isotopes can yield substantial isotopic fractionation (up to 20‰) confirmed by experiments and field investigations, depending on various biotic or abiotic redox transformations. Therefore, it is expected that redox changes in the ancient oceans would induce significant isotopic fractionation, and the Se isotopic signatures recorded in old sedimentary rocks might provide new insight into how the redox state of the ancient ocean has evolved. However, previous studies have shown that Se is slightly enriched in the lighter isotope relative to the bulk earth values in most deposited conditions (oxic, anoxic, and even sulfidic). Here, our results reveal that ferruginous conditions can result in excessive accumulation of Se in sediments with an elevated Se/S ratio and significant isotope fractionation (about 6‰), which leads us to propose that Se isotopes are an appropriate geochemical proxy to trace unique oceanic conditions over times. Accordingly, Se isotopic variations measured in three Early Cambrian formations in southern China suggest that anoxic waters with ferruginous conditions must have been present in early Cambrian ocean along the eastern margin of the Yangtze platform, and oceanic circulation was stepwise reorganized. This may have triggered biological diversification from the Ediacaran to the Early Cambrian.


      PubDate: 2014-11-15T04:38:19Z
       
  • U(VI) and Sr(II) batch sorption and diffusion kinetics into mesoporous
           silica (MCM-41)
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): David M. Singer , Hua Guo , James A. Davis
      Important reactive phenomena that affect the transport and fate of a radioactive material such as uranium (U) and strontium (Sr) in the environment occur at the mineral–water interface, particularly in mesoporous materials which are ubiquitous in surface and near-surface environments, and typically dominate the reactive surface area of geologic media. Ion sorption and physical bonding forces (including electrostatic forces) can be significantly modified within these confined pore spaces, leading to preferential enrichment of trace elements in mesopores. Pore space confinement may also lead to kinetic restraints on thermodynamically favorable sorption/desorption, precipitation/dissolution, and redox reactions, due to slow migration of metals out of mesopores, chemical gradients within the pore space, or steric constraints for inward migration of larger molecules. Using a combination of benchtop sorption experiments, TEM, and synchrotron-based X-ray absorption spectroscopy, U(VI) and Sr(II) uptake on mesoporous silica (MCM-41) with a 4.67nm pore diameter was measured in batch conditions at pH4.0 and 9.8 as a function of time and metal speciation. Uptake of U was determined for U-hydrolysis, U–CO3, and U–CO3–Ca aqueous species. This suite of techniques enabled determination of the rate of metal sorption and precipitation in the pore spaces, and identification of the reaction products. Our results indicate that Sr and U (at less than 10μM total U) rapidly diffuse into MCM-41. U at a higher concentration than 10μM also rapidly diffuses in, but the higher pore volume U concentration eventually leads to polymerization and precipitation of nano-U-bearing phases. The steady state U sorption maximum after 48h of exposure to MCM-41 prior to precipitation was dependent on the size and charge of the dominant U species in solution, where the trend is: UO2(OH)3 − >UO2 +2 >UO2(CO3)3 −4 ≈CaUO2(CO3)3 −2. Precipitation of a U-bearing phase within the silica pore spaces occurred only after a threshold time point and indicated that U uptake was both thermodynamically and kinetically controlled. Initial diffusion and adsorption were controlled by aqueous speciation and precipitation was controlled by the buildup of sorption species that subsequently created a bottleneck effect near pore openings. Acidic solutions wee more efficient at extracting U than carbonate solutions once the U has diffused into the mesopore region, and this may explain frequent observations of this behavior in extractions of natural sediments. A nitric acid wash was not completely effective at desorbing U and Sr from the mesoporous silica, where there was 5% (initial pH4.0) and 20% (pH9.8) remaining even after 500h of exposure to the nitric acid wash. The results show that U and Sr diffusion and sorption into mesopores result in a recalcitrant pool of ions that are sequestered in deep internal pore spaces, and this may have significant impacts on attempts to clean up contaminated soils and sediments.
      Graphical abstract image

      PubDate: 2014-11-15T04:38:19Z
       
  • Effects of U–Th-rich grain boundary phases on apatite helium ages
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Kendra E. Murray , Devon A. Orme , Peter W. Reiners
      Single-grain apatite (U–Th–Sm)/He ages (He ages) from non-detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion, such as grain size, radiation damage, parent zonation, fracturing, or intracrystalline inclusions. We present observations and model age bias effects of an additional source of apatite He age dispersion observed in some cases: U–Th-rich grain boundary phases (GBPs) precipitated on apatite crystal surfaces. Apatite grains from several samples with dispersed apatite He ages are coated or partially coated by reddish-orange GBPs rich in Fe, U, and Th. These GBPs are heterogeneous in thickness, grain coverage, and composition, and have effective U concentrations ([eU]) up to ~1000ppm. These phases may have large effects on the bulk [eU] and 4He compositions of apatite grains and can produce significant He age biases. The direction and magnitude of this age bias is primarily a function of four factors: 1) the host grain's size, 2) the GBPs' thickness and [eU] relative to the host grain, 3) the timing of GBP formation relative to the cooling age of the host grain, and 4) whether the GBPs are preserved and analyzed with the grain during He dating. Some of the most severe effects occur when GBPs formed before or near the time of the apatite cooling age but were lost just prior to analysis, for example during mineral separation. In this case, GBPs of commonly observed thicknesses (1–10μm) with [eU] 2–10 times that of the host apatite grain implant enough 4He into typical-sized apatite grains to produce hundreds of percent positive age biases. In contrast, when U–Th-rich GBPs are preserved and analyzed with apatite grains, the resulting He ages can be negatively biased. Though the heterogeneity, variable preservation, and ambiguous formation ages of GBPs preclude a general, quantitative, and practical solution to this problem, our model demonstrates that observed age dispersions in some samples are consistent with the effects of U–Th-rich grain boundary phases and illustrates the conditions under which, and by how much, they will bias He ages.


      PubDate: 2014-11-15T04:38:19Z
       
  • Assessing redox zones and seawater intrusion in a coastal aquifer in South
           Korea using hydrogeological, chemical and isotopic approaches
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Duk-Min Kim , Seong-Taek Yun , Man Jae Kwon , Bernhard Mayer , Kyoung-Ho Kim
      A shallow (<25m), coastal alluvial groundwater system underneath a paddy field in the Yangyang area of South Korea was investigated to examine the occurrence of redox processes. The aquifer is affected by seawater intrusion, and is characterized by a highly reducing environment facilitated by high organic matter in the sediments. Hydrochemical data with δ34S and δ18O of sulfate were examined for depth-specific groundwater from two multilevel samplers that were installed at seaward (YY2) and landward (YY1) locations. Shallow groundwater showed distinct patterns of redox zoning. Evidence of significant bacterial sulfate reduction (BSR) was observed throughout the nearly entire depths of the two boreholes, while at the depths of active seawater intrusion in YY2, conditions suitable for methanogenesis were never reached. Thus, at YY2 the deep zone of intense BSR was overlain by a zone in which methanogenesis occurred in a low-sulfate environment. In contrast, concurrent BSR and methanogenesis in YY1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive BSR. Considerable BSR in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of δ34Ssulfate up to 99.9‰. The inferred sulfur isotopic enrichment factor (ε) for BSR in the lower part of YY2 was −12.3‰, while ε at YY1 was much higher (−45.9‰). In addition, the observed trends of δ18Osulfate at YY1 indicated significant oxygen isotope exchange of sulfate-oxygen with ambient water, likely because of lower cell-specific rates of BSR and higher sulfur isotope fractionation as indicated by the δ34S. In contrast, there was little evidence of oxygen isotope exchange between water and SO4 2− at YY2. This study indicates that in coastal aquifers with sulfate-reducing activity, δ34S and δ18O of sulfate can reveal zones of active seawater intrusion and of trapped seawater. This study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic-rich coastal environment.


      PubDate: 2014-11-15T04:38:19Z
       
  • Accurate analysis of shallowly implanted solar wind ions by SIMS backside
           depth profiling
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Veronika S. Heber , Kevin D. McKeegan , Donald S. Burnett , Jean Duprat , Yunbin Guan , Amy J.G. Jurewicz , Chad T. Olinger , Stephen P. Smith
      A method to quantitatively determine the fluences of shallowly-implanted solar wind ions returned to Earth by the Genesis Discovery mission is described. Through backside depth-profiling, we recover nearly complete depth profiles of implanted solar wind for several nonvolatile elements, including Mg, Al, Ca, Cr, and, to a lesser extent, Na, in silicon targets that collected bulk solar wind and solar wind from specific velocity regimes. We also determine the fluences of the volatile elements C, N, and O in silicon targets that collected bulk solar wind. By the use of appropriately calibrated ion implanted standards, fluences as low as 2×1010 atoms cm−2 can be determined with precision and accuracy typically in the few percent range. Specific approaches to sample preparation, sputtering artifacts during depth profiling by secondary ion mass spectrometry, and quantification including the production of ion implant standards are discussed.


      PubDate: 2014-11-10T04:06:55Z
       
  • Zn isotope fractionation during interaction with phototrophic biofilm
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Aude Coutaud , Merlin Meheut , Jérôme Viers , Jean-Luc Rols , Oleg S. Pokrovsky
      This study addresses Zn isotopic fractionation during its sorption and excretion with a mature phototrophic biofilm using both a batch reactor and an open drip flow reactor (DFR). Short-term batch adsorption experiments demonstrated an enrichment in heavier isotopes larger than that induced by biofilm's Zn uptake, in general agreement with previous results for aquatic micro-organisms. Presumably, the adsorption of heavy isotopes was controlled by the formation of strong covalent bonds with the surface moieties rather than the kinetics of diffusion through the boundary layer to the biofilm surface. The rapid physico-chemical passive desorption was driven by the exchange of H+/Zn2+ ions with a highly negative isotopic enrichment factor (e.g. ε66Znexcretion of −1.4±0.35‰). The Zn efflux dominated by diffusion due to the osmotic biofilm–solution gradient favored lighter isotope release (e.g. ε66Znexcretion of −0.40±0.07‰ for wet biofilm in the DFR). The passive physico-chemical processes of Zn diffusion and H+/Zn2+ ion exchange are capable of inducing greater isotopic separation between solution and biomass compared to the active (intracellular) uptake. Regardless of the nature of aquatic microorganisms, the biomass enrichment with heavy isotopes during adsorption and uptake and preferential release of light isotopes seem to be general characteristics of aquatic biota.
      Graphical abstract image

      PubDate: 2014-11-07T03:50:48Z
       
  • Influence of radiation damage on Late Jurassic zircon from southern China:
           Evidence from in situ measurements of oxygen isotopes, laser Raman,
           U–Pb ages, and trace elements
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Xiao-Lei Wang , Matthew A. Coble , John W. Valley , Xu-Jie Shu , Kouki Kitajima , Michael J. Spicuzza , Tao Sun
      The influence of radiation damage on chemical alteration and in situ elemental and isotopic analyses in zircons is evaluated in three ~155Ma granites that are associated with W–Sn polymetallic deposits in the Nanling area of southern China. A combined approach is described using SEM imaging, laser Raman spectroscopy, and ion microprobe analysis of oxygen isotope ratios, U–Pb ages, and trace elements, including H. Domains within single zircons can be classified into two groups based on cathodoluminescence (CL) intensity and U–Th concentrations. Type-I are single crystals or clearly defined cores with bright CL, having U<1400ppm and Th<800ppm; type-II are rims surrounding type-I cores or single crystals with dark or intermediate CL-intensity and have 2100–30,000ppm U and 900–6500ppm Th. Both types of zircon display oscillatory zonation. Type-I zircons show a narrow range of δ18Ozircon values (8.5–9.2‰), while type-II zircons have much greater variation of apparent δ18Ozircon values (1.4–8.6‰). Relative to type-I, type-II zircons are characterized by elevated trace-element concentrations (including U, Th, rare earth elements, Y, Hf, Fe and Ti, LaN >10), high cumulative α-dose and background-corrected 16OH/16O, decreased Th/U and Hf/Yb ratios, and deviation in 206Pb/238U ages (up to 70Ma) from the mean age (ca. 155Ma) of type-I zircons. Moreover, laser Raman results show that type-II zircons have much broader FWHM (full width half maximum=6.3–37.8cm−1) and decreased amplitudes for the ~1005cm−1 Raman peak, indicating that they are affected by significant radiation damage. The Raman-measured preserved radiation damage (D α P =0.01–0.49×1015 α-decay events/mg) is less than the total alpha-dose (D α T =0.1–1.72×1015 α-decay events/mg) for each zircon of type-I and type-II with FWHM<10cm−1, indicating that 30–80% of the total radiation damage experienced by most of these zircons has been annealed. We attribute the negative correlation of δ18O versus U, Th, Y, Hf, La and cumulative α-dose for type-II zircons to open-system behavior resulting from radiation damage of the zircon crystal structure. We interpret type-II zircons to originally represent crystallization from a late-stage granite melt that was enriched in magmatic fluids and incompatible elements. The wide range and low δ18O values recorded by most type-II zircon domains are the result of secondary alteration by fluids (perhaps hydrothermal) facilitated by radiation damage and open system chemical and isotopic exchange. This work shows that a combined approach using imaging, laser Raman and trace element analysis is effective for evaluating the influence of radiation damage and alteration of zircon. This protocol is necessary to evaluate the reliability of in situ oxygen isotopic and U–Pb ages. The 16OH/16O ratio is readily measured during SIMS analysis of oxygen isotopes and can be an effective parameter to monitor alteration of radiation-damaged domains in zircon.


      PubDate: 2014-11-02T03:26:06Z
       
  • Carbon- and oxygen-stable isotopic signatures of methanogenesis,
           temperature, and water column stratification in Holocene siderite varves
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Chad Wittkop , Jane Teranes , Brady Lubenow , Walter E. Dean
      Manganoan siderite ([Fe,Mn]CO3) occurs in abundance of up to 19% (dry weight) as the sole endogenic carbonate within a succession of Holocene, organic-rich, varved sediments from freshwater Otter Lake (OL), Michigan. Radiocarbon dating and varve counts from a 7-m piston core constrain periods of major siderite accumulation to sediments older than 1200calyr BP. Sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet. Bulk-sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial-scale cycles of siderite accumulation. Siderite δ13C and δ18O are enriched when the mineral is abundant and depleted when it is least abundant. Samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water, but are δ13C enriched relative to modern waters. Samples from intervals of low siderite abundance are δ13C and δ18O depleted relative to equilibrium with modern lake water. These data suggest that abundant siderite precipitation occurred when the OL water column was ferruginous (iron meromictic), allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon (DIC) enriched in δ13C below the chemocline, where methanogenesis in waters and sediments influenced DIC composition. Seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution. Manganese substitution in siderite lowered kinetic barriers to low-temperature mineral precipitation. Climate changes and basin filling influenced lake stratification and the rate of siderite precipitation. The siderite occurrence reported here displays remarkable similarity to its occurrence in Paleogene maar lake deposits, suggesting that further studies of Holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments.


      PubDate: 2014-11-02T03:26:06Z
       
  • Fingerprints of metamorphism in chromite: New insights from minor and
           trace elements
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Vanessa Colás , José M. González-Jiménez , William L. Griffin , Isabel Fanlo , Fernando Gervilla , Suzanne Y. O'Reilly , Norman J. Pearson , Thomas Kerestedjian , Joaquín A. Proenza
      A suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in chromite grains from ophiolitic chromitites subjected to high-pressure metamorphism defines a metamorphic signature. A two-stage process associated with the infiltration of fluids during retrograde metamorphism from eclogite- to amphibolite-facies has produced four types of chromites: (1) porous chromite strongly enriched in Cr and Fe2+ but depleted in Al and Mg, with abundant chlorite filling the pores; (2) non-porous chromite strongly enriched in Fe3+ (i.e., ferrian chromite); (3) partly altered chromite with primary cores surrounded by chlorite-bearing porous chromite; and (4) zoned chromite made up of primary cores surrounded by non-porous rims of ferrian chromite. Compared to spinels from unmetamorphosed chromitites the cores of partly altered chromites after primary high-Cr chromite are enriched in Zn, Co and Mn but strongly depleted in Ga, Ni and Sc. This distribution of minor- and trace-elements is related to a decrease in Mg# [Mg/(Mg+Fe2+)] and Al, produced by the crystallization of chlorite in the pores of porous chromite. Non-porous chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned chromites have cores depleted in Ga, Ni and Sc but are progressively enriched in Zn, Co and Mn as Mg# and Al decrease toward the rims; they have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian chromite. The complex variation of the minor- and trace-elements vs Fe3+/(Fe3+ +Fe2+) in the different types of chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims. The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear “unaltered” in terms of major elements. The use of the major elements as indicators of magmatic processes therefore must be linked to careful evaluation of metamorphic effects, using LA-ICP-MS analysis of minor and trace elements.


      PubDate: 2014-11-02T03:26:06Z
       
  • Characterization of cores from an in-situ recovery mined uranium deposit
           in Wyoming: Implications for post-mining restoration
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): G. WoldeGabriel , H. Boukhalfa , S.D. Ware , M. Cheshire , P. Reimus , J. Heikoop , S.D. Conradson , O. Batuk , G. Havrilla , B. House , A. Simmons , J. Clay , A. Basu , J.N. Christensen , S.T. Brown , D.J. DePaolo
      In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7m of core of the formerly the ISR-mined Smith Ranch–Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (>1wt.%) and clay (>6–17wt.%) contents yielded higher 234U/238U activity ratios (1.0–1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374–11,534ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234U/238U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining. EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U–O distances of ~2.05Å were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from UO at 1.79Å, which indicates that U is partially oxidized. There is no apparent U–S or U–Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite. Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of their limited access to fluid flow, these areas might not contribute significantly to post-mining U release or attenuation. Areas with pyrite that are accessible to fluids seem to be more reactive and could act as reductants and facilitate U reduction and accumulation, limiting its migration.


      PubDate: 2014-11-02T03:26:06Z
       
  • Sr/Ca differences within and among three Tridacnidae species from the
           South China Sea: Implication for paleoclimate reconstruction
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Hong Yan , Da Shao , Yuhong Wang , Liguang Sun
      In this study, eight Tridacnidae specimens of three different species (Tridacna gigas, Tridacna derasa and Hippopus hippopus) were collected from Xisha Islands of South China Sea. The Sr/Ca ratio, which has been widely used as sea surface temperature (SST) proxy in coral samples, was determined on these specimens using ICP-OES to examine their potential for paleoclimate reconstruction. While within-species differences were found to be insignificant, the mean Sr/Ca ratios of the three species (2.24±0.1, 1.77±0.1 and 1.91±0.14mmol/mol for modern T. gigas, T. derasa and H. hippopus specimens, respectively) are statistically different. Species identification, therefore, is necessary before using Tridacnidae Sr/Ca ratios as paleoclimate recorders. While the relationship between local SST and Sr/Ca ratios of H. hippopus was unclear, the Sr/Ca profiles of modern T. gigas and T. derasa specimens are significantly correlated to local SST, implying that they can be used as SST proxies. The range of Sr/Ca ratios measured in this study is comparable to the Tridacnidae Sr/Ca measurements obtained in previous studies from other areas, but there are also substantial differences in detail, indicating that calibration for regional differences is also needed before paleoclimate reconstruction is attempted. In addition, although the mean Sr/Ca values of T. gigas, T. derasa and coral samples in Xisha Islands were significantly different, their Sr/Ca-SST calibration slopes were comparable, pointing to a similar thermodynamic fractionation and highlighting the potential of the combination of Tridacnidae and coral samples for paleo-SST seasonality reconstruction.


      PubDate: 2014-10-30T03:15:29Z
       
  • Structure and stability of arsenate adsorbed on α-Al2O3
           single-crystal surfaces investigated using grazing-incidence EXAFS
           measurement and DFT calculation
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Meiyi Zhang , Guangzhi He , Gang Pan
      Direct characterization of contaminants on single-crystal planes is required because the specific adsorption characteristics on different exposed crystal planes constitute their actual behavior at water–mineral interfaces in aquifers. Here, the structure and stability of arsenate on α-Al2O3 (0001) and ( 11 2 ¯ 0 ) surfaces were characterized by using a combination of grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) spectra and periodic density functional theory (DFT) calculation. The combined results indicated that arsenate was mainly adsorbed as inner-sphere monodentate and bidentate complexes on both surfaces, but the orientational polar angles on the (0001) surface were commonly 10–20° greater than that on the ( 11 2 ¯ 0 ) surface. The DFT calculation showed that the large polar angle was more favorable for arsenate stabilized on the alumina surfaces. Based on the spectroscopic and computational data, the dominant bonding modes of arsenate on the two crystal planes of α-Al2O3 were identified as bidentate binuclear structures, and the (0001) surface displayed a stronger affinity toward arsenate.
      Graphical abstract image

      PubDate: 2014-10-26T02:53:07Z
       
  • Adhesion of Pseudomonas putida onto kaolinite at different growth phases
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Huayong Wu , Wenli Chen , Xingmin Rong , Peng Cai , Ke Dai , Qiaoyun Huang
      Bacterial adhesion to minerals in soils and sediments is of fundamental importance in mineral weathering and formation, soil aggregate stability, organic matter degradation and the fate of pollutants. Bacterial surface properties are considered to govern adhesion, and these properties likely change as a function of bacterial growth phase. However, the effect of growth stage on bacterial adhesion to clay minerals remains unclear. This work examined the influence of growth phase on the adhesion of Pseudomonas putida to kaolinite-coated coverslips. Fluorescence microscopy, together with a bacterial viability stain, was used to directly quantify surface cell density and viability of adhered P. putida. In situ attenuated total reflectance Fourier transform infrared spectroscopy was applied to yield molecular information about the characteristics of the bacteria, and the adsorption and desorption kinetics. Stationary-phase cells exhibited a higher adsorption density on kaolinite surfaces than mid-exponential-phase cells under static deposition conditions. Compared with the mid-exponential-phase cells, the stationary-phase cells displayed higher saturation coverage, and we fitted the results using a pseudo-first-order kinetics equation. The greater extent of adhesion of the stationary-phase cells was probably due to their smaller cell size and less negative surface charges compared with the cells from other growth stages, which resulted in deeper secondary energy minima and lower energy barriers for adhesion. The results from this study suggest that growth phase may strongly influence cell mobility and biofilm formation in aqueous geochemical environments.


      PubDate: 2014-10-26T02:53:07Z
       
  • Reliability of LA-ICP-MS U-Pb dating of zircons with high U
           concentrations: A case study from the U-bearing Douzhashan Granite in
           South China
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): Kui-Dong Zhao , Shao-Yong Jiang , Hong-Fei Ling , Martin R. Palmer
      Granite-hosted uranium ore deposits are the most important commercial U-producers in South China. These U-bearing granites generally contain high-U-concentration zircons. Zircons in the U-bearing Douzhashan Granite show black rims in cathodoluminescence images, which have very high U concentrations of up to >25,000ppm. The high-U black rim domains of zircons show similar REE patterns to those of low-U mantle domains with oscillatory zoning, but have higher REE concentrations and weaker positive Ce anomalies. The high-U black rim domains of zircons may have formed from late-stage evolved U-rich and H2O-rich magma. We carried out a SHRIMP U-Pb dating for these zircons and the results show that the high-U (12,007–26,706ppm) rim domains of the zircons always yield older 206Pb/238U ages (268±9Ma, n=5, MSWD=2.7) than the low-U (558–3667ppm) mantle domains (228±11Ma, n=4, MSWD=3.1), and both of these ages are older than the emplacement age (211±3Ma) of the U-barren Xiangchaoping Granite, which was intruded by the Douzhashan Granite. Hence, these results demonstrate a matrix effect associated with SHRIMP U-Pb analyses of high-U zircons. The reverse discordance for SHRIMP results of high-U zircons was produced by analytical artifact. However, our LA-ICP-MS analyses show that the black rim domains (12,840–26,018ppm U) of the zircons have the same U-Pb ages as the low-U mantle domains (1215–4075ppm U). Both domains yield a weighted mean 206Pb/238U age of 203±4Ma (n=13, MSWD=1.4), which is consistent with the 40Ar-39Ar age (207±4Ma) of muscovite from this granite. It thus indicates that LA-ICP-MS U-Pb analyses for high-U zircons (up to 26,000ppm U) show an insignificant matrix effect and likely yield more reliable U-Pb ages for the high-U zircons than SHRIMP analyses. In order to get reliable U-Pb ages using the LA-ICP-MS method for the high-U zircons, a pulse-analog cross calibration for the ICP-MS collector should be performed. Our precise U-Pb dating and geochemical studies indicate that the U-bearing Douzhashan Granite was emplaced during the late Indosinian stage and formed in a post-collisional setting of the Indosinian Orogeny. It is distinct from most U-bearing Granites in South China, which were largely emplaced during the early Indosinian stage and formed in a syn-collisional setting.


      PubDate: 2014-10-26T02:53:07Z
       
  • High-precision Mg isotope analyses of low-Mg rocks by MC-ICP-MS
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Yajun An , Fei Wu , Yuanxin Xiang , Xiaoyun Nan , Xun Yu , Jinhui Yang , Huimin Yu , Liewen Xie , Fang Huang
      We present a method for precise measurement of Mg isotope ratios for low-Mg rock samples (where MgO <1wt.%) by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The efficiency of Mg purification is significantly improved by using a newly calibrated HNO3 +HF step to remove undesired matrix elements (such as Ti, Al, Fe, and K) in low-Mg samples. We also establish that increasing the amount of Mg loaded to the chromatographic column minimized blank effects of organics leached from cation resin. All parameters that could affect the accuracy and precision of Mg isotope analyses were rigorously examined by two independent laboratories in Beijing and Hefei. The δ26Mg of mono-elemental Mg standard CAM-1 measured in the two laboratories were −2.597±0.042‰ (2σ, n=49) and −2.598±0.039‰ (2σ, n=79), respectively; in house standard IGGMg1 were −1.742±0.041‰ (2σ, n=53) and −1.749±0.049‰ (2σ, n=72), respectively. The average δ26Mg over ten months of two synthetic standards, made by doping IGGMg1 and IGGMg2 with matrix elements, agrees well with their recommended values, within error. The robustness of our method was further assessed by replicated analyses of sixteen rock standards with MgO contents from 0.28wt.% to 49.4wt.%. The δ26Mg of USGS rhyolite standards RGM-1 and RGM-2 are −0.188±0.031‰ (2σ, n=35) and −0.182±0.041‰ (2σ, n=72), respectively; granite standard GA is −0.165±0.038‰ (2σ, n=57), G-2 is −0.129±0.045‰ (2σ, n=34), GS-N is −0.204±0.059‰ (2σ, n=33), GSP-2 is −0.042±0.020‰ (2σ, n=15), and GSR-1 is −0.234±0.016‰ (2σ, n=17). Based on repeated analyses of standards, the long-term external precision of our method is better than ±0.05‰ for δ26Mg. This precision allows us to distinguish the fractionation of Mg isotopes in low-Mg granites and rhyolites as well as that between mantle minerals.


      PubDate: 2014-10-26T02:53:07Z
       
  • Testing provenance diagrams: Lessons from the well-constrained Cariaco
           Basin
    • Abstract: Publication date: 11 December 2014
      Source:Chemical Geology, Volume 389
      Author(s): A. Riboulleau , V. Bout-Roumazeilles , N. Tribovillard , F. Guillot , P. Recourt
      The fine-grained sediments of the Cariaco Basin, Venezuela, of the last 130ky, whose deposition history is well characterized, were analyzed geochemically in order to test the validity of sediment bulk geochemistry as an indicator of detrital provenance. Several binary and ternary diagrams as well as the chemical index of alteration (CIA) were tested for their capacity to discriminate the poorly contrasted detrital sources to the Cariaco Basin, and to describe the temporal evolution of the contributions of these different sources. Most of the diagrams tested did not allow a good discrimination of sources or, when sources were well discriminated, did not allow an interpretation of the temporal variations consistent with the known history. A relatively good discrimination of sources and a consistent interpretation of temporal variations were however obtained using Hf vs. Th and La/Yb vs. Gd/Yb binary diagrams, as well as Ti–Zr–Th, Ti–Zr–La, and Lu–Hf–Th ternary diagrams. Compared to the previous studies of the detrital content of the Cariaco Basin sediments, the geochemical approach permitted the recognition of a sediment contribution eroded from the Unare platform and Gulf of Cariaco during rapid sea level oscillations, and the contribution of Saharan eolian particles during the Younger Dryas–Preboreal and MIS6-5 transition. The choice of plotted elements was determined after considering carrier minerals, so that different elements may be informative in different sedimentary contexts. Overall, mineral sorting during transport appears as a major limit to quantitative estimation of the different contributions. In particular mineral sorting leads to the selective enrichment of elements associated with clays (Al, Rb, Th and LREE) in sediments deposited in the basin. Unless the geochemical effect of mineral sorting can be measured, it appears that quantitative provenance analysis should be performed on fractions of similar grain size instead of bulk sediment.


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


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


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


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


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


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


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


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


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


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


      PubDate: 2014-10-02T01:21:03Z
       
  • Climate-forced change in Hudson Bay seawater composition and temperature,
           Arctic Canada
    • Abstract: Publication date: 21 November 2014
      Source:Chemical Geology, Volume 388
      Author(s): Uwe Brand , Rosemarie E. Came , Hagit Affek , Karem Azmy , Randy Mooi , Kara Layton
      Global climate change impacts marine ecosystems, directly and indirectly, especially in the Arctic and Antarctic. We show the first long-term (1920–2011) time-series of oceanographic change in Hudson Bay, an arctic marine ecosystem, based on coupled brachiopod-calcite stable and clumped isotope results. Long-term decrease in brachiopod δ13C parallels that of seawater-DIC in Hudson Bay, and after considering its seasonal sea ice coverage, it is similar to that of the 13C-Suess effect observed in the North Atlantic and other regions. Acidification of Hudson Bay seawater leads warming by about 10–20years, and with intensified warming from the 1970s to 2010s closely coupled to earlier sea-ice breakup. Post-industrial warming of Hudson Bay is initially slow, but in later years, faster and of greater magnitude than of the coeval global oceans. Our observations for the past 90years suggest that climate-forced change contributed to an average increase of about 0.1°C and 3.6°C in sea-surface water temperature of Hudson Bay over the first 50 and subsequent 40years, respectively. This 3.7°C post-industrial warming of Hudson Bay seawater is about six times the 0.67°C increase observed during the past 100years in global ocean sea-surface temperature, which is about double the postulated increase of about 2°C for polar regions. Our results are consistent with the general notion that polar marine environments, such as Hudson Bay, can serve as sensitive indicators of change in climate, and of change still to come for lower latitude ecosystems.


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


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


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


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


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


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


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

      PubDate: 2014-09-25T00:58:03Z
       
 
 
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