for Journals by Title or ISSN
for Articles by Keywords
help
  Subjects -> EARTH SCIENCES (Total: 645 journals)
    - EARTH SCIENCES (466 journals)
    - GEOLOGY (71 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (60 journals)

EARTH SCIENCES (466 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal  
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 4)
Advances in High Energy Physics     Open Access   (Followers: 16)
Advances In Physics     Hybrid Journal   (Followers: 9)
Aeolian Research     Hybrid Journal   (Followers: 3)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 4)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 2)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 18)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 12)
Anthropocene     Hybrid Journal  
Anthropocene Review     Hybrid Journal   (Followers: 3)
Applied Clay Science     Hybrid Journal   (Followers: 3)
Applied Geochemistry     Hybrid Journal   (Followers: 8)
Applied Geomatics     Hybrid Journal   (Followers: 5)
Applied Geophysics     Hybrid Journal   (Followers: 8)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 12)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 35)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 21)
Asian Review of Environmental and Earth Sciences     Open Access  
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: 6)
Atmospheric and Climate Sciences     Open Access   (Followers: 15)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 12)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 11)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 10)
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: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 15)
Canadian Mineralogist     Full-text available via subscription   (Followers: 2)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 18)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 11)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 2)
Ciencia del suelo     Open Access   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 23)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 7)
Computational Geosciences     Hybrid Journal   (Followers: 13)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 8)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 8)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 3)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 1)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 2)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 72)
Earth and Space Science     Open Access   (Followers: 1)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 4)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 14)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 6)
Electromagnetics     Hybrid Journal   (Followers: 2)

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [11 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2801 journals]
  • The continental Si cycle and its impact on the ocean Si isotope budget
    • Abstract: Publication date: Available online 28 January 2016
      Source:Chemical Geology
      Author(s): Patrick J. Frings, Wim Clymans, Guillaume Fontorbe, Christina De La Rocha, Daniel J. Conley
      The silicon isotope composition of biogenic silica (δ30SiBSi) in the ocean is a function of the δ30Si of the available dissolved Si (DSi; H2SiO4), the degree of utilisation of the available DSi, and, for some organisms, the concentration of DSi. This makes δ30SiBSi in sediment archives a promising proxy for past DSi concentrations and utilisation. At steady-state, mean δ30SiBSi must equal a weighted average of the inputs, the majority of which are of continental origin. Variation in the functioning of the continental Si cycle on timescales similar to the residence time of DSi in the ocean (~10ka) may therefore contribute to downcore variability in δ30SiBSi on millennial or longer timescales. The direction and magnitude of change in published δ30SiBSi records over the last few glacial cycles are consistent among ocean basins and between groups of silicifiers. They document glacial values that are typically 0.5 to 1.0‰ lower than interglacial values and together hint at coherent and predictable glacial–interglacial variability in whole-ocean δ30Si driven by a change in mean δ30Si of the inputs. In this contribution, we review the modern inputs of DSi to the ocean and the controls on their isotopic composition, and assess the evidence for their variability on millennial-plus timescales. Today, 9.55×1012 molyr−1 DSi enters the ocean, of which roughly 64% and 25% are direct riverine inputs of DSi, and DSi from dissolution of aeolian and riverborne sediments, respectively. The remainder derives from alteration or weathering of the ocean crust. Each input has a characteristic δ30Si, with our current best estimate for a weighted mean being 0.74‰, although much work remains to be done to characterise the individual fluxes. Many aspects of the continental Si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean δ30Si of DSi entering the ocean. These changes relate to i) a cooler, drier glacial climate, ii) lowered sea level and the exposure of continental shelves, iii) the presence of large continental ice-sheets, and iv) altered vegetation zonation. Using a simple box-model with a Monte-Carlo approach to parameterisation, we find that a transition from a hypothesised glacial continental Si cycle to the modern Si cycle can drive an increase in whole ocean δ30Si of comparable rate and magnitude to that recorded in δ30SiBSi. This implies that we may need to revisit our understanding of aspects of the Si cycle in the glacial ocean. Although we focus on the transition from the last glacial, our synthesis suggests that the continental Si cycle should be seen as a potential contributory factor to any variability observed in ocean δ30SiBSi on millennial or longer timescales.


      PubDate: 2016-02-01T14:08:56Z
       
  • The pseudomorphic replacement of marble by apatite: The role of fluid
           composition
    • Abstract: Publication date: Available online 28 January 2016
      Source:Chemical Geology
      Author(s): Elisabete Trindade Pedrosa, Christine V. Putnis, Andrew Putnis
      The replacement of a natural carbonate rock (Carrara marble) by apatite was used as a model to study the role of fluid chemistry in replacement reactions, focusing on the mineralogy, chemical composition, and porosity of the replacement product. Carrara marble was reacted with ammonium phosphate solutions ((NH4)2HPO4), in the presence and absence of four salt solutions (NH4Cl, NaCl, NH4F, and NaF) at different ionic strengths, at 200°C and autogenous pressure. The replacement products were analyzed using powder X-ray diffraction, Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and Raman spectroscopy. The reaction in all samples resulted in pseudomorphic replacements and shared the characteristics of an interface-coupled dissolution–precipitation mechanism. Increasing the ionic strength of the phosphate fluid increased the replacement rates. With a fixed concentration of phosphate, replacement rates were reduced with the addition of NH4Cl and NaCl and increased significantly with the addition of NaF and NH4F. The addition of different salts resulted in specific porosity structures resulting from the formation of different phosphate phases. Chlorine-containing fluids showed a higher degree of fluid percolation through grain boundaries. This study illustrates the significant impact that small differences in solvent composition can have in the progress of replacement reactions, the nature of the products and the resultant porosity.


      PubDate: 2016-02-01T14:08:56Z
       
  • Modeling biogeochemical processes and isotope fractionation of enhanced in
           situ biodenitrification in a fractured aquifer
    • Abstract: Publication date: Available online 28 January 2016
      Source:Chemical Geology
      Author(s): Paula Rodríguez-Escales, Albert Folch, Georgina Vidal-Gavilan, Boris M. van Breukelen
      Enhanced in situ biodenitrification (EIB) is a feasible technology to clean nitrate-polluted groundwater and reach drinking water standards. Aimed at enabling a better monitoring and management of the technology at the field scale, we developed a two-dimensional reactive transport model (RTM) of a cross section (26.5×4m) of a fractured aquifer composed of marls involving both biogeochemical processes and associated isotope fractionation. The RTM was based on the upscaling of a previously developed batch-scale model and on a flow model that was constructed and calibrated on in situ pumping and tracer tests. The RTM was validated using the experimental data provided by Vidal-Gavilan et al. (2013). The model considers several processes including (i) exogenous and endogenous microbial nitrate and sulfate respiration coupled to ethanol oxidation and linked to microbial growth and decay, and (ii) geochemical interactions (dissolution/precipitation of calcite), and (iii) isotopic fractionation of the reaction network (15N–NO3, 18O–NO3, 13C–DIC, 13C–ethanol, 13C–biomass, and 13C–calcite). Most of the calibrated microbiological parameter values at field scale did not change more than one order of magnitude from those obtained at batch scale, which indicates that parameters determined at the batch scale can be used as initial estimates to reproduce field observations provided that groundwater flow is well known. In contrast, the calcite precipitation rate constant increased significantly (fifty times) with respect to batch scale. The incorporation of isotope fractionation into the model allowed to confirm the overall consistency of the model and to test the practical usefulness of assessing the efficiency of EIB through the Rayleigh equation approach. The large underestimation of the Rayleigh equation of the extent of EIB (from 10 to 50%) was caused by the high value of hydrodynamic dispersion observed in this fractured aquifer together with the high reaction rates.


      PubDate: 2016-02-01T14:08:56Z
       
  • An assessment of monazite from the Itambé pegmatite district for use
           as U–Pb isotope reference material for microanalysis and
           implications for the origin of the “Moacyr” monazite
    • Abstract: Publication date: 10 April 2016
      Source:Chemical Geology, Volume 424
      Author(s): Guilherme O. Gonçalves, Cristiano Lana, Ricardo Scholz, Ian S. Buick, Axel Gerdes, Sandra L. Kamo, Fernando Corfu, Moacyr M. Marinho, Alexandre O. Chaves, Claudio Valeriano, Hermínio A. Nalini
      Large quantities of monazite from different pegmatite bodies of the Itambe pegmatite district were investigated to assess their suitability as U–Pb and Sm–Nd isotope reference materials for LA-ICP-MS and to track the origin of a piece of the Moacyr monazite (termed here Itambé), a widely used reference material for LA-ICP-MS U–Pb geochronology. Monazite from the largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraíso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U–Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7±1.3 (207Pb⁎/235U) and 513.6±1.2Ma (206Pb⁎/238U)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508Ma. The Bananeiro monazite was assessed as a LA-ICP-MS U–Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials. MREE/HREE fractionation (ie, (La/Gd)N and (Gd/Lu)N values), Eu/Eu⁎ and the chondrite-normalized REE patterns suggest that the “Itambé” monazite aliquot is very similar to that from the Coqueiro pegmatite. This similarity is likewise apparent in their Sm–Nd isotope compositions. Moreover, the εNdi values of the “Itambé” monazite fragment (εNdi=−4.2) and those from all the major pegmatites in the district, are distinct from other reference materials (eg, Managountry; εNdi=−22.3) as well as gem-quality monazite from c. 490–520Ma pegmatites from the Araçuaí Orogen, further to the south. The εNdi can provide a further distinction for tracing Brazillian gem-quality monazite reference materials.


      PubDate: 2016-02-01T14:08:56Z
       
  • Clarifying the zircon Hf isotope record of crust–mantle evolution
    • Abstract: Publication date: Available online 29 January 2016
      Source:Chemical Geology
      Author(s): Jeffery D. Vervoort, Anthony I.S. Kemp
      Zircon has played a critically important role in our understanding of the growth and evolution of the Earth. The U–Pb isotope system as preserved in zircon, more than any other mineral or method, has provided the most precise geochronological constraints for timing of geological events and processes on the Earth. More recently, technological advances have allowed for the precise determination of the Hf isotope composition of zircon, a geochemical tracer that has provided important details on the Earth's chemical evolution, in particular the evolution of the crust–mantle system. When combined, U–Pb ages and Hf isotopes in zircons hold the promise of providing unprecedented resolution in the timing and processes of planetary differentiation. Nowhere is this more true than for the early history of the Earth, where younger tectonothermal processes have compromised the isotope information in bulk rock samples. With the promise of this integrated technique, however, lies numerous potential pitfalls in the acquisition and interpretation of these data. In this paper we review several important issues related to unraveling the complexities of integrated U–Pb age and Hf isotope datasets, especially with respect to understanding crust–mantle evolution. In particular, we address the potential difficulty of assigning accurate initial Hf isotope compositions as well as some of the inherent problems associated with so-called “depleted-mantle model ages”. Finally, we make some suggestions regarding the optimum analytical approach and presentation of the Hf (and Nd) isotope data to obtain the clearest record of Earth's chemical evolution.


      PubDate: 2016-02-01T14:08:56Z
       
  • Chromium fluxes and speciation in ultramafic catchments and global rivers
    • Abstract: Publication date: Available online 30 January 2016
      Source:Chemical Geology
      Author(s): C.N. McClain, K. Maher
      The role of ultramafic rock weathering in global chromium (Cr) budgets and how these may be linked to changes in Earth's climate over geological time was investigated by compiling dissolved Cr speciation and fluxes in (i) 73 of the worlds large rivers, representing ~35% of global river discharge to oceans, and in (ii) an ultramafic catchment (Putah Creek watershed) in the California Coast Range Mountains, USA. Weathering of ultramafic rocks creates ideal conditions for Cr release and redox cycling. Alkaline river water draining ultramafic catchments is naturally enriched in Cr (up to 582nmol/L). Chromium concentrations increase with the extent of rock–water interaction (as indicated by correlations with Mg and HCO3 − concentrations and pH), whereas Cr cycling depends on the availability of electron donors and acceptors (e.g. Fe(II), organics, Mn(III/IV)-oxides). Thus, dissolved Cr is exported from ultramafic watersheds as both the soluble hexavalent Cr(VI) species (MgCrO4, CrO4 2−), and also as trivalent Cr(III) species (CrOH(CO3)2−), Cr(OH)3). The latter Cr(III) species were previously thought to have low solubility. Ultramafic catchments have higher area normalized Cr and major ion fluxes for a given runoff when compared to global rivers and may thus have a disproportional impact on global Cr-budgets, long-term carbon cycling and moderation of the Earth's climate over million-year timescales. Riverine export fluxes of Cr are linearly correlated with fluxes of Ca+Mg and alkalinity in both ultramafic catchments and global rivers. This suggests that silicate weathering is a key control on Cr fluxes and that major element weathering fluxes may be a useful proxy for estimating Cr and other trace element fluxes. Globally, present-day riverine dissolved Cr fluxes to oceans are spatially variable and estimated to be 1.7×109 mol/yr, three times higher than previously reported. However, when geochemical reactions in estuaries are considered riverine Cr fluxes may be lower. Throughout Earth history, Cr weathering fluxes also may have varied with changes in the global distribution of ultramafic rocks, Cr concentrations in continental rocks, or climactic conditions (e.g. atmospheric CO2 levels or runoff). Such variation should be considered when interpreting both modern and past Cr seawater isotopic compositions and residence times.
      Graphical abstract image

      PubDate: 2016-02-01T14:08:56Z
       
  • Structural and dynamical properties of supercritical H2O–SiO2 fluids
           studied by ab initio molecular dynamics
    • Abstract: Publication date: Available online 30 January 2016
      Source:Chemical Geology
      Author(s): Georg Spiekermann, Max Wilke, Sandro Jahn
      In this study we report the structure of supercritical H 2O–SiO 2 fluid composed of 50mol% H 2O and 50mol% SiO 2 at 3000K and 2400K, investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 1.88g/cm 3, which yields a pressure of 4.3GPa at 3000K and 3.6GPa at 2400K. Throughout the trajectories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH=SiOSi+H 2O. The calculation of the reaction constant K =[OH-]2/[H2O][O2-] is carried out on the basis of the experimentally relevant Q n species notation and agrees well with an extrapolation of experimental data to 3000K. After quench from 3000K to 2400K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H 2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H 2O–silica interaction dynamics is described on the basis of the bond autocorrelation function.


      PubDate: 2016-02-01T14:08:56Z
       
  • Geochemistry of formation waters from the Wolfcamp and “Cline”
           shales: Insights into brine origin, reservoir connectivity, and fluid flow
           in the Permian Basin, USA
    • Abstract: Publication date: Available online 30 January 2016
      Source:Chemical Geology
      Author(s): Mark A. Engle, Francisco R. Reyes, Matthew S. Varonka, William H. Orem, Lin Ma, Adam J. Ianno, Tiffani M. Schell, Pei Xu, Kenneth C. Carroll
      Despite being one of the most important oil producing provinces in the United States, information on basinal hydrogeology and fluid flow in the Permian Basin of Texas and New Mexico is lacking. The source and geochemistry of brines from the basin were investigated (Ordovician- to Guadalupian-age reservoirs) by combining previously published data from conventional reservoirs with geochemical results for 39 new produced water samples, with a focus on those from shales. Salinity of the Ca–Cl-type brines in the basin generally increases with depth reaching a maximum in Devonian (median=154g/L) reservoirs, followed by decreases in salinity in the Silurian (median=77g/L) and Ordovician (median=70g/L) reservoirs. Isotopic data for B, O, H, and Sr and ion chemistry indicate three major types of water. Lower salinity fluids (<70g/L) of meteoric origin in the middle and upper Permian hydrocarbon reservoirs (1.2–2.5km depth; Guadalupian and Leonardian age) likely represent meteoric waters that infiltrated through and dissolved halite and anhydrite in the overlying evaporite layer. Saline (>100g/L), isotopically heavy (O and H) water in Leonardian [Permian] to Pennsylvanian reservoirs (2–3.2km depth) is evaporated, Late Permian seawater. Water from the Permian Wolfcamp and Pennsylvanian “Cline” shales, which are isotopically similar but lower in salinity and enriched in alkalis, appear to have developed their composition due to post-illitization diffusion into the shales. Samples from the “Cline” shale are further enriched with NH4, Br, I and isotopically light B, sourced from the breakdown of marine kerogen in the unit. Lower salinity waters (<100g/L) in Devonian and deeper reservoirs (>3km depth), which plot near the modern local meteoric water line, are distinct from the water in overlying reservoirs. We propose that these deep meteoric waters are part of a newly identified hydrogeologic unit: the Deep Basin Meteoric Aquifer System. Chemical, isotopic, and pressure data suggest that despite over-pressuring in the Wolfcamp shale, there is little potential for vertical fluid migration to the surface environment via natural conduits.


      PubDate: 2016-02-01T14:08:56Z
       
  • Molybdenum sources and isotopic composition during early stages of
           pedogenesis along a basaltic climate transect
    • Abstract: Publication date: Available online 1 February 2016
      Source:Chemical Geology
      Author(s): E.K. King, A. Thompson, O.A. Chadwick, J.C. Pett-Ridge
      Molybdenum (Mo) is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes. Globally, riverine δ98Mo values (based on 98Mo/95Mo relative to NIST SRM 3134) are heavier relative to bedrock, suggesting weathering processes preferentially retain light Mo isotopes, however, the mechanisms governing this process in soils are poorly understood. We investigated soil Mo abundance and isotopic composition as a function of climate along a well-constrained climosequence on a 10kyr lava flow in Hawaii receiving 660 to 2100mmyear−1 mean annual precipitation (MAP). We assessed Mo abundance and isotopic composition as a function of soil organic matter (OM) content, short-ranged-ordered (SRO) iron (Fe) (oxyhydr)oxide abundance, and Mo loss/gain. We find net accumulation of Mo across all soils (up to +139% gain) that is positively correlated with increasing MAP. The highest Mo gains are in surface soil horizons, and are correlated with high OM content. The isotopic composition of soil Mo, deviates from underlying basaltic bedrock, which ranges from δ98Mo values of −0.11‰ to −0.26‰. Soil Mo isotopic values are lightest at the dry sites (δ98Mo=−0.63‰) and become heavier with increasing precipitation (up to δ98Mo=+0.03‰). Samples with the heaviest δ98Mo values are from horizons with the largest net gains of Mo relative to bedrock and have higher OM content. In order to further constrain Mo fluxes into and out of the soil system, we measured Mo isotope ratios in local rainwater (average δ98Mo=+1.11‰), groundwater (average δ98Mo=+0.14‰), and vegetation (δ98Mo values between −0.18‰ and +0.64‰). Based on these data, we propose that Mo in these soils is substantially augmented by additions from precipitation, volcanic fog, and potentially anthropogenic inputs of Mo, and that retention of these inputs within soils is likely related to Mo–OM interactions. The large atmospheric inputs of Mo strongly modulate the δ98Mo values of dissolved Mo fluxes from soil to values only slightly heavier than bedrock. These patterns illustrate the potential for Mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy δ98Mo values in the global riverine Mo flux.


      PubDate: 2016-02-01T14:08:56Z
       
  • Nano-structural features of barite crystals observed by electron
           microscopy and atom probe tomography
    • Abstract: Publication date: Available online 23 January 2016
      Source:Chemical Geology
      Author(s): Juliane Weber, Juri Barthel, Felix Brandt, Martina Klinkenberg, Uwe Breuer, Maximilian Kruth, Dirk Bosbach
      For the first time a barite sample was investigated by applying electron microscopy and atom probe tomography in a complementary approach to characterize its inner microstructure. Pores covering the size range from a few nanometers to a few micrometers were identified inside particles of a commercially available high purity barite. The barite matrix appeared to be a single crystal. A particular feature of the observed porous structure is a system of layers containing a high number of homogeneously distributed nano-scale pores, alternating with layers containing a limited number of larger pores. High-resolution chemical analyses indicate that the pores contain a solution of water and sodium chloride. The filling of these nano-scale inclusions was interpreted as residua of the initial supersaturated solution used for particle growth by precipitation. These findings explain the high reactivity observed in previous Ra uptake studies on the same type of barite and provide further implications for the interpretation of these results. Our results offer a new analytical approach and necessary reference data for future investigations on changes of the internal barite structure during these Ra uptake experiments.


      PubDate: 2016-01-28T08:32:40Z
       
  • CO2 solubility in multi-component brines containing NaCl, KCl, CaCl2 and
           MgCl2 at 297K and 1–14MPa
    • Abstract: Publication date: Available online 26 January 2016
      Source:Chemical Geology
      Author(s): Ruth Jacob, Beverly Z. Saylor
      Carbon capture and storage (CCS) is a strategy to reduce greenhouse gas emissions by capturing CO2 from point sources and injecting it into deep saline formations. Over time, the injected CO2 dissolves into the saline pore waters forming an aqueous solution that is both a negatively buoyant fluid (sinks) and geologically reactive. Deep saline aquifers are geochemically complex and experimental studies have several gaps in solubility measurements for the multi-component brines that are needed to model the fate and reaction of CO2. New experimental data are presented here for three of the most common salts found in potential carbon storage locations, NaCl, KCl and MgCl2, as well as multi-salt brines including these salts and CaCl2. These experimental data are compared with available literature data and models used to predict CO2 solubility in single- and multi-component brines. Comparison of the experimental data with the models indicates that the reactor does work in the range of temperature, pressure and salinity presented here and that available models can accurately predict the solubility of CO2 in various salt solutions.


      PubDate: 2016-01-28T08:32:40Z
       
  • Thermodynamic description of H2S–H2O–NaCl solutions at
           temperatures to 573K and pressures to 40MPa
    • Abstract: Publication date: 10 April 2016
      Source:Chemical Geology, Volume 424
      Author(s): Nikolay N. Akinfiev, Vladimir Majer, Yuri V. Shvarov
      Reliable experimental results were selected from the literature (using over 700 data) to develop a thermodynamic model for calculating the solubility of hydrogen sulfide (H2S) in pure water and in aqueous NaCl solutions between 283 and 573K, 0.1–40MPa and m s 0–6mol·kg−1. Thermodynamic properties of the pure components were calculated using highly accurate multiparametric equations of state for H2S (Lemmon and Span, 2006) and for H2O (Wagner and Pruss, 2002). Thermodynamic properties of H2S(aq) at infinite dilution were based on the Henry's law constants generated from the SOCW model (Sedlbauer et al., 2000) and reported by Majer et al. (2008). The determined activity coefficients of H2S in pure water and in NaCl solutions were treated using the Pitzer interaction model. The Pitzer parameters for H2S in binary and ternary solutions were newly determined while those for NaCl(aq) in the H2S-free system were adopted from the review of Archer (1992). The experimental solubilities selected for correlation are reproduced by the model with mean relative deviations of 5.2% and 6.1% for the H2S–H2O and for H2S–H2O–NaCl systems, respectively. These values are comparable to the experimental uncertainty of the solubility data. The new model allows a thermodynamically consistent description of numerous other properties of the liquid phase in the ternary H2S–H2O–NaCl system, including the activity coefficients of H2S and NaCl, the osmotic coefficients, the Setchenow constants, and the molar volume and density of the bulk liquid. These properties can be calculated for any H2S and NaCl concentrations up to halite saturation. The model is available as a computer code that is freely distributed.


      PubDate: 2016-01-28T08:32:40Z
       
  • Silicon isotope compositions of euhedral authigenic quartz crystals:
           Implications for abiotic fractionation at surface temperatures
    • Abstract: Publication date: 20 March 2016
      Source:Chemical Geology, Volume 423
      Author(s): Xinyang Chen, Henry S. Chafetz, Rasmus Andreasen, Thomas J. Lapen
      Silicon (Si) isotopes have been demonstrated as proxies for the paleoenvironmental conditions of various silica deposits. In an effort to investigate the petrogenesis and paleoenvironments of Phanerozoic silica deposits, a suite of euhedral megaquartz crystals in the Cretaceous Edwards Formation from central Texas was petrographically analyzed and their Si isotopic compositions analyzed by bulk and in situ techniques. Petrographic analysis shows a close relationship between megaquartz and evaporite minerals. The lithologic association of silica with evaporite-bearing dolomitized carbonate strata suggests that the silicification probably developed in a back-reef tidal-flat environment in which quartz crystals formed after the primary calcite cementation and partial dissolution of evaporites. Silicon isotopic mapping across the megaquartz crystals show that δ30Si(NBS28) values range from −2.90 to +2.94‰. These values span the majority of published silicon isotopic values observed in nature and indicate complex growth histories. The negative δ30Si values are attributed to the dissolution of sponge spicules, which likely act as the primary source of the authigenic megaquartz. The observed range of δ30Si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re-precipitated as megaquartz in a closed system during diagenesis. This Rayleigh-type fractionation model also adds a new level of insight into the abiotic fractionation factor between dissolved and precipitated silica. Based on temperature estimates of 20 to 50°C for megaquartz precipitation, the fractionation factor was determined to be between −1.8 and −2.1‰. The estimated average δ30Si value of Early Cretaceous seawater is +2.7 to +3.0‰, significantly higher than modern seawater.


      PubDate: 2016-01-28T08:32:40Z
       
  • Variability in magnesium, carbon and oxygen isotope compositions of
           brachiopod shells: Implications for paleoceanographic studies
    • Abstract: Publication date: 20 March 2016
      Source:Chemical Geology, Volume 423
      Author(s): C. Rollion-Bard, S. Saulnier, N. Vigier, A. Schumacher, M. Chaussidon, C. Lécuyer
      This study presents the carbon, oxygen, and magnesium isotope compositions of two modern brachiopods, Terebratalia transversa and Frieleia halli, and one fossil specimen (2.3Ma), Terebratula scillae. The aim of this study is to investigate the variability of these isotopic compositions and to evaluate the potential of brachiopods as a proxy of past seawater δ26Mg values. The two investigated brachiopod shells present the same range of δ26Mg variation (up to 2‰). This variation cannot be ascribed to changes in environmental parameters (temperature or pH). As has been previously observed, the primary layer of calcite shows the largest degree of oxygen and carbon isotope disequilibrium relative to seawater. In contrast, the δ26Mg value of the primary layer is comparable to that of the secondary calcite layer value. In both T. scillae and T. transversa, negative trends are observable between magnesium isotopic compositions and oxygen and carbon isotopic compositions. These trends can be explained by kinetic effects linked to changes in growth rate during the brachiopod life. The innermost calcite layer of T. transversa is in isotopic equilibrium for both oxygen and magnesium and could therefore be the best target for reconstructing past δ26Mg values of seawater.


      PubDate: 2016-01-28T08:32:40Z
       
  • Copper and zinc isotope fractionation during deposition and weathering of
           highly metalliferous black shales in central China
    • Abstract: Publication date: Available online 22 January 2016
      Source:Chemical Geology
      Author(s): Yiwen Lv, Sheng-Ao Liu, Jian-Ming Zhu, Shuguang Li
      Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ65Cu (+0.14±0.09‰, 1σ) but heavy δ66Zn (0.51±0.11‰, 1σ). The elevated δ66Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems. The weathered shales and cherts have an extreme δ65Cu range from −6.42‰ to +19.73‰ and a modest δ66Zn range of +0.25‰ to +0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ66Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans.


      PubDate: 2016-01-28T08:32:40Z
       
  • The role of Te(IV) and Bi(III) chloride complexes in hydrothermal mass
           transfer: An X-ray absorption spectroscopic study
    • Abstract: Publication date: Available online 22 January 2016
      Source:Chemical Geology
      Author(s): Barbara E. Etschmann, Weihua Liu, Allan Pring, Pascal V. Grundler, Blake Tooth, Stacey Borg, Denis Testemale, Dale Brewe, Joël Brugger
      Tellurium (Te) and bismuth (Bi) and are two metal(loid)s often enriched together with gold (Au) in hydrothermal deposits; however the speciation and transport properties for these two metals in hydrothermal systems are poorly understood. We investigated the effect of chloride on the speciation of Te(IV) and Bi(III) in hydrothermal solutions using in-situ XAS spectroscopy. At ambient temperature, oxy-hydroxide complexes containing the [TeO3] moiety (e.g., H3TeO3 + under highly acidic conditions) predominate in salty solutions over a wide range in pH and salt concentrations. Te(IV)-Cl complexes only appear at pH25˚C ≤2 and high Cl- activity (≥10). The highest order Te(IV) chloride complex detected is TeCl4(aq), and contains the [TeCl4] moiety. Upon heating to 199˚C, the Te(IV)-Cl complexes become more stable; however they still required highly acidic conditions which are likely to exist only in very limited environments in nature. At ambient temperature, Bi(III) is coordinated to 5.5(5)Cl atoms in high salinity, acidic (HCl≥0.5m) chloride solutions. This, combined with large EXAFS-derived structural disorder parameters, suggests that the Bi(III) complex is most likely present as both BiCl5 2- and BiCl6 3-. The number of Cl atoms coordinated to Bi(III) decreases with increasing temperature; at around 200 oC and above, Bi(III) is coordinated to three Cl atoms. Overall the data show that Te(IV) chloride complexes can be ignored in predicting Te mobility under oxidizing conditions in most geological environments, but that Bi(III) chloride complexes are expected to account for Bi mobility in acidic brines. New thermodynamic properties for Bi(III) chloride complexes are provided to improve reactive transport modeling of Bi up to 500˚C. Although higher order complexes such as BiCl5 2- and BiCl6 3- exist at ambient temperature, the BiCl3(aq) complex becomes the predominant chloride complex in saline solutions at T ≥ 200˚C.


      PubDate: 2016-01-28T08:32:40Z
       
  • Characterization, dissolution and solubility of cadmium–calcium
           hydroxyapatite solid solutions at 25°C
    • Abstract: Publication date: 20 March 2016
      Source:Chemical Geology, Volume 423
      Author(s): Yinian Zhu, Zongqiang Zhu, Xin Zhao, Yanpeng Liang, Liuqin Dai, Yanhua Huang
      A series of cadmium–calcium hydroxyapatite solid solutions [Cd–Ca-HAP, (Ca1−xCdx)5(PO4)3(OH)] was synthesized and characterized, and then the dissolution of the prepared solids in aqueous solutions was experimentally studied. The element concentrations in the aqueous solutions were considerably influenced by the Cd/(Cd+Ca) atomic ratio of the Cd–Ca-HAP solids (XCd). For solids with high XCd (>0.6), the aqueous Ca2+ concentration increased gradually with time and reached a stable state after approximately 5040h of dissolution; the aqueous Cd2+ concentration increased rapidly with time and reached a peak value within 480–720h, and then decreased gradually and attained a stable state after 5040h of dissolution. For solids with low XCd (≤0.6), the aqueous Ca2+ concentration increased rapidly with time and reached a peak value after approximately 240–360h of dissolution, and then decrease slightly and are relatively stable after 720h; the aqueous Cd2+ concentration increased quickly with time and attained a peak value within 1h and then decreased gradually and attained a stable state after 480h of dissolution. The dissolution process of the solid samples with high XCd (>0.6) was different from that of the solids with low XCd (≤0.6), which can be related to a slight tendency of Cd2+ in preferring the M(2) sites and Ca2+ in occupying the M(1) sites in the apatite structure. The average solubility products (K sp) were estimated for Cd5(PO4)3OH to be 10–64.62 (10–64.53–10–64.71) at 25°C and for Ca5(PO4)3OH to be 10–57.65 (10–57.56–10–57.79) at 25°C. The corresponding free energies of formation (ΔG f o ) were −3970.47kJ/mol and −6310.45kJ/mol. The solubility of the solid solution (CdxCa1−x)5(PO4)3(OH) decreased with increasing XCd. For the dissolution at an initial pH of 2 and 25°C, plotting of the experimental data on the Lippmann diagram indicated that (Cd0.71Ca0.29)5(PO4)3(OH) dissolved stoichiometrically during the initial period and approached gradually up to the Lippmann solutus before overshooting it. Finally, the data points moved along the Lippmann solutus curve from right to left. The Cd5(PO4)3(OH)-rich solid phase was in equilibrium with the Cd-poor aqueous solution. These findings provided a better understanding of the processes controlling the solubility, transport and distribution of cadmium in the environment.


      PubDate: 2016-01-22T14:10:45Z
       
  • Crystallization kinetics and rheology of leucite-bearing tephriphonolite
           magmas from the Colli Albani volcano (Italy)
    • Abstract: Publication date: Available online 21 January 2016
      Source:Chemical Geology
      Author(s): S. Campagnola, A. Vona, C. Romano, G. Giordano
      We have investigated the rheology of liquid and crystal-bearing tephriphonolite magmas from the Colli Albani volcanic district. High (1124–1569°C) and low (690–800°C) temperature anhydrous liquid viscosities were determined by a combination of concentric cylinder (101.0 to 103.6 Pas) and micropenetration (109.2 to 1012.1 Pas) viscometry. Comparison with literature data reveals that at high temperatures, viscosity seems to be related to the melts degree of polymerization (NBO/T), while at low temperatures the dependency is not linear with values of viscosity higher than expected. Subliquidus isothermal crystallization experiments and viscosity determinations were carried out at high temperature (1150–1240°C) in air using a concentric cylinder apparatus at constant shear strain rate ( γ ̇ =0.1s−1). The overall crystal fraction varies between ϕ=0.06 at 1240°C (leucite) and ϕ=0.34 at 1150°C (leucite ϕ=0.32+plagioclase ϕ=0.02), with a direct linear increase of crystal content with decreasing temperature which parallels the viscosity increase. The inspection of products quenched at the end of the crystallization stage, defined when viscosity reaches a constant value, reveals strong evidence of leucite clustering. After the first segment of the experiment, performed at a constant shear rate, a second stage of experiments at variable shear rate was performed, comprised of an up-ramp ( γ ̇ =0.1s−1–0.9s−1) and a down-ramp ( γ ̇ =0.9s−1–0.1s−1) segment. At the end of the down-ramp, leucite crystals appear sub-spherical and unclustered. For the same applied shear rate, the viscosity values of the up-ramp are not recovered within the experimental time-scale, indicating strain and strain-rate dependent rheology for these suspensions. While the down-ramp viscosity data are shown to be in perfect agreement with literature models, discrepancies between the up-ramp data and pre-existing predicting models have been observed. We suggest that this complex behavior is related to the clustering of leucite crystals during the crystallization process, and their breakage during the higher shear rate viscosity measurements. The kinetics of crystallization (in terms of nucleation and growth rates) seems to be strongly affected by the degree of undercooling and dynamic stirring conditions. We conclude that any modeling of tephriphonolite magma flow (either in conduit or subaerial) should take into account the effects of strain, strain-rate and internal textures (i.e. clustering).


      PubDate: 2016-01-22T14:10:45Z
       
  • Salinity and density modifications of synthetic H2O and H2O-NaCl fluid
           inclusions in re-equilibration experiments at constant temperature and
           confining pressure
    • Abstract: Publication date: Available online 21 January 2016
      Source:Chemical Geology
      Author(s): Ronald J. Bakker, Gerald Doppler
      Experiments have been performed to elucidate post-entrapment modifications of natural fluid inclusions. Synthetic H2O and H2O-NaCl fluid inclusions in quartz were re-equilibrated at 599.7–600.7°C and 332.6–338.3MPa in hydrothermal autoclaves, and subjected to H2O fugacity gradients at similar temperature and hydrostatic pressure conditions to those of the original syntheses. Individual inclusions in specific assemblages were analysed in detail, i.e. size, shape, depth, homogenization and ice-dissolution temperatures, before and after the re-equilibration experiments. Accurate loading and unloading of the samples along isochoric T–p paths requires monitoring the temperature at the sample within the autoclaves. Multiple loading and un-loading of the samples along these paths does not affect the properties of fluid inclusions. Synthetic H2O fluid inclusions are not modified after re-equilibration in a pure H2O fluid, but reveal significantly higher homogenization temperatures after re-equilibration in 20 mass% NaCl solution. Synthetic NaCl-H2O fluid inclusions with 10, 16.3 and 19.8 mass% NaCl reveal higher salinities after re-equilibration in a pure H2O fluid, and highly variable homogenization temperatures: positive as well as negative modifications. The magnitude of modifications indicates that two processes must have operated simultaneously to obtain the observed homogenization and dissolution temperatures in individual inclusions: (1) preferential H2O loss via diffusion; and (2) total volume loss by diffusion of quartz into the former inclusion volume. These processes are inconsistent with the expected H2O diffusion into inclusions according to the applied fugacity gradients in the experimental setup. These simultaneously operating processes are suggested to be the main modification method of natural fluid inclusions in a variety of experimental settings and in geological environments.


      PubDate: 2016-01-22T14:10:45Z
       
  • Chromium isotope fractionation during subduction-related metamorphism,
           black shale weathering, and hydrothermal alteration
    • Abstract: Publication date: 20 March 2016
      Source:Chemical Geology, Volume 423
      Author(s): Xiangli Wang, Noah J. Planavsky, Christopher T. Reinhard, Huijuan Zou, Jay J. Ague, Yuanbao Wu, Benjamin C. Gill, Esther M. Schwarzenbach, Bernhard Peucker-Ehrenbrink
      Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. However, many geological reservoirs and isotope fractionation processes are still not well understood. The purpose of this contribution is to move forward our understanding of (1) the Earth's high temperature Cr isotope inventory and (2) Cr isotope fractionations during subduction-related metamorphism, black shale weathering and hydrothermal alteration. The examined basalts and their metamorphosed equivalents yielded δ53Cr values falling within a narrow range of −0.12±0.13‰ (2SD, n=30), consistent with the previously reported range for the bulk silicate Earth (BSE). Compilations of currently available data for fresh silicate rocks (43 samples), metamorphosed silicate rocks (50 samples), and mantle chromites (39 samples) give δ53Cr values of −0.13±0.13‰, −0.11±0.13‰, and −0.07±0.13‰, respectively. Although the number of high-temperature samples analyzed has tripled, the originally proposed BSE range appears robust. This suggests very limited Cr isotope fractionation under high temperature conditions. Additionally, in a highly altered metacarbonate transect that is representative of fluid-rich regional metamorphism, we did not find resolvable variations in δ53Cr, despite significant loss of Cr. This work suggests that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. Oxidative weathering of black shale at low pH creates isotopically heavy mobile Cr(VI). However, a significant proportion of the Cr(VI) is apparently immobilized near the weathering surface, leading to local enrichment of isotopically heavy Cr (δ53Cr values up to ~0.5‰). The observed large Cr isotope variation in the black shale weathering profile provides indirect evidence for active manganese oxide formation, which is primarily controlled by microbial activity. Lastly, we found widely variable δ53Cr (−0.2‰ to 0.6‰) values in highly serpentinized peridotites from ocean drilling program drill cores and outcropping ophiolite sequences. The isotopically heavy serpentinites are most easily explained through a multi-stage alteration processes: Cr loss from the host rock under oxidizing conditions, followed by Cr enrichment under sulfate reducing conditions. In contrast, Cr isotope variability is limited in mildly altered mafic oceanic crust.


      PubDate: 2016-01-18T11:33:59Z
       
  • Luminescence of uranium-bearing opals: Origin and use as a pH record
    • Abstract: Publication date: 20 March 2016
      Source:Chemical Geology, Volume 423
      Author(s): Guillaume Othmane, Thierry Allard, Thomas Vercouter, Guillaume Morin, Mostafa Fayek, Georges Calas
      Fluorescence of minerals has been long used for U exploration. It is proposed here that opal fluorescence can be used as a probe of the pH of the formation solution, bringing constraints for modeling U speciation and sequestration at the Earth's surface. We present a study of fluorescence spectroscopy of U in opals resulting from low-temperature (29°C) alteration of mineralized rhyolitic lavas (Nopal I U deposit, Sierra Peña Blanca, Mexico). These opals show green fluorescence with a concentric distribution at microscopic scale, which is unambiguously assigned to uranyl groups (oxidized form of U). Spectra appear typical of uranyl in opal as reported in literature for other localities according to peak positions. When considering also the lifetime of fluorescence spectra (with time resolved laser fluorescence spectroscopy), fingerprinting indicates that uranyl occurs either as phosphate or hydroxo-polynuclear complexes trapped onto the opal internal surface. Data are indicative of a pH of opal formation around 8, as derived from both conditions of laboratory experiments and reference to calculated diagrams of speciation. This pH value is consistent with the Nopal geological formations that were potentially in contact with the aquifer involved in the hydrothermal process. In addition, the microscopic zoning of U suggests that the speciation has been stable since opal formation more than 50ka ago, which allows the record of pH that prevailed at that time.


      PubDate: 2016-01-14T12:21:30Z
       
  • Tracing water cycle in regulated basin using stable
           δ18O–δ2H isotopes: The Ebro river basin (Spain)
    • Abstract: Publication date: 1 March 2016
      Source:Chemical Geology, Volume 422
      Author(s): Philippe Négrel, Emmanuelle Petelet-Giraud, Romain Millot



      PubDate: 2016-01-10T11:03:54Z
       
  • Eclogitic diamonds from variable crustal protoliths in the northeastern
           Siberian craton: Trace elements and coupled δ13C–δ18O
           signatures in diamonds and garnet inclusions
    • Abstract: Publication date: Available online 2 January 2016
      Source:Chemical Geology
      Author(s): Dmitry Zedgenizov, Daniela Rubatto, Vladislav Shatsky, Alexey Ragozin, Victoria Kalinina
      Diamonds of eclogitic assemblages are dominant in the placer diamond deposits of the northeastern Siberian platform. In this study we present new trace elements and stable isotopes (δ13C and δ18O) data for alluvial diamonds and their garnet inclusions from this locality. Cr-rich garnets of peridotitic affinity in the studied diamonds have a narrow range of δ18O values from 5.7‰ to 6.2‰, which is largely overlapping with the accepted mantle range. This narrow range suggests that the garnet inclusions showing different REE patterns and little variations in oxygen isotopes may have formed by different processes involving fluid/melts that, however, were in oxygen isotopic equilibrium with the mantle. The trace element composition of the eclogitic garnet inclusions supports a crustal origin for at least the high-Ca garnets, which show flat HREE patterns and in some cases a positive Eu-anomaly. High-Ca eclogitic garnets generally show heavier oxygen isotope compositions (δ18O 6.5–9.6‰) than what is observed in low-Ca garnets (δ18O 5.7–7.4‰). The variability in oxygen isotopes and trace elements is suggested to be inherited from contrasting crustal protoliths. The relationship between the high δ18O values of inclusions and the low δ13C values of the host diamonds implies that the high-Ca garnet inclusions were derived from intensely hydrated (e.g., δ18O >7‰) and typically oxidised basaltic rock close to the seawater interface, and that the carbon for diamonds was closely associated with this protolith.
      Graphical abstract image

      PubDate: 2016-01-06T11:01:17Z
       
  • Ca, Sr and Ba stable isotopes reveal the fate of soil nutrients along a
           tropical climosequence in Hawaii
    • Abstract: Publication date: 1 March 2016
      Source:Chemical Geology, Volume 422
      Author(s): Thomas Bullen, Oliver Chadwick
      Nutrient biolifting is an important pedogenic process in which plant roots obtain inorganic nutrients such as phosphorus (P) and calcium (Ca) from minerals at depth and concentrate those nutrients at the surface. Here we use soil chemistry and stable isotopes of the alkaline earth elements Ca, strontium (Sr) and barium (Ba) to test the hypothesis that biolifting of P has been an important pedogenic process across a soil climosequence developed on volcanic deposits at Kohala Mountain, Hawaii. The geochemical linkage between these elements is revealed as generally positive site-specific relationships in soil mass gains and losses, particularly for P, Ba and Ca, using the ratio of immobile elements titanium and niobium (Ti/Nb) to link individual soil samples to a restricted compositional range of the chemically and isotopically diverse volcanic parent materials. At sites where P is enriched in surface soils relative to abundances in deeper soils, the isotope compositions of exchangeable Ca, Sr and Ba in the shallowest soil horizons (<10cm depth) are lighter than those of the volcanic parent materials and trend toward those of plants growing on fresh volcanic deposits. In contrast the isotope composition of exchangeable Ba in deeper soil horizons (>10cm depth) at those sites is consistently heavier than the volcanic parent materials. The isotope compositions of exchangeable Ca and Sr trend toward heavier compositions with depth more gradually, reflecting increasing leakiness from these soils in the order Ba<Sr<Ca and downward transfer of light biocycled Ca and Sr to deeper exchange sites. Given the long-term stability of ecosystem properties at the sites where P is enriched in surface soils, a simple box model demonstrates that persistence of isotopically light exchangeable Ca, Sr and Ba in the shallowest soil horizons requires that the uptake flux to plants from those near-surface layers is less than the recycling flux returned to the surface as litterfall. This observation implicates an uptake flux from an additional source which we attribute to biolifting. We view the heavy exchangeable Ba relative to soil parent values in deeper soils at sites where P is enriched in surface soils, and indeed at all but the wettest site across the climosequence, to represent the complement of an isotopically light Ba fraction removed from these soils by plant roots consistent with the biolifting hypothesis. We further suggest that decreasing heaviness of depth-integrated exchangeable Ba in deeper soils with increasing median annual precipitation across the climosequence reflects greater reliance on shallow nutrient sources as site water balance increases. While the Ca, Sr and Ba isotopes considered together were useful in confirming an important role for nutrient biolifting across the climosequence, the Ba isotopes provided the most robust tracer of biolifting and have the greatest potential to find application as an isotopic proxy for P dynamics in soils.


      PubDate: 2016-01-06T11:01:17Z
       
  • Ca isotopic analysis of laser-cut microsamples of (bio)apatite without
           chemical purification
    • Abstract: Publication date: 1 March 2016
      Source:Chemical Geology, Volume 422
      Author(s): Qiong Li, Matthew Thirlwall, Wolfgang Müller
      We report methodological innovations and an initial application to retrieve Ca isotopic (δ44/40Ca) compositions of (bio)apatite at high spatial resolution. We utilize laser-microsampling via laser-cut polygons followed by extraction of μg-sized bioapatite fragments; and apply a 1010 Ω resistor for 40Ca and parafilm-dam loading to minimize in-run Ca isotope fractionation for TIMS analysis. We obtain identical internally-normalized TIMS Ca isotopic ratios for natural calcite, apatite and bone meal SRM1486 without chemical separation, suggesting no need of chemical purification of these materials for such analysis. Our 42Ca–48Ca double spike (DS) corrected δ44/40Ca values of standard solutions HPSnew, Fisher07 and bone meal SRM1486 are 0.70±0.18‰, 1.03±0.20‰, and −1.03±0.19‰ (2SD), relative to SRM915a, all well in line with published data. Solutions prepared from both laser-cut and non-laser cut Durango apatite have similar δ44/40Ca values (0.70±0.17‰, 2SD), suggesting no resolvable Ca isotope fractionation effect induced from laser-cutting. A δ44/40Ca profile of a third molar from a modern human female, laser-sampled in the growth direction of the enamel apatite, reveals a 0.5‰ increase from age ~11 to ~11.5, followed by a 0.7‰ drop at age ~12 and a final return to enriched Ca-isotope compositions until age ~14. We surmise that the initial increase is related to her known phase-out of dairy products, whereas the drop may coincide with hormonal changes at the onset of her menstruation. The latter hints at a relationship between physiological change and δ44/40Ca variation, and raises the need for proper evaluation of physiological effects on Ca isotopes before reliable environmental signals can be extracted from the large Ca-isotope variability previously observed in skeletal tissues. We also investigated the potential for Ca isotopic analysis using an IsoProbe MC-ICPMS in both solution and laser ablation (LA) modes. While the use of a collision cell almost completely removes 40Ar+ interference on 40Ca+, partially-resolved hydrocarbon interferences result in only ~30% useable peak flats, that potentially cause problems with peak jumping needed for analysis of 46Ca and 48Ca. Furthermore, it is difficult to correct for Ti isobaric interferences so that analysis without chemical separation is challenging. In-situ Ca isotope analysis of natural calcite (SRM915b), aragonite and apatite by LA-MC-ICPMS shows substantial matrix sensitivity, and therefore requires close matrix matching.


      PubDate: 2016-01-02T10:49:51Z
       
  • Systematic variations of trace element and sulfur isotope compositions in
           pyrite with stratigraphic depth in the Skouriotissa volcanic-hosted
           massive sulfide deposit, Troodos ophiolite, Cyprus
    • Abstract: Publication date: Available online 30 December 2015
      Source:Chemical Geology
      Author(s): Manuel Keith, Karsten M. Haase, Reiner Klemd, Stefan Krumm, Harald Strauss
      The Troodos ophiolite represents one of the best-preserved fossil analogs of modern oceanic crust and includes numerous volcanic-hosted massive sulfide deposits. The Skouriotissa deposit can be separated into a stockwork ore zone and an overlying massive sulfide lens that is covered by metalliferous sediments representing the former sulfide–seawater interface. Pyrite is the dominant sulfide mineral within these ores. The trace element composition of pyrite varies systematically with stratigraphic depth (down to ~150mbsf) probably reflecting fluid temperature variations and effects of phase separation (Co, Ni, Se, Te, Bi and Cu). Metal remobilization due to hydrothermal zone refining (Zn, Sb and Pb) and fluid-seawater mixing at the seafloor (Mo) represent further important processes controlling the pyrite chemistry. Massive sulfide-hosted sphalerite and euhedral pyrite probably formed from hot fluids (~400°C), while the occurrence of colloform pyrite indicates lower precipitation temperatures (<400°C). Similar δ18O quartz-fluid equilibration temperatures (~400°C) in the stockwork zone suggest that the Skouriotissa fluids did not cool significantly during the final 150m of fluid ascent to the seafloor. The δ34S composition of deep stockwork pyrite (−1.4‰) suggests that an isotopically light magmatic volatile phase (<0‰) was added to the hydrothermal system of Skouriotissa. During further fluid ascent about 38% of Cretaceous seawater (δ34S=18–19‰) was added leading to the precipitation of stockwork pyrite with positive δ34S values (6.1‰). In addition, the chemical and textural similarities between Skouriotissa and modern seafloor vent systems and massive sulfide deposits suggest that the Skouriotissa hydrothermal system has a modern analog.


      PubDate: 2016-01-02T10:49:51Z
       
  • Formation of high-Al komatiites from the mesoarchaean quebra osso group,
           Minas Gerais, Brazil: Trace elements, HSE systematics and os isotopic
           signatures
    • Abstract: Publication date: Available online 31 December 2015
      Source:Chemical Geology
      Author(s): D. van Acken, J.E. Hoffmann, J.H.D. Schorscher, T. Schulz, A. Heuser, A. Luguet
      We report highly siderophile element data combined with Re-Os isotopes and major and trace elements of the ca. 2.7–3.0Ga komatiites from the Quebro Osso Group, Minas Gerais, Brazil. These komatiites resemble the rare high Al-type, characterized by high Al2O3/TiO2 ratios (26.7–59.8). These geochemical similarities are shared with the 3.33Ga Commondale and 3.26Ga Weltevreden komatiites from the eastern Kaapvaal Craton pointing to a similar origin of these suites. While anhydrous melting in an unusually hot mantle was inferred for the Weltevreden komatiites, the Commondale komatiites were suggested to have formed by hydrous, multi-stage melting. Significant depletion in LREE is coupled with subchondritic Re/Os, unradiogenic to radiogenic 187Os/188Os and and fractionated HSE, with enrichments in Ru, Pt, and Pd over Os and Ir. The combination of these signatures suggests minor late-stage crustal influence. Potential late-stage alteration overprint, assimilation of ambient mantle material during magma ascent and complex phase relationships of HSE-hosting phases make it difficult to estimate the composition of the source of the Quebra Osso komatiites and to place constraints on the nature of the late Archaean mantle. However, the Quebra Osso komatiites are unlikely to have formed in a single-stage plume setting or in a supra-subduction zone setting. Instead we suggest a multi-stage melting history of the komatiite source to explain the origin of their peculiar geochemical characteristics, as has been suggested for other high-Al2O3/TiO2 komatiite suites.


      PubDate: 2016-01-02T10:49:51Z
       
  • Editorial Board
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421




      PubDate: 2016-01-02T10:49:51Z
       
  • Experimental calibration of silicon and oxygen isotope fractionations
           between quartz and water at 250°C by in situ microanalysis of
           experimental products and application to zoned low δ30Si quartz
           overgrowths
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Anthony D. Pollington, Reinhard Kozdon, Lawrence M. Anovitz, R. Bastian Georg, Michael J. Spicuzza, John W. Valley
      The interpretation of silicon isotope data for quartz is hampered by the lack of experimentally determined fractionation factors between quartz and fluid. Further, there is a large spread in published oxygen isotope fractionation factors at low temperatures, primarily due to extrapolation from experimental calibrations at high temperature. We present the first measurements of silicon isotope ratios from experimentally precipitated quartz and estimate the equilibrium fractionation vs. dissolved silica using a novel in situ analysis technique applying secondary ion mass spectrometry to directly analyze experimental products. These experiments also yield a new value for oxygen isotope fractionation. Quartz overgrowths up to 235μm thick were precipitated in silica–H2O–NaOH–NaCl fluids, at pH12–13 and 250°C. At this temperature, 1000lnα30Si(Qtz–fluid)=0.55±0.10‰ and 1000lnα18O(Qtz–fluid)=10.62±0.13‰, yielding the relations 1000lnα30Si(Qtz–fluid)=(0.15±0.03)*106/T2 and 1000lnα18O(Qtz–fluid)=(2.91±0.04)*106/T2 when extended to zero fractionation at infinite temperature. Values of δ30Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to −5.4‰. Paired δ18O and δ30Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport. The application of the experimental quartz growth results to observations from natural sandstone samples suggests that precipitation of quartz at low temperatures in nature is dominated by kinetic, rather than equilibrium, processes.


      PubDate: 2016-01-02T10:49:51Z
       
  • Surface adsorption and precipitation of inositol hexakisphosphate on
           calcite: A comparison with orthophosphate
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Biao Wan, Yupeng Yan, Fan Liu, Wenfeng Tan, Xiuhua Chen, Xionghan Feng
      Unraveling the processes of adsorption–precipitation of organic phosphates (OPs) on calcite is important for understanding the transformation, mobility and bioavailability of phosphorus (P) in calcareous soils. To elucidate these processes, the interaction between myo-inositol hexakisphosphate (IHP) and calcite was studied using macroscopic sorption experiments and a variety of analytical approaches. The experiments were also conducted with orthophosphate (P i ) for comparison. Calcite presented a similar sorption capability for IHP and P i through the rapid formation of surface precipitates based on sorption experiments and the results of Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. After interacting with IHP/P i , two different types of precipitates on a calcite surface could be directly observed by SEM images and analyzed by energy dispersive spectroscopy (EDS): a sphere-shaped precipitate considered typical for the poorly crystallized calcium phytate and a plate-shaped precipitate considered typical for the crystalline hydroxylapatite. This study suggests that active calcite strongly influences the species and behavior of IHP via a rapid surface precipitation reaction in a natural environment and advances our knowledge for predicting the fate of dissolved OP species in a variety of calcareous soils and geological settings enriched with calcite.
      Graphical abstract image

      PubDate: 2015-12-29T09:02:44Z
       
  • U–Pb, Lu–Hf and trace element characteristics of zircon from
           the Felbertal scheelite deposit (Austria): New constraints on timing and
           source of W mineralization
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Michael Kozlik, Johann G. Raith, Axel Gerdes
      Combined in-situ U–Pb, Lu–Hf and trace element LA–ICP–MS analyses were performed on zircons from the W-mineralized K1–K3 orthogneiss and associated aplite gneiss from the Felbertal scheelite deposit. The textural and trace element characteristics suggest the presence of magmatic and hydrothermal zircon. Magmatic zircons have low concentrations of W (0.108–3.77ppm), Nb (5.04–48ppm), U (691–3079ppm), B (0.26–6.57ppm), and REE+Y (640–4576ppm) and form euhedral CL-bright cores with distinct oscillatory zoning (zone a). Saturation of P in the granitic melt causing apatite crystallization explains the presence of two sub-types of CL-bright zircon cores distinguished by distinct P and Ca concentrations and variable positive Ce-anomalies. Higher trace element abundances and lower Zr/Hf ratios occur in CL-dark zircons with weak oscillatory zoning (zone b) overgrowing the zircon cores and in granular textured zircons restricted to the aplite gneiss. The increase in trace elements in zircon can be linked to the differentiation of the granitic melt and prolonged zircon crystallization during the transition from the late magmatic to the hydrothermal stage. CL-dark unzoned (texture-less) domains (zone b′) in K1–K3 orthogneiss zircons and granular textured zircons from the aplite gneiss have supra-chondritic Y/Ho ratios (29–62) indicating zircon growth in the presence of F-rich hydrothermal fluids exsolved from the evolved K1–K3 melt. Zone b′ zircons have the highest concentrations of W (4.3–11.4ppm), Nb (63–405ppm), B (7.3–135ppm), REE+Y (0.31–1.87 mass%) and U (1.23–5.56mass%); they are interpreted as hydrothermal and related to W-mineralizing fluids. Magmatic and hydrothermal zircon has identical U–Pb and Lu–Hf isotopic signature indicating the same source. The in-situ U–Pb concordia ages of zircons of four K1–K3 orthogneisses and one aplite gneiss confine the emplacement period of this granitic series between 341.0Ma and 336.2Ma. The initial ɛHft values ranging from −7.6 to −4.3 indicate a crustal protolith. The Felbertauern augengneiss is similar; it yielded a U–Pb concordia age of 338.5±1.3Ma and initial ɛHft between −6.8 and −5.3. On the contrary, zircons from the Granatspitz gneiss yielded discordant U–Pb ages (Late Carboniferous to Early Permian) and the ɛHft values scatter between −3.1 and +2.5 excluding any relation with the K1–K3 orthogneiss. Our new LA–ICP–MS data corroborate the genetic relationship of the W-mineralization at Felbertal with the Early Carboniferous K1–K3 orthogneiss series. The K1–K3 orthogneiss was part of a larger magmatic suite that developed during the Variscan orogeny in the pre-Alpine basement of the Tauern Window in the Viséan.
      Graphical abstract image

      PubDate: 2015-12-29T09:02:44Z
       
  • Determination of the stable iron isotopic composition of sequentially
           leached iron phases in marine sediments
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Susann Henkel, Sabine Kasten, Simon W. Poulton, Michael Staubwasser
      Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), the adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for δ56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine–HCl, reducible oxides (e.g. goethite and hematite) by dithionite–citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore δ56Fe trend. This iron pool was most depleted in 56Fe close to the sediment–water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide range of environmental settings.


      PubDate: 2015-12-29T09:02:44Z
       
  • Investigating the effects of growth rate and temperature on the B/Ca ratio
           and δ11B during inorganic calcite formation
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Karina Kaczmarek, Gernot Nehrke, Sambuddha Misra, Jelle Bijma, Henry Elderfield
      To deconvolve the effect of growth rate and temperature on the boron partitioning into calcite and its isotope fractionation, seeded calcite precipitation experiments were performed at a constant temperature and various growth rates and at a constant growth rate and various temperatures. We show that boron partitioning increases with increasing growth rate and decreases with increasing temperature. The B isotope fractionation between calcite and B(OH)4 − increases with increasing growth rate favoring the lighter B isotope for incorporation into calcite whereas no effect of temperature was observed within the temperature range investigated (12°C to 32°C). At the lowest temperature and growth rate δ11B of the calcite almost equals that of B(OH)4 − in solution. Applying the surface entrapment model (SEMO) of Watson and Liang (1995) to our data, we demonstrate that the observed effects of temperature and growth rate on B concentration can be explained by processes in the near surface layer of the calcite crystal.


      PubDate: 2015-12-29T09:02:44Z
       
  • Discerning primary versus diagenetic signals in carbonate carbon and
           oxygen isotope records: An example from the Permian–Triassic
           boundary of Iran
    • Abstract: Publication date: Available online 29 December 2015
      Source:Chemical Geology
      Author(s): Martin Schobben, Clemens Vinzenz Ullmann, Lucyna Leda, Dieter Korn, Ulrich Struck, Wolf Uwe Reimold, Abbas Ghaderi, Thomas J. Algeo, Christoph Korte
      Sedimentary successions across the Permian–Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C–O isotopes and Mn–Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C–δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of −1.2(±0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‘Boundary Clay’ in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by −2.1(±0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~50–80°C and water/rock ratios of <10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon — at a global scale — that has received relatively little attention to date.
      Graphical abstract image

      PubDate: 2015-12-29T09:02:44Z
       
  • Helium and carbon isotope signatures of gas exhalations in the westernmost
           part of the Pannonian Basin (SE Austria/NE Slovenia): Evidence for active
           lithospheric mantle degassing
    • Abstract: Publication date: Available online 29 December 2015
      Source:Chemical Geology
      Author(s): Karin Bräuer, Wolfram H. Geissler, Horst Kämpf, Samuel Niedermannn, Nina Rman
      We present new isotope (C, He, Ne, Ar, partly N) and compositional data from the free gas phase of fourteen degassing sites in the westernmost part of the Pannonian Basin near the Austria/Slovenia borderline. Based on these data, the origin of the gases and the degree of modification of the gas signatures due to interaction processes during migration are evaluated. The isotope signatures indicate an origin of helium and CO2 predominantly in the subcontinental mantle. Measured 3He/4He ratios from 4.95 to 6.32 Ra include the highest ones recorded in the whole Pannonian Basin system. Only at three locations in the periphery of the degassing center, a substantial admixture of crustal helium was found. The CO2 in the mofette gases and at the sites with the highest 3He/4He ratios (~6.3 Ra) is characterized by δ13C values of −3.5‰. In comparison with MORB (Mid-ocean Ridge Basalt), it is thus slightly enriched in 13C. The 3He/4He isotope ratios within the range typical for the subcontinental lithospheric mantle (SCLM) point to a fast, localized fluid transport from the magmatic reservoir to the surface. There are only few sites in European non-active volcanic regions where free gases with unmodified SCLM helium isotope signature escape at the surface. A comparison of the elemental and isotopic geochemical characteristics of gases with SCLM-helium signature from four different regions (Massif Central/France, Eifel/Germany, Eger Rift/Czech Republic and the westernmost part of the Pannonian Basin system) indicates that the European SCLM in general is characterized by a reservoir more enriched in 13C compared to MORB.


      PubDate: 2015-12-29T09:02:44Z
       
  • Extracting thermal histories from the near-rim zoning in titanite using
           coupled U-Pb and trace-element depth profiles by single-shot
           laser-ablation split stream (SS-LASS) ICP-MS
    • Abstract: Publication date: Available online 29 December 2015
      Source:Chemical Geology
      Author(s): M.A. Stearns, J.M. Cottle, B.R. Hacker, A.R.C. Kylander-Clark
      A method for depth profiling using single-shot laser-ablation split stream (SS-LASS) ICP-MS is developed to simultaneously measure U-Pb age and trace-element concentrations in titanite. Simple semi-infinite, 1-D half-space diffusion models were applied to near-rim, trace-element zoned domains in titanite to distinguish between cooling and (re)crystallization ages and investigate the potential for preservation of thermally mediated diffusive loss profiles. These data illustrate the need to measure multiple trace elements with varying diffusivities to interpret a mineral's thermal history resulting from the non-unique nature of 1-D diffusion models where both temperature and time are unknown. A case study of titanites from two Pamir Plateau gneiss domes indicates they underwent ≥25 Myr of amphibolite and granulite facies metamorphism yet did not experience significant volume diffusion modification following (re)crystallization. The interpretation of prolonged (re)crystallization rather than diffusion allows for high-resolution, near-rim temperature-time histories to be extracted using U-Pb dates and Zr4+ apparent temperatures by SS-LASS.


      PubDate: 2015-12-29T09:02:44Z
       
  • Copper and zinc isotope fractionation during deposition and weathering of
           highly metalliferous black shales in central China
    • Abstract: Publication date: Available online 29 December 2015
      Source:Chemical Geology
      Author(s): Yiwen Lv, Sheng-Ao Liu, Jian-Ming Zhu, Shuguang Li
      Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ65Cu (+0.14±0.0.09‰, 1σ) but heavy δ66Zn (0.51±0.11‰, 1σ). The elevated δ66Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems. The weathered shales and cherts have an extreme δ65Cu range from −6.42‰ to +19.73‰ and a modest δ66Zn range of +0.25‰ to +0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ66Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans.


      PubDate: 2015-12-29T09:02:44Z
       
  • The impact of evolving mineral–water–gas interfacial areas on
           mineral–fluid reaction rates in unsaturated porous media
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Anna L. Harrison, Gregory M. Dipple, Ian M. Power, K. Ulrich Mayer
      The distribution and evolution of mineral–water–gas interfacial areas exert a fundamental yet poorly documented control on mineral–fluid reactions in the unsaturated zone. Here, we explore the impact of changing mineral reactive surface area, water content, and gas distribution on the reaction of brucite [Mg(OH)2] with CO2 gas to form hydrated Mg-carbonate minerals in partially water saturated meter-scale column experiments. Brucite surface area, which is inferred to exert a direct control on mineral dissolution rates, demonstrates a complex evolution including roughening, fracturing and passivation that is inconsistent with conventional models of geometric evolution. Mineral–fluid reaction in the interior of single brucite grains maintains surface area at near-constant values despite the decreasing volumetric brucite content, until solid carbonate precipitates passivate brucite surfaces. The evolution of reactive surface area during passivation also does not follow simple geometric processes. A porous amorphous carbonate phase permits ready access to brucite surfaces for reaction until recrystallization of the amorphous carbonate into bladed, low-porosity nesquehonite [MgCO3·3H2O] abruptly quenches reaction. The varied water content of the experiments illustrates that the extent of mineral–gas reaction is limited by the abundance of water available to facilitate precipitation of hydrated carbonate minerals. Conversely, at high bulk water saturation, the development of preferential gas flow paths limited the exposure of reactive minerals to CO2 and reduced the overall extent of reaction. Thus, bulk mineral–fluid reaction rates were reduced at both high and low bulk water contents.
      Graphical abstract image

      PubDate: 2015-12-25T09:01:04Z
       
  • Gradual changes in upwelled seawater conditions (redox, pH) from the late
           Cretaceous through early Paleogene at the northwest coast of Africa:
           Negative Ce anomaly trend recorded in fossil bio-apatite
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Lászlό Kocsis, Emmanuel Gheerbrant, Mustapha Mouflih, Henri Cappetta, Alex Ulianov, Massimo Chiaradia, Nathalie Bardet
      Marine vertebrate fossils from late Cretaceous–early Paleogene shallow marine phosphorites of Morocco were investigated for their trace element compositions and neodymium isotopic ratios in order to constrain marine and depositional conditions. The various analyzed fossils are separated into two groups with different geochemical compositions: (1) shark tooth enameloid with generally lower Cu, Ba, rare earth elements (REEs) and U, and higher Na, Zn and Sr concentrations than (2) dentine, bone, and coprolites. These differences are related to the originally different structures of these fossils. All the fossils revealed very similar shale-normalized REE patterns, with negative Ce anomaly and heavy REE enrichment mimicking the REE pattern of modern oxic-seawater. The results therefore suggest REE uptake by the fossils from an early diagenetic pore fluid that was dominated by seawater. Importantly, the Ce anomaly does not vary among the different types of fossil remains and the values are very similar in a given layer. Moreover, a step-wise shift towards lower Ce/Ce* values from older to younger beds is apparent. Nd isotope analyses across the phosphorite succession yielded minor variation with an average εNd(t) value of −6.2±0.4 (n=12) indicating no major changes in the REE source during the studied interval. The relatively radiogenic values reflect Tethyan connection and/or important contribution of a mafic Nd source from weathering in the region. The age-related trend in the Ce anomaly is interpreted to be due to changes in redox and/or pH conditions of the upwelling seawater on the shallow shelf areas. The driving force of these changes was probably an extended global oceanic circulation, especially enhanced connection and water exchange between the North and South Atlantic.


      PubDate: 2015-12-25T09:01:04Z
       
  • Chlorite dissolution kinetics at pH3–10 and temperature to
           275°C
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Megan M. Smith, Susan A. Carroll
      Sheet silicates and clays are ubiquitous in geothermal environments. Their dissolution is of interest because this process contributes to scaling reactions along fluid pathways and alteration of fracture surfaces which could affect reservoir permeability. In order to better predict the geochemical impacts on long-term performance of engineered geothermal systems, we have measured chlorite dissolution and developed a generalized kinetic rate law applicable over an expanded range of solution pH and temperature. Chlorite, (Mg,Al,Fe)12(Si,Al)8O20(OH)16, commonly occurs in many geothermal host rocks as either a primary mineral or alteration product. A combination of new rate data from this study (collected using an Mg-rich chlorite variety, at 100–275°C and pH>5.5) as well as all available published chlorite dissolution datasets results in a kinetic rate equation that is valid over temperatures of 25–275°C and 3≤pH≤10: R = 1 ⋅ 10 − 4 ⋅ e − 30 RT ⋅ a H + 0.74 + 4.7 ⋅ 10 − 11 ⋅ e − 13 RT + 1.5 ⋅ 10 − 9 ⋅ e − 15 RT ⋅ a O H − 0.43 ⋅ 1 − e Δ G r RT The form of this equation, which includes a reaction affinity term to slow reaction as equilibrium is approached, can be incorporated into most existing reactive transport codes for use in prediction of rock–water interactions in engineered geothermal systems.


      PubDate: 2015-12-25T09:01:04Z
       
  • Editorial Board
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420




      PubDate: 2015-12-21T04:39:32Z
       
  • Geochemical constraints on the protoliths of eclogites and blueschists
           from North Qilian, northern Tibet
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Long Zhang, Ren-Xu Chen, Yong-Fei Zheng, Zhaochu Hu, Yueheng Yang, Lijuan Xu
      An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-temperature eclogites and blueschists from the North Qilian orogen in northern Tibet. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts rather than mid-ocean ridge basalts as commonly thought. These high pressure metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts-like, mid-ocean ridge basalts-like to oceanic island basalts-like, respectively, in trace element distribution diagrams. Such variable compositions are comparable with those of backarc basin basalts at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, and steep HREE patterns, with protolith U–Pb ages of 486–496Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U–Pb ages of 463±10Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to a localized fluid source for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that a backarc basin was transformed to an oceanic subduction zone where eclogite-facies metamorphism occurred.


      PubDate: 2015-12-05T13:46:41Z
       
  • Vanadium isotope measurement by MC-ICP-MS
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Fei Wu, Yuhan Qi, Huimin Yu, Shengyu Tian, Zhenhui Hou, Fang Huang
      We present a method to measure vanadium (V) isotopic composition for terrestrial rocks in this study. Vanadium was efficiently separated from matrix elements by a chromatographic technique using cation- and anion-exchange resin columns, avoiding the expensive TRU Spec resin. Vanadium isotope ratios were measured using a Thermo Scientific Neptune Plus MC-ICP-MS employing a sample–standard bracketing method. The increase in instrument sensitivity significantly reduces the amount of V required for the isotope analysis. Potential effects of acid molarities and concentration mismatch on instrumental analyses were rigorously evaluated. In addition, we performed Cr- and Ti-doping experiments to ensure the precision and accuracy of V isotope measurement. The δ51V values of mono-elemental V standards (BDH and USTC-V) relative to an Alfa Aesar (AA) standard solution (defined as δ51V=[(51 V/50 V)sample /(51V/50V)AA −1]×1000) measured in our laboratory were −1.23±0.08‰ (2SD, n =197) and 0.07±0.07‰ (2SD, n =112), respectively. Analyses of synthetic standard solutions (element doping+matrix spiking) obtained the same δ51V for the pure V solutions with a precision better than ±0.1‰ (2SD). Vanadium isotopic compositions of 12 reference materials, including igneous rocks (with mafic to felsic compositions) and manganese nodules, were measured using this method. These reference materials including basalts: BCR-2, −0.78±0.08‰ (2SD, n =36); BHVO-2, −0.83±0.09‰ (2SD, n =22); BIR-1, −0.92±0.09‰ (2SD, n =52); JB-2, −0.87±0.06‰ (2SD, n =20); diabase: W-2, −0.94±0.08‰ (2SD, n =15); andesites: AGV-1, −0.71±0.10‰ (2SD, n =6); AGV-2, −0.70±0.10‰ (2SD, n =37); JA-2, −0.80±0.07‰ (2SD, n =15); quartz latite: QLO-1, −0.61±0.03‰ (2SD, n =3); granodiorite: GSP-2, −0.62±0.07‰ (2SD, n =26); and manganese nodules: NOD-P, −1.65±0.06‰ (2SD, n =10); NOD-A, −0.99±0.10‰ (2SD, n =19). Based on repeated analyses of the rock standards, the long-term external precision of our method is better than ±0.1‰ (2SD) for δ51V. Such precision allows us to identify V isotope fractionation in high-temperature terrestrial samples, suggesting that V isotope geochemistry can be more widely used to study magmatism as well as supergene processes.


      PubDate: 2015-12-05T13:46:41Z
       
  • Stable isotope (B, H, O) and mineral-chemistry constraints on the magmatic
           to hydrothermal evolution of the Varuträsk rare-element pegmatite
           (Northern Sweden)
    • Abstract: Publication date: 10 February 2016
      Source:Chemical Geology, Volume 421
      Author(s): Karin Siegel, Thomas Wagner, Robert B. Trumbull, Erik Jonsson, Gabriella Matalin, Markus Wälle, Christoph A. Heinrich
      The internal evolution of the Varuträsk rare-element pegmatite (Skellefte District, Northern Sweden) has been investigated using stable isotope (B, H, O) geochemistry of tourmaline and coexisting micas, feldspar and quartz. Varuträsk is a classic and typical example of highly fractionated LCT-type pegmatites, with a pronounced concentric zoning pattern composed of well-developed border, wall and intermediate zones and a quartz core. The pegmatite displays considerable rare-element enrichment, culminating in the formation of albite-lepidolite and pollucite units in the innermost zones. Major and trace element variations in tourmaline from the main pegmatite zones correlate well with the internal zoning pattern. Mineral compositions record an abrupt change in fractionation trends between the barren outer and intermediate zones and the inner, late-stage assemblages that carry rare-element mineralization. This change is also shown by the B-isotope variations of tourmaline. Early and mid-stage tourmalines record a systematic increase in δ11B from −14.6‰ to −6.2‰ which can be explained by closed-system melt-mineral isotope fractionation whereby crystallization of large amounts of muscovite preferentially removes 10B from the residual melt. In contrast, tourmaline from late-stage assemblages in the inner zones and cross-cutting veinlets shows a reversal in the B isotope trend, with a decrease in δ11B from −8‰ to −14.1‰. This reversal cannot be explained by mineral-melt isotope fractionation, but requires fluid-melt partitioning and partial fluid loss. Hydrogen isotope variations in mica support this model. The systematic increase in δD from −75‰ in the outer zones (muscovite) to −63‰ and −53‰ in the inner zones (Li-micas) cannot be explained by closed-system variations in temperature or melt-mica fractionation, but it is consistent with late fluid exsolution. Oxygen isotope compositions of tourmaline (δ18O from 9.7‰ to 11.6‰), quartz (13.3‰ to 14‰) and mica (10.3‰ to 11.3‰) show good agreement with equilibrium partitioning and yield temperatures in the range 450°C to 600°C. Combining this with the stability fields of Li-aluminosilicates petalite and spodumene indicates crystallization pressures of 2–3kbar. Taken together, the stable isotope and mineral chemistry data demonstrate that rare-element enrichment in the innermost fractionated assemblages in the Varuträsk pegmatite was associated with the transition from purely magmatic crystallization to conditions where a separate aqueous fluid phase became important.


      PubDate: 2015-12-05T13:46:41Z
       
  • Hydrothermal exploration of mid-ocean ridges: Where might the largest
           sulfide deposits be forming?
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): Christopher R. German, Sven Petersen, Mark D. Hannington
      Here, we review the relationship between the distribution of modern-day seafloor hydrothermal activity along the global mid-ocean ridge crest and the nature of the mineral deposits being formed at those sites. Since the first discovery of seafloor venting, a sustained body of exploration has now prospected for one form of hydrothermal activity in particular – high temperature “black smoker” venting – along >30% of the global mid-ocean ridge crest. While that still leaves most of that ~60,000km continuous network to be explored, some important trends have already emerged. First, it is now known that submarine venting can occur along all mid-ocean ridges, regardless of spreading rate, and in all ocean basins. Further, to a first approximation, the abundance of currently active venting, as deduced from water column plume signals, can be scaled linearly with seafloor spreading rate (a simple proxy for magmatic heat-flux). What can also be recognized, however, is that there is an “excess” of high temperature venting along slow and ultra-slow spreading ridges when compared to what was originally predicted from seafloor spreading/magmatic heat-budget models. An examination of hydrothermal systems tracked to source on the slow spreading Mid-Atlantic Ridge reveals that no more than half of the sites responsible for the “black smoker” plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast-spreading ridges. The other half of all currently known active high-temperature submarine systems on the Mid-Atlantic Ridge are hosted under tectonic control. These systems appear both to be longer-lived than, and to give rise to much larger sulfide deposits than, their magmatic counterparts — presumably as a result of sustained fluid flow. A majority of these tectonic-hosted systems also involve water–rock interaction with ultramafic sources. Importantly, from a mineral resource perspective, this subset of tectonic-hosted vent-sites also represents the only actively-forming seafloor massive sulfide deposits on mid-ocean ridges that exhibit high concentrations of Cu and Au in their surface samples (>10wt.% average Cu content and >3ppm average Au). Along ultraslow-spreading ridges, first detailed examinations of hydrothermally active sites suggest that sulfide deposit formation at those sites may depart even further from the spreading-rate model than slow-spreading ridges do. Hydrothermal plume distributions along ultraslow ridges follow the same (~50:50) distribution of “black smoker” plume signals between magmatic and tectonic settings as the slow spreading MAR. However, the first three “black smoker” sites tracked to source on any ultra-slow ridges have all revealed high temperature vent-sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings. Further, deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of Cu and Au, respectively. An important implication is that ultra-slow ridges may represent the strongest mineral resource potential for the global ridge crest, despite being host to the lowest magmatic heat budget.


      PubDate: 2015-11-23T14:59:43Z
       
  • Closing in on the marine 238U/235U budget
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): M.B. Andersen, D. Vance, J.L. Morford, E. Bura-Nakić, S.F.M. Breitenbach, L. Och
      Significant redox-driven variability in the ratio between the two long-lived U parent isotopes, 238U and 235U, has recently been discovered. Thus, the 238U/235U system provides a promising new tool to evaluate redox changes in the past using the geological record. For such reconstruction to be successful, however, the modern marine U isotope cycle needs to be quantified. Here we compile U isotope ratios for the global dissolved riverine U flux. A total of 30 river samples covering a range of catchment sizes, latitudes, climates and continents were measured. Although variability is observed, the mean 238U/235U in the dissolved riverine U flux entering the ocean is close to the average for the continental crust, suggesting only minor 238U/235U fractionation during U weathering and transport. This riverine data is complemented by U isotope data in reducing sediments, the main oceanic U sink. Reducing sediments from an oxygen minimum zone off the coast of Washington State USA, show authigenic U accumulation with elevated 238U/235U compared to seawater, consistent with observations in other reducing marine sediments. However, these data also highlight that isotope partitioning between sediment and pore-water can cause variability in the 238U/235U of the accumulated authigenic U. The new data from this study, placed in the context of the growing body of data on modern marine U isotope cycling, suggest that U could be at steady-state in the modern ocean.


      PubDate: 2015-11-19T13:20:48Z
       
  • The influence of chromite on osmium, iridium, ruthenium and rhodium
           distribution during early magmatic processes
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): Philippe Pagé, Sarah-Jane Barnes
      Chromite-rich plutonic rocks are enriched in Os, Ir, Ru (IPGE) and Rh which is usually attributed to the presence of micron-size inclusions of platinum-group minerals. It is also known that volcanic rocks show positive correlations between Cr and IPGE and Rh with the most primitive lavas (olivine+chromite phyric) being enriched in these elements compared to the more fractionated lavas suggesting that chromite phenocrysts somehow influence and concentrate the IPGE and Rh. Previous results from in situ analysis of chromite phenocrysts from komatiites and from oxidized arc lavas showed that they contain IPGE and Rh in their structure. Our new results confirm these previous observations and show that the enrichment is a common phenomenon observed in volcanic rocks from several geological settings including MORB, boninite, komatiite, picrite and Hawaiian tholeiite. In contrast, in situ analysis of plutonic chromites show that they contain low concentrations of IPGE and Rh. Mass balance calculations indicate that chromite phenocrysts from volcanic samples are not the major hosts of Os, Ir and Rh accounting for <40%, <25% and <30% of the whole rock budgets, respectively, but they account for ≥30% of the whole rock Ru budget. As in the case of volcanic chromites, plutonic chromites do not have a great influence on Os, Ir and Rh whole rock budget in accounting for <25%, in contrast with volcanic chromites, plutonic chromites account for ≤10% of the whole rock Ru budget. These new results show that chromite from rapidly cooled environments can act as the main Ru-carrier phase but has a minor role in hosting Os, Ir and Rh. Overall, plutonic chromite has a minor role in hosting IPGE and Rh. Chromites from Bushveld ultramafic sills have also been analyzed. They show IPGE and Rh enrichments comparable to chromites from plume related magmas (komatiite and picrite) whereas chromites from Bushveld chromitites show low IPGE and Rh concentrations like other chromitite samples despite the fact that they crystallized from magmas of similar composition. This clearly indicates a change in Os, Ir, Ru and Rh behavior between rapidly cooled and slowly cooled environments.


      PubDate: 2015-11-19T13:20:48Z
       
  • Influence of glaciation on mechanisms of mineral weathering in two high
           Arctic catchments
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): Ruth S. Hindshaw, Tim H.E. Heaton, Eric S. Boyd, Melody R. Lindsay, Edward T. Tipper
      In order to investigate the effect of glaciation on mineral weathering, the stream water chemistry and the bacterial community composition were analysed in two catchments containing nominally identical sedimentary formations but which differed in the extent of glaciation. The stream waters were analysed for major ions, δ34S, δ18OSO4 and δ18OH2O and associated stream sediments were analysed by 16S rRNA gene tagged sequencing. Sulphate comprised 72–86% and 35–45% of the summer anion budget (in meq) in the unglaciated and glaciated catchments respectively. This indicates that sulfuric acid generated from pyrite weathering is a significant weathering agent in both catchments. Based on the relative proportions of cations, sulphate and bicarbonate, the stream water chemistry of the unglaciated catchment was found to be consistent with a sulphide oxidation coupled to silicate dissolution weathering process whereas in the glaciated catchment both carbonates and silicates weathered via both sulfuric and carbonic acids. Stable isotope measurements of sulphate, together with inferences of metabolic processes catalysed by resident microbial communities, revealed that the pyrite oxidation reaction differed between the two catchments. No δ34S fractionation relative to pyrite was observed in the unglaciated catchment and this was interpreted to reflect pyrite oxidation under oxic conditions. In contrast, δ34S and δ18OSO4 values were positively correlated in the glaciated catchment and were positively offset from pyrite. This was interpreted to reflect pyrite oxidation under anoxic conditions with loss of S intermediates. This study suggests that glaciation may alter stream water chemistry and the mechanism of pyrite oxidation through an interplay of biological, physical and chemical factors.


      PubDate: 2015-11-19T13:20:48Z
       
  • Effect of preservation state of planktonic foraminifera tests on the
           decrease in Mg/Ca due to reductive cleaning and on sample loss during
           cleaning
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): Heather J.H. Johnstone, William Lee, Michael Schulz
      Four species of planktic foraminifera from core-tops spanning a depth transect on the Ontong Java Plateau were prepared for Mg/Ca analysis both with (Cd-cleaning) and without (Mg-cleaning) a reductive cleaning step. Reductive cleaning caused etching of foraminiferal calcite, focused on Mg-rich inner calcite, even on tests which had already been partially dissolved at the seafloor. Despite corrosion, there was no difference in Mg/Ca of Pulleniatina obliquiloculata between cleaning methods. Reductive cleaning decreased Mg/Ca by an average (all depths) of ~4% for Globigerinoides ruber white and ~10% for Neogloboquadrina dutertrei. Mg/Ca of Globigerinoides sacculifer (above the calcite saturation horizon only) was 5% lower after reductive cleaning. The decrease in Mg/Ca due to reductive cleaning appeared insensitive to preservation state for G. ruber, N. dutertrei and P. obliquiloculata. Mg/Ca of Cd-cleaned G. sacculifer appeared less sensitive to dissolution than that of Mg-cleaned. Mg-cleaning is adequate, but SEM and contaminants (Al/Ca, Fe/Ca and Mn/Ca) show that Cd-cleaning is more effective for porous species. A second aspect of the study addressed sample loss during cleaning. Lower yield after Cd-cleaning for G. ruber, G. sacculifer and N. dutertrei confirmed this to be the more aggressive method. Strongest correlations between yield and Δ[CO3 2−] in core-top samples were for Cd-cleaned G. ruber (r=0.88, p=0.020) and Cd-cleaned P. obliquiloculata (r=0.68, p=0.030). In a down-core record (WIND28K) correlation, r, between yield values >30% and dissolution index, XDX, was −0.61 (p=0.002). Where cleaning yield <30% most Mg-cleaned Mg/Ca values were biased by dissolution.


      PubDate: 2015-11-19T13:20:48Z
       
  • Diffusion in mineral geochronometers: Present and absent
    • Abstract: Publication date: 20 January 2016
      Source:Chemical Geology, Volume 420
      Author(s): Igor M. Villa
      In solid-state physics and materials science Fick's Law diffusion is a well-established process. In Earth Sciences, laboratory experiments on garnet, olivine and other anhydrous minerals do document the intra-grain element concentration gradients that follow the functional form required by Fick's Law. Natural gradients in minerals have rarely been analyzed with the necessary spatial resolution. Reports of actual observations of diffusion profiles of element concentrations are rare in the literature, and diffusion profiles of isotope ratios in minerals used for geochronology are absent. An in-depth re-examination of recent and older literature suggests that isotope transport in minerals is instead often dominated by fluid-mediated retrogression reactions. Imaging microtextures by cathodoluminescence or back-scattered electron maps provides ubiquitous evidence of patchy or dendritic replacement structures, which correspond to multiple growth stages, in zircon, monazite, muscovite, biotite, K-feldspar, etc. The U–Pb, K–Ar and Rb–Sr systems in these partly retrogressed minerals show isotopic inheritance (that survived the retrograde reactions at least in part) in close correspondence with the petrologic relicts. Depending on the relative rates of the petrological processes relevant for isotope transport, geochronometers can be grouped in two classes: Class I (thermochronometers) are those for which the diffusivity of a given radiogenic isotope is faster than the rate of dissolution and/or reprecipitation, and Class II (hygrochronometers) are those for which aqueous dissolution/reprecipitation is the faster process. All of the abovementioned geochronometers, for which patchy/dendritic textures formed by diachronous mineral generations and isotopic inheritance were observed, must be assigned to Class II. Class II samples in petrologic equilibrium can (but need not) record purely thermal diffusion of daughter isotopes. The isotope record of Class II minerals in petrologic disequilibrium, being controlled by inheritance and retrogression reactions, depends chiefly on the reaction-promoting factors, water activity and strain. The dependence of Class II mineral ages on thermal diffusion is subordinate and never unique.


      PubDate: 2015-11-15T06:15:03Z
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2015