for Journals by Title or ISSN
for Articles by Keywords
help
  Subjects -> EARTH SCIENCES (Total: 649 journals)
    - EARTH SCIENCES (467 journals)
    - GEOLOGY (73 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (21 journals)
    - OCEANOGRAPHY (61 journals)

EARTH SCIENCES (467 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Open Access   (Followers: 6)
Acta Geotechnica     Hybrid Journal   (Followers: 6)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 3)
Advances in High Energy Physics     Open Access   (Followers: 16)
Advances In Physics     Hybrid Journal   (Followers: 10)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 12)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anadolu University Journal of Science and Technology     Open Access  
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 3)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Full-text available via subscription   (Followers: 12)
Annals of GIS     Hybrid Journal   (Followers: 17)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene     Hybrid Journal   (Followers: 2)
Anthropocene Review     Hybrid Journal   (Followers: 4)
Applied Clay Science     Hybrid Journal   (Followers: 4)
Applied Geochemistry     Hybrid Journal   (Followers: 11)
Applied Geomatics     Hybrid Journal   (Followers: 3)
Applied Geophysics     Hybrid Journal   (Followers: 7)
Applied Ocean Research     Hybrid Journal   (Followers: 4)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 29)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 33)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 10)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 16)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 8)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Asian Review of Environmental and Earth Sciences     Open Access   (Followers: 1)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 2)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 24)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 10)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 20)
Bulletin of Volcanology     Hybrid Journal   (Followers: 17)
Cadernos de Geociências     Open Access  
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 14)
Canadian Mineralogist     Full-text available via subscription   (Followers: 3)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 19)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 4)
Chemical Geology     Hybrid Journal   (Followers: 14)
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: 3)
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: 12)
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: 10)
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: 4)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 10)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 65)
Earth and Space Science     Open Access   (Followers: 7)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 19)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 9)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 6)
Earthquake Science     Hybrid Journal   (Followers: 9)
Earthquake Spectra     Full-text available via subscription   (Followers: 17)
Ecohydrology     Hybrid Journal   (Followers: 9)
Ecological Questions     Open Access   (Followers: 5)
Electromagnetics     Hybrid Journal   (Followers: 3)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 21)
Environmental Geology     Hybrid Journal   (Followers: 9)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Hybrid Journal   (Followers: 3)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 17)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 10)
Exploration Geophysics     Hybrid Journal   (Followers: 3)
Facies     Hybrid Journal   (Followers: 9)
Fieldiana Life and Earth Sciences     Full-text available via subscription   (Followers: 1)
Física de la Tierra     Open Access  
Folia Musei rerum naturalium Bohemiae occidentalis. Geologica et Paleobiologica     Open Access  
Folia Quaternaria     Open Access  
Forestry Chronicle     Full-text available via subscription   (Followers: 10)
Frontiers in Earth Science     Open Access   (Followers: 5)
Frontiers in Geotechnical Engineering     Open Access   (Followers: 2)
Frontiers of Earth Science     Hybrid Journal   (Followers: 8)
Fundamental and Applied Limnology / Archiv für Hydrobiologie     Full-text available via subscription   (Followers: 4)
GEM - International Journal on Geomathematics     Hybrid Journal   (Followers: 1)
Geo-Marine Letters     Hybrid Journal   (Followers: 7)
Geoacta     Open Access   (Followers: 4)
Geobiology     Hybrid Journal   (Followers: 7)
Geocarto International     Hybrid Journal   (Followers: 15)
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 24)
Geochronometria     Hybrid Journal   (Followers: 3)
Geoderma Regional : The International Journal for Regional Soil Research     Full-text available via subscription   (Followers: 3)
Geodinamica Acta     Hybrid Journal   (Followers: 3)
Geodynamics & Tectonophysics     Open Access  
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Hybrid Journal   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 20)
Géographie physique et Quaternaire     Full-text available via subscription  
Geography and Natural Resources     Hybrid Journal   (Followers: 4)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 6)
Geological Journal     Hybrid Journal   (Followers: 14)
Geological Magazine     Hybrid Journal   (Followers: 16)
Geology Today     Hybrid Journal   (Followers: 18)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 8)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 23)
Geophysical & Astrophysical Fluid Dynamics     Hybrid Journal   (Followers: 2)
Geophysical Journal International     Hybrid Journal   (Followers: 31)
Geophysical Prospecting     Hybrid Journal   (Followers: 7)
Geophysics     Full-text available via subscription   (Followers: 18)
GeoResJ     Hybrid Journal  
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards     Hybrid Journal   (Followers: 8)
Geoscience Canada : Journal of the Geological Association of Canada / Geoscience Canada : journal de l'Association Géologique du Canada     Full-text available via subscription   (Followers: 3)
Geoscience Data Journal     Open Access   (Followers: 2)
Geoscience Frontiers     Open Access   (Followers: 8)
Geoscience Letters     Open Access  
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 9)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 2)
Geoscientific Model Development     Open Access   (Followers: 2)
Geostandards and Geoanalytical Research     Hybrid Journal   (Followers: 2)
Geosystem Engineering     Hybrid Journal   (Followers: 1)
Geotectonic Research     Full-text available via subscription   (Followers: 4)
Geotectonics     Hybrid Journal   (Followers: 7)
GISAP : Earth and Space Sciences     Open Access   (Followers: 1)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 3)
Global and Planetary Change     Hybrid Journal   (Followers: 11)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 11)
Gondwana Research     Hybrid Journal   (Followers: 6)
Grassland Science     Hybrid Journal   (Followers: 1)
Ground Water     Hybrid Journal   (Followers: 23)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 15)
GSA Today     Partially Free  
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 3)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 16)
Hydrological Processes     Hybrid Journal   (Followers: 23)
Hydrology and Earth System Sciences     Open Access   (Followers: 26)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 51)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 2)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 5)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 29)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 2)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 23)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 27)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 10)
International Journal of Earth Sciences     Hybrid Journal   (Followers: 30)
International Journal of Forest, Soil and Erosion     Open Access   (Followers: 4)
International Journal of Geo-Engineering     Open Access  
International Journal of Geographical Information Science     Hybrid Journal   (Followers: 52)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [14 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2969 journals]
  • Compound-specific carbon and nitrogen isotopic compositions of chlorophyll
           a and its derivatives reveal the eutrophication history of Lake Zurich
           (Switzerland)
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Sebastian Naeher, Hisami Suga, Nanako O. Ogawa, Carsten J. Schubert, Kliti Grice, Naohiko Ohkouchi
      To reconstruct the impact of eutrophication on phototrophic communities and the biogeochemical cycling of carbon and nitrogen in the surface water, we investigated the distributions and carbon and nitrogen isotopic compositions (δ13C and δ15N values) of chlorins in the sediments of Lake Zurich. The chlorin distributions were dominated by chlorophyll a (Chl a) and its derivatives, which reflect rapid degradation to the pheopigments in the water column and sediments of the lake. The δ13C values of these sedimentary chlorins followed the historical trends of eutrophication and reoligotrophication, except in the surface sediments, which were characterised by higher relative contributions of aged, redeposited organic matter (OM). The δ13C values of the sedimentary chlorins together with bulk sediment δ13C values and C/N ratios indicate that the phototrophic communities in the lake used a 13C-depleted carbon source, which is mainly of aquatic origin. The δ15N values of chlorins reflect the predominance of nitrate assimilating phototrophs, especially the non-N2-fixing cyanobacterium Planktothrix rubescens prevalent during sediment deposition. Shifts in δ15N values of Chl a followed mostly the trends in eutrophication and reoligotrophication, but were also affected by community assemblage shifts to diatoms and/or other cyanobacteria at the end of the 19th century and during the eutrophication maximum in the 1970s. The lower δ15NChl-a values in the surface sediments coincide with increasing nitrogen to phosphorus ratios and reduced water column mixing that characterise the recent reoligotrophication period and may explain the predominance of P. rubescens in Lake Zurich. In contrast, the higher contributions of laterally transported OM explains the large offset of δ15N values of the pheopigments relative to Chl a, which is supported by the high radiocarbon age of the surface sediments.


      PubDate: 2016-08-24T02:49:41Z
       
  • Oxygen isotope and trace element geochemistry of zircons from porphyry
           copper system: Implications for Late Triassic metallogenesis within the
           Yidun Terrane, southeastern Tibetan Plateau
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): De-Xin Kong, Ji-Feng Xu, Jian-Lin Chen
      It is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore-bearing ones from barren ones, especially by examining the oxygen isotopic and trace element compositions of zircon. The oxygen isotopic composition and trace element abundances of zircons from the mineralized Pulang porphyries and the coeval Disuga and Lannitang andesites of the Yidun Terrane, eastern Tibetan Plateau, have been determined by secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) in this study, respectively. Analyzed zircons have relatively high but invariant δ18O values (~5.8‰–6.9‰) that are indicative of derivation from homogeneous mantle or mantle-derived magmas together with contribution of mature crustal materials. We propose a model for the Late Triassic magmatism in the Yidun Terrane involving magmas derived from partial melting of subduction-metasomatized mantle peridotites that subsequently experienced melting–assimilation–storage–homogenization (MASH) processes. The low calculated Ti-in-zircon temperatures (644°C–863°C, generally between 640°C and 690°C) for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas, and partly controlled by the crystallization of amphibole. Zircon grains from porphyry copper system display small negative europium anomalies (mostly Eu/Eu*>0.5), contrasting with pronounced negative Eu anomalies in zircons (mostly Eu/Eu*<0.5) associated with magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W deposits. Calculated REE compositions of the melt that was in equilibrium with zircons from porphyry copper system display depletion in the middle rare earth elements (MREE) without negative Eu anomaly, while that of zircons from magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W system show depletion in the MREE with negative Eu anomaly. It is speculated that the parental melts of zircon have decreasing magmatic oxidation state in the following sequence: porphyry copper system>magmato-hydrothermal quartz-vein Cu-Mo-W deposit>magmato-hydrothermal quartz-vein Mo-W deposit. This study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits.


      PubDate: 2016-08-24T02:49:41Z
       
  • Geochemistry of dissolved aluminum at low pH: Hydrobasaluminite formation
           and interaction with trace metals, silica and microbial cells under anoxic
           conditions
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Javier Sánchez-España, Iñaki Yusta, William D. Burgos
      The precipitation of dissolved aluminum is among the most important geochemical processes affecting the water chemistry and trace metal dynamics of acid mine drainage (AMD). In this study, we focus on the in situ formation of Al precipitates and their subsequent interaction with trace metals, dissolved silica and microbial cells under anoxic conditions (e.g., deep strata of pit lakes, flooded underground mines). In the presence of sulfate (0.1–0.2M), dissolved aluminum precipitates at pH ~4.0 and forms an amorphous, globular hydroxysulfate precipitate with composition corresponding to hydrobasaluminite. This proto-hydrobasaluminite shows an important capacity to incorporate SiO2 (aq), divalent metal cations (FeII, PbII, BeII), and many anion-forming trace elements (As, Cr, Sb, V, Se, U) by coprecipitation. Removal efficiencies obtained experimentally with freshly formed hydrobasaluminite under anoxic and oxic conditions were consistent with metal profiles obtained in the pit lakes and suggest that the behaviour of many toxic metals is closely linked to the Al particles. Detailed transmission electron microscopy (TEM) shows that hydrobasaluminite may coexist with more crystalline minerals like gibbsite, bayerite and/or allophane, which are present as nanocrystals embedded in the amorphous matrix and appear to form by transformation of the former. Hydrobasaluminite precipitation is favoured by the activity of iron- and sulfate-reducing microbes in the water column and at the sediment/water interface. Our study shows that, at solubility equilibrium, microbial cells often serve as nuclei for Al precipitation. Further, chemical mapping of Al-coated cells by scanning/TEM shows an important incorporation of SiO2, Fe2+ (valence confirmed by electron energy loss spectroscopy) and other metal cations (e.g., Mg2+) in the Al cell coatings as compared to Al particles presumed to form via abiotic precipitation. These Al cell coatings are amorphous to X-rays and their compositions likely result from coprecipitation of SiO2 and Fe2+ resulting in Si-rich and Fe(II)-rich hydrobasaluminite. The disparity between field and laboratory data, the serendipitous detection of nontronite-like smectite in precipitates formed in the acidic waters under laboratory conditions, and the apparent saturation with respect to different clay minerals (e.g., kaolinite, halloysite) are all evidences suggesting thermodynamic feasibility of aluminosilicate precursor formation in these systems.
      Graphical abstract image

      PubDate: 2016-08-24T02:49:41Z
       
  • Behavior of U, Th and Ra isotopes in soils during a land cover change
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Sophie Rihs, Adrien Gontier, Eric Pelt, David Fries, Marie-Pierre Turpault, François Chabaux
      Over the last decades, the U- and Th-series nuclides were successfully used to determine weathering rates in various environments. The objective of this study is to assess the potential impact of the vegetation change on the U- and Th-series signal recorded in forested soils. This study was carried out from the experimental forest site of Breuil-Chenue (Morvan, France) developed by the INRA-BEF team. The native forest of the site was partially clear-felled in 1976 and replaced by monospecific plantations stands (Oak and Douglas fir). U- and Th-series disequilibria were measured in 2011 in the podzolic soils developed under the native forest, and in the two replanted stands. Separation of primary minerals (biotite, muscovite and perthitic feldspar) and selective extractions of the Fe and Al oxides were performed to investigate the distribution of U and Th among these soil fractions. The selective extractions suggest that a significant part of U and Th is primarily held by Fe-bearing silicates. Our results suggest that the tree substitution seems to produce a large dissolution of these minerals under the Oaks, resulting to a release of U and Th. However, below 25cm no impact of this release was observed on U-series disequilibria. A scenario allowing to reconcile the significant mobilization of U and the constancy of U-series disequilibria is proposed. Above 25cm, additional pedogenic redistribution of U and Th isotopes occurs in all the profiles, inducing some discrepancies between U-series disequilibria. A clear correlation between the (230Th/234U) ratios and the proportions of amorphous and interlayer Al hydroxides has been highlighted. This correlation suggests a mobility of U and Th isotopes strongly associated to Al dynamics in these soils rather than Fe, despite the primary location of U in the Fe-bearing silicates and the overwhelming reported control of UVI by Fe-oxides in oxidized environments. These pedogenic processes make the shallowest horizons of podzolic soils unsuitable for U-series dating. In contrast, a soil production rate can be deduced from the deepest soil layers which do not show such effects on the U-series nuclides. The reproducible U-series disequilibria measured in four whole-profile replicates emphasize the robustness and the significance of the “long-lived” U-series disequilibria in deep soil layers relative to long-term weathering rates, independent of transient perturbations such as land cover changes. Finally, because Ra can strongly accumulates in plants, the (226Ra/230Th) ratios in the different soils were affected by the flux of 226Ra released by litter degradation. The use of this ratio as a long-term chronometer should therefore be performed with caution in such contexts. No direct impact of the vegetation type on the (228Ra/232Th) ratios was identified, due to the short 228Ra half-life.
      Graphical abstract image

      PubDate: 2016-08-24T02:49:41Z
       
  • Elevated uranium concentrations in Lake Baikal sediments: Burial and early
           diagenesis
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Lawrence M. Och, Beat Müller, Christian März, Adrian Wichser, Elena G. Vologina, Michael Sturm
      The water column of Lake Baikal (Siberia) is pervasively oxic and O2 penetrates several cm into the sediment, followed by distinct layers of Fe/Mn oxide that undergo reductive-dissolution/oxidative-precipitation cycles. Uranium (U) contents of the oxic surface sediment layers were ~15μgg−1, which is unparalleled in oxygenated lakes. To understand the processes leading to this enrichment we investigated the geochemical composition of the particulate matter and pore water of four sediment cores from different locations in the lake and performed mass balance calculations based on sediment mass accumulation rates and published loads from major tributaries. The comparison of loads and export of U in Lake Baikal suggested that current estimates of loads are too low by a factor of about 3 compared to sediment mass accumulation rates. Peak loads during spring ice melt in tributaries that are difficult to monitor and quantify might be the main cause for the deviation. The high U concentrations in the lake sediments originated from the scavenging of U in the water column through association with settling organic particles and particulate Fe(III)- and, to a lesser extent, Mn(IV)-oxides. We outline the hypothesis that two distinct U phases, lithogenic and non-lithogenic U reach the lake sediment and that authigenic U is subsequently formed under reducing conditions within the sediment. In some cores we found that most U was remobilized during the degradation of organic matter, in particular within the top oxygenated layer of the sediment. Significant enrichments prevailed due to U adsorption to and/or co-precipitation with Fe-oxides. When Fe-oxides and, to a lesser extent, Mn-oxides were reductively dissolved, they released U to the pore water, leading to peak dissolved U concentrations in the anoxic sediment, which in turn, precipitated as authigenic U under predominantly sulphate-reducing conditions. The onset of the accumulation of authigenic U coincided with the formation of distinct Fe/Mn oxide layers above. We argue that the resilience of Fe-oxides (especially crystalline goethite and hematite), in association with phosphate, even within reducing (but non-sulfidic) sediments support the burial of substantial amounts of U.


      PubDate: 2016-08-24T02:49:41Z
       
  • Simultaneous quantitative analysis of Ni, VO, Cu, Zn and Mn geoporphyrins
           by liquid chromatography-high resolution multistage mass spectrometry:
           Method development and validation
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Martijn Woltering, Svenja Tulipani, Chris J. Boreham, John Walshe, Lorenz Schwark, Kliti Grice
      A method is described for the identification and quantification of Cu, Ni, VO, Zn and Mn metalloporphyrins in geological samples using a high performance liquid chromatography (HPLC) system coupled to a high resolution Thermo Orbitrap XL mass spectrometer (MS). The linear correlation of the detector response to the compound concentrations in Cu, Ni, VO, Zn and Mn porphyrin standards (R2 values between 0.9975 and 0.9994), verified the suitability of the methodology for the quantification of these compounds. Furthermore, the method was validated by the analysis of complex porphyrin distributions in geological sample isolates from the Australian Toolebuc Formation and Bight Basin. By using the high resolution of the Orbitrap MS detector it was not only possible to reproduce the porphyrin distributions reported from previous analyses of the same isolates, but also to identify and resolve a range of additional compounds such as an iso-butyl C34 VO porphyrin indicative of palaeoenvironmental photic zone euxinia and several Cu and Zn porphyrins. The methodology described here provides a new high resolution tool for routine analysis of complex metalloporphyrin distributions in geological sample extracts, enabling the simultaneous quantitative analysis of Cu, Ni, VO, Zn and Mn porphyrins without the need of prior de-metalation or further fractionation of the porphyrin extract. The high resolution of the Orbitrap MS combined with the ability to perform multistage mass spectrometry leads to a significant improvement in compound detection and identification, which shows a high potential in the analysis of low abundance porphyrins, such as high-molecular-weight porphyrins with extended alkyl side-chains.


      PubDate: 2016-08-24T02:49:41Z
       
  • Aluminous gneiss derived by weathering of basaltic source rocks in the
           Neoarchean Storø Supracrustal Belt, southern West Greenland
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Kristoffer Szilas, Kate Maher, Dennis K. Bird
      The origin of amphibolite-facies aluminous gneiss from the gold-hosting Neoarchean Storø Supracrustal Belt in the Nuuk region of southern West Greenland is investigated in this study. An improved understanding of the formation of such aluminous gneiss has implications for genetic models (epithermal vs. orogenic style) for a local gold occurrence, which is hosted by sheeted quartz-veins within amphibolite in the hanging wall adjacent to the aluminous gneiss on the island of Storø. The aluminous gneiss mainly consists of garnet, plagioclase, sillimanite, quartz and biotite, which suggest a pelitic protolith. However, it has previously been postulated that the aluminous gneiss represents a hydrothermal alteration product, formed by leaching of a mafic precursor that was subsequently transformed to the current mineral assemblage during later regional metamorphism. In support of this hypothesis are decimeter-scale relicts of amphibolite, found within the decameter-wide aluminous gneiss, that share similar ratios of commonly fluid immobile elements, such as Al, Ti, Zr, Hf, Nb and Lu. Metasedimentary rocks sensu stricto are also present within the Storø Supracrustal Belt. These mostly comprise biotite schist with a similar mineralogy as the aluminous gneiss, and are located adjacent to the latter in the footwall. The aluminous gneiss has high bulk-rock Al2O3 contents and low SiO2 relative to the biotite schist, which is less aluminous, has higher SiO2, and is more potassic than the aluminous gneiss. The immobile element ratios of the biotite schist are significantly different from those of the amphibolite and the aluminous gneiss. Additionally, the biotite schist yields distinct detrital zircon age populations, whereas only metamorphic zircon has been found in the aluminous gneiss and amphibolite. In the present study, the isocon method was applied to provide mass balance constraints on the alteration of basalt to the protolith of the aluminous gneiss prior to regional metamorphism of the entire Storø supracrustal sequence. Accepting minor fractionation among otherwise immobile elements, an error of at least ±20% is estimated for the isocon mass-balance model. The results indicate that all major elements, except for K2O were leached from the basaltic precursor, resulting in a net mass loss ranging from −20 to −40wt.%. However, despite the similar trace element patterns and ratios of the aluminous gneiss and the adjacent amphibolite, in situ alteration or weathering of the basaltic precursor rock is not possible given the small, but persistent, fractionation among fluid immobile elements, due to the unusual accumulation of Cr, U, Ni and Th, as well as the observed modal layering within the aluminous gneiss. Instead, this points to physical transport during sedimentary reworking of a mafic protolith and potentially the addition of redox-sensitive elements such as U and Cr from the water column, and therefore suggest that this aluminous gneiss simply represents a metasediment with a mafic provenance. The implication of this study for gold exploration within the Archean supracrustal belts of the SW Greenland is that aluminous gneiss is unlikely to represent an indicator of acidic hydrothermal alteration as previously postulated. Instead, aluminous gneisses within these supracrustal belts are likely of sedimentary origin and may provide a venue to further understand the exogenous environments of the Archean Earth, and thus further geochemical studies of such rocks are recommended in order to place constraints on the composition of the hydrosphere at that time.


      PubDate: 2016-08-19T18:19:08Z
       
  • Multiple sulfur isotopes (δ34S, Δ33S) and trace elements (Mo, U,
           V) reveal changing palaeoenvironments in the mid-Carboniferous Chokier
           Formation, Belgium
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Katharina Siedenberg, Harald Strauss, Ralf Littke
      In this study, multiple sulfur isotopes are combined with widely-used redox-sensitive trace element concentrations (Mo, V, and U) in order to further validate the use of δ 34S and Δ 33S as a paleoredox proxy. The approach is applied to the mid-Carboniferous Chokier Formation, for which a detailed microfacies study identified a marked change in the environmental conditions during sediment deposition. Sediments from the upper part of the Chokier Formation exhibit negative δ 34S (mean −24.6‰) and positive Δ 33S (mean +0.040‰) values that are attributed to microbial sulfate reduction (MSR) and/or microbial sulfur disproportionation (MSD). Samples from the lower part of the Chokier Formation display less negative δ 34S (mean −9.7‰) and negative Δ 33S (mean −0.029‰) values. Paired δ 34S-Δ 33S values are located on a mixing curve between two endmembers: (1) sulfur generated by MSR and/or MSD, and (2) seawater sulfate. This latter observation is attributed to a restricted seawater exchange that limited the sulfate supply, thus, promoting closed-system conditions. Consequently, sulfur isotope values approached the isotopic composition of contemporaneous seawater. Trace element abundances exhibit a stronger enrichment in the upper Chokier Formation (EFMo =9–77, EFV =1–4; EF=Enrichment Factor) than in the lower Chokier Formation (EFMo =16–21, EFV ≈1). However, a hydrographic rather than a redox control is inferred for both sections based on Mo/TOC≤15 and Mo/U ratios well below seawater values. For the upper Chokier Formation, Mo/TOC≈15 indicates a moderately restricted basin, whereas the lower part is more strongly restricted (Mo/TOC≪15). Multiple sulfur isotopes and trace element abundances/ratios reveal a consistent picture of restricted basinal conditions during deposition of the lower Chokier Formation and more open conditions for the upper Chokier Formation. Hence, it can be concluded that results support the application of multiple sulfur isotope analyses for revealing paleoenvironmental conditions.


      PubDate: 2016-08-19T18:19:08Z
       
  • Trace elements (Li, B, Mn and Ba) as sensitive indicators for salinization
           and freshening events in coastal aquifers
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): A. Russak, O. Sivan, Y. Yechieli
      The current global intrusion of seawater into coastal aquifers causes salinization of groundwater and thus significant degradation of its quality. This study quantified the effect of seawater intrusion and freshening events in coastal aquifers on trace elements (Li, B, Mn and Ba) across the fresh-saline water interface (FSI) and their possible use as indicators for these events. This was done by combining field data and column experiments simulating these events. The experiments enabled quantification of the processes affecting the trace element composition and examination of whether salinization and freshening events are geochemically reversible, which has been seldom investigated. The dominant process affecting trace element composition during salinization and freshening is ion exchange. The results of the experiments show that the concentrations of major cations and Li+ were reversible during salinization and freshening, whereas B, Mn2+ and Ba2+ were not. During salinization, Li+ and B were depleted due to sorption by 10 and 100μmol·L−1, respectively, to about half of their expected conservative concentrations. The relative depletion of Li+ increased with distance from the shore, representing the propagation of salinization. Ba2+ and Mn2+ were desorbed from the sediment during salinization and enriched by tenfold in the aqueous phase compared to their concentration in seawater (~0.1 μeq·L−1). During freshening both were depleted by almost tenfold compared to their concentration in fresh groundwater (~0.7 μeq·L−1). The depletion of Mn2+ is a sensitive marker for freshening because Mn2+ has a strong affinity to the solid phase. Moreover, this study shows that both Mn2+ and Ba2+ can be used as sensitive hydrogeochemical tools to distinguish between salinization and freshening events in the FSI zone in coastal aquifers.


      PubDate: 2016-08-19T18:19:08Z
       
  • Crystal-melt elemental partitioning in silicic magmatic systems: An
           example from the Peach Spring Tuff high-silica rhyolite, Southwest USA
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Abraham J. Padilla, Guilherme A.R. Gualda
      Partition coefficients (K d ) are critical for the quantitative modeling of the evolution of magmatic systems. High-silica rhyolites (HSR) are characterized by saturation in numerous phases, such that a comprehensive K d database is necessary. While there exists a large body of published K d values, various methods have been used by different authors over time, focusing on different compositions and conditions, and most investigations are limited to small suites of elements (e.g. rare earth elements - REE), typically in few (2–3) mineral phases. In this study, we present K d values for a suite of 45 elements in 8 mineral phases from a HSR pumice clast from the Peach Spring Tuff (PST): sanidine, plagioclase, biotite, amphibole, titanite, apatite, zircon, and chevkinite. We determine K d by measuring crystal rim compositions and unaltered glass from a single volcanic sample, which reflect equilibration between crystals and melt in the system at or near the time of eruption. We measure compositions using a combination of SEM-EDS and LA-ICPMS analysis. We show that titanite strongly partitions REE from the melt, with a significant preference for middle (MREE) over light (LREE) and heavy-REE (HREE). Apatite and amphibole share a similar MREE-enriched pattern, though with absolute K d values more than an order of magnitude lower than titanite; while apatite displays a slight preference for LREE over HREE, amphibole shows a higher affinity for HREE than LREE. Zircon strongly partitions HREE, Hf, and U, with little affinity for LREE. Chevkinite concentrates LREE and Th significantly more than any other phase we analyzed. Biotite K d are generally low (<10), with the highest values for transition metals and Ba. Sanidine and plagioclase display a strong preference for Sr, Ba, and Eu. Our dataset is largely consistent with published studies that use similar (in-situ) techniques in samples of similar composition (HSR). We observe a large variation in other studies that we attribute in part to contamination from inclusions in crystals, particularly when measuring compositions by whole-crystal methods. Our dataset suggests that accessory minerals play a dominant role in the partitioning of trace elements. They collectively control the distribution of REE and high field strength elements (HFSE) in the system. On the other hand, large ion lithophile elements (LILE) are most dominantly influenced by feldspars, but, if abundant, phases such as biotite and apatite may also influence the partitioning of LILE from the melt. We analyze our K d values in light of the theoretical crystal lattice strain model (LSM), by fitting our data to curves that have the expected form in ln(K d ) versus ionic radius space (Onuma diagrams). The fact that we are successful in finding good fits for our K d values, especially the REE, reinforces the idea that mineral-melt equilibrium was attained and analytical data are of appropriate quality. Additionally, we use the fitted curves to estimate Ce3+/Ce and Eu2+/Eu ratios in the melt, which are controlled by the oxidation state of the magma. We estimate that >97% of all Ce in the PST melt existed as Ce3+. The proportion of Eu2+ in the melt is not as well constrained; we estimate Eu2+ ranges mainly from ~10 to ~50% of the total Eu.


      PubDate: 2016-08-15T02:56:05Z
       
  • SIMS and NanoSIMS analyses of well-preserved microfossils imply
           oxygen-producing photosynthesis in the Mesoproterozoic anoxic ocean
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Xiaotong Peng, Zixiao Guo, Christopher H. House, Shun Chen, Kaiwen Ta
      Well-preserved microfossils in the stromatolitic cherts from the Gaoyuzhuang Formation (~1500Ma), which have one of the best state of preservation in the Proterozoic Era, may play key roles in systematizing information about the evolution of early life in the Proterozoic Ocean. Here, a combination of light microscopy (LM), scanning electron microscopy (SEM), nano-scale secondary ion mass spectrometry (NanoSIMS), secondary ion mass spectrometry (SIMS) and geochemical techniques was employed to characterize the morphology, elemental distributions and carbon isotope values of individual Gaoyuzhuang microfossils, and their growing environment. Light microscopy analyses show that abundant filamentous and coccoid microfossils, classified as oscillatoriacean and chroococcacean cyanobacteria, respectively, are exceptionally well preserved in chert. NanoSIMS analyses show that metabolically important elements, such as 12C−, 13C−, 12C14N−, 32S−, and 34S−, are concentrated in these microfossils and that the variations in the concentrations of these elements are similar to each other, establishing the elemental patterns in undoubtedly biogenic microstructures. The carbon isotope (δ13C) values of individual microfossils range from −31.7‰±0.9‰ to −23.1‰±1.0‰ (weighted mean=−28.5‰±0.1‰), consistent not only with a Calvin Cycle-based photosynthesis, but typically for chroococcacean cyanobacteria as well. Our results, for the first time, provide the element distributions and cell specific carbon isotope values on convincing Mesoproterozoic cyanobacterial fossils, supporting sustained oxygen-producing photosynthesis in the Proterozoic Ocean. The geochemical data indicate these fossil microorganisms may grow in an anoxic seawater, potentially supporting the delayed oxygenation on the Earth. In this regard, this integrated approach may be a step towards a more comprehensive picture of the evolution of early life on the Earth.


      PubDate: 2016-08-15T02:56:05Z
       
  • Divergent drivers of carbon dioxide and methane dynamics in an
           agricultural coastal floodplain: Post-flood hydrological and biological
           drivers
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Jackie R. Webb, Isaac R. Santos, Douglas R. Tait, James Z. Sippo, Ben C.T. Macdonald, Barbara Robson, Damien T. Maher
      Many coastal floodplains have been artificially drained for agriculture, altering hydrological connectivity and the delivery of groundwater-derived solutes including carbon dioxide (CO2) and methane (CH4) to surface waters. Here, we investigated the drivers of CO2 and CH4 within the artificial drains of a coastal floodplain under sugarcane plantation and quantify the contribution of groundwater discharge to CO2 and CH4 dynamics over a flood event (290mm of rainfall). High temporal resolution, in situ observations of dissolved CO2 and CH4, carbon stable isotopes of CH4 (δ13C-CH4), and the natural groundwater tracer radon (222Rn) allowed us to quantify CO2, CH4 and groundwater dynamics during the rapid recession of a flood over a five day period. Extreme super-saturation of free CO2 ([CO2*]) up to 2,951μM (25,480% of atmospheric equilibrium) was driven by large groundwater input into the drains (maximum 87cmday−1), caused by a steep hydraulic head in the adjacent water table. Groundwater input sustained between 95 and 124% of the surface [CO2*] flux during the flood recession by delivering high carbonate alkalinity groundwater (DIC=10,533μM, ~pH=7.05) to acidic surface water (pH <4), consequently transforming all groundwater-derived DIC to [CO2*]. In contrast, groundwater was not a major direct driver of CH4 contributing only 14% of total CH4 fluxes. A progressive increase in CH4 concentrations of up to ~2400nMday−1 occurred as a combination of increased substrate availability delivered by post-flood drainage water and longer residence times, which allowed for a biogenic CH4 signal to develop. The progressive enrichment in δ13C-CH4 values (−70‰ to −48‰) and increase in CH4 concentrations (46–2460nM) support coupled production-oxidation, with concentrations and δ13C values remaining higher (2,798nM and -47‰) than pre-flood conditions (534nM and -55‰) three weeks after the flood. Our findings demonstrate how separate processes can drive the aquatic CO2 and CH4 response to a flood event in a drained coastal floodplain, and the key role groundwater had in post-flood [CO2*] evasion to the atmosphere, but not CH4.
      Graphical abstract image

      PubDate: 2016-08-15T02:56:05Z
       
  • Stable hydrogen and oxygen isotopes in mineral-bound water and the
           indication for chemical weathering intensity
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): Chengfan Yang, Shouye Yang, Ni Su
      Chemical weathering plays a key role in the long-term climate change and global biogeochemical cycle. In this study, a 5.1m-long basalt-developed weathering profile from Hunan Province, China, was selected for the analyses of major elements, grain size, pH, total organic carbon, thermogravimetric (TG) and differential thermogravimetric properties (DTG), as well as stable isotopes (δD and δ18O) in mineral-bound waters. The major purpose is to distinguish different types of mineral-bound water and to verify whether hydrogen and oxygen isotopes of mineral-bound water can indicate chemical weathering intensity. Based on the TG and DTG results, different types of mineral-bound water were extracted by a sequential heating extraction technique (40–120°C, 120–300°C, 300–600°C and 600–900°C). The mineral-bound water lost below 120°C is absorbed water, a mixture of interlayer and crystal waters occurs at 120–300°C, and above 300°C is mostly constitution water. Hydrogen and oxygen isotopic ratios at top ~2m are subject to the influences of eolian and pedogenic processes. The absorbed water has been exchanged and reached isotopic equilibrium with ambient soil water, while the crystal water and interlayer water cannot be separated by heating method in this study. Hydrogen and oxygen isotopes of constitution water can indicate chemical weathering, showing positive relationships with weathering intensity. Although the mechanism for their indication to chemical weathering remains further investigation, we suggest that the isotopic fractionation between the formation of clay minerals and surface water should be taken into account. This research sheds new light on mineral-bound water phase and a potential proxy for tracing the process of chemical weathering.


      PubDate: 2016-08-15T02:56:05Z
       
  • Spatial and temporal variations of base cation release from chemical
           weathering on a hillslope scale
    • Abstract: Publication date: 21 November 2016
      Source:Chemical Geology, Volume 441
      Author(s): M. Erlandsson, E.H. Oelkers, K. Bishop, H. Sverdrup, S. Belyazid, J.L.J. Ledesma, S.J. Köhler
      Cation release rates to catchment runoff from chemical weathering were assessed using an integrated catchment model that included the soil's unsaturated, saturated and riparian zones. In-situ mineral dissolution rates were calculated in these zones as a function of pH, aluminum and dissolved organic carbon (DOC) concentrations along a hillslope in Northern Sweden where soil water was monitored over nine years. Three independent sets of mineral dissolution equations of varying complexity were used: PROFILE, Transition-State Theory (TST), and the Palandri & Kharaka database. Normalization of the rate-coefficients was necessary to compare the equations, as published rate-coefficients gave base cation release rates differing by several orders of magnitude. After normalizing the TST- and Palandri & Kharaka-rate coefficients to match the base cation release rates calculated from the PROFILE-equations, calculated Ca2+ and Mg2+ release rates are consistent with mass balance calculations, whereas those of Na+ and K+ are overestimated. Our calculations further indicate that a significant proportion of base cations are released from the organic soils in the near-stream zone, in part due to its finer texture. Of the three sets of rate equations, the base cation release rates calculated from the normalized TST-equations were more variable than those calculated using the other two sets of equations, both spatially and temporally, due to its higher sensitivity to pH. In contrast, the normalized Palandri & Kharaka-equations were more sensitive to variations in soil temperature.


      PubDate: 2016-08-15T02:56:05Z
       
  • Sr partitioning in the benthic foraminifera Ammonia aomoriensis and
           Amphistegina lessonii
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Gerald Langer, Aleksey Sadekov, Silke Thoms, Nina Keul, Gernot Nehrke, Antje Mewes, Mervyn Greaves, Sambuddha Misra, Gert-Jan Reichart, Lennart Jan de Nooijer, Jelle Bijma, Henry Elderfield
      The shallow water benthic foraminifera Ammonia aomoriensis and Amphistegina lessonii were grown at different seawater Sr/Ca and the test Sr/Ca ratio was determined by Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry. A. aomoriensis test Sr/Ca is positively correlated with seawater Sr/Ca. The linear regression has a slope of 0.17, representing the overall Sr partitioning coefficient KDSr. The slope remains unchanged, if seawater Ca concentration is changed in order to change the seawater Sr/Ca. In the case of A. lessonii, the slopes of the linear regressions representing KDSr differ from one another i.e., 0.16 if the Sr concentration is changed and 0.32 if the Ca level is changed. This difference in KDSr can be explained by the, relative to A. aomoriensis, high Mg content of A. lessonii (ca. 40mmol/mol), distorting the calcite lattice and weakening the discrimination against Sr. The Mg content of A. aomoriensis is too low (ca. 4mmol/mol) to observe the influence on Sr partitioning. All data are compatible with a recently developed model for minor element partitioning in foraminifera (Nehrke et al., 2013; Mewes et al., 2015a). On the whole, our data confirm the model (applicability of the model to Sr and dependence of the calcite Sr/Ca on seawater Sr/Ca, as opposed to seawater Sr concentration), and improve our understanding of the model (influence of calcite Mg/Ca on Sr partitioning).


      PubDate: 2016-08-11T15:36:17Z
       
  • Numerical simulation of long-term storage of CO2 in Yanchang shale
           reservoir of the Ordos basin in China
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Danqing Liu, Yilian Li, Ramesh K. Agarwal
      Shale reservoirs have attracted increasing attention in recent years as a new and promising geological repository for carbon dioxide (CO2) storage. However, their long-term storage mechanism is unclear and the interactions between CO2 adsorption to shale and the CO2-water-rock reactions have not been investigated. A 2-D simplified reactive transport model is developed in this paper based on the properties of a shale reservoir in the Ordos basin in China which has been identified as a promising location for shale gas production and CO2 storage. The model is used to study the CO2-water-rock reactions coupled with gas adsorption and to identify the long-term trapping mechanisms of CO2 in shale gas. Results show that in short (several decades) to middle term (hundreds of years) CO2 storage in a shale reservoir, >60% CO2 is trapped in the supercritical and adsorbed phases, but in the long term (thousands of years), mineral trapping gradually plays a dominant role among other storage mechanisms. The displacement of CH4 with CO2 during injection is also analyzed. Results show that this process can be divided into two stages: the pre-displacement stage during which the adsorbed concentrations of both CH4 and CO2 increase due to pressure build-up, but due to low concentration of CO2 displacement does not occur at this stage; and the displacement stage during which the CO2 concentration is high enough to displace the CH4 from the host rock. Also, higher pressure facilitates the displacement of CH4 by CO2 in this stage. Study of the interaction between gas adsorption and CO2-water-rock reactions shows that by buffering the reservoir pressure during CO2 injection into the subsurface, gas adsorption can impact the solubility trapping process so as to interfere with the CO2-water-rock reactions. Furthermore, this interference is intensified with reduction in CO2 concentration in the reservoir over time. Sensitivity analysis indicates that variations in the CO2 adsorption isotherm, abundance of smectite and chlorite, reaction rate and reactive surface area of smectite can significantly affect the CO2 storage by different trapping mechanisms. The high CO2 adsorption capacity of shale is very beneficial for CO2 injection into a shale reservoir; the amount of adsorbed CO2 is proportional to the CO2 adsorption isotherm, while CO2 in other phases (gas/supercritical phase, dissolved phase and mineral phase) is inversely related to the CO2 adsorption isotherm. Smectite and chlorite are the key minerals that influence the CO2-water-rock reactions in Yanchang shale.


      PubDate: 2016-08-11T15:36:17Z
       
  • Tracing the fluid source of heavy REE mineralisation in carbonatites using
           a novel method of oxygen-isotope analysis in apatite: The example of
           Songwe Hill, Malawi
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Sam Broom-Fendley, Timothy Heaton, Frances Wall, Gus Gunn
      Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of δ18O=+2.5 to +6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates. Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher δ18O values (+7.8 to +26.7‰, VSMOW), with a slight increase in δ13C (−4.6 to −0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+2.5 to −0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature.


      PubDate: 2016-08-11T15:36:17Z
       
  • Stable and radiogenic isotope constraints on the magmatic and hydrothermal
           evolution of the Nechalacho Layered Suite, northwest Canada
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Volker Möller, Anthony E. Williams-Jones
      The agpaitic nepheline syenites of the Nechalacho Layered Suite represent the youngest intrusive phase of the Blatchford Lake Igneous Complex, Canada, and host hydrothermally altered eudialyte-rich layers that constitute one of the world's largest rare earth element (REE) and niobium deposits. Three distinct peripheral rare metal pegmatites, the T-, R- and S-Zones, contain high-grade Li, Be, REE and Nb deposits. Based on stable and radiogenic isotope data, we constrain the age of the Nechalacho Layered Suite, identify its magmatic source and interpret the hydrothermal alteration that modified the ore and gangue mineralogy. In situ Laser ICP-MS U-Pb dating of igneous zircon yielded a concordia age of 2176.0±2.7Ma for the Nechalacho Layered Suite. Initial εNd2176Ma between 1.1 and 3.0 and averaging 2.0 indicates a derivation from mantle magma that was isotopically similar to modern ocean island basalt. The δ18Omelt values calculated for magmatic aegirine, annite and zircon at 700°C range from 5.1 to 7.8‰; δDmelt is −60‰. For the earlier alkaline granite and quartz syenites of the Blatchford Lake Igneous Complex (Grace Lake Granite, Thor Lake Syenite, Rim Syenite), δ18Omelt calculated for fayalite, riebeckite, richterite and quartz ranges from 6.7 to 8.1‰ and δDmelt from −99 to −93‰. Together with the Nd isotope ratios, this indicates a common, deep-seated feeder system of mantle magma that evolved rapidly to generate granites, quartz syenites, and finally nepheline syenites, with little or no crustal contamination. The Nechalacho Layered Suite exsolved a late-magmatic aqueous-carbonic fluid that caused auto-metasomatism associated with ankerite-bastnäsite-(Ce) mineralization, producing hydrothermal carbonates with δ13C from −6.3 to −2.7‰ and δ18O from 11.3 to 15.0‰. Similar ratios previously reported for ankerite from the T-Zone demonstrate a genetic link to the pegmatites at the magmatic-hydrothermal interface. Hydrothermal monazite-(Ce) from the Basal Zone of the Nechalacho deposit yielded a U-Pb concordia age of 1871±14Ma and 208Pb/232Th ages of 1852±14, 1835±57 and 1854±27Ma. This indicates monazite formation during a late regional hydrothermal event associated with faulting and emplacement of the Compton Intrusive Suite along the southern margin of the Slave Craton. The temperature determined for this late overprint using oxygen isotopes of quartz and magnetite is ~300°C and the fluids had δ18OH2O of 3.1–9.3‰ and δD of −24‰, indicating the presence of a regionally pervasive amagmatic crustal fluid.


      PubDate: 2016-08-11T15:36:17Z
       
  • Editorial Board
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439




      PubDate: 2016-08-07T20:34:19Z
       
  • Holocene climate controls on water isotopic variations on the northeastern
           Tibetan Plateau
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Yuxin He, Cheng Zhao, Zhonghui Liu, Huanye Wang, Weiguo Liu, Zicheng Yu, Yan Zhao, Emi Ito
      On the Tibetan Plateau, applications of δD and δ18O values in paleoclimate studies tend to be complicated due to multiple processes influencing isotopic compositions in paleoclimatic archives. In this study, isotopic compositions of modern waters in the eastern Qaidam Basin on the northeastern Tibetan Plateau, and δD values of n-fatty acids (n-FA δD) from a sediment core at Hurleg Lake were systematically analyzed to infer hydroclimate controls during the Holocene. The modern water isotopic results show a major contribution of snowmelt water originating from high-elevation mountains to the north of the Qaidam Basin via river and groundwater discharge, and the importance of evaporation in affecting lake water budget in this region. n-C26 FA δD values tend to be more negative at millennial-scale warm-dry periods during the Holocene, and vice versa, opposite to what is commonly expected. Assisted with modern water isotopic results, we infer amplified contribution of snowmelt water to the soil water around this open lake system at warm-dry periods. Meanwhile, changes in n-C16 FA δD values at Hurleg Lake reflect the evolution of isotopic compositions of lake water, thus we use the isotopic difference between n-C26 and n-C16 FA (ΔDC16-C26) to infer hydroclimate and evaporation variations in this region. Based on our data, relatively low n-C26 FA δD and n-C16 FA δD values at 10–6calkaBP indicate large contribution of snowmelt water into the lake during the Holocene Climate Optimum. After 6calkaBP, changes in evaporation became the major control on lake hydrology and led to larger fluctuations of ΔDC16-C26. Our study highlights the importance of systematic analysis on modern processes before using stable isotopes for paleoclimate reconstructions, and demonstrates that δD difference between long-chain and short-chain n-FA might be an effective way to better understand the controlling factor of hydrological variations in a climatic complex region like the Tibetan Plateau.


      PubDate: 2016-08-07T20:34:19Z
       
  • The effect of titanium on the partitioning behavior of high-field strength
           elements between silicates, oxides and lunar basaltic melts with
           applications to the origin of mare basalts
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Felipe P. Leitzke, Raúl O.C. Fonseca, Lina T. Michely, Peter Sprung, Carsten Münker, Alexander Heuser, Henrik Blanchard
      A specific feature of some basaltic lunar rocks is that their TiO2 contents can reach concentrations as high as 16wt.%. The high-field strength elements (HFSE) group, which includes Ti, may provide valuable information of the processes that occurred in the lunar mantle to generate high-Ti mare basalts. To assess the effect of such high TiO2 concentrations on the partitioning of Zr, Hf, Nb, Ta, U, Th, Mo and W between major silicate and oxide phases and silicate melts, we present results from experiments at one atmosphere and 1100°C–1305°C, under controlled oxygen fugacity. With the exception of Nb, all D HFSE cpx/melt show a strong negative correlation with the TiO2 content of the silicate melt. Olivine/Silicate melt partition coefficients for Zr, Hf, Nb, Ta and Th decrease slightly from 0 to ca. 5wt.% TiO2, above which they remain constant up to ca. 20wt.% TiO2 in the silicate glass. In addition, redox sensitive elements, i.e. U, Mo, and W show clearly distinct D M silicates/melt at different fO2, implying that these elements are relatively more compatible at reduced (ca. IW −1.8) than at oxidized (FMQ and air) environments. Iron-rich and Mg-rich armalcolite show contrasting patterns of D M crystal/melt , with the latter exhibiting slightly higher values of partition coefficient for all analyzed elements, except Th, which is equally incompatible in both end-members. Finally, the new dataset of D HFSE crystal/melt was used to perform simple melting models of the lunar mantle cumulates. Results indicate that to reproduce the fractionation of W from the HFSE, as well as U and Th observed in lunar mare basalts, metal saturation and the presence of Fe–Ti oxides in the mantle sources is required.


      PubDate: 2016-08-07T20:34:19Z
       
  • Chemical and oxygen isotope composition of gem-quality apatites:
           Implications for oxygen isotope reference materials for secondary ion mass
           spectrometry (SIMS)
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Yadong Sun, Michael Wiedenbeck, Michael M. Joachimski, Christoph Beier, Fabian Kemner, Christoph Weinzierl
      Oxygen isotope ratios (δ18O) of biogenic apatite have become a widely used tool for reconstructing palaeoenvironmental conditions in the past. Ongoing improvements in secondary ion mass spectrometry (SIMS) technology have made in situ δ18O analyses on sub-nanogram domains within single microfossil samples possible; however this method requires calibration with reference materials (RMs) with a matrix “similar” to that of the samples. Here we evaluated five sources of gem-quality, abiogenic apatites to assess their potentials as SIMS RMs. Our results show that all these gem-quality apatites are low-REEs calcium fluorapatites with δ18OVSMOW values between 6.6 and 11.4‰. Large variations in δ18O have been found for between crystals from a single deposit as well as within individual crystals. Durango apatite has an inter-crystal δ18O range of 4.4‰ (6.6–11.0‰, N =9 crystals). Madagascar Green apatite, Madagascar 1st Mine apatite and Ipira apatite have inter-crystal variations in δ18O of 4.1‰ (7.3–11.4‰, N =15 crystals), 3.5‰ (7.6–11.1‰, N =9 crystals) and 3.1‰ (7.1–10.2‰, N =11 crystals), respectively. South Africa Blue apatite has a smaller inter-crystal δ18O range of only 0.9‰ (8.7–9.6‰, N =6 crystals), though this might be an artefact due to the restricted number of samples studied. Intra-crystal δ18O variations of studied crystals generally range from 0.8 to 1.8‰. However, several gem apatite crystals from Madagascar have minor δ18O variation within 0.5‰ and represent most promising candidate RM. Gem-quality abiogenic apatites can be used as RMs for calibrating SIMS δ18O analyses, however these require homogeneity testing at sub-millimetre scale in advance. Durango crystals, commonly used as RMs in earlier SIMS-based studies, display a conspicuous heterogeneity in δ18O, with intra-crystal variations ranging from 0.7 to 2‰ as detected by both gas source isotope ratio mass spectrometry and large geometry SIMS. Thus, Durango apatite cannot be considered as suitable for SIMS calibration and alternative reference materials need to be sought.


      PubDate: 2016-08-03T09:55:14Z
       
  • Transformation of silicon in a sandy beach ecosystem: Insights from stable
           silicon isotopes from fresh and saline groundwaters
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Claudia Ehlert, Anja Reckhardt, Janek Greskowiak, Bianca T.P. Liguori, Philipp Böning, Ronja Paffrath, Hans-Jürgen Brumsack, Katharina Pahnke
      Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736μM, but homogeneous δ30Si of +1.0±0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23–192μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5μmolSig−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210molSiyr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.


      PubDate: 2016-08-03T09:55:14Z
       
  • Impact of diagenetic alteration on brachiopod shell magnesium isotope
           (δ26Mg) signatures: Experimental versus field data
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Sylvia Riechelmann, Vasileios Mavromatis, Dieter Buhl, Martin Dietzel, Anton Eisenhauer, Adrian Immenhauser
      Due to their thermodynamically stable low-Mg calcite mineralogy, the shells of brachiopods are often counted among the most reliable archives of the physicochemical conditions that occurred during the Phanerozoic in marine waters. Consequently, traditional and non-traditional isotope and elemental proxy data from brachiopod valves have been analyzed in numerous studies and results obtained have been placed in context with ancient seawater properties. This paper tests the sensitivity of brachiopod shell magnesium isotope (δ26Mg) data to diagenetic alteration. We apply a dual approach by: (i) performing hydrothermal alteration experiments using meteoric, marine, and burial reactive fluids; and (ii) comparing these data to naturally altered, ancient brachiopod shells. The degree of alteration of individual shells is assessed by a combination of fluorescence and cathodoluminescence microscopy. The absence of luminescence might indicate both well-preserved shell material, but also the secondary enrichment of quenching elements such as iron along diagenetic pathways. Complementary oxygen isotope data provide insight into the question of open versus closed system behavior of brachiopod shells. Brachiopod shell magnesium isotope values respond to differential fluid temperature, chemistry, and experiment durations. The patterns observed are complicated by the interplay of kinetic and thermodynamic patterns and the presence of variable amounts of water soluble and water insoluble organic matter within these biominerals. Generally, the range in bulk δ26Mg from experimentally altered (1.52‰) and that of bulk samples from ancient, diagenetically altered brachiopod valves (1.53‰) exceed the geochemical variability of δ26Mgbrachiopod bulk values of most recent specimens (1.26‰) in the lower and upper range. More 26Mg enriched (0.8‰) and more 26Mg depleted (0.7‰) values, respectively, are found in altered shells in comparison to unaltered ones. The data shown here are considered significant for those aiming to reconstruct palaeoenvironmental parameters based on brachiopod archives. Consequently, we propose tentative guidelines for magnesium isotope research applied to ancient carbonates.


      PubDate: 2016-08-03T09:55:14Z
       
  • U-series disequilibria in subduction zone lavas: Inherited from subducted
           slabs or produced by mantle in-growth melting?
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Fang Huang, Juan Xu, Julin Zhang
      U-series disequilibria of subduction zone lavas have been used to provide temporal constraints on flux transfer in convergent margins. There are debates on whether the disequilibria are inherited from the source modified by slab materials (fluids or sediments) or produced by in-growth melting of the mantle. After filtering out the effect of crustal modification on U-series data, (226Ra)/(230Th) is not positively correlated with Sr/Th and Ba/Th in arc basalts with SiO2 <52wt%. This invalidates the key evidence for addition of slab-fluids to the mantle producing 226Ra excess in subduction zone lavas. Sediment melts with a zircon-free residue may have 231Pa excess, but it is not consistent with the overwhelming Zr-Hf depletions in subduction zone lavas, which instead require a zircon-saturated residue during sediment melting. With appropriate parameters applied, in-growth melting in the mantle followed by modification in the crust can provide a self-consistent explanation of the U-series disequilibria and the relationships with other geochemical signatures in most subduction zone lavas except the few Tonga-Kermadec lavas with slight 231Pa deficit. On the basis of melting experiments of hydrous peridotites, we further propose that most mantle-derived melts and thus U-series disequilibria should be mainly produced by melting in the hot zone of the mantle wedge, as controlled by thermal structure of the convergent margins. In order to produce enough short-lived nuclides, the timescales of mass transfer and magmatism in the subduction zone should at least range from a few to a few hundred millennia. Future studies on more lavas, numerical modelling, and partition coefficients of U-series nuclides will help for better application of U-series disequilibria into subduction zone magmatism.


      PubDate: 2016-08-03T09:55:14Z
       
  • ArAR — A software tool to promote the robust comparison of
           K–Ar and 40Ar/39Ar dates published using different decay, isotopic,
           and monitor-age parameters
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Cameron M. Mercer, Kip V. Hodges
      The comparison of K–Ar and 40Ar/39Ar geochronologic data published by different laboratories is markedly hindered by the inconsistent use of the parameters necessary to convert isotopic analyses to dates. This problem is particularly acute when we try to evaluate the significance of datasets obtained prior to the development of community consensus values for basic decay constants, isotopic abundances, and the ages of common monitor minerals. Unfortunately, the effect of using different parameters for the same dataset can sometimes exceed the quoted analytical precision of derived dates. We created the Argon Age Recalculator, or ArAR, to help researchers account for such discrepancies in a simple, efficient manner, allowing for more robust comparisons among datasets and more effective compilation of existing datasets using self-consistent parameter sets. ArAR is freely available as a platform independent executable application at: http://group18software.asu.edu.


      PubDate: 2016-07-28T08:04:27Z
       
  • Simulation of geochemical banding I: Acidization-precipitation experiments
           in a ferruginous limestone rock
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Maysam Msharrafieh, Mazen Al-Ghoul, Farah Zaknoun, Houssam El-Rassy, Samia El-Joubeily, Rabih Sultan
      This paper is the first among two articles which aim at exploring the possible similarities between the well-known Liesegang banding phenomenon in precipitate systems, and the stripe formation observed in a large number of rocks. In the present (first) article, we review a comprehensive and long study wherein patterning experiments were performed in-situ (real rock systems), to simulate the band formation through the acidization of a ferruginous limestone rock, causing dissolution and precipitation reactions. The results are analyzed by microscopy, AA and XRD techniques. The correlation between the Liesegang gel experiment and the processes taking place inside the rock medium is established. In the second part (following paper), a theoretical model is set forth to support our experiments. The fractal nature of the contours of the various regions will be explored.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Evidence against an ancient non-chondritic mantle source for North
           Atlantic Igneous Province lavas
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): James M.D. Day
      North Atlantic Igneous Province (NAIP) lavas host olivine with the highest 3He/4He ever measured for terrestrial igneous rocks (up to 50 RA, or 4He/3He=~15,300). The relationship of high-3He/4He with Pb isotope compositions close to the terrestrial geochron and 143Nd/144Nd plausibly consistent with supra-chondritic mantle Sm/Nd in Baffin Island and West Greenland lavas has been interpreted to reflect an ancient ‘non-chondritic’ mantle source signature. Alternatively, assimilation of continental crustal rocks with unradiogenic Pb isotope compositions and low 143Nd/144Nd, into magmas with high-3He/4He, and derived from variably depleted mantle sources, could impart similar geochemical signatures. Radiogenic and stable isotope data for NAIP lavas are consistent with origins as melts from upper mantle sources that contain low-18O/16O recycled lithosphere and/or hydrothermally altered crust, or that have experienced pervasive contamination by crustal gneisses. Olivines from NAIP lavas with 3He/4He spanning from 8 to 48 RA have δ18O ranging from 3.5 to 5.5‰. These compositions are consistent with sources of ambient mantle and low-δ18O recycled lithosphere, or with concomitant crustal assimilation and He-loss during fractional crystallization. Limited assimilation (≤1%) of incompatible element rich crustal gneisses with low 206Pb/204Pb and 143Nd/144Nd by melts from variably depleted mantle sources can explain Nd-Pb isotope compositions of Baffin Island and West Greenland picrites. Icelandic lavas provide supporting evidence that the ancestral mantle plume responsible for generating NAIP magmatism sampled variably enriched and depleted mantle, with no evidence for ancient non-chondritic mantle sources. Pervasive crustal contamination and partial melting of heterogeneous mantle sources, generated by plate tectonic processes, can account for the compositions of continental flood basalts (CFB), without the requirement of a non-chondritic terrestrial reservoir. Combined with evidence that the 142Nd/144Nd composition of the bulk silicate Earth is due to nucleosynthetic S-process deficits in chondrite meteorites, these observations cast doubt that NAIP lavas sampled a non-chondritic mantle source with Sm/Nd higher than in chondrites. If short-lived radiogenic (e.g., 146Sm-142Nd, 182Hf-182 W) isotope anomalies are found in CFB, they must either reflect assimilation of isotopically anomalous crustal materials, or partial melting of early-formed mantle heterogeneities produced by differentiation and late accretion.


      PubDate: 2016-07-24T06:13:25Z
       
  • Tracing provenance and sediment fluxes in the Irrawaddy River basin
           (Myanmar)
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Eduardo Garzanti, Jiang-Gang Wang, Giovanni Vezzoli, Mara Limonta
      This study illustrates the petrographic, heavy-mineral, geochemical and geochronological signatures of sand transported by various branches of the Irrawaddy (Ayeyarwadi) River, one of the first in the world for sediment flux. Intrasample and intersample compositional variability, weathering and hydraulic-sorting controls are also discussed. Feldspatho-quartzose sand in Irrawaddy headwaters is largely derived first-cycle from mid-crustal metamorphic and plutonic rocks of the Mogok Belt and Lohit complex, whereas feldspatho-litho-quartzose Chindwin sand is largely recycled from supracrustal, sedimentary and very low-grade metasedimentary units. Additional mafic to ultramafic detritus is derived from ophiolites and blueschists exposed from the Indo-Burman Ranges to the Jade Mines and Myitkyina belts, linked northward to the Yarlung-Tsangpo suture of the Himalaya. Volcanic detritus derived from the Popa-Wuntho arc or recycled from forearc-basin strata also occurs. Decreasing concentration of most chemical elements along the Irrawaddy reflects progressive addition of detritus recycled from sedimentary rocks, most evident downstream of the Chindwin confluence. REE patterns with LREE enrichment and negative Eu anomaly reflect the occurrence of allanite, largely derived from granitoid rocks in the Mali catchment. Chemical indices indicate moderate weathering in the monsoon-dominated climate of Myanmar. Young U-Pb ages (15–170Ma) represent 85% of detrital zircons in Irrawaddy headwater branches, reflecting long-lasting subduction-related magmatism along a ring of fire connecting with the southern and central Lhasa batholiths in Tibet and polyphase metamorphism in the Mogok belt. Chindwin sand contains larger amounts of finer-grained, recycled pre-Mesozoic zircons, also yielding early Mesoproterozoic to Archean ages. Such different petrographic, heavy-mineral, geochemical and geochronological fingerprints characterizing sand in different river branches allowed us to calculate bulk-sediment and zircon-provenance budgets that converge to indicate equivalent sand supply from the Nmai and Mali Rivers to the upper Irrawaddy, and from the Chindwin and upper Irrawaddy to the lower Irrawaddy. This implies that despite of higher erosion potential indicated by stream-profile analysis in high-relief Irrawaddy headwaters, sediment yields and erosion rates are detectably higher in the Chindwin catchment, which is mainly ascribed to higher erodibility of widely exposed siliciclastic rocks. Quantifying sediment provenance and defining erosion patterns based on an integrated compositional database in a big-river system such as the mighty Irrawaddy allows us to expand our understanding of sediment-generation processes with the ultimate goal to increase our capacity to read into the stratigraphic record.


      PubDate: 2016-07-24T06:13:25Z
       
  • Hf–Nd isotope decoupling in bulk abyssal peridotites due to
           serpentinization
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Carl Frisby, Michael Bizimis, Soumen Mallick
      Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization (Frisby et al. (2016)). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH)3 during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid–liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 εHf, while Nd isotopes in the bulk peridotite are up to 8 εNd units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater — peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of >100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1–34% of the bulk rock Nd budget is seawater derived, but only 1–6% of Sm and <2% of Lu and Hf in these samples. Therefore serpentinization will lower the Sm/Nd relative to Lu/Hf ratio in a peridotite and this may lead to decoupling between Hf and Nd isotopes upon recycling and aging, assuming no additional fractionation of these elements during the subduction process. Alternatively, if Nd becomes fluid-mobile upon dehydration during subduction then the excess Nd in the serpentinized peridotites may become an additional reservoir for Nd in arc volcanism.


      PubDate: 2016-07-24T06:13:25Z
       
  • The origin of N2-H2-CH4-rich natural gas seepages in ophiolitic context: A
           major and noble gases study of fluid seepages in New Caledonia
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Eric Deville, Alain Prinzhofer
      The study of natural gas seepages in New Caledonia has shown the occurrence of two gas families, one is N2-H2-CH4-rich, the other is N2-rich. The N2-H2-CH4-rich gases are bubbling in hyperalkaline springs (OH–-rich) known in the peridotite massif of the southern part of the island. This family of gas shows contents of N2 between 50 and 62%, H2 between 26 and 36%, and CH4 between 11 and 16%. δ13C values of methane are ranging from −39 to −32‰. We interpret the origin of H2 as a product of fluid-rock interaction between basic-ultrabasic rocks and water with oxidation of Fe2+ and reduction of underground water in the fracture system of the peridotites nappe. Methane is interpreted as the result of a reduction of dissolved inorganic and/or organic carbon in subsurface aquifers. The second family of gas emissions was found in thermal springs in the sedimentary units located structurally below the peridotites nappe. The gas is composed mostly of N2 (between 97 and 98%) associated with relatively high concentration of He. Both gas families show notably high N2 vs fossil noble gas contents (36Ar, 20Ne, 84Kr) suggesting that N2 is not directly issued from gas dissolved in aquifers equilibrated with atmosphere but most likely finds its origin in a deep source, probably within metamorphic sediments which are tectonically buried below the ophiolitic nappe. We interpret the N2-H2-CH4-rich family of gas as a result of a mixing between two end-members, (1) a H2-CH4-rich pole issued from the weathering of peridotite rocks and (2) a N2-rich pole which would be issued from the metamorphosed sediments buried below the ophiolitic units. An unusual inverse correlation between the atmospheric noble gas isotopes 20Ne and 36Ar is interpreted as the result of a degassing of a relatively shallow confined aquifer related to a bubble flow from depth.


      PubDate: 2016-07-24T06:13:25Z
       
  • High-precision measurement and standard calibration of triple oxygen
           isotopic compositions (δ18O, Δ′17O) of sulfate by F2 laser
           fluorination
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Benjamin R. Cowie, David T. Johnston
      A new approach for measuring the triple oxygen isotope composition of sulfate minerals was developed using fluorine gas and an infrared laser to generate O2. A correction for the mass-dependent isotope effect observed during barite fluorination was rigorously calibrated. Analyte gas purification was performed via numerous cryofocus steps and the introduction of an in-line gas chromatograph. As with previous methods, our fluorination technique still requires pairing with independent δ 18O measurements made by TC/EA conversion to carbon monoxide, but leads to higher yields than BrF5-based methods. We first calibrated our method against known silicate standards (UWG-2, SCO, NBS-28). Following from this, we report high-precision triple oxygen isotope ratios for international sulfate standards (IAEA-SO-5, IAEA-S O-6, NBS-127), and an internal laboratory standard, JMG. Replicate analyses of JMG yielded a Δ ′17O value of −0.057 ± 0.004‰ (standard error), demonstrating per-meg level precision that approaches instrumental limits. We provide δ 17O and δ 18O values that can be translated into any preferred reference frame for comparison with gases, water, or rocks/minerals. Quantification of differences in triple oxygen isotope composition of the standard reference materials further enables application of this measure and approach on environmental and geological materials.


      PubDate: 2016-07-24T06:13:25Z
       
  • Geochemical anatomy of a spheroidally weathered diabase
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Anupam Banerjee, Ramananda Chakrabarti, Sourav Mandal
      Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2–41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements. Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions. Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2–1.3Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average εNd (1.2 Ga) of the weathered samples coincides with the εNd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.


      PubDate: 2016-07-24T06:13:25Z
       
  • The controversial role of inter-diffusion in glass alteration
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): S. Gin, L. Neill, M. Fournier, P. Frugier, T. Ducasse, M. Tribet, A. Abdelouas, B. Parruzot, J. Neeway, N. Wall
      Current kinetic models for nuclear waste glasses (e.g. GM2001, GRAAL) are based on a set of mechanisms that have been generally agreed upon within the international waste glass community. These mechanisms are: hydration and ion exchange reactions (the two processes are referred as inter-diffusion), hydrolysis of the silicate network, and condensation/precipitation of partly or completely hydrolyzed species that produces a porous and amorphous layer and crystalline phases on surface of the altered glass. Recently, a new idea with origins in the mineral dissolution community has been proposed that excludes inter-diffusion processes as a potential rate-limiting mechanism. To understand how the newly proposed interfacial dissolution/precipitation model can change the current understanding of glass corrosion, a key experiment used to account for this model was replicated to further revisit the interpretation. This experiment was performed far from saturation, at 50°C, with SON68 glass, in static mode, deionized water, and a S/V ratio of 10m−1 for 6months. Results were repeatable and showed that glass dissolution rate progressively dropped by ~1 order of magnitude compared to the forward rate, suggesting that a dense surface layer was under construction. According to previous and new solids characterizations, it is concluded that neither a simple inter-diffusion model nor the interfacial dissolution/precipitation model can account for the observed elemental profiles within the alteration layer. More generally, far-from- and close-to-saturation conditions must be distinguished. This argument is bolstered by literature where evidence shows that inter-diffusion takes place in acidic conditions and far from saturation. However, closer to saturation, when a sufficiently dense layer is formed, a new approach is proposed requiring a full description of chemical reactions taking place within the alteration layer and an accurate budget of hydrous species along the profile as it is thought that the access of a sufficient amount of water to the pristine glass is the rate-limiting process in these conditions.


      PubDate: 2016-07-24T06:13:25Z
       
  • Basin redox and primary productivity within the Mesoproterozoic Roper
           Seaway
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Grant M. Cox, Amber Jarrett, Dianne Edwards, Peter W. Crockford, Galen P. Halverson, Alan S. Collins, André Poirier, Zheng-Xiang Li
      The ca. 1.4Ga Roper Group of the greater McArthur Basin in northern Australia comprises the sedimentary fill of one of the most extensive Precambrian hydrocarbon-bearing basins preserved in the geological record. It is interpreted to have been deposited in a large epeiric sea known as the Roper Seaway. Trace element data suggest that the redox structure of the basin was a shallow oxic layer overlying deeper suboxic to anoxic waters along with a prominent episode of euxinia. These anoxic and sulfidic conditions, as inferred by Mo, V, and U concentrations (molybdenum, vanadium and uranium), developed due to high organic carbon loading consistent with models that suggest that euxinic conditions cannot develop until the flux of organic matter is significantly greater than the flux of bioavailable iron, which permits sulphate reduction to proceed. Considering the high reactive iron and molybdenum contents of these shales and the requirement for S/Fe ratios>2 for euxinia to develop, suggests that sufficient atmospheric O2 was available for oxidative scavenging of S and Mo from the continents. This is further supported by prominent negative cerium anomalies within these shales, indicative of active oxidative redox cycling of cerium. We propose that the high organic matter flux was the result of increased nutrient loading to the Roper Seaway from weathering of the continental hinterland. Data from both major and high-field strength elements (niobium, tantalum, zirconium and, hafnium) together with neodymium isotopes (143Nd/144Nd) indicate that a likely mechanism for this enhanced nutrient delivery was a shift in sedimentary provenance to a more primitive (i.e. mafic) precursor lithology. This switch in provenance would have increased phosphorus delivery to the Roper Seaway, contributing to high primary productivity and the onset of euxinia. This dataset and model serve as a basis for understanding the temporal evolution of the deepest sections of the Roper Seaway and finer scale changes in the environment at this time.


      PubDate: 2016-07-24T06:13:25Z
       
  • How sulfate-driven anaerobic oxidation of methane affects the sulfur
           isotopic composition of pyrite: A SIMS study from the South China Sea
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Zhiyong Lin, Xiaoming Sun, Jörn Peckmann, Yang Lu, Li Xu, Harald Strauss, Haoyang Zhou, Junli Gong, Hongfeng Lu, Barbara M.A. Teichert
      Sulfate-driven anaerobic oxidation of methane (SO4-AOM) in marine sediments commonly leads to the precipitation of pyrite. It is, however, frequently challenging to unequivocally unravel the entire history of pyritization, because of the common coexistence of SO4-AOM derived pyrite with pyrite resulting from organiclastic sulfate reduction (OSR). To better understand how SO4-AOM affects pyritization in methane-bearing sediments and how this can be identified, we applied secondary ion mass spectroscopy (SIMS) to analyze the sulfur isotope composition (δ34S) of authigenic pyrite in addition to sulfur isotope measurements of bulk sulfide and hand-picked pyrite aggregates from the two seafloor sites, HS148 and HS217, in the Shenhu seepage area, South China Sea. Authigenic, mostly tubular pyrite aggregates from these sites consist of three types of pyrite: framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals. Framboids with low SIMS δ34S values (as low as −41.6‰ at HS148, and −38.8‰ at HS217) are dispersed throughout the cores, but are especially abundant in the shallow part of the sedimentary column (i.e. above 483cmbsf in HS148; above 670cmbsf in HS217). These patterns are interpreted to reflect the dominance of OSR during early diagenetic processes in the shallow sediments. With increasing depth, both δ34S values of bulk sulfide minerals and hand-picked pyrite aggregates increase sharply at 483cmbsf in core HS148, and at 700cmbsf in core HS217, respectively. Radial pyrite overgrowths and euhedral crystals become abundant at depth typified by high δ34S values for hand-picked pyrite. Moreover, SIMS analysis reveals an extreme variability of δ34S values for the three pyrite types on a small scale in these zones. Besides some moderately 34S enriched framboids, most of the overgrowths and euhedral crystals display extremely high SIMS δ34S values (as high as +114.8‰ at HS148, and +74.3‰ at HS217), representing the heaviest stable sulfur isotope composition of pyrite ever reported to the best of our knowledge. Such an abrupt and extreme increase in δ34Spyrite values with depth is best explained by an enrichment of 34S in the pool of dissolved sulfide caused by SO4-AOM in the sulfate methane transition zone (SMTZ). The increase in δ34S values from framboidal cores to overgrowth layers and euhedral crystals indicates continuous, and finally near to complete exhaustion of dissolved sulfate at the SMTZ following a Rayleigh distillation process. SO4-AOM allowed for subsequent growth of later stage pyrite over the initial framboids, part of which formed earlier and at shallower depth by OSR. The combination of a detailed petrographic study of authigenic pyrite with SIMS analysis of stable sulfur isotopes in organic-rich strata proves to be a powerful tool for reconstructing the dynamics of sulfur cycling in modern and, potentially, ancient sedimentary sequences.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Celestine in a sulfidic spring barite deposit - A potential biomarker?
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): David M. Singer, Elizabeth M. Griffith, John M. Senko, Kaci Fitzgibbon, Inoka H. Widanagamage
      We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.
      Graphical abstract image

      PubDate: 2016-07-24T06:13:25Z
       
  • Redox conditions across the G–L boundary in South China: Evidence
           from pyrite morphology and sulfur isotopic compositions
    • Abstract: Publication date: 15 November 2016
      Source:Chemical Geology, Volume 440
      Author(s): Hengye Wei, Xuemei Wei, Zhen Qiu, Huyue Song, Guo Shi
      The end-Guadalupian (middle Permian) mass extinction recorded the disappearance of shelf faunas such as fusulinids, small foraminifers, brachiopods, rugose corals which flourished in the warm shallow marine habitats after the late Pennsylvanian–early Permian glaciation. The causes for this mass extinction are still unclear. Marine anoxia, often as a cause for other mass extinctions, has been paid less attention in this extinction. Here we present a comprehensive study including pyrite morphology and sulfur isotopic compositions of pyrite across the Guadalupian–Lopingian (middle–upper Permian; G–L) boundary at Tianfengping in northern Yangtze Platform and at Penglaitan in southern Yangtze Platform in South China. Our results show that the sulfur isotope ratios of pyrite mainly track the framboid size distributions in these two sections. Anoxia and intermittent euxinia occur at the G–L boundary evidenced by the coincidence between reduction of framboid size and the extremely negative sulfur isotope values of pyrite involving disproportionation, and corresponds to the main mass extinction of the end-Guadalupian, suggesting a causal link between them. Long-term dysoxic conditions during the middle Capitanian are evidenced by the relative small framboid size and lower sulfur isotope ratios of pyrite, and coincide with the first pulse of the end-Guadalupian mass extinction, also suggesting a causal link between them. The oxygen-depletion conditions in the Capitanian during the global sea level falling may be the main cause for the end-Guadalupian mass extinction.


      PubDate: 2016-07-24T06:13:25Z
       
  • Chemical abrasion-SIMS (CA-SIMS) U-Pb dating of zircon from the late
           Eocene Caetano caldera, Nevada
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Kathryn E. Watts, Matthew A. Coble, Jorge A. Vazquez, Christopher D. Henry, Joseph P. Colgan, David A. John
      Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6Myr older than the 40Ar/39Ar sanidine eruption age (34.00±0.03Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3Myr younger. U-Pb ages do not correlate with U (~100–800ppm), Th (~50–300ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~1.6μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures.


      PubDate: 2016-07-05T14:49:38Z
       
  • Divalent metal cation adsorption onto Leptothrix cholodnii SP-6SL
           bacterial cells
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Ryan M. Nell, Jennifer E.S. Szymanowski, Jeremy B. Fein
      Bacteriogenic iron oxide (BIOS) particles have a high affinity to adsorb inorganic nutrients such as Ca, Mg, Zn, and Ni, raising the question of how Fe(II) oxidizing bacteria (FeOB) compete for these same cations while the BIOS are in such close proximity to the cells. Answering this question requires a detailed understanding of metal binding properties of FeOB. In this study, the adsorption behaviors of aqueous Ni, Cu, Zn, Sr, Cd, and Pb onto the sheathless FeOB Leptothrix cholodnii SP-6SL were measured separately, and the adsorption behaviors were compared to that of Bacillus subtilis in order to determine if the iron oxidizing bacterial species exhibits similar binding properties to those of most previously studied bacterial species. The experiments for both species were performed aerobically; ionic strength was held constant with 0.1M NaClO4; the experiments were conducted as a function of pH over the range of 2 to 9; and biomass and metal concentrations were 10g (wet weight)/L and 2ppm, respectively. Our results show that the two studied bacterial species exhibit similar adsorption of the tested metals, and that although some iron oxides formed during L. cholodnii SP-6SL liquid culture growth, they are present in low enough concentrations not to significantly affect the extent of metal adsorption. We apply a linear free-energy approach to define relationships between the stability constants for site-specific metal-bacterial surface complexes and corresponding metal-acetate binding constants. We use these relationships to estimate binding constants for L. cholodnii for some metals that have not been studied, and we apply these results to calculate the ability of L. cholodnii to compete for nutrients with BIOS particles that form during Fe(II) oxidation by the bacteria.


      PubDate: 2016-07-05T14:49:38Z
       
  • Extreme differences in 87Sr/86Sr between Samoan lavas and the magmatic
           olivines they host: Evidence for highly heterogeneous 87Sr/86Sr in the
           magmatic plumbing system sourcing a single lava
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): A.A. Reinhard, M.G. Jackson, J. Harvey, C. Brown, J.M. Koornneef
      Investigations of mantle heterogeneity in ocean island basalts (OIB) frequently compare heavy radiogenic isotopes (i.e. 87Sr/86Sr), often measured in whole rock powders, with 3He/4He and δ18O, commonly measured in olivines. However, the 87Sr/86Sr in the olivines, which is dominated by Sr in melt inclusions, may not be in equilibrium with the 87Sr/86Sr in the whole rock. Here we present new 87Sr/86Sr measurements made on Samoan magmatic olivines, where multiple olivine crystals are aggregated for a single isotopic measurement. The olivines host abundant melt inclusions, and yielded relatively large quantities of Sr (13.0 to 100.6ng) in 19 to 185mg aliquots of fresh olivine, yielding high Srsample/Srblank ratios (≥427). These new data on olivines show that samples can exhibit significant 87Sr/86Sr disequilibrium: in one extreme sample, where the basaltic whole rock 87Sr/86Sr (0.708901) is higher than several different aliquots of aggregate magmatic olivines (0.707385 to 0.707773), the whole rock-olivine 87Sr/86Sr disequilibrium is >1590ppm. The 87Sr/86Sr disequilibrium observed between whole rocks and bulk olivines relates to the isotopic disequilibrium between whole rocks and the average 87Sr/86Sr of the population of melt inclusions hosted in the olivines. Therefore, a population of olivines in a Samoan lava must have crystallized from (and trapped melts of) a different 87Sr/86Sr composition than the final erupted lava hosting the olivines. A primary question is how melts with different 87Sr/86Sr can exist in the same magmatic plumbing system and contribute heterogeneous 87Sr/86Sr to a lava and the magmatic olivines it hosts. We explore potential mechanisms for generating heterogeneous melts in magma chambers. The reliance, in part, of chemical geodynamic models of the relationships between isotopic systems measured in whole rocks (87Sr/86Sr) and systems measured in olivines (3He/4He and δ18O) means that whole rock-olivine Sr-isotopic disequilibrium will be important for evaluating relationship among these key isotopic tracer systems. Moving forward, it will be important to evaluate whether whole rock-olivine Sr-isotopic disequilibrium is a pervasive issue in OIB globally.


      PubDate: 2016-07-01T13:05:17Z
       
  • Ancient depleted mantle as a source of boninites in the Izu-Bonin-Mariana
           arc: Evidence from Os isotopes in Cr-spinel and magnetite
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Ryoko Senda, Kenji Shimizu, Katsuhiko Suzuki
      Boninite is a volcanic rock derived from shallow melting of highly depleted hydrous mantle, fluxed with water from subducted slabs. The eruption of boninite early in the history of the Izu-Bonin-Mariana (IBM) arc (~48–45Ma), suggests generation by melting of upper mantle material that was relatively unmodified by subducted components. Thus, the boninite composition should largely reflect that of the sub-arc mantle. For better understanding of the mantle sources of nascent arc settings and the contributions of different components to arc melts, we analyzed Os isotope ratios (187Os/188Os) of bulk rocks and mineral separates (euhedral Cr-spinel from boninites and euhedral Cr-spinel/magnetite mixtures from tholeiites younger than 45Ma that erupted after boninites) from the Bonin Islands and Guam. The age-corrected (initial) Os isotope ratios of the whole-rock samples (0.1179–0.2050) were more radiogenic and variable than those of the mineral separates, possibly because of contamination with crustal materials during magma ascent or alteration after emplacement. The age-corrected Os isotope ratios of euhedral Cr-spinel in boninite from the Bonin Islands (0.1187–0.1254) and from Guam (0.1220–0.1269) are unradiogenic relative to primitive mantle, and those of the Cr-spinel/magnetite mixtures from the tholeiites from the Bonin Islands are similar to or slightly more radiogenic (0.1224–0.1382). The most depleted Os isotope ratio of the Cr-spinel from boninite yielded a model Re depletion (TRD) age of 1.4Ga, suggesting that the mantle source of the boninite experienced melt extraction prior to 1.4Ga. The source of the boninites is interpreted to be mostly highly depleted mantle with a small contribution of slab flux arising from altered oceanic crust that has radiogenic Os components, with or without contributions from components with relatively unradiogenic Os such as volcaniclastics of oceanic island basalt affiliation or very young mid-ocean ridge basalt.
      Graphical abstract image

      PubDate: 2016-07-01T13:05:17Z
       
  • Oxidative biotransformation of biotite and glauconite by alkaliphilic
           anaerobes: The effect of Fe oxidation on the weathering of phyllosilicates
           
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Daria G. Zavarzina, Natalya I. Chistyakova, Alexey V. Shapkin, Alla V. Savenko, Tatyana N. Zhilina, Vadim V. Kevbrin, Tatiana V. Alekseeva, Andrey V. Mardanov, Sergey N. Gavrilov, Andrey Yu. Bychkov
      Two alkaliphilic anaerobic bacteria, namely, the dissimilatory iron-reducer Geoalkalibacter ferrihydriticus and the fermentative hydrolytic Clostridium alkalicellulosi, along with their co-cultures, are studied to examine their ability to release Si and Fe from two main Fe-containing phyllosilicates in Earth's crust: biotite and glauсonite. The formation of magnetically ordered phase(s) within 200days of incubation was only observed in the presence of G. ferrihydriticus whether in a mono- or co-culture but not in the abiotic controls or a pure culture of C. alkalicellulosi. The co-culture of these organisms could represent a simple trophic chain in which C. alkalicellulosi decomposed microcrystalline cellulose to produce organic acids and ethanol, while G. ferrihydriticus, as we expected, utilized these products and reduces Fe(III) in phyllosilicate lattices. Unexpectedly, G. ferrihydriticus did not utilize but instead produced an additional 3mM of acetate during growth with phyllosilicates. An analysis of the Mössbauer spectra of biotite and glauconite that were weathered in the presence of G. ferrihydriticus revealed magnetically ordered phases that formed by Fe2+ oxidation rather than by Fe3+ reduction. The only possible explanation of this phenomenon could be in the ability of G. ferrihydriticus to produce acetate during anaerobic Fe2+ oxidation with carbonate as an electron acceptor. Thermodynamic calculations show the possibility of such a reaction. Thus, microorganisms with respiratory metabolism could play an active role in the bioweathering of phyllosilicates under alkaline anaerobic conditions. The bacterial anaerobic oxidation of ferrous iron with carbonate as an electron donor is supposed to have played a significant role in ancient environments, serving as one of the causes of banded iron formations.
      Graphical abstract image

      PubDate: 2016-06-26T15:23:47Z
       
  • Os isotopic constraints on crustal contamination in Auckland Volcanic
           Field basalts, New Zealand
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Jenni L. Hopkins, Christian Timm, Marc-Alban Millet, André Poirier, Colin J.N. Wilson, Graham S. Leonard
      The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that <1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os=0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.


      PubDate: 2016-06-26T15:23:47Z
       
  • Lithium isotope fractionation during incongruent melting: Constraints from
           post-collisional leucogranite and residual enclaves from Bengbu Uplift,
           China
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): He Sun, Yongjun Gao, Yilin Xiao, Hai-ou Gu, John F. Casey
      Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-rich mafic enclaves and wall rock Wuhe gneisses from the southeast margin of North China Craton (NCC) were investigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucogranites have distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher than their presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace element modeling of REE and Sr/Y suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt+Qz+Pl+H2O=Grt+melt, leaving mainly Grt+Bt with minor allanite in the residuum. The mafic enclaves show identical Sr-Nd isotopic compositions with their host leucogranites, contrasting with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thus interpreted as entrained residual phases formed by this incongruent partial melting. The Jingshan leucogranites show relatively high δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7–11.3ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show low δ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118ppm). Garnet separated from residual enclaves is characterized by a narrow range of low δ7Li values (−1.5‰ to −0.1‰) with high Li concentrations from 32.9 to 81.7ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+15.0‰ to +16.6‰) with very low Li concentrations (~1ppm). Biotite from both leucogranite and residual enclaves shows high Li concentrations (195–382ppm) and relatively heavy Li isotope compositions (+3.2‰ to +7.5‰). The Li elemental and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed with leucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator as commonly suggested.


      PubDate: 2016-06-22T21:05:18Z
       
  • A new method for estimating parent rock trace element concentrations from
           zircon
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): James B. Chapman, George E. Gehrels, Mihai N. Ducea, Nicky Giesler, Alex Pullen
      Zircon/bulk rock REE partition coefficients from natural samples correlate with REE concentration in zircon. The correlation is the strongest for the LREE and diminishes with decreasing ionic radius. The relationship between partition coefficient and REE concentration in zircon can be modeled as a power law and the coefficient (α) and exponent (β) terms for each of the REE are empirically determined using new and previously published data. A series of independent tests show that using variable partition coefficients based on the reported α and β terms commonly results in more accurate estimates of bulk rock REE concentrations than average partition coefficients, particularly for the LREE that have previously been difficult to constrain. These results provide a way to account for highly variable REE concentrations in zircon, which may be controlled by numerous processes such as surface enrichment, complex substitution mechanisms, or accidental sampling of sub-microscopic inclusions. The proposed method for estimating bulk rock REE concentrations is especially well-suited to detrital zircon investigations where there is no information available on the parent rock composition.
      Graphical abstract image

      PubDate: 2016-06-22T21:05:18Z
       
  • Beryllium desorption from minerals and organic ligands over time
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Vanessa Boschi, Jane K. Willenbring
      Beryllium isotopes sorbed to sediments have provided useful tools in the field of geochronology and geomorphology over the last few decades. The use of beryllium isotopes relies on the premise that beryllium sorbed to sediments is unaltered over large timescales. Changes in the environmental chemistry, either in-situ or en route from soil to fluvial system, to the ocean, can cause beryllium desorption and may preclude some beryllium isotopic applications. Four mechanisms were tested to determine the relative desorption potential of beryllium including a reduction in pH, an increase in ionic strength (NaCl) and complexation by soluble organic (malonic acid) and inorganic species (NaF). To assess the relative effect of each mechanism on beryllium desorption from both organic and mineral fractions, we prepared separate solutions of beryllium bound to minerals and organic compounds and measured beryllium concentrations in solution before and after each chemical perturbation. We conclude a reduction in pH resulted in the greatest amount of desorption among the four treatments, removing 97% and 75% of sorbed beryllium from illite and montmorillonite, respectively, and none from the organic ligands tested. The addition of malonic acid and increasing the ionic strength also resulted in desorption from montmorillonite. Although increasing the ionic strength did remove 32% and 8.4% of beryllium from montmorillonite and sulfonate, respectively, the presence of sodium significantly enhanced sorption to illite. The addition of NaF did not result in any beryllium desorption. Our results demonstrate that various chemical processes can promote the exchange of beryllium between solid and dissolved phases, the extent to which depends on the composition of the system. We also related differences in beryllium desorption behavior to complexation mechanisms driving retention among organic and mineral species. We estimate inner sphere complexation is the predominant sorption mechanism among the organic ligands tested due to the minimal amounts of desorption and the large stability constants previously reported in the literature. Additionally, we found that different complexation processes are involved in beryllium sorption to illite versus montmorillonite. Because beryllium desorbed from montmorillonite due to changes in pH, ionic strength and organic acid complexation, we hypothesize that a portion of beryllium-montmorillonite associations involve outer sphere processes, driven by weaker electrostatic attractions. However, beryllium exhibited a unique relationship with illite in that sorption not only involves inner sphere processes but also physical inclusion within collapsed interlayer spaces.


      PubDate: 2016-06-22T21:05:18Z
       
  • Sr isotopic compositions of the interstitial water and carbonate from two
           basins in the Gulf of Mexico: Implications for fluid flow and origin
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Yan-Ping Li, Shao-Yong Jiang
      Strontium isotopic compositions of the interstitial water and carbonate from marine sediments sampled during IODP 308, in two basins (the normally-pressured Brazos-Trinity Basin IV and the over-pressured Ursa Basin) on the northern slope of the Gulf of Mexico, are present in this study. In the Brazos-Trinity Basin IV, the 87Sr/86Sr ratios of the interstitial water range from 0.70917 to 0.70954, with carbonates sharing similar or slightly lower values from 0.70851 to 0.70952. The interstitial water above 31m shows similar 87Sr/86Sr ratio close to that of seawater, whereas the interstitial water below 31m shows more radiogenic 87Sr/86Sr ratios, possibly indicating a water/rock interaction between the fluid and silicate component in the deep basin sediments. The Sr-isotope ratios of the carbonates are less radiogenic than the seawater, which may reflect a terrestrial carbonate input (such as limestone) transported through the Brazos and Trinity rivers. In the Ursa Basin, the 87Sr/86Sr ratios of the interstitial water range from 0.70887 to 0.70999, those of the carbonate vary from 0.70808 to 0.70930. Both the interstitial water and carbonate show a similar trend of 87Sr/86Sr ratios throughout the depth. The 87Sr/86Sr ratios of the interstitial water decrease from the sea floor surface to the minimum at the Seismic Reflector S10. A lateral fluid incursion with less radiogenic 87Sr/86Sr ratios along the Seismic Reflector S10 can be inferred in the Ursa Basin. It is suggested that the lateral fluid incursion shows a seawater origin, modified by diagenetic reactions including the dissolution of halite. Between the Seismic Reflector S10 and S40, 87Sr/86Sr ratios of the interstitial water increase linearly to the maximum. Along the Seismic Reflector S40, a lateral fluid incursion with more radiogenic 87Sr/86Sr ratios is again recognized. The fluid may have also originated from seawater, but modified by the diagenesis of terrigenous sediments, likewise characterized by highly radiogenic 87Sr/86Sr compositions. A two-dimension fluid-flow model in the Ursa Basin is established.


      PubDate: 2016-06-17T18:00:47Z
       
  • A comparison of Fe(III) reduction rates between fresh and aged biogenic
           iron oxides (BIOS) by Shewanella putrefaciens CN32
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): Tarek Najem, Sean Langley, Danielle Fortin
      This study compared the chemical and physical properties of freshwater biogenic iron oxides (BIOS) aged at 4°C for ~5years in the dark to simulate early diagenesis to previously published findings from fresh samples. In addition, the reactivity of aged BIOS in the presence of a well-characterized iron reducing bacterium, Shewanella putrefaciens CN32, was investigated. The composition and proportion of the iron minerals were examined using XRD, Fe k-edge EXAFS, and chemical extractions, and results were similar in both fresh and aged BIOS. Aged BIOS were dominated by 2-line ferrihydrite indicating that the intermixed bacterial cells and their exudates inhibited the transformation of 2-line ferrihydrite into crystalline phases. However, despite the similarity of mineralogy between aged and fresh BIOS, the rates of bacterial iron reduction in aged BIOS were found to be significantly lower than that of their fresh counterparts. Further characterization of aged BIOS by TEM revealed unique morphologies which were not previously observed in fresh BIOS. TEM also showed thickly coated bacterial exudates and presumably those formed as a result of continued accretion or aggregation of iron oxide nanoparticles. Aggregation of the iron oxide particles and subsequent decrease in solubility are likely responsible for the low rates of bacterial iron reduction in aged BIOS. Phase stability and aggregation in BIOS have important implications for the long-term reactivity of BIOS and the preservation of organic matter in the environment.


      PubDate: 2016-06-17T18:00:47Z
       
  • Hydrothermal transport, deposition, and fractionation of the REE:
           Experimental data and thermodynamic calculations
    • Abstract: Publication date: 7 November 2016
      Source:Chemical Geology, Volume 439
      Author(s): A. Migdisov, A.E. Williams-Jones, J. Brugger, F.A. Caporuscio
      For many years, our understanding of the behavior of the REE in hydrothermal systems was based on semi-empirical estimates involving extrapolation of thermodynamic data obtained at 25°C (Haas et al., 1995; Wood, 1990a). Since then, a substantial body of experimental data has accumulated on the stability of aqueous complexes of the REE. These data have shown that some of the predictions of Haas et al. (1995) are accurate, but others may be in error by several orders of magnitude. However, application of the data in modeling hydrothermal transport and deposition of the REE has been severely hampered by the lack of data on the thermodynamic properties of even the most common REE minerals. The discrepancies between the predictions of Haas et al. (1995) and experimental determinations of the thermodynamic properties of aqueous REE species, together with the paucity of data on the stability of REE minerals, raise serious questions about the reliability of some models that have been proposed for the hydrothermal mobility of these critical metals. In this contribution, we review a body of high-temperature experimental data collected over the past 15 years on the stability of REE aqueous species and minerals. Using this new thermodynamic dataset, we re-evaluate the mechanisms responsible for hydrothermal transport and deposition of the REE. We also discuss the mechanisms that can result in REE fractionation during their hydrothermal transport and deposition. Our calculations suggest that in hydrothermal solutions, the main REE transporting ligands are chloride and sulfate, whereas fluoride, carbonate, and phosphate likely play an important role as depositional ligands. In addition to crystallographic fractionation, which is based on the differing affinity of mineral structures for the REE, our models suggest that the REE can be fractionated hydrothermally due to the differences in the stability of the LREE and HREE as aqueous chloride complexes.


      PubDate: 2016-06-17T18:00:47Z
       
 
 
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 © 2009-2016