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EARTH SCIENCES (471 journals)                  1 2 3 | Last

Showing 1 - 200 of 371 Journals sorted alphabetically
Acta Geochimica     Hybrid Journal   (Followers: 3)
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: 20)
Advances In Physics     Hybrid Journal   (Followers: 18)
Aeolian Research     Hybrid Journal   (Followers: 5)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 4)
AMBIO     Hybrid Journal   (Followers: 14)
Anales del Instituto de la Patagonia     Open Access   (Followers: 1)
Andean geology     Open Access   (Followers: 13)
Annales Henri Poincaré     Hybrid Journal   (Followers: 3)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access  
Annals of Geophysics     Open Access   (Followers: 13)
Annals of GIS     Hybrid Journal   (Followers: 21)
Annals of Glaciology     Full-text available via subscription   (Followers: 3)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal   (Followers: 3)
Anthropocene Review     Hybrid Journal   (Followers: 6)
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: 5)
Applied Petrochemical Research     Open Access   (Followers: 2)
Applied Remote Sensing Journal     Open Access   (Followers: 34)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 34)
Arctic Science     Open Access   (Followers: 6)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites : The Journal of Space Research Centre of Polish Academy of Sciences     Open Access   (Followers: 17)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 20)
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: 10)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 12)
Atmospheric and Climate Sciences     Open Access   (Followers: 28)
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: 11)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 21)
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: 21)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 5)
Chemical Geology     Hybrid Journal   (Followers: 18)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 4)
Chinese Geographical Science     Hybrid Journal   (Followers: 5)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 4)
Ciencia del suelo     Open Access   (Followers: 2)
Ciencias Espaciales     Open Access  
Climate and Development     Hybrid Journal   (Followers: 12)
Coastal Management     Hybrid Journal   (Followers: 25)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 8)
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: 3)
Continental Shelf Research     Hybrid Journal   (Followers: 9)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 10)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 3)
Coral Reefs     Hybrid Journal   (Followers: 16)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 3)
Diatom Research     Hybrid Journal   (Followers: 2)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 11)
E&S Engineering and Science     Open Access  
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 107)
Earth and Space Science     Open Access   (Followers: 11)
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 5)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 4)
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: 67)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 10)
Earthquake Spectra     Full-text available via subscription   (Followers: 18)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 7)
Electromagnetics     Hybrid Journal   (Followers: 5)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Hybrid Journal   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 24)
Environmental Geology     Hybrid Journal   (Followers: 20)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Hybrid Journal   (Followers: 5)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (Followers: 18)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (Followers: 33)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (Followers: 13)
European Journal of Remote Sensing     Open Access  
Exploration Geophysics     Hybrid Journal   (Followers: 4)
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: 8)
Geocarto International     Hybrid Journal   (Followers: 22)
Geochemical Perspectives     Hybrid Journal  
Geochemistry : Exploration, Environment, Analysis     Hybrid Journal   (Followers: 7)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26)
Geochimica et Cosmochimica Acta     Hybrid Journal   (Followers: 47)
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   (Followers: 1)
Geoenvironmental Disasters     Open Access   (Followers: 3)
Geofluids     Open Access   (Followers: 4)
Geoforum     Hybrid Journal   (Followers: 21)
Géographie physique et Quaternaire     Full-text available via subscription   (Followers: 1)
Geography and Natural Resources     Hybrid Journal   (Followers: 7)
Geoheritage     Hybrid Journal   (Followers: 1)
Geoinformatica Polonica : The Journal of Polish Academy of Arts and Sciences     Open Access  
Geoinformatics & Geostatistics     Hybrid Journal   (Followers: 8)
Geological Journal     Hybrid Journal   (Followers: 14)
Geology Today     Hybrid Journal   (Followers: 25)
Geology, Geophysics and Environment     Open Access   (Followers: 1)
Geomagnetism and Aeronomy     Hybrid Journal   (Followers: 3)
Geomatics, Natural Hazards and Risk     Hybrid Journal   (Followers: 9)
Geomechanics for Energy and the Environment     Full-text available via subscription   (Followers: 1)
GEOmedia     Open Access   (Followers: 1)
Geomorphology     Hybrid Journal   (Followers: 27)
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: 9)
Geoscience Letters     Open Access   (Followers: 1)
Geoscience Records     Open Access  
Geosciences     Open Access   (Followers: 2)
Geosciences Journal     Hybrid Journal   (Followers: 10)
Geoscientific Instrumentation, Methods and Data Systems     Open Access   (Followers: 4)
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: 2)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 4)
Global and Planetary Change     Hybrid Journal   (Followers: 17)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15)
Gondwana Research     Hybrid Journal   (Followers: 7)
GPS Solutions     Hybrid Journal   (Followers: 17)
Grassland Science     Hybrid Journal  
Ground Water     Hybrid Journal   (Followers: 31)
Ground Water Monitoring & Remediation     Hybrid Journal   (Followers: 18)
Groundwater for Sustainable Development     Full-text available via subscription   (Followers: 1)
Helgoland Marine Research     Open Access   (Followers: 3)
History of Geo- and Space Sciences     Open Access   (Followers: 4)
Hydrobiologia     Hybrid Journal   (Followers: 18)
Hydrogeology Journal     Hybrid Journal   (Followers: 17)
Hydrological Processes     Hybrid Journal   (Followers: 27)
Hydrology and Earth System Sciences     Open Access   (Followers: 28)
ICES Journal of Marine Science: Journal du Conseil     Hybrid Journal   (Followers: 54)
IEEE Journal of Oceanic Engineering     Hybrid Journal   (Followers: 11)
Indian Geotechnical Journal     Hybrid Journal   (Followers: 3)
Indonesian Journal on Geoscience     Open Access   (Followers: 2)
Interdisciplinary Environmental Review     Hybrid Journal   (Followers: 3)
International Geology Review     Hybrid Journal   (Followers: 14)
International Journal of Advanced Geosciences     Open Access  
International Journal of Advanced Remote Sensing and GIS     Open Access   (Followers: 34)
International Journal of Advancement in Earth and Enviromental Sciences     Open Access   (Followers: 3)
International Journal of Advancement in Remote Sensing, GIS, and Geography     Open Access   (Followers: 27)
International Journal of Applied Earth Observation and Geoinformation     Hybrid Journal   (Followers: 35)
International Journal of Coal Geology     Hybrid Journal   (Followers: 2)
International Journal of Disaster Risk Reduction     Hybrid Journal   (Followers: 15)

        1 2 3 | Last

Journal Cover Chemical Geology
  [SJR: 2.346]   [H-I: 145]   [18 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [3031 journals]
  • Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar
           ages in perlitised obsidian
    • Abstract: Publication date: Available online 19 May 2017
      Source:Chemical Geology
      Author(s): Stephanie Flude, Hugh Tuffen, Sarah C. Sherlock
      In situ laser ablation Ar-isotope analyses of variably hydrated and devitrified obsidian from the ~27Ma Cochetopa Dome, San Juan, USA, reveal complex interplay between degassing of initial Ar and absorption of atmospheric Ar. These processes have locally modified the Ar-isotope composition of the obsidian and led to spurious, spatially-heterogeneous Ar-isotope and 40Ar/39Ar age data. Small perlite beads exhibit older apparent Ar-ages at the rims than the cores. This is interpreted as an apparent excess of 40Ar at the rims, produced either by a) diffusion of excess 40Ar into the bead during flushing of the lava with excess 40Ar-bearing volcanic gas, or by b) isotopic fractionation during degassing of initial Ar, causing preferential loss of 36Ar over 40Ar at the bead rims. The second interpretation is favoured by a relative enrichment of 36Ar in the core of a perlite bead along a microlite-free (poorly degassed) flow band, and by a lack of age variation in a larger, fresh, well-degassed perlite bead. These isotopic gradients were later overprinted during glass hydration by absorption of Ar with near-atmospheric composition, resulting in elevated 36Ar and reduced radiogenic 40Ar* yields at the rims of perlite beads. These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high 36Ar yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples.

      PubDate: 2017-05-22T03:39:31Z
  • Ancient depleted and enriched mantle lithosphere domains in northern
           Madagascar: Geochemical and isotopic evidence from
           spinel-to-plagioclase-bearing ultramafic xenoliths
    • Abstract: Publication date: Available online 19 May 2017
      Source:Chemical Geology
      Author(s): Ivana Rocco, Alberto Zanetti, Leone Melluso, Vincenzo Morra
      Mantle xenoliths hosted in Cenozoic alkaline rocks of northern Madagascar (Massif d'Ambre and Bobaomby volcanic fields) are spinel lherzolites, harzburgites and rare websterites. Petrography, electron microprobe, LA-ICP-MS and thermal ionization mass spectrometry techniques allowed to recognize domains characterized by variable degree of partial melting and extent of re-enrichment processes: 1) refractory spinel-to-spinel+plagioclase-lherzolites, with clinopyroxenes having marked LREE (Light Rare Earth Elements) depletion ((La/Yb)N ~0.2) and very high 143Nd/144Nd (0.513594), which represent a limited and shallow portion of old mantle that suffered low degree partial melting (2–3%) and was later accreted to the lithosphere. These lherzolites acted as a low-porosity region, being, in places, percolated by small volumes of melts shortly before eruption; 2) lherzolites and harzburgites that suffered variable degrees of partial melt extraction (up to 15%), assisted and/or followed by pervasive, porous flow infiltration of alkaline melts in a relatively large porosity region, leading to the creation of a wide area rich in secondary mineral phases (i.e. olivine, clinopyroxene and pargasitic amphibole), enriched in incompatible elements (e.g., LaN/YbN in clinopyroxene up to 15) and having radiogenic Sr and unradiogenic Nd; 3) websterites and wehrlite-bearing samples that record differentiation processes of alkaline melts highly enriched in Th, U and LREE, not yet documented in the erupted volcanics of northern Madagascar. The mantle xenoliths of northern Madagascar show a regional decrease of the equilibration temperature from to SW (up to 1180°C, Nosy Be Archipelago) to the NE (up to 900°C, Bobaomby district). A significant lithologic and geochemical variation of the shallow lithospheric mantle beneath northern Madagascar is noted, in contrast with the relatively uniform geochemical and isotopic composition of the host alkali basalt and basanite lavas.
      Graphical abstract image

      PubDate: 2017-05-22T03:39:31Z
  • Building biogenic beachrock: Visualizing microbially-mediated carbonate
           cement precipitation using XFM and a strontium tracer
    • Abstract: Publication date: Available online 19 May 2017
      Source:Chemical Geology
      Author(s): Jenine McCutcheon, Luke D. Nothdurft, Gregory E. Webb, Jeremiah Shuster, Linda Nothdurft, David Paterson, Gordon Southam
      The fate of reef islands is a topic of ongoing debate in the face of climate change-induced sea-level rise and increased cyclone intensity. Increased erosion and changes to the supply of reef-derived sediment may put sand reef cays at risk of dramatic morphological changes. These changes may have negative implications for the existence of reef cay environments, which host vital sea turtle and bird rookery habitats. Beachrock, consolidated carbonate beach sediment in the intertidal zone, forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand. In spite of the critical role beachrock plays in stabilizing some reef cay shores, the method of beachrock formation is still incompletely understood. In this investigation, beachrock was synthesized using beach sand and beachrock samples from Heron Island (Great Barrier Reef, Australia) in aquarium experiments in which natural beachrock formation conditions were simulated. Beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock ‘inoculum’, whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration. The new cements in the synthesized beachrock were analyzed using synchrotron-based X-ray fluorescence microscopy and were identified using a Sr-tracer added to the experimental seawater. The resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated, demonstrating the fundamental role microbes play in beachrock formation. These results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays, and in turn, protect the unique habitats reef islands support.
      Graphical abstract image

      PubDate: 2017-05-22T03:39:31Z
  • The origin of hydrothermal chlorite- and anhydrite-rich sediments in the
           middle Okinawa Trough, East China Sea
    • Abstract: Publication date: Available online 19 May 2017
      Source:Chemical Geology
      Author(s): Hebin Shao, Shouye Yang, Susan Humphris, Di Cai, Feng Cai, Jiangtao Li, Qing Li
      During the Integrated Ocean Drilling Program (IODP) Expedition 331, five sites were drilled into the Iheya North Knoll hydrothermal system in the Okinawa Trough (OT) — a back-arc basin characterized by thick terrigenous sediment. Following up on the previous study by Shao et al. (2015), we present new mineralogical, geochemical, and Sr-Nd isotope data to investigate the origin of the hydrothermal sediments and characterize the hydrothermal system. The substrate at the Iheya North Knoll is dominated by pumiceous sediment and other volcanoclastic materials interbedded with hemipelagic (terrigenous and biogenous) sediments. Impermeable layers separate the hydrothermal sediments into distinct units with depth that are characterized by various assemblages of alteration materials, including polymetallic sulfides, sulfates, chlorite- and kaolinite-rich sediments. The rare earth elements (REEs) and Nd isotope data suggest that the chlorite-rich and kaolinite-rich layers primarily resulted from the alteration of pumiceous materials in different chemical and physical conditions. Kaolinite-rich sediment likely reflects low pH and low Mg concentration fluids, while chlorite-rich sediment formed from fluids with high pH and increased Mg contents, likely at higher temperatures. The Sr isotopic compositions of subsurface anhydrite reflect high seawater/hydrothermal fluid ratios in the mid-OT hydrothermal area. Compared with chlorite-rich sediments from other sediment-covered or felsic-hosted hydrothermal systems, the chlorite-rich sediments in the mid-OT are characterized by lower concentrations of Al and Fe but much higher Y, Zr, Hf, Th and REEs, indicative of the distinct nature of the precursor rocks in this region.

      PubDate: 2017-05-22T03:39:31Z
  • The diffusion coefficients of noble gases (HeAr) in a synthetic basaltic
           liquid: One-dimensional diffusion experiments
    • Abstract: Publication date: Available online 18 May 2017
      Source:Chemical Geology
      Author(s): Julien Amalberti, Pete Burnard, Laurent Tissandier, Didier Laporte
      Diffusion coefficients of noble gases in silicate liquids are poorly known, and, as a result, it is difficult to quantify the importance of kinetic fractionation of noble gas abundances and isotopic compositions during magmatic processes. Nevertheless, diffusive fractionation has been invoked to explain noble gas signatures in MORBs and OIBs, with important implications for magma degassing and noble gas mantle geochemistry. In order to investigate the diffusion of noble gases in magmas, we developed an experimental protocol based on uniaxial diffusion of He and Ar through a column of synthetic basaltic liquid in a Pt tube. At the end of the experiment, the column of silicate liquid was rapidly quenched to a glass recording the diffusion profile. The glass cylinder was cut into a series of slices, which were analyzed for noble gases by traditional noble gas mass spectrometry following total gas extraction by fusion. Using this protocol, we measured He and Ar diffusivities in CMAS glass G1 (50mol% SiO2, 9mol% Al2O3, 16mol% MgO, 25mol% CaO) at temperatures of 1673K and 1823K: D He =2.75±0.25×10−6 cm2·s−1 (T =1673K); D He =4.77±0.42×10−6 cm2·s−1 (T =1823K); and D Ar =9.3±1.3×10−7 cm2·s−1 (T =1673K). Combining these new high temperature data with diffusion coefficients measured on the same composition just above the glass transition temperature, we determined the activation energy E a and the pre-exponential factor D 0 for He and Ar diffusion in silicate liquids: D 0 =1.72±0.9×10−2 cm2·s−1 and E a =136.5±3.2kJ/mol for Ar; D 0 =1.8±0.5×10−4 cm2·s−1 and E a =57.6±1.5kJ/mol for He. Because He and Ar have very different activation energies for diffusion in the liquid state, the ratio DHe/DAr is strongly sensitive to temperature, decreasing from 145 at the glass transition temperature (1005K) to 2 at 1823K. The implication is that the kinetic fractionation of He relative to Ar in magmas is likely to be more important during the cooling stages than during the earlier, high temperature stages of magmatic history.

      PubDate: 2017-05-22T03:39:31Z
  • Noble gas signals in corals predict submarine volcanic eruptions
    • Abstract: Publication date: Available online 18 May 2017
      Source:Chemical Geology
      Author(s): A.M. Álvarez-Valero, R. Burgess, C. Recio, V. de Matos, O. Sánchez-Guillamón, M. Gómez-Ballesteros, G. Recio, E. Fraile-Nuez, H. Sumino, J.A. Flores, M. Ban, A. Geyer, M.A. Bárcena, J. Borrajo, J.M. Compaña
      Deep-water corals growing in close proximity to the 2011 submarine eruption at Tagoro (El Hierro Island; Canary Archipielago, Spain) have revealed their ability to record the magmatic helium (3He) signal during a period of months prior to the eruption with magmatic 3He/4He ratios of 3.6–5.0 RA This is similar to the range of He isotope values (3–10 Ra) obtained from olivine samples from basinites erupted during the 2011 Tagoro eruption. Whip-like black corals (Anthozoa: Antipatharia: Antipathidae: Stichopathes) growing on basaltic rocks trapped 3He within their skeletons. We used a theoretical growth rate to estimate the time of trapping of 3He between release from the magmatic source at depth to trapping by the aquatic organisms. Our findings suggest that magmatic 3He emission occurred a few months before the beginning of the seismic activity in the region, the latter occurring four months in advance of the beginning of the eruption. This discovery implies that corals living in submarine volcanic areas may act as archives of noble gases released before the beginning of an eruption, and that the continuous monitoring of 3He may help to constrain the arrival time of a subaqueous eruption with potential applications in volcano eruption forecasting.

      PubDate: 2017-05-22T03:39:31Z
  • Spatial distribution of helium isotopes in Icelandic geothermal fluids and
           volcanic materials with implications for location, upwelling and evolution
           of the Icelandic mantle plume
    • Abstract: Publication date: Available online 17 May 2017
      Source:Chemical Geology
      Author(s): Sunna Harðardóttir, Sæmundur Ari Halldórsson, David R. Hilton
      The distribution of helium isotope ratios (3He/4He) in Icelandic geothermal fluids, volcanic glasses and phyric lavas is investigated. Along with presenting a new helium isotope dataset using phyric lavas largely from off-rift regions, we compiled published data and constructed a database of all available helium isotope data from Iceland. The new dataset reveals an exceptionally high 3He/4He ratio from a phyric lava in NW-Iceland (47.5 RA, where RA is the 3He/4He ratio of air), which is among the highest values measured in any mantle-derived magma to date. Modifications of primary (i.e., mantle-derived) helium isotope ratios, due to additions of air-derived helium and He from radiogenic ingrowth, were evaluated and the database was filtered accordingly. The geographical information system ArcGIS (ESRI) was used to perform spatial analysis on the filtered database and the interpolation method, Natural Neighbor, was used to calculate representative helium isotope ratios for all parts of Iceland, including off-rift regions. The results show that helium isotope ratios for the whole of Iceland vary from 5.1 to 47.5 RA. However, this study allows for a fine-scale distinction to be made between individual rift segments and off-rift regions. The results clearly reveal that each rift zone has its own distinctive mean isotope signature: 12–17 RA in the Western Rift Zone, 8–11 RA in the Northern Rift Zone and 18–21 RA in the Eastern Rift Zone. Our isoscape map places new constraints on a previously inferred high-helium plateau region in central Iceland (Breddam et al., 2000). The plateau continues southward along the propagating Eastern Rift Zone and through to the South Iceland Seismic Zone and the Mid-Iceland belt. Its location coincides with many geological features, e.g., eruption rates, location of abandoned rift segments, seismic velocity and gravity anomalies. Such high helium isotope ratios have been associated with undegassed and primordial mantle sources that have been isolated in the lower mantle over Earth's history. Thus, high-helium domains throughout Iceland are interpreted to mark the loci of present and past plume conduits which help explain the considerable spatial variation in the sampling of a primordial mantle He component beneath the Iceland hotspot.

      PubDate: 2017-05-22T03:39:31Z
  • Branched GDGT-based paleotemperature reconstruction of the last
           30,000years in humid monsoon region of Southeast China
    • Abstract: Publication date: Available online 17 May 2017
      Source:Chemical Geology
      Author(s): Mengyuan Wang, Zhuo Zheng, Meiling Man, Jianfang Hu, Quanzhou Gao
      The use of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) to reconstruct mean annual air temperatures (MAATs) and sediment pH from soils has sparked significant interest in the terrestrial paleoclimate community. However, the reconstruction of these climate proxies from peat sediments is rare in monsoonal regions of the East Asia. This research was carried out on a core from the Shuizhuyang (SZY) peat bog located in Fujian Province. Branched GDGT (brGDGT) indexes were used for reconstructing the paleoclimate of the last 30calka. The aim was to evaluate quantitatively the MAAT (mean annual air temperature) and pH values since the Last Glacial Maximum (LGM) in the subtropical zone of China. Results show that the CBT-MBT′-derived MAAT at MIS 3 is about 15.6°C on average, which is followed by a significant fall at the LGM (11.7–12.1°C). The temperature difference between the LGM and the present-day value is as high as 5.8°C. The synchronous variation of biomarker and pollen proxies indicates that replacement of subtropical evergreen broadleaved forests by cold-tolerant, deciduous broadleaved forests was driven by the significant drop in air temperature. Our results also indicate that the Younger Dryas event lasted from about 12.9 to about 11.3calka, and cooling events at 3.2calka in the late Holocene were detected, showing the sensitivity of peat bogs to rapid cooling. Our pH reconstructions indicate that the pH of the bog rose during Heinrich 1 and Bølling-Allerød periods, probably due to low precipitation, and were lowest in the Holocene thermal maximum between 8ka and 2.5ka, probably due to higher precipitation. The decoupling of reconstructed MAAT and pH during particularly deglaciation and YD periods supports the hypothesis that the variations in temperature and precipitation are not always synchronous.

      PubDate: 2017-05-22T03:39:31Z
  • Skeletal growth controls on Mg/Ca and P/Ca ratios in tropical Eastern
           Pacific rhodoliths (coralline red algae)
    • Abstract: Publication date: Available online 16 May 2017
      Source:Chemical Geology
      Author(s): Hillary R. Sletten, David P. Gillikin, Jochen Halfar, C. Fred T. Andrus, Héctor M. Guzmán
      The Mg/Ca ratios of non-geniculate coralline red algae (CA) are often assumed to be solely dependent on the sea surface temperature (SST) in which they grow, suggesting that they are ideal proxies for SST reconstruction. Mg/Ca ratio cyclicity is also commonly used as a marker for annual growth band identification in many CA species. However, other controls on elemental uptake such as light, calcification mechanisms, and vital effects may control elemental ratios (e.g. Mg/Ca and P/Ca) in certain genera or species of CA, thus complicating the use of Mg/Ca and P/Ca ratios as a proxy source for SST and paleo-environmental reconstructions. Here we present evidence showing that the Mg/Ca and P/Ca ratio cyclicity in unattached living tropical Lithothamnion sp. (n=8) rhodoliths correlate >90% to the number of sub-annual growth bands (n=1 to 20) formed during a 172-day controlled growth experiment and not to temperature or other environmental conditions (e.g. salinity, pH, etc.). This pattern is also observed in the wild, pre-experimental growth further supporting that the bands are not annual. The Mg/Caalga (R2 =0.67; p-value=0.013; n=8) and P/Caalga (R2 =0.64; p-value=0.018; n=8) significantly correlate to skeletal extension during the experiment. This work has implications for trace elemental proxy development using CA as well as the influence of skeletal extension on elemental incorporation in biogenic carbonates, particularly rhodoliths of Lithothamnion sp. from the Gulf of Panama, Panama.

      PubDate: 2017-05-17T03:28:31Z
  • Atmospheric helium isotope composition as a tracer of volcanic emissions:
           A case study of Erta Ale volcano, Ethiopia
    • Abstract: Publication date: Available online 15 May 2017
      Source:Chemical Geology
      Author(s): Christine Boucher, Tefang Lan, Bernard Marty, Peter G. Burnard, Tobias P. Fischer, Dereje Ayalew, Jennifer Mabry, J. Maarten de Moor, Michael E. Zelenski, Laurent Zimmermann
      The composition of atmospheric helium is generally considered to be constant (3He/4He=1.39×10−6) on a large spatial scale. However, local variations may arise in tectonically active areas due to focussed degassing of one of its two isotopes, for example degassing of mantle-derived 3He or crustal-derived 4He. If detected, such variations have the potential to trace open conduit degassing of magmatic bodies and/or diffusive emissions from volcanic and/or crustal sources. Here, we test the possibility of detecting such variations in the Afar region of north-eastern Ethiopia, which is located over a well-developed rift system. Special attention was paid to the Erta Ale volcanic edifice, where both magma lake activity and strong degassing occur. We conducted high-precision 3He/4He ratio measurements of air samples from this region using an analytical facility at the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Nancy (France) that was specially designed for high-precision noble gas analyses. Within the precision of our measurements (0.2–0.4%, 95% confidence interval), the helium isotopic compositions of air from both the Afar rift zone and the crater zone of Erta Ale are similar to the composition of air collected at Brabois Park in Villers-les-Nancy, France (labelled here as RBB). An additional air sample collected in a large tank (500cm3) in Afar in 2015 permitted replicate analysis (n=8) and improved precision. The 3He/4He ratio of this additional sample was also identical to RBB within 0.19% (95% confidence interval, CI). However, a clear excess of 3He (1.32±0.64%, 95% CI, relative to RBB based on a weighted mean of two samples) was detected in air collected above the active lava lake located in the central pit crater of Erta Ale volcano. Such excess requires a 3He flux of 0.15±0.09mol/yr from the crater lava lake to be sustained. A similar 3He flux of 0.12±0.06mol/yr is calculated from SO2 flux measurements and fumerolic gas data obtained during the same field trip. At several sites in the rim of the Northern crater, we conducted soil flux measurements using an accumulation chamber. Both the CO2 contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO2 and 3He fluxes outside the lava lake area. These fluxes were found to be minor (~1%) compared to the lava lake flux. The CO2/3He ratio of (3.1±0.7)×109 of soil gases is comparable to that of the high temperature (1084°C) fumaroles sited in the north pit crater. Using this ratio and our estimated 3He flux, we determined a CO2 flux of 4.6±3.0×108 mol/yr for the lava lake, which is about 105 times lower than the global volcanic subaerial CO2 flux. Based on this pilot study, we suggest that 3He excesses in air could provide another means with which to evaluate the fluxes of CO2 and other volatile species in specific environments, such as highly active volcanic areas.

      PubDate: 2017-05-17T03:28:31Z
  • A new approach to laser-ablation inductively-coupled-plasma
           mass-spectrometry (LA-ICP-MS) using the flexible map interrogation tool
    • Abstract: Publication date: Available online 12 May 2017
      Source:Chemical Geology
      Author(s): J.A. Petrus, D.M. Chew, M.I. Leybourne, B.S. Kamber
      A new flexible interrogation tool for 2-dimensional LA-ICP-MS element and isotope maps is presented. Called Monocle, the tool works as an add-on in the software Iolite. Initial inspection of map data is undertaken with a size and shape adjustable loupe that is moved across the map. As the loupe is panned over the map, histograms and density/probability plots of the element concentration (or ratio, or age) contained within the area under the loupe are displayed. The data of any loupe position can be saved as a region of interest (ROI) in a table. For maps with rare earth element (REE) data a live normalised REE graph can be shown. Similarly, live graphs (e.g. Wetherill or Tera-Wasserburg concordiae) can be displayed for U/Pb data. The second step of map interpretation refines the initial data selection, for example by drawing polygons over irregular zones of similar chemistry. Where areas are separated (e.g. by cracks) several polygons can be nested into one ROI to improve counting statistics. Additional means of data pooling exist. Firstly, by growing a ROI from a seed, whereby the data distribution in the seed is used to find adjacent pixels that are within a user-defined statistical boundary (e.g. 1 standard deviation). Secondly, multiple criteria can be set (e.g. element A>0.1ppm and element B<40wt%) to determine which pixels (connected or unconnected) are included in a ROI. This new approach is illustrated with several examples. A Mariana Arc oceanic Mn crust is used to show how elemental maps resolve alternating periods of precipitation from variably intense hydrothermal input and how their REE systematics reflect mixing between two distinct sources. A map of a garnet from a lherzolite illustrates that with modern LA-ICP-MS instruments it is possible to obtain reproducible REE data from strongly depleted mantle minerals (e.g. La≈14ppb) with a 20×20μm beam and how mineral inclusions, metasomatism and serpentinisation can be avoided to obtain data from the most pristine parts of a grain. A map of a zoned augite crystal from Mt. Etna was used to examine the distribution of Ni and Cr, whose interrelationship in the strong oscillatory zoning can be modelled with extreme fractional crystallisation. A U/Pb map of a complex zircon was used to separate data from the complex inherited core and the much younger magmatic rim. Extraction of data from 2-dimensional maps (<1μm deep) avoids limitations of conventional LA-ICP-MS analysis that typically ablates much deeper. In combination with fast aerosol transfer systems that permit high repetition rate mapping (50–80Hz), extraction of quantitative data with map analysis tools heralds a new era of in situ chemical and isotopic analysis.
      Graphical abstract image

      PubDate: 2017-05-17T03:28:31Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 June 2017
      Source:Chemical Geology, Volume 460

      PubDate: 2017-05-11T21:43:23Z
  • Microprecipitation and δ18O analysis of phosphate for paleoclimate
           and biogeochemistry research
    • Abstract: Publication date: 5 June 2017
      Source:Chemical Geology, Volume 460
      Author(s): A.H. Mine, A. Waldeck, G. Olack, M.E. Hoerner, S. Alex, A.S. Colman
      Phosphate oxygen isotope ratios provide a powerful tool for paleoclimate reconstruction and the study of phosphorus biogeochemistry in aquatic systems. The temperature dependent offset between the oxygen isotope composition of phosphate and water in biogenic minerals and in solutions has been used extensively in paleoclimate research and the study of phosphorus cycling in aquatic and soil environments. Sample sizes are small in many applications, e.g., serial sampling of mammalian tooth enamel for paleoseasonality studies, the use of conodont elements for climate and ocean δ18O reconstruction, or the isolation and analysis of dissolved phosphate from marine and freshwater environments in biogeochemistry studies. This has pushed the development of techniques that allow for processing and analysis of a few micromoles of phosphate. Current approaches to phosphate oxygen isotopic composition (δ18O p ) determination require purification of phosphate as silver phosphate (Ag3PO4). The techniques for the final precipitation of Ag3PO4 fall into two categories: slow vs. rapid microprecipitations. We have tested both and identify artifacts that impact measured δ18O p values resulting from commonly used methods for rapid Ag3PO4 precipitation. In particular, commonly used methods are prone to (a) incomplete precipitation of phosphate, with associated isotopic fractionation, and (b) the production of silver oxide contaminants whose oxygen isotope composition is sensitive to the δ18O of water used to prepare the precipitation reagent solution. These artifacts are commonly on the order of 0.5 to 2‰, which is subtle enough to go undetected in many studies, but large enough to cause errors in temperature estimates on the order of 2–8°C. Slow microprecipitations lead to complete precipitation of phosphate without contaminating O-bearing phases. We develop a rapid precipitation technique that avoids the problems noted above. Both the slow microprecipitation and the rapid microprecipitation developed herein lead to δ18O p measurements that are indistinguishable from those made on conventional large-batch precipitations of Ag3PO4.

      PubDate: 2017-05-11T21:43:23Z
  • Element/Ca, C and O isotope ratios in modern brachiopods: Species-specific
           signals of biomineralization
    • Abstract: Publication date: 5 June 2017
      Source:Chemical Geology, Volume 460
      Author(s): C.V. Ullmann, R. Frei, C. Korte, C. Lüter
      Fossil brachiopods are of major importance for the reconstruction of palaeoenvironmental conditions, particularly of the Paleozoic. In order to better understand signals of ancient shell materials, modern analogue studies have to be conducted. Here we present C and O isotope data in conjunction with Mg/Ca, Sr/Ca, Mn/Ca and Fe/Ca data for nine modern rhynchonellid and terebratulid brachiopod species from tropical to intermediate latitudes and shallow to very deep marine settings. C and O isotope signals of most species suggest formation of secondary shell layers near or in isotopic equilibrium with ambient seawater. Some species – especially in the suborder Terebratellidina – show partly distinct disequilibrium signals, suggesting some degree of phylogenetic control on the expression of vital effects. Mn/Ca and Fe/Ca ratios measured in the modern species form a baseline to assess fossil preservation, but also yield environmental information. Mg/Ca and Sr/Ca ratios follow previously observed patterns, with all studied brachiopod species comprising low-Mg calcite. Strong covariation of Sr/Ca ratios with Mg/Ca ratios is only observed in rhynchonellids and possibly one terebratulid species, potentially linking the incorporation behaviour of alkaline earth metals to phylogeny. Sr/Ca show a strong negative correlation with δ13C values in terebratellidinid species which exhibit major isotopic disequilibrium and also combined data from three localities for which two species were studied indicate such a negative relation. The observed covariation of Sr/Ca ratios with δ13C values may therefore become a useful tool to detect δ13C disequilibrium and to robustly estimate δ13C values of ambient DIC in deep time.

      PubDate: 2017-05-11T21:43:23Z
  • Diagenesis of carbonate associated sulfate
    • Abstract: Publication date: Available online 9 May 2017
      Source:Chemical Geology
      Author(s): Vanessa Fichtner, Harald Strauss, Adrian Immenhauser, Dieter Buhl, Rolf D. Neuser, Andrea Niedermayr
      Carbonate associated sulfate (CAS) is a proxy for the seawater redox conditions of ancient oceans. Despite its frequent utilization from altered carbonate archives, the impact of diagenetic and anchimetamorphic overprint on CAS itself has not yet been calibrated in a systematic manner. In the present study, CAS abundances and sulfur and oxygen isotopic compositions from early diagenetic sabkha type dolomicrites (Triassic Dolomia Principale Formation) were compared with geochemical (87Sr/86Sr, δ13Ccarb, δ18Ocarb), optical (cathodoluminescence), and crystallographical data to assess the impact of burial alteration. Data shown here document that δ34SCAS withstands burial diagenesis or anchimetamorphosis (350°C) and reliably preserves a record of ambient seawater sulfate and early diagenetic redox processes. In contrast, δ18OCAS ratios of early diagenetic dolomites exposed to burial temperatures of 200°C and more are enriched in 18O likely due to exchange between δ18OCAS and δ18Ocarb. Carbonate associated sulfate concentrations are directly affected by burial conditions: Under increasing burial depth, temperature, and cation order in the increasingly stoichiometric dolomite crystal lattice, CAS concentration decreases linearly from a mean value of 470ppm in samples that experienced a burial temperature of 100°C to values below analytical detection in samples exposed to burial fluids with temperatures in excess of 350°C. Significant variations of δ34SCAS in samples from the northern Alps are further attributed to local sulfide oxidation triggered by the influence of meteoric water as based on the δ34S and δ18O values of the water soluble sulfate (WSS). The data shown here are of significance for those concerned with the chemical parameters of ancient oceans and shed light on processes during dolomite formation and diagenetic pathways.

      PubDate: 2017-05-11T21:43:23Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 25 May 2017
      Source:Chemical Geology, Volume 459

      PubDate: 2017-05-07T00:55:40Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 15 May 2017
      Source:Chemical Geology, Volume 458

      PubDate: 2017-05-07T00:55:40Z
  • Temperature versus hydrologic controls of chemical weathering fluxes from
           United States forests
    • Abstract: Publication date: 15 May 2017
      Source:Chemical Geology, Volume 458
      Author(s): Peter A. Raymond
      Chemical weathering is a dominant control on modern inland water chemistry and global element budgets over geologic time scales. Due to its central role in the earth's biogeochemistry it remains an intense area of interest. Understanding the controls on chemical weathering is difficult because it has many drivers with overlapping temporal and spatial signals. Of particular interest is the relationship between chemical weathering fluxes and global temperatures due to a negative feedback loop where warmer temperatures leads to more chemical weathering and its associated atmospheric CO2 consumption (Berner et al., 1983). Recently it has been proposed that this negative feedback loop is indirect where the acceleration of the hydrologic cycle by increased global temperatures leads to higher chemical weathering and atmospheric CO2 consumption (Maher and Chamberlain, 2014). Here, fluxes of all major cations and anions are calculated for 150 forested watersheds smaller than 500km2 in order to explore controls on chemical weathering from United States forests. Relationships between watershed hydrology, ion ratios and pH are reported that explain a large amount of across site variation in bicarbonate fluxes. Furthermore, across all watersheds ~32% of the cation flux is not balanced by bicarbonate but by sulfate. Paired alkalinity, temperature and discharge data are used to determine a temperature sensitivity of chemical weathering across 51 forested watersheds with a minimum of 70 measurements. The temperature sensitivity of bicarbonate fluxes is absent at low flow conditions because long residence times leads to chemical equilibrium and transport limitation. As discharge increases and residence time decreases, the temperature sensitivity of chemical weathering is released from transport limitation. The temperature sensitivity then increases with discharge until it levels off at high discharges as the system becomes reaction limited. Records of daily discharge, watershed discharge to flux relationships, and the temperature sensitivity are then used to estimate how chemical fluxes would change with a 20% increase in discharge, and 10° increase in temperature global change scenario. Not surprisingly it is found that increased discharge leads to higher weathering fluxes. Interestingly, wetter years have a higher temperature sensitivity due to a release of the temperature sensitivity from transport limitation. This coupled with a strong direct temperature effect leads to an approximately equal response from temperature and increased discharge using this scenario of global change. Thus periods of time and regions that are subject to both warm and wet conditions may have a particularly strong control on weathering fluxes.
      Graphical abstract image

      PubDate: 2017-05-07T00:55:40Z
  • Inside front cover (Editorial Board)
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456

      PubDate: 2017-05-07T00:55:40Z
  • Electron transfer at microbe-humic substances interfaces: Electrochemical,
           microscopic and bacterial community characterizations
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Yong Yuan, Xixi Cai, Yueqiang Wang, Shungui Zhou
      Soluble and colloidal humic substances (HSs) involved in microbial extracellular electron transfer (EET) have been extensively investigated. However, little is known regarding EET at HS adlayers that are bound or complexed to the surface of natural solids. Here, we investigated EET process on seven types of HS adlayers by forming electroactive biofilms (EABs) and monitoring the biocurrent generation. The HS immobilized surfaces were carried out by firstly modifying glassy carbon surfaces via electrochemical reduction of aryl diazonium salts to create amino-functionalized surfaces, and then various HSs were covalently bonded to these amino groups via linkage molecules. The results showed that the HS adlayers formed from Leonardite, Elliott Soil and Pahokee Peat humic acids (HAs) facilitated EET by forming more active EABs, whereas the Suwannee River and Aldrich HAs, Suwannee River and Pahokee Peat fulvic acids (FAs) hindered EET. The EET positively correlated with electron accepting capacity and wettability of the HSs, and negatively correlated with the polarity of the HSs, and only weakly correlated with electron donating capacity and zeta potential of the HSs. Microscopic images showed that the thickness and viability of the biofilms varied based on the HS properties. A microbial community structure analysis showed that all of the biofilms were dominated by Proteobacteria and selectively enriched in Firmicutes and Bacteroidetes. Our study indicated that the HS adlayers significantly affected microbial EET, and these results provide a more comprehensive understanding of the HSs involved biogeochemical and biotransformation processes.
      Graphical abstract image

      PubDate: 2017-05-07T00:55:40Z
  • Trace element homogeneity from micron- to atomic scale: Implication for
           the suitability of the zircon GJ-1 as a trace element reference material
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Sandra Piazolo, Elena Belousova, Alexander La Fontaine, Christopher Corcoran, Julie M. Cairney
      The quality of a chemical reference material relies on the fact that the composition of the material is homogeneous across all scales. A series of different techniques have been used to evaluate the trace element homogeneity of the GJ-1 reference zircon from the micron- to atomic scale. Cathodoluminescence imaging was conducted along with quantitative crystallographic orientation analysis and trace element analysis using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The nanometre-scale homogeneity was evaluated by analysing five mineral tips using atom probe tomography, which provides atomic scale three dimensional chemical reconstructions with unprecedented spatial resolution. Results show that the GJ-1 reference zircon is homogeneous at all scales, both structurally and chemically. Crystallographic orientation data confirms that this gem quality zircon has no detectable internal crystallographic orientation changes such as crystal-plastic deformation features or cracks. No mineral inclusions were found. Atom probe tomography shows that there is a lack of any chemical clustering or other modes of spatially defined elemental accumulation or depletion for the most abundant trace elements such as Y, Yb and Hf. This finding is supported by LA-ICPMS data revealing homogeneity within the analytical precision. Trace elements of significant abundance include P, Yb, Y, U and Hf, with contents of 30±6, 65±2, 238±5, 284±14 and 6681±57ppm, respectively. Hence, the GJ-1 zircon used as a reference zircon for UPb and Hf-isotopic studies is also a suitable zircon reference material for trace element analyses.
      Graphical abstract image

      PubDate: 2017-05-07T00:55:40Z
  • Statistics for LA-ICP-MS based fission track dating
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Pieter Vermeesch
      Despite the conceptual elegance and simplicity of the External Detector Method (EDM) for fission track dating, an increasing number of laboratories are switching to LA-ICP-MS as a means of measuring the uranium content of apatite, zircon and sphene. LA-ICP-MS based fission track (LAFT) dating offers shorter turnaround times, removes the need to handle radioactive materials and hydrofluoric acid, and facilitates double-dating with the U-Pb method. This paper aims to bring the statistical treatment of LAFT data on an equal footing with the EDM by formulating four different analytical protocols, depending on the accuracy and reproducibility of the uranium measurements. Under the ‘absolute’ dating approach, the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation, assuming that both of these measured quantities have been determined accurately, and that the fission decay constant and equivalent isotopic track length are known. The ‘zeta calibration’ approach avoids making these assumptions by normalisation to a reference material of known age. Uranium zoning has a detrimental effect on the accuracy of LAFT ages. This effect can be removed by counting only those fission tracks located within the laser ablation pit. Alternatively, the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains, using a (lognormal) distributional assumption for the uranium concentration. LAFT dating is arguably less well suited than the EDM to young and U-poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning. Such samples occasionally contain no fission tracks at all, resulting in infinite analytical uncertainties. This paper introduces a pragmatic solution to this problem, in which the uranium measurement are converted to ‘virtual’ induced fission track counts to produce strictly positive ages with finite standard errors. With the age equation and zero-track strategy in place, LAFT ages can be subjected to more sophisticated statistical analysis. Using a logarithmic transformation, these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures. The methods introduced in this paper have been implemented in a software package called IsoplotR that is available free of charge at

      PubDate: 2017-05-07T00:55:40Z
  • Subduction channel fluid–rock interaction and mass transfer: Constraints
           from a retrograde vein in blueschist (SW Tianshan, China)
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Ji-Lei Li, Timm John, Jun Gao, Reiner Klemd, Xin-Shui Wang
      One widely neglected part of the subduction zone fluid cycle is the retrograde fluid flow along the plate interface. This fluid flow may facilitate the exhumation of slab-derived rocks within the subduction channel. However, our understanding on the nature and behavior of these retrograde fluids is still insufficient. Retrograde veins preserved in HP–UHP rocks can provide valuable information about fluid activities and mass transfer at the plate interface and exhumation processes. In this study, we present a petrological and geochemical investigation of a typical retrograde vein and its blueschist host rock, which occurs throughout the whole Akeyazi (U)HP terrane in the Chinese southwestern Tianshan. The magnetite-bearing albite–calcite vein, representing an external fluid pathway, crosscuts the host blueschist that mainly consists of glaucophane, garnet, epidote and titanite. Along the fluid conduit the immediate wall-rock experienced greenschist-facies overprinting as displayed by an albite-rich selvage that grades into a chlorite–biotite selvage towards the host rock. The replacements of garnet by chlorite and glaucophane by albite are thought to have occurred due to an interface-coupled dissolution-precipitation mechanism. Mass balance calculations show that during fluid–rock interaction large amounts of LILE (K–Rb–Ba–Cs and Pb), Cu and CO2 were added to the selvages (mass gain >200%), whereas the altered wall-rock released significant amounts of Ca, Li, Sr, REE and transition metal elements (V–Mn–Co–Ni–Zn) (mass loss mainly between 40 and 100%) due to the dissolution of glaucophane, garnet and epidote. The chlorite–biotite selvage experienced additional SiO2 and Na2O mass losses as well as FeO, MgO and H2O mass gains, whereas the albite-rich selvage underwent exactly the opposite mass transfer with respect to these components. The Al, Ti, Nb, Ta and Cr contents were constant during the alteration. Therefore, the retrograde fluid causing greenschist-facies overprinting of the blueschist host in the subduction channel (at ca. 30km depth) is proposed to be oxidized and enriched in LILE, Cu and CO2 components, whereas Ca, Li, Sr, REE, Y and transition metal elements (V–Mn–Co–Ni–Zn) are thought to have been mobilized during fluid-rock interaction. In addition, the main fluid sources (sediments, oceanic crust or serpentinites) diversely contribute to retrograde fluids at different depths in the subduction channel.

      PubDate: 2017-05-07T00:55:40Z
  • Trace elements in anatectic products at the roof of mid-ocean ridge magma
           chambers: An experimental study
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Martin Erdmann, Lydéric France, Lennart A. Fischer, Etienne Deloule, Jürgen Koepke
      At fast-spreading mid-ocean ridges (MORs), the horizon between the axial melt lens (AML) and the overlying sheeted dikes is characterized by extensive anatectic processes. The heat flux of the AML in combination with hydrothermal fluids from above causes high-grade contact metamorphism, which may result in anatexis of the roof rocks above the AML. The products of this process are silica-rich anatectic melts that have the potential to contaminate MOR basalts and residual hornfels. Here, we simulate the complex igneous and metamorphic processes occurring at the AML roof by hydrous partial melting experiments and provide corresponding trace element partition coefficients between melt and residues, which are useful to quantify those processes. We present trace element patterns from experimental anatectic felsic melts and the related residue produced by hydrous partial melting of various types of AML roof rocks. The starting materials used are sheeted dikes and hornfelses from Hole 1256D drilled by the Integrated Ocean Drilling Program. Results are compared with directly-related natural lithologies (i.e., felsic veins and granoblastic hornfels) from the same site. The trace element contents generally overlap with natural examples and experimental melts produced at low water activity (aH2O<0.5) can be highly enriched in trace elements despite relatively low SiO2 contents (58.9 to 65.7wt%). A low aH2O is required to reproduce the low Al2O3 contents observed in natural silica-rich rocks. However, low aH2O implies that the presence of residual amphibole is not required for anatectic processes Even though residual amphibole is often used as an important phase for explaining trace element characteristics in relevant felsic rocks formed at MORs when modeling anatexis. Because amphibole is lacking in any experimental residue, which is in agreement with natural hornfelses from the dike/gabbro transition at Site 1256, we assume that partial melting within the AML roof rocks proceeds without the participation of amphibole as residual phase. We present a comprehensive set of trace element compositions as well as bulk and mineral/melt trace element partition coefficients obtained from our amphibole-free experimental results for different potential protoliths over a large range of temperature and at different aH2Os.

      PubDate: 2017-05-07T00:55:40Z
  • Coupled control of land uses and aquatic biological processes on the
           diurnal hydrochemical variations in the five ponds at the Shawan Karst
           Test Site, China: Implications for the carbonate weathering-related carbon
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Bo Chen, Rui Yang, Zaihua Liu, Hailong Sun, Hao Yan, Qingrui Zeng, Sibo Zeng, Cheng Zeng, Min Zhao
      High-resolution hydrochemical data from five spring-fed ponds are presented to demonstrate the effect of different land uses and aquatic biological processes on the carbon cycle at a karst-analog test site. The results show that hydrochemical parameters including pH and the concentrations of HCO3 −, Ca2+, NO3 −, partial pressures of CO2 (pCO2) and dissolved O2 (DO) as well as carbon isotopic compositions (δ13C) of HCO3 – in the pond water all displayed distinct diurnal variations, while those of the spring water itself were rather stable. The coupled dynamic behaviors of pCO2, DO and NO3 − indicate a significant influence from the metabolism of submerged plants in the ponds. In the afternoon, when photosynthesis is the strongest, the pCO2 of the five pond waters was lower even than that of the ambient atmosphere, demonstrating the existence of a “biological carbon pumping (BCP) effect”, similar to that in the oceans. It was determined that, in October (autumn), the BCP fluxes in the five spring-fed ponds were 156±51tCkm−2 a−1 in P1 (Pond 1 – adjoining a bare rock shore), 239±83tCkm−2 a−1 in P2 (adjoining uncultivated soil), 414±139tCkm−2 a−1 in P3 (adjoining land cultivated with corn), 493±165tCkm−2 a−1 in P4 (adjoining grassland) and 399±124tCkm−2 a−1 of P5 (adjoining brushland), indicating the potentially significant role of aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink. In addition, by comparing the DIC concentrations and fluxes of DIC transformed into autochthonous organic matter (AOC) in the five ponds, the so-called “DIC fertilization effect” was found in which more AOC is produced in pond waters with higher concentrations of DIC. This implies that the carbon cycle driven by aquatic biological processes can be regulated by changing land use and cover, the latter determining the DIC concentrations. Further, the rock weathering-related carbon sink is underestimated if one only considers the DIC component in surface waters instead of both DIC and AOC.

      PubDate: 2017-05-07T00:55:40Z
  • Impact of low denudation rates on soil chemical weathering intensity: A
           multiproxy approach
    • Abstract: Publication date: 5 May 2017
      Source:Chemical Geology, Volume 456
      Author(s): Yolanda Ameijeiras-Mariño, Sophie Opfergelt, Jérôme Schoonejans, Veerle Vanacker, Philippe Sonnet, Jeroen de Jong, Pierre Delmelle
      Quantifying the influence of denudation, i.e., physical erosion and chemical weathering, on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles. The relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings. Here, in the Betic Cordillera (southern Spain), we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil: the Total Reserve in Bases (TRB), the content in Fe-oxides, the quartz and clay content, the soil cation exchange capacity (CEC), and the silicon (Si) isotope composition of the clay-sized fraction. Our multiproxy approach demonstrates that in this semi-arid environment at low denudation rates, an increasing denudation rate decreases the soil weathering intensity, whereas Si mobility remains limited. Our results converge with previous conclusions based on chemical mass balance methods in the same geological setting. Mass balance methods, and particularly Chemical Depletion Fractions (CDF), are based on the immobility of a refractory element (commonly zirconium, Zr) relative to major cations in soils. Interestingly, our study suggests that a weathering index such as the TRB may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential Zr mobility in the soil or by heterogeneity of Zr concentrations in the bedrock.

      PubDate: 2017-05-07T00:55:40Z
  • Resolving the conundrum in origin of the Manipur Ophiolite Complex,
           Indo-Myanmar range: Constraints from Nd isotopic ratios and elemental
           concentrations in serpentinized peridotite
    • Abstract: Publication date: Available online 26 April 2017
      Source:Chemical Geology
      Author(s): Oinam Kingson, Rajneesh Bhutani, J.K. Dash, Sibin Sebastian, S. Balakrishnan
      Major, trace and rare earth element (REE) abundances along with Nd isotopic ratios are measured in serpentinized peridotite samples from the Manipur Ophiolite Complex (MOC), Indo-Myanmar range (IMR). An attempt is made to resolve the conundrum caused by apparently contradictory petrogenetic models and tectonic settings reported in literature. Nd isotope ratios, reported for the first time, range from −7.6 to +10.4 in epsilon units in the whole-rock samples of serpentinized harzburgite and lherzolite. Variation of La/Yb with εNdt is consistent with the progressive addition of a subduction derived fluid to a depleted mantle source. Literature data from the mafic extrusive, intrusive and ultramafic rocks together with new data generated during this study can be explained by a two stage petrogenetic model. Non-modal dynamic melting in the mantle-wedge subsequent to the mixing of fluids derived from the subducted slab explains the observed range of elemental abundances in the MOC mafic and ultramafic rocks. This model also explains the observed variability of Nb and Th in the mafic rocks without invoking different sources for the different types of rocks. Taking into account field, petrographic and geochemical evidences, we propose, that MOC comprises a buoyant fore-arc-mantle-wedge system along with the subducted slab that was obducted during the terminal stage of subduction of the Neotethys below the Burmese plate.

      PubDate: 2017-05-01T00:48:15Z
  • U-Pb geochronology of grossular-andradite garnet
    • Abstract: Publication date: Available online 26 April 2017
      Source:Chemical Geology
      Author(s): S. Seman, D.F. Stockli, N.M. McLean
      This study presents a new laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) based U-Pb geochronometric method for dating grossular-andradite (grandite) garnet. Grandite is a primary skarn mineral, therefore dating its growth directly dates hydrothermal activity. As zircon U-Pb geochronometry provides a high-resolution record of magmatic processes, grandite U-Pb dating has the potential to provide a complementary record for hydrothermal systems. This study characterizes four garnets of variable grossular-andradite content and age as potential reference materials for LA-ICPMS U-Pb geochronology: Willsboro Andradite (~1020Ma, Adirondacks, USA), Red and Yellow Mali Grandite (~200Ma, Southern Mali) and Lake Jaco Grossular (~35Ma, Coahuila, Mexico). Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb analyses of Willsboro andradite yield a weighted mean 206Pb/238U date of 1022±16Ma. We use Willsboro Andradite as a primary reference material for LA-ICPMS U-Pb characterization of the two other garnets. The non-radiogenic Pb (Pbc) concentration in grandite is variable between specimens. Willsboro Andradite and Mali Grandites contain almost purely radiogenic Pb (Pb*), however Lake Jaco Grossular has a higher Pbc concentration comparatively and yields discordant U/Pb ratios. For specimens with highly variable Pb*/Pbc, linear regression is employed to derive a lower-intercept age. LA-ICPMS U-Pb dates for Yellow Mali Grandite (202±2Ma; 206Pb/238U weighted mean) and Lake Jaco Grossular (35±2Ma; lower intercept) agree with independent ID-TIMS U-Pb (202.0±1.2Ma) and (U-Th)/He dates (35±5Ma) derived for each specimen. The precision of LA-ICPMS U-Pb dates varies between 1–10% (2σ) for the analyzed garnets. Considering the short estimated lifespans of pluton-related hydrothermal systems (tens of thousands to a few million years), Neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels. As a performance test for the method, we present new U-Pb andradite data from a Late Miocene skarn system on Serifos Island, Greece. This garnet yields a lower-intercept age of 9.15±0.36Ma, in agreement with biotite Rb-Sr and zircon U-Pb age data from the causative pluton.

      PubDate: 2017-05-01T00:48:15Z
  • Ligand exchange adsorption and coordination structure of Pd on δ-MnO2
           in NaCl solution
    • Abstract: Publication date: Available online 25 April 2017
      Source:Chemical Geology
      Author(s): Kazuya Tanaka, Masato Tanaka, Naoko Watanabe, Kohei Tokunaga, Yoshio Takahashi
      Palladium is highly accumulated in ferromanganese (Fe-Mn) nodules and crusts relative to its concentration in seawater but the mechanism by which Pd(II) is incorporated remains poorly understood, probably because the Pd concentration in Fe-Mn nodules and crusts is fairly low. In this study, we investigated the local coordination structure of Pd(II) adsorbed on δ-MnO2, using X-ray absorption fine structure spectroscopy to clarify the mechanism by which Pd is incorporated into Fe-Mn nodules and crusts. Adsorption experiments of Pd(II) on δ-MnO2 were conducted in 0.1 and 0.5M NaCl solutions, in which the negatively charged chloride complexes PdCl4 2−, PdCl3 −, and PdCl3OH2− were predominant species. The distribution coefficient of Pd between δ-MnO2 and solution, Kd(Pd)=[Pd]solid /[Pd]solution (L/kg), increased from ~2×103 to ~4×106 with increasing pH. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicated that Pd was adsorbed on δ-MnO2 through ligand exchange, from Cl coordination to O coordination. Furthermore, curve fitting of EXAFS spectra demonstrated the formation of two different inner-sphere complexes, bidentate-mononuclear and bidentate-binuclear complexes, and this finding was supported by density functional theory calculations. The formation of inner-sphere complexes is reasonable given the relatively large Kd(Pd) values obtained from adsorption experiments and the large apparent Kd(Pd) values calculated from reported data on Fe-Mn nodules and crusts and seawater.

      PubDate: 2017-05-01T00:48:15Z
  • On the effect of aqueous barium on magnesite growth – A new route for
           the precipitation of the ordered anhydrous Mg-bearing double carbonate
    • Abstract: Publication date: Available online 24 April 2017
      Source:Chemical Geology
      Author(s): Michael Lindner, Giuseppe D. Saldi, Guntram Jordan, Jacques Schott
      The easiness of norsethite [BaMg(CO3)2] precipitation at ambient conditions is in vast contrast to the growth problems of other anhydrous Mg containing carbonate minerals such as magnesite [MgCO3] and dolomite [CaMg(CO3)2]. The striking structural similarity of dolomite and norsethite makes the different growth behavior even more noteworthy. In order to investigate this remarkable discrepancy, this study focuses on the fate and behavior of aqueous barium during growth of magnesite as magnesium endmember in the BaCO3-MgCO3 system. Knowledge of barium partitioning between carbonate minerals and solution, moreover, has important implications for the assessment of the mobility of toxic elements such as radium. Growth experiments have been conducted on magnesite seeds in hydrothermal mixed-flow reactors (T=100°C, pH~7.8, 0–100μM Ba2+, supersaturations Ω with respect to magnesite: ~100–200) and by hydrothermal atomic force microscopy (T=100°C, pH~8.2, 0–50μM Ba2+, Ω~60–90). The experiments showed that aqueous barium leaves magnesite growth rates unaffected but leads to norsethite precipitation. At the conditions of the experiments, norsethite growth rates were found to be controlled by the aqueous Ba2+ concentration. Given enough Ba2+, Mg2+ withdrawal from solution by norsethite clearly exceeded the withdrawal by magnesite growth. The logarithm of the solubility product of norsethite was found to be lower than −18.2 at 100°C. Microscopic investigations of the growth on the (104) surface of magnesite did not reveal any signs of Ba2+ incorporation yielding a partitioning coefficient of barium between magnesite and solution in the range of 10−2 or smaller. Preferential formation of the ordered phase norsethite over a solid solution is presumably facilitated by the large difference in Mg2+ and Ba2+ ionic radii. Due to the likely very high free energy of formation of the solid solution, ordering into distinct Ba- and Mg-layers is the only way to combine both cations within one phase. The lack of a solid solution, therefore, is one important prerequisite for the direct and unimpeded growth of an ordered double carbonate from additive free solution at ambient conditions. This behavior is in great contrast to the CaCO3-MgCO3 system where solid solution occurrence is common and growth of the ordered double carbonate (i.e. dolomite) is largely inhibited below 120°C.
      Graphical abstract image

      PubDate: 2017-05-01T00:48:15Z
  • Effects of natural organic matter on the binding of arsenate and copper
           onto goethite
    • Abstract: Publication date: Available online 22 April 2017
      Source:Chemical Geology
      Author(s): Alba Otero-Fariña, Sarah Fiol, Florencio Arce, Juan Antelo
      Goethite is a common iron oxide present in the environment. It is used as a model system to represent the mineral fraction of the soil, whereas humic acid is used as a model system to represent natural organic matter (NOM). In this study, we tried to understand the behaviour of anionic and cationic pollutants in a model system, which could easily represent the organic-mineral interaction in the environment. Specifically, the retention of arsenate and copper over different organo-mineral systems was studied. The obtained results show an increase in the adsorption of copper as the pH and the C content in the organo-mineral composites was raised, whereas in the case of the arsenate, the opposite behaviour was observed. The adsorption behaviour was compared with that of the end-members goethite and humic acid. Furthermore, the NOM-CD model (natural organic matter-charge distribution model) was used to describe the reactivity of the pollutants in the solid/solution interphase and to predict the surface and aqueous speciation. To simulate the behaviour of the organo-mineral composites, a surface component was included to describe the chemical and electrostatic interactions of the NOM with the goethite in the modelling of the solid/solution interphase. This approximation allows the quantification of the effective organic matter, which can be used to describe the copper and arsenate reactivity in organo-mineral systems.

      PubDate: 2017-04-24T13:38:39Z
  • The boron isotopic evolution of the Little Three pegmatites, Ramona, CA
    • Abstract: Publication date: Available online 20 April 2017
      Source:Chemical Geology
      Author(s): James L. Maner, David London
      Tourmaline, mica, hambergite, danburite, and axinite from three pegmatite dikes (named Main, Swamp, and Spessartine) at the Little Three mine near Ramona, California, have been analyzed by secondary ion mass spectrometry (SIMS) for their boron isotopic compositions (δ11B). Tourmaline is the sole host of B in the massive footwall and hanging wall sections of the dikes, and the principal sink for B in the miarolitic cavities. In the thinner Spessartine and Swamp dikes, the pocket mineralogy includes foitite, spessartine, muscovite, axinite, danburite, and rare beryl. The thicker Main dike culminates in cavities containing topaz, elbaite, lepidolite, boromuscovite, hambergite, and rare stibiotantalite. Values of δ11BTur from the Little Three pegmatites are strikingly heavy (−0.1‰ to +13.8‰) compared to other pegmatitic tourmaline (~−27 to +9‰), which is best explained by inheritance from an 11B-enriched source material such as altered oceanic crust. Flat patterns of δ11BTur from margin-to-core of each pegmatite attest to a negligible fractionation of boron isotopes between tourmaline-granitic melt and granitic melt-aqueous fluid. Tourmaline samples from intermediate zones and miarolitic cavities have identical boron isotopic compositions, except for miarolitic cavities that contain axinite and danburite, whose B is in 4-fold structural coordination. In these cavities, the δ11B of tourmaline increases relative to tourmaline from the intermediate zone. Interpretations of δ11BTur from margin-to-core in these and other pegmatites are hampered by uncertainties in the boron isotopic fractionation factors for melt-aqueous fluid and for tourmaline-granitic melt. Based on modeled cooling and crystallization of these dikes, the growth of tourmaline should have been fast relative to the diffusivity of B through granitic melt. Hence, an equilibrium distribution of 11B and 10B between tourmaline, bulk melt, and an aqueous solution (i.e. Rayleigh fractionation process) is unlikely. The negligible isotopic fractionation of boron in tourmaline from margins to center, therefore, probably records only the bulk isotopic composition of the pegmatite-forming melts.

      PubDate: 2017-04-24T13:38:39Z
  • Molybdenum isotope fractionation and speciation in a euxinic
           lake—Testing ways to discern isotope fractionation processes in a
           sulfidic setting
    • Abstract: Publication date: Available online 18 April 2017
      Source:Chemical Geology
      Author(s): Tais W. Dahl, Stefanie B. Wirth
      The molybdenum (Mo) isotope composition in euxinic shales has been used as a proxy for the global distribution of anoxic conditions in ancient oceans, and since more recently also as a proxy for sulfide concentrations in depositional environments. However, there is currently no way to distinguish isotope fractionation at low bottom water sulfide concentrations in ‘local’ basins from ‘global’ secular isotope variations associated with changing seawater composition. This uncertainty is challenging the use of Mo isotopes for paleoceanographic reconstructions. To explore this further, we present new data from sediments deposited over the past ~9800years in one of the best studied euxinic localities in the world: Lake Cadagno in Switzerland. The sample set allows us to test ways to discern isotope fractionation processes at play in a highly restricted euxinic basin. Most of our drill core samples (n=18) show high δ98Mo values similar to previously studied shallow sediments, indicative of quantitative Mo removal from the water column (Dahl et al. 2010a). However, a few samples (n=3) deposited between about 1200 and 3400years ago carry low δ98Mo values and have been isotopically fractionated in the lake. Sedimentological and geochemical characterizations show that these δ98Mo-fractionated sediments formed during times of frequent injection of O2- and sediment-rich river water into the deep sulfidic water column. A positive correlation between δ98Mo and sedimentary Mo contents suggests that isotope fractionation occurred during times of non-quantitative Mo removal, although Mn-oxide cycling at the chemocline might also contribute a subordinate proportion of (98Mo-depleted) molybdenum into the sulfidic zone. Sedimentary Mo/U enrichments relative to oxic lake water further supports the hypothesis that a particulate Mo shuttle was most efficient during times of quantitative Mo removal. Therefore, periods with inefficient Mo capture are ascribed to incomplete conversion of molybdate to particle reactive Mo species when bottom water H2S levels were low or less stable than today. Using XAFS spectroscopy, we found that the two distinct Mo compounds predominating in the sediments (MoIV-S and MoVI-OS) are not diagnostic for isotope fractionation that has occurred in Lake Cadagno. Instead, we infer that δ98Mo-fractionated products (forming via a low-sulfide Mo pathway) can be subsequently altered with little or no isotopic imprint during remobilization and re-precipitation (e.g., at higher sulfide levels in the sediments) as well as during post-depositional oxidation. Future work could investigate local δ98Mo-fractionation processes expressed in other euxinic settings and explore other sedimentary metrics to constrain the steps involved in the euxinic burial pathway(s). One tantalizing prospect of this is to distinguish between local bottomwater sulfide levels and variations in the fraction of global seafloor anoxia from the Mo isotope composition in ancient euxinic mudrocks.

      PubDate: 2017-04-24T13:38:39Z
  • A new approach for electron microprobe zircon fission track
    • Abstract: Publication date: Available online 18 April 2017
      Source:Chemical Geology
      Author(s): A.N.C. Dias, F. Chemale, C.J. Soares, S. Guedes
      Fission track thermochronology (FTT) has been applied for decades to quantify rates and timing of processes in the shallow crust. The most widely used approach is the external detector method (EDM). In this conventional approach, an age is obtained by counting both the fossil (238U spontaneous fission) and the induced (235U induced fission) tracks using an optical microscope. The induced tracks are obtained through the irradiation of the sample with thermal neutrons in the nuclear reactor, which causes fission of 235U. Based on the studies carried out by Gombosi et al. (2014), we present an alternative method of dating zircons using electron probe microprobe analysis (EPMA) to measure uranium concentration [U]. The electron microprobe analysis fission track (EPMA-FT) method was applied to three samples of rapidly cooled zircons: the Fish Canyon Tuff, Poços de Caldas (syenite) and Serra Geral zircons. The analyses were performed using two approaches: 1) using the age equation described in Gombosi et al. (2014) and 2) using a new age equation calibration developed for this work. The results using the Gombosi et al. (2014) age equation were 26.7±4.1Ma, 80.6±12.8Ma and 130.9±20.1Ma, respectively, and the results using the age equation from this work were 27.8±1.9Ma, 83.8±7.7Ma and 136±12Ma, respectively. The uncertainty of the age is affected mainly by 238U concentration and ρ S (the spontaneous fission track density) determinations. Other factors can affect the uncertainty of the age, but their contributions are smaller. For all samples, the yield ages found by the two methods are consistent and overlap within two standard deviations of published reference ages determined from other radiometric techniques (i.e., K/Ar, 40Ar/39Ar, and/or U/Pb) and traditional FTT by the EDM.

      PubDate: 2017-04-24T13:38:39Z
  • Origin of calcium in pedogenic carbonate nodules from silicate watersheds
           in the Far North Region of Cameroon: Respective contribution of in situ
           weathering source and dust input
    • Abstract: Publication date: Available online 18 April 2017
      Source:Chemical Geology
      Author(s): Fabienne Dietrich, Nathalie Diaz, Pierre Deschamps, Benjamin Ngounou Ngatcha, David Sebag, Eric P. Verrecchia
      Significant amounts of pedogenic Ca-carbonate nodules have been observed in the Far North Region of Cameroon in the carbonate-free watershed of the Mayo Tsanaga, thus a priori not favourable for carbonate nodules accumulations. These nodules are associated with a Clay-Rich Parent Material (CRPM), covering either a granitic (upstream) or a greenstone bedrock (downstream). In this peculiar context, the amount of pedogenic carbonate nodules represents large quantities of Ca and C. Therefore, determining the Ca sources for pedogenic carbonate nodules is a key point regarding the mechanisms leading to carbonate nodule precipitation and their role in the biogeochemical cycle of Ca. Three sites, two on granite and one on greenstone, were studied by combining Sr et Nd isotope systematics in order to assess the Ca sources of carbonate nodules and the origin of the CRPM associated with nodules. Carbonate nodules have a distinct Sr isotopic composition in each profile, pointing to the contribution of a local source for Ca. Sr isotopic compositions of plagioclases and carbonate nodules display a concomitant variation in each profile, indicating that plagioclases acted as a Sr, and thus Ca, source for the carbonate nodules. Nevertheless, carbonate nodules have a higher Sr isotopic signature than plagioclases, implying the contribution of another more radiogenic Sr source. Sr and Nd isotope data from the CRPM show that it is a mixture of weathered bedrock compounds and Saharan dust. This result highlights two other potential Sr and Ca sources: biotite and Saharan dust. Calculations of their respective contributions demonstrate that in situ weathering significantly adds to the Ca source of nodules. This result contrasts with many other studies from similar settings that highlight the importance of allochthonous - marine, aeolian - inputs. This conclusion points out that a substantial part of Ca is transferred from the local bedrock to the carbonate nodules. This mechanism seems to have been possible as peculiar edaphic conditions preserved the primary Ca sources, making in situ weathering a significant process for providing Ca.

      PubDate: 2017-04-24T13:38:39Z
  • In situ Nanoscale Observations of Gypsum Dissolution by Digital
           Holographic Microscopy
    • Abstract: Publication date: Available online 17 April 2017
      Source:Chemical Geology
      Author(s): Pan Feng, Alexander S. Brand, Lei Chen, Jeffrey W. Bullard
      Recent topography measurements of gypsum dissolution have not reported the absolute dissolution rates, but instead focus on the rates of formation and growth of etch pits. In this study, the in situ absolute retreat rates of gypsum (010) cleavage surfaces at etch pits, at cleavage steps, and at apparently defect-free portions of the surface are measured in flowing water by reflection digital holographic microscopy. Observations made on randomly sampled fields of view on seven different cleavage surfaces reveal a range of local dissolution rates, the local rate being determined by the topographical features at which material is removed. Four characteristic types of topographical activity are observed: 1)smooth regions, free of etch pits or other noticeable defects, where dissolution rates are relatively low; 2)shallow, wide etch pits bounded by faceted walls which grow gradually at rates somewhat greater than in smooth regions; 3)narrow, deep etch pits which form and grow throughout the observation period at rates that exceed those at the shallow etch pits; and 4)relatively few, submicrometer cleavage steps which move in a wave-like manner and yield local dissolution fluxes that are about five times greater than at etch pits. Molar dissolution rates at all topographical features except submicrometer steps can be aggregated into a continuous, mildly bimodal distribution with a mean of 3.0 μmol m −2 s −1 and a standard deviation of 0.7 μmol m −2 s −1.

      PubDate: 2017-04-24T13:38:39Z
  • Microbe-clay interactions as a mechanism for the preservation of organic
           matter and trace metal biosignatures in black shales
    • Abstract: Publication date: Available online 17 April 2017
      Source:Chemical Geology
      Author(s): Tiffany Playter, Kurt Konhauser, George Owttrim, Cheryl Hodgson, Tyler Warchola, Aleksandra M. Mloszewska, Bruce Sutherland, Andrey Bekker, J.-P. Zonneveld, S. George Pemberton, Murray Gingras
      Organic-rich, fine-grained sedimentary rocks, such as black shales, are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3billionyears. While biological productivity and sedimentation rates greatly affect the organic matter content in these rocks, mechanisms linking these two processes remain poorly resolved. Here, we examine the interactions of clay minerals with the marine planktonic cyanobacterium Synechococcus sp. PCC 7002. We suggest that clays settling through the water column could influence carbon and trace metal burial in three ways: (1) the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude; (2) reactive bacterial cells become completely encased within a clay shroud, enhancing the preservation potential of this organic matter; and (3) the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments. Significantly, our findings imply that the chemical composition of ancient, organic-rich, fine-grained deposits are not only archives of ancient seawater composition and redox state, but they also provide a record of the degree of biological activity in the water column through geological time.

      PubDate: 2017-04-24T13:38:39Z
  • Characterization and differentiation of rock varnish types from different
           environments by microanalytical techniques
    • Abstract: Publication date: Available online 13 April 2017
      Source:Chemical Geology
      Author(s): D.S. Macholdt, K.P. Jochum, C. Pöhlker, A. Arangio, J.-D. Foerster, B. Stoll, U. Weis, B. Weber, M. Müller, M. Kappl, M. Shiraiwa, A.L.D. Kilcoyne, M. Weigand, D. Scholz, G.H. Haug, A. Al-Amri, M.O. Andreae
      We investigated rock varnishes collected from several locations and environments worldwide by a broad range of microanalytical techniques. These techniques were selected to address the challenges posed by the chemical and structural complexity within the micrometer- to nanometer-sized structures in these geological materials. Femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS), scanning transmission X-ray microscopy-near edge X-ray adsorption fine structure spectroscopy (STXM-NEXAFS) in combination with scanning electron microscopy (SEM) of focused ion beam (FIB) ultra-thin (100–200nm) sections, conventional and polarization microscopy, as well as electron paramagnetic resonance (EPR) measurements were used to obtain information about these rock varnishes. Rock varnishes from different environments, which cannot readily be distinguished based on their macroscopic appearance, differ significantly in their constituent elemental mass fractions, e.g., of Mn, Fe, Ni, Co, Ba, and Pb, and their rare earth element (REE) patterns. Structural characteristics such as the particle sizes of embedded dust grains, internal structures such as layers of Mn-, Fe-, and Ca -rich material, and structures such as cavities varied between varnishes from different environments and regions in the world. The EPR spectra were consistent with aged biogenic Mn oxides in all samples, but showed subtle differences between samples of different origin. Our observations allow us to separate rock varnishes into different types, with differences that might be indicators of distinct geneses. Five different types of rock varnish could be distinguished, Type I–V, of which only Type I might be used as potential paleoclimate archive. Each varnish type has specific characteristics in terms of their elemental composition, element distribution, and structures. The combination of element ratios (Mn/Ba, Al/Ni, Mn/REY, Mn/Ce, Mn/Pb, LaN/YbN, and Ce/Ce*), total REE contents, and structures can be used to separate the different types of rock varnish from each other.

      PubDate: 2017-04-17T13:33:52Z
  • Thermodynamic properties in the Fe(II)-Fe(III)-As(V)-HClO4–H2O and
           Fe(II)-Fe(III)-As(V)-HCl–H2O systems from 5 to 90°C
    • Abstract: Publication date: Available online 13 April 2017
      Source:Chemical Geology
      Author(s): Xiangyu Zhu, D. Kirk Nordstrom, R. Blaine McCleskey, Rucheng Wang, Xiancai Lu
      Fe-As mineral solubility and associated aqueous species have been intensively studied because of the environmental need to immobilize arsenic. The thermodynamic data for aqueous iron-arsenic species are inadequately characterized, however. The Gibbs free energy, enthalpy, entropy, and heat capacity and activity coefficients were refined in the Fe(II)-Fe(III)-As(V)-HClO4-H2O and Fe(II)-Fe(III)-As(V)-HCl-H2O systems using redox potential measurements from 5 to 90°C. The association constants for FeHAsO4 + and FeH2AsO4 2+ at 25 °C were 1010.28 and 104.02 and the corresponding association reaction enthalpies and heat capacities were 25.74 and 8.73kJmol−1 and 843.1 and −529.6J − 1mol−1, respectively. Activity coefficients for H+, ClO4 −, Fe2+, Fe3+, HAsO4 2−, and H2AsO4 − at 25°C in the form of the Hückel equation were derived for ionic strengths up to 1mol−1 kg−1. Newly derived activity coefficients and thermodynamic data were incorporated into PHREEQCI to calculate the Eh of laboratory solutions. The differences between calculated and measured Eh were all within 10 mV and relative differences were all lower than 1.5%.

      PubDate: 2017-04-17T13:33:52Z
  • Spectroscopic and DFT study on the species and local structure of arsenate
           incorporated in gypsum lattice
    • Abstract: Publication date: Available online 12 April 2017
      Source:Chemical Geology
      Author(s): Shaofeng Wang, Danni Zhang, Xu Ma, Guoqing Zhang, Yongfeng Jia
      The incorporation of arsenate (As(V)) in the gypsum structure is an important process for arsenic (As) fixation during industrial effluent treatment and may influence the mobility and bioavailability of As in surface environment. However, spectroscopic evidence is still lacking for its species and local structure. The species and local structure of As(V) in a gypsum lattice were investigated using Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) modelling, and full-potential multiple scattering (FPMS) simulations. Ascorbic acid-treated As(V)-gypsum co-precipitates were used to avoid the influence of amorphous calcium arsenate on the characterization. The lack of the FTIR band in the range of 750–860cm−1 was an indicative that no AsO4 3− species was incorporated into the gypsum structure. DFT calculations proved that the incorporation of AsO4 3− was energetically much harder than HAsO4 2− species. The FPMS structural refinement yielded the optimal AsO interatomic distances of 1.77, 1.67, 1.65, and 1.66Å, with an average of 1.69±0.057Å, in agreement with the DFT and EXAFS results. Our work conclusively showed that HAsO4 2− dominated as the species of As(V) incorporated into the gypsum lattice, with the H atom in the HAsO4 2− group adjacent to water layer, regardless of pH.

      PubDate: 2017-04-17T13:33:52Z
  • Revisiting the hydrothermal geochemistry of europium(II/III) in light of
           new in-situ XAS spectroscopy results
    • Abstract: Publication date: Available online 7 April 2017
      Source:Chemical Geology
      Author(s): Weihua Liu, Barbara Etschmann, Artas Migdisov, Hakim Boukhalfa, Denis Testemale, Harald Müller, Jean-Louis Hazemann, Joël Brugger
      Knowledge of the mobility of Rare Earth Elements (REE) in crustal fluids is important for understanding both the formation of REE deposits and their use as geochemical tracers. This is particularly true for europium (Eu), which exists in both Eu(II) and Eu(III) states in natural fluids, making it a sensitive probe of hydrothermal redox processes. Currently, our understanding of Eu complexation in hydrothermal fluids relies mainly on extrapolations from room-temperature data, as there are no high-temperature experimental data available for Eu(II) species. In this study we conducted in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to investigate the complexing of Eu(II) and Eu(III) in chloride- and bromide-rich acidic solutions and Eu(III) sulphate complexes, at 35–400°C, 600bar. For Eu(III) complexes, the total number of ligands decreased and the ratio of H2O:Cl ligands in the first coordination shell decreased with increasing temperature. For the Eu(II) system, our data confirm Sverjensky's (1984, EPSL 67, 70–78) theoretical predictions that Eu(II) species become increasingly stable (relative to Eu(III)) at elevated temperatures. However the new data indicate that complexation between Eu(II) and chloride is much weaker than Haas's (1995, GCA 59, 4329–4350) theoretical estimates. The thermodynamic properties of Eu(II) chloride complexes have been reassessed in light of the new data. Thermodynamic calculations reveal that the identity of the Eu(III) aqueous complexes (e.g., chloride or hydroxide species) have a significant impact on the oxidation state of Eu in hydrothermal fluids at elevated temperatures.

      PubDate: 2017-04-10T07:36:43Z
  • Formation of Rotliegend Ca-Cl brines in the North German Basin compared to
           analogues in the geological record
    • Abstract: Publication date: Available online 6 April 2017
      Source:Chemical Geology
      Author(s): P. Möller, V. Lüders, M. De Lucia
      The formation of terrestrial Ca-Cl brines is controlled by two major processes: chemical reactions between halite brines and igneous and metamorphic rocks and/or their debris in sedimentary basins, and the abstraction of H2O by evaporation, freezing or formation of hydrous minerals. Albitization dominates in environments of low H2O activity, i.e., halite saturated systems. Chloritization, serpentinization and formation of smectites seems to be related to less saline systems, i.e., enhanced water activity. The Rotliegend brines of the North German Basin evolved by interaction of infiltrating halite brines with volcanic debris of the Permian acid volcanic rocks by albitization and chloritization of plagioclase. At present their temperatures are between 130 and 150°C at 3000–4000m depth. Although they developed in a different environment they chemically and isotopically resemble the basinal brines from the Mississippi Salt Dome basin or even the deep brines from the Red Sea. All these brines have in common that halite brines interacted with plagioclase (An+SiO2 +2Na+ ⇔2Alb+Ca2+) or possibly with clay minerals (Ca-clay+2Na+ ⇔Na2-clay+Ca2+) at enhanced temperatures. The trends of stable isotopes show slopes in the δD-δ18O diagram which are typical for hydrothermal reactions of evaporation brines with rock or sediments. The genesis of Rotliegend brines differ from Ca-Cl brines in the Canadian Shield, Siberian Platform, the Dry Valleys in Antarctica and even the local brines from the Chilean salars and the Californian Bristol Dry Lake. The Antarctic Dry Valley brines seem to be dominated by melting of ice-cements of their permafrost sections. The Ca-Cl brines of springs in the Chilean salars and the Bristol Dry Lake ascend due to thermal convection along tectonic faults. Although the Canadian Shield brines chemically resemble those of the Siberian Platform their stable isotope fractionation suggest that they developed by different processes of water abstraction. Exotic stable isotope values in brines are gained by freezing or ultrafiltration processes in the precursor sediments of the present crystalline rocks of the Canadian Shield and by small volume ratios of liquids and metamorphic rocks such as of KTB/VB.
      Graphical abstract image

      PubDate: 2017-04-10T07:36:43Z
  • Volatiles (CO2, S, F, Cl, Br) in the dike-gabbro transition zone at IODP
           Hole 1256D: Magmatic imprint versus hydrothermal influence at
           fast-spreading mid-ocean ridge
    • Abstract: Publication date: Available online 6 April 2017
      Source:Chemical Geology
      Author(s): Chao Zhang, Lian-Xun Wang, Michael A.W. Marks, Lydéric France, Juergen Koepke
      The dike-gabbro transition zone of mid-ocean ridges (MORs) is a critical site for the accretion of oceanic crusts as it is the main zone of interactions between the hydrothermal and magmatic systems. In this study, volatiles contents of CO2, S, F, Cl and Br for a variety of lithologies from the dike-gabbro transition zone of an intact oceanic crust were investigated in order to examine the magmatic imprint versus hydrothermal influence. The studied samples include plutonic rocks (gabbros, diorites and tonalites), hornfelses (i.e. granoblastic dikes), an amphibole-rich vein and albitites. These rocks were recently sampled by the IODP (Integrated Ocean Drilling Program) at Site 1256, representing parts of an oceanic crust that formed at the fast-spreading ridge of the East Pacific Rise. The bulk CO2 contents (500–10,000ppm) show no lithological dependence and are similar to those of fresh lavas. Highly variable bulk S concentrations (3–900ppm) show significant depletions compared to undegassed MORB melts, which might be caused by exsolution of magmatic fluids during crystallization. The plutonic samples have Cl concentrations of 500–1000ppm, remarkably higher than the hornfelses (200–600ppm Cl) and erupted lavas (average ~200ppm Cl). Except for the albitites that contain very low F (20–30ppm) and the hydrothermal amphibole-rich vein that contains extremely high F (~1000ppm), the other lithologies have similar F concentrations within 100–350ppm. The Br concentrations of different lithologies are largely overlapping within 0.5–3.5ppm. Petrological and mineralogical evidence indicates that bulk F concentrations are dominated by the presence of amphibole and apatite, whereas Cl and Br are mainly hosted in fluid inclusions rather than in hydrous minerals. The variation of F/Cl and Br/Cl ratios may trace the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids. Our data support the current S budget of oceanic crust, but indicate that the budgets of C and Cl may have been underestimated.

      PubDate: 2017-04-10T07:36:43Z
  • In-situ determination of the kinetics and mechanisms of nickel adsorption
           by nanocrystalline vernadite
    • Abstract: Publication date: Available online 31 March 2017
      Source:Chemical Geology
      Author(s): Sylvain Grangeon, Alejandro Fernandez-Martinez, Francis Claret, Nicolas Marty, Christophe Tournassat, Fabienne Warmont, Alexandre Gloter
      In-situ kinetics and mechanisms of Ni2+ uptake by synthetic vernadite were determined at pH5.8 and I =0.1M NaCl using wet chemistry, atomic-resolution scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and synchrotron high-energy X-ray scattering (HEXS) in both the Bragg-rod and pair distribution function formalisms. The structural formula of the initial solids was TCMn3+ 0.05Na+ 0.23(H2O)0.69H+ 0.06[(Mn4+ 0.86Mn3+ 0.04vac0.1)O2], where species under brackets form the layer having “vac” layer vacancies, and where other species are present in the interlayer, with TC standing for “triple-corner sharing” configuration. According to HEXS and STEM-EELS, adsorbed Ni2+ adopted mainly a TC configuration, and had a Debye-Waller factor about four times higher than layer Mn. Steady-state was reached after ~2.2h of contact time, and the final structural formula of the solid was TCNi2+ 0.12 TCMn3+ 0.05Na+ 0.12H2O0.36H+ 0.01[(Mn4+ 0.87vac0.13)O2]. Atomic-scale imaging of the solids also evinced the presence of minor Ni adsorbed at the crystal edge. The retention coefficient RD =103.76±0.06 Lkg−1, computed from PDF data modelling and solution chemistry results, was in agreement with those available in the literature.
      Graphical abstract image

      PubDate: 2017-04-03T13:36:29Z
  • Isotopic variation of dissolved and colloidal iron and copper in a
           carbonatic floodplain soil after experimental flooding
    • Abstract: Publication date: Available online 31 March 2017
      Source:Chemical Geology
      Author(s): Charirat Kusonwiriyawong, Moritz Bigalke, Florian Abgottspon, Marina Lazarov, Stephan Schuth, Stefan Weyer, Wolfgang Wilcke
      Many floodplain soils worldwide are contaminated by present and past industrial and mining activities. During flooding redox potential decreases, triggering the release of dissolved and colloidal metals. We used an anaerobic microcosm incubation to simulate flooding of a carbonate-rich floodplain soil for 40days. Soil solution samples were extracted to determine the release of dissolved (<0.02μm) and colloidal fractions (0.02–10μm). We analyzed stable isotope ratios of colloidal and dissolved Fe and Cu representing two groups of metals with different release behavior; release of Fe was steadily increasing, while Cu peaked sharply after flooding and decreased afterwards. The temporal trend of δ56Fe values of total Fe in solution indicated dissimilatory iron reduction. The apparent isotopic fractionation between dissolved and colloidal Fe (Δ56Fedissolved-colloidal =δ56Fedissolved −δ56Fecolloidal) varied between 0.31±0.04‰ and −1.86±0.26‰. Low δ56Fecolloidal (−1.16±0.04‰) values on day 4 of the experiment suggested colloid formation by precipitation of dissolved Fe, while the strong temporal variation in Δ56Fedissolved-colloidal indicated subsequent changes in colloid mineralogy, sorption to soil components and/or electron transfer-atom exchange. The variations in δ65Cu values (Δ65Cudissolved-colloidal from 0.81±0.03‰ to 1.58±0.09‰) are probably linked to changing oxidation state of colloidal Cu. While at the beginning of the experiment colloidal Cu and solid soil Cu exchange, these systems decouple after the onset of sulfate reduction in the second half of the experiment. The experimental results fit well to findings from redoximorphic soils described in the literature and highlight the importance of colloids for metal release and the isotopic pattern in carbonatic soils.
      Graphical abstract image

      PubDate: 2017-04-03T13:36:29Z
  • The complex rheology of megacryst-rich magmas: The case of the mugearitic
           “cicirara” lavas of Mt. Etna volcano
    • Abstract: Publication date: Available online 29 March 2017
      Source:Chemical Geology
      Author(s): Alessandro Vona, Andrea Di Piazza, Eugenio Nicotra, Claudia Romano, Marco Viccaro, Guido Giordano
      Multiphase magma rheological properties play a fundamental role on lava flow transport, emplacement and morphology. To date, however, the three-phase (melt+crystals+bubbles) rheology of natural magma remains relatively understudied. We present here a series of high-temperature experiments designed to investigate the multiphase rheology of a mugearitic megacryst-bearing lava from Mt. Etna. A peculiar textural feature of this magma is the abundance of cm-size plagioclase crystals (megacrysts) together with smaller size crystals (phenocrysts and microlites), yielding a very wide crystal size distribution. We combined different experimental techniques (rotational and compressional rheometry) to investigate the rheology of this natural lava under different degrees of partial melting at subliquidus conditions. Results indicate that natural megacryst-bearing mugearite magmas from Mt. Etna display a wide range of behaviors as a function of temperature (T =1000–1200°C) and crystal content (ϕX =0.2–0.7). In the investigated T range, the deformation mechanism of these magmas varies from mainly brittle (T <1050°C) to mainly ductile (T >1085°C). At T =1075°C, both ductile and brittle behavior have been observed. In the ductile regime, these magmas behave as non-Newtonian fluids (at least up to T =1100°C) showing marked apparent shear thinning behavior. The observed rheological behavior is due to a complex response related to a non-homogenous deformation of the natural sample (e.g. viscous and/or brittle shear localization), favored by the presence of bubbles. Consequently, the obtained flow parameters can be considered as representative of the bulk rheology of natural magmas, commonly characterized by similar non-homogeneous deformation styles. We applied the obtained data to discuss the flow and emplacement conditions of these peculiar lava flows. We demonstrated that at eruptive temperatures, the presence of a pre-eruptive crystal cargo and bubbles facilitates the achievement of critical crystal content during flow and cooling ultimately controlling lava transport and emplacement. Flow conditions can be maintained in the presence of an efficient insulation and, importantly, of deformable vesicles promoting and enhancing shear localization.

      PubDate: 2017-04-03T13:36:29Z
  • Quantification of oxygen isotope SIMS matrix effects in olivine samples:
           Correlation with sputter rate
    • Abstract: Publication date: Available online 29 March 2017
      Source:Chemical Geology
      Author(s): J. Isa, I.E. Kohl, M.-C. Liu, J.T. Wasson, E.D. Young, K.D. McKeegan
      We investigated the magnitude and reproducibility of instrumental mass-dependent fractionation of oxygen isotopes in secondary ion mass spectrometry (SIMS) analyses of olivine crystals of different major element chemistry (from Mg-rich to Fe-rich) in order to improve the accuracy of in-situ O-isotope measurements in geochemical/cosmochemical olivine samples. We found that oxygen isotope SIMS matrix effects are reproducible, and developed a model curve that can be used for correcting instrumental mass fractionation of olivine samples of intermediate chemical composition. The changes in instrumental mass fractionations were likely caused by differing Cs concentrations in the near surface regions of the samples due to different sample sputtering rates.

      PubDate: 2017-04-03T13:36:29Z
  • Towards the reconciliation of viscosity change and CO2-induced
           polymerization in silicate melts
    • Abstract: Publication date: Available online 28 March 2017
      Source:Chemical Geology
      Author(s): Yann Morizet, Michael Paris, David Sifre, Ida Di Carlo, Sandra Ory, Fabrice Gaillard
      Deep mantle melts contain massive amounts of CO2 but three critical issues related to the effect of CO2 on molecular structure and physical properties of magmatic melts remain poorly constrained: 1) there is no exact picture of CO2 dissolution mechanism in silicate melt, 2) the silicate melt polymerization upon CO2 dissolution has not been reliably quantified and 3) the effect of CO2 on silicate melt viscosity has never been measured adequately. We synthesized a series of 17O and 29Si-enriched CO2-bearing melilitite (SiO2 ~35wt%) silicate glasses at high temperature and pressure. Using NMR spectroscopy, we have interrogated both anionic and cationic networks for quantifying the change in the degree of polymerization associated to CO2 incorporation in the melt. Increasing CO2 content induces a strong increase in the degree of polymerization. CO2 dissolution follows a complex mechanism involving the formation of Free Ionic Carbonate (FIC) Ca2+·CO3 2– species. This carbonate subnetwork is the precursor to the immiscibility process between a carbonate liquid and a silicate liquid. Glass transition temperature (Tg) measurements show that increasing CO2 content induces a decrease in Tg implying a decrease in viscosity for the studied low silica melt composition. This result appears in complete contradiction with the melt polymerization induced by CO2 as quantified by NMR. We propose a model that reconciles both aspects. CO2 induces silicate subnetwork polymerization resulting in a viscosity increase but it also induces a competing effect by forming a carbonate subnetwork having a low viscosity. The overall result appears dominated by the carbonate subnetwork resulting in a slight decrease in melt viscosity in agreement with existing studies.

      PubDate: 2017-04-03T13:36:29Z
  • Geochemical systematics of Pb isotopes, fluorine, and sulfur in melt
           inclusions from São Miguel, Azores
    • Abstract: Publication date: Available online 27 March 2017
      Source:Chemical Geology
      Author(s): E.F. Rose-Koga, K.T. Koga, M. Moreira, I. Vlastelic, M.G. Jackson, M.J. Whitehouse, N. Shimizu, N. Habib
      Pb isotopic measurements in olivine-hosted melt inclusions of ocean island basalts (OIBs) from São Miguel, Azores, reflect the high 207Pb/204Pb and 208Pb/204Pb at a given 206Pb/204Pb of their host lavas. The data are consistent with mixing between two endmembers: the first (the Central Group) has relatively high 208Pb/206Pb and is similar to the component sampled by Graciosa and Faial, and the second is a radiogenic endmember (with low 208Pb/206Pb) that we refer to as the Nordeste component. F and Cl concentrations measured in the same melt inclusions from São Miguel represent parental abundances unmodified by crustal assimilation. Based on undegassed F and Cl concentrations, the source beneath São Miguel is volatile-rich. Pb-isotope composition of the melt inclusions correlates positively with S concentration: we attribute this correlation to the dissolution of oceanic crust sulfides in the S-undersaturated basaltic melt of the Nordeste component. These blebs were then assimilated by the São Miguel magma and the magma droplets were later trapped in olivines. When comparing melt inclusions from worldwide OIBs representing mantle endmembers, we find an overall negative correlation between F/Nd and 208Pb/206Pb for volcanic glasses and melt inclusions. The high F/Nd (up to 40) endmember is represented by HIMU melt inclusions and the low F/Nd (down to 14) by EM1 Pitcairn glasses. São Miguel melt inclusions have an intermediate F/Nd of 23.1±3.4.

      PubDate: 2017-04-03T13:36:29Z
  • Modeling controls on the chemical weathering of marine mudrocks from the
           Middle Jurassic in Southern Germany
    • Abstract: Publication date: Available online 22 March 2017
      Source:Chemical Geology
      Author(s): Zhongwen Bao, Christina M. Haberer, Uli Maier, Richard T. Amos, David W. Blowes, Peter Grathwohl
      Chemical weathering of sedimentary rocks is of great importance in determining seepage water chemistry, carbon, iron, calcium and sulfur turnover, as well as mineral transformation. In this study, we used the numerical code MIN3P to investigate controls on seepage water chemistry during chemical weathering of marine mudrocks. In particular, we focused on the pyrite- and kerogen-bearing formation, Opalinus Clay (with outcrops in the area of the Swabian and Franconian Alb in Southern Germany), a typical fine-grained sedimentary mudrock that had been deposited during the Middle Jurassic in a shallow marine environment. In the geochemical model we considered four reactive minerals, i.e., pyrite, kerogen, calcite and siderite (assuming silicate minerals to be stable), and ran model scenarios over a time period of 10kyrs (since the last ice age). Our numerical results show that chemical weathering of Opalinus Clay is driven by oxygen ingress (which depends on effective gas diffusion, and thus on water saturation). Due to oxidation of pyrite and kerogen seepage water acidifies, which leads to dissolution of carbonate minerals, i.e., calcite and siderite. As a consequence, porosity and groundwater alkalinity increase, and CO2 is released into the atmosphere at early decades. Following the consumption of primary reactive minerals, iron oxides precipitate in the oxic zone. We compared our model results with field data of water saturation, porosity, and water chemistry. The overall reasonable fit between model results and field data demonstrates the applicability of the numerical code MIN3P to quantify chemical weathering of pyrite-bearing sedimentary mudrocks and to predict seepage water chemistry that is impacted by geochemical water-rock interactions.
      Graphical abstract image

      PubDate: 2017-03-27T07:54:43Z
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