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  Subjects -> EARTH SCIENCES (Total: 621 journals)
    - EARTH SCIENCES (458 journals)
    - GEOLOGY (67 journals)
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EARTH SCIENCES (458 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 9)
Aeolian Research     Hybrid Journal   (Followers: 2)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
AMBIO     Hybrid Journal   (Followers: 12)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 1)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 2)
Annals of Geophysics     Full-text available via subscription   (Followers: 10)
Annals of GIS     Hybrid Journal   (Followers: 17)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene Review     Hybrid Journal   (Followers: 2)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 7)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 9)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic Science     Open Access  
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 18)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 7)
Atmospheric and Climate Sciences     Open Access   (Followers: 17)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 11)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 18)
Bulletin of Volcanology     Hybrid Journal   (Followers: 15)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 10)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 11)
Coastal Management     Hybrid Journal   (Followers: 18)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 11)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 6)
Contemporary Trends in Geoscience     Open Access   (Followers: 1)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 8)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 5)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 4)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 221)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 9)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 2)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 13)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 2)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 13)
Ecohydrology     Hybrid Journal   (Followers: 11)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 11)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)

        1 2 3 4 5 | Last

Journal Cover   Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [12 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2588 journals]
  • High-resolution line-scan analysis of resin-embedded sediments using laser
           ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)
    • Abstract: Publication date: Available online 14 March 2015
      Source:Chemical Geology
      Author(s): Rick Hennekam , Tom Jilbert , Paul R.D. Mason , Gert J. de Lange , Gert-Jan Reichart
      Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) line-scanning is a promising technique for producing high-resolution (μm-scale) geochemical records on resin-embedded sediments. However, this approach has not yet been thoroughly tested on sediment samples of known elemental compositions. Here, we address this through the analysis of resin-embedded quartz, calcite, and clay (montmorillonite) sediments spiked with Al, V, Mo, and Ba across a range of concentrations. LA-ICP-MS spot analyses on these samples were compared to data independently obtained by conventional techniques: solution nebulization-ICP-optical emission spectroscopy (SN-ICP-OES), SN-ICP-MS, and X-Ray fluorescence on fused glass beads. Data were reported as (log-)ratios of the analyte elements normalized to Ca, Si, or Al for calcite, quartz and clay respectively to correct for varying ablation yields. Our LA-ICP-MS data demonstrate close agreement to within 6% of the reference values determined by the conventional techniques, with high correlation coefficients (R2≈1.0) across the full range of concentrations, and high precision (<3 % relative standard deviations after three repeated analyses). Barium was hosted in both aluminosilicates and carbonates in the clay matrix, giving differing yields (elemental fractionation), and leading to variable accuracy (mean deviation of 15 %). A selection of the spiked sediments was used to produce artificial laminated sediments to further test the effects of line scanning on LA-ICP-MS accuracy and precision in resin-embedded samples. It appears that LA-ICP-MS line-scan analyses have good accuracy (deviations from the reference values generally <5%) and high precision (relative standard deviations of repeated analyses on the same sample generally <5%) for our target elements. Moreover, the LA-ICP-MS line-scanning closely record the alternating geochemical profiles of our artificial laminations. However, calcium showed a clear tailing signal at the transition from a calcite to a quartz layer, indicating that geochemical signals can be smeared at transitions to sediment layers devoid of analyte element(s). We also analyzed resin-embedded natural sediments for comparison of LA-ICP-MS line-scan data to parallel sub-samples measured by SN-ICP-OES and SN-ICP-MS. In general, the results from LA-ICP-MS line-scans of natural sediments correspond well with reference values and with high reproducibility, corroborating the results of the artificial laminations. The results for Ca, V, Mn, Sr, Mo, Ba, and U (in log-ratios to Al) suggest that these elements can be analyzed semi-quantitatively, showing correlation coefficients (R2) ranging from 0.59 to 0.85 compared to conventional analytical techniques. However, we show that using non-matrix matched calibration standards such as NIST SRM 610, may induce deviations from the LA-ICP-MS values to the references values. Consequently, we recommend a simple protocol for further data correction in which binned-mean LA-ICP-MS values are calibrated to a parallel series of discrete samples analyzed by these conventional techniques.


      PubDate: 2015-03-17T03:28:44Z
       
  • Experimental anatexis, fluorine geochemistry and lead-isotope constraints
           on granite petrogenesis in the Seridó Belt, Borborema Province,
           northeastern Brazil
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Ricardo Sallet , Jonathan D. Price , Marly Babinski , Robert Moritz , Zorano S. Souza , Massimo Chiaradia
      The Neoproterozoic Seridó Belt of northeastern Brazil is characterized by low-pressure, high-temperature metamorphism, large transcurrent shear zones and voluminous plutonic activity. The belt consists of a thick sequence of Neoproterozoic metasedimentary rocks overlying Paleoproterozoic metaigneous basement rocks. Both units were intruded by plutonic rocks related to two main magmatic episodes: large Neoproterozoic high-K calc-alkaline bimodal plutons and small intrusions of peraluminous pegmatitic granites and zoned pegmatites of Cambrian age. In this study, we partially melted samples of the potential protoliths (orthogneiss, paragneiss, mica schist and micaceous quartzite) as analogs for the sources of the magmas belonging to the felsic intrusions. Short-duration (24h) experiments assessed the composition of disequilibrium melts in both H2O-fluxed and fluid-absent conditions. Experiments with added water at 0.4 and 1.0GPa and 725°C produced strongly peraluminous and Ca-rich rhyolitic glass; these compositions are not observed in the natural rocks. In contrast, fluid-absent melting at 0.6GPa and 930°C produced mildly peraluminous rhyolitic glass with major-element compositions similar to those of the felsic intrusions in the Seridó Belt. The elevated F content of the glasses approximates that of the Neoproterozoic granites but is higher than that of the Cambrian pegmatitic granites. Mass balance calculations modeled the fractionation from initially enriched melts into exsolving fluids during the late-stage crystallization of the pegmatitic granites. The Pb-isotope compositions of the Seridó Belt felsic rocks show substantial variation from non-radiogenic mantle-like to high-radiogenic signatures. The basement gneisses and the metasedimentary belt rocks can be considered as the principal lead sources for several pegmatitic granite intrusions and zoned pegmatites. However, the high-radiogenic Neoproterozoic Acari granite and Cambrian pegmatitic granites are derived from an unknown lithologic unit and/or to the dissolution of zircon and monazite in high-temperature anatectic melts. These results place constraints on the crustal origin of most of the SiO2-rich plutons in the region.


      PubDate: 2015-03-17T03:28:44Z
       
  • The effect of particle density on the sources, distribution, and
           degradation of sedimentary organic carbon in the Changjiang Estuary and
           adjacent shelf
    • Abstract: Publication date: Available online 10 March 2015
      Source:Chemical Geology
      Author(s): Jinpeng Wang , Peng Yao , Thomas S. Bianchi , Dong Li , Bin Zhao , Xingqian Cui , Huihui Pan , Tingting Zhang , Zhigang Yu
      Hydrodynamic sorting processes have a significant influence on the dispersal of sedimentary organic carbon (OC) in large-river delta-front estuaries (LDEs). Density fractionation was conducted on 6 sediments from the Changjiang LDE to investigate the sources, distribution, and degradation of OC among density fractions. Mass, elemental composition (C and N) and stable carbon isotope ratios, lignin, and specific surface area (SSA) of density fractions and bulk sediments were determined. Mass was concentrated in high density fractions (2.0-2.5gcm−3 and >2.5gcm−3) (up to 67.8%) while OC was dominated in 2.0-2.5gcm−3 fraction (up to 77.3%) due to extremely low OC in the highest density fraction (%OC<0.10%). The less dense fractions (<1.6gcm−3 and 1.6-2.0gcm−3) contained the highest OC (up to 21.4%) and lignin concentrations (up to 3.17mgg−1 or 2.85mg/100mg OC), and thus most depleted δ13C (down to −25.2‰). A three end-member mixing model using δ13C and lignin content (Ʌ8) as source markers and Monte-Carlo simulation, to distinguish the relative importance of terrestrial vascular plant, soil, and marine plankton OC sources, showed that less dense fractions were dominated by terrestrial OC (the sum of soil- and vascular plant-derived OC), whereas marine OC was the predominant OC source in high density fractions. Distributions of OC from different sources in density fractions and bulk sediments both indicated a differential transport and deposition of OC along the coast, showing a gradual decreasing of terrestrial OC and addition of marine OC. Ratios of vanillic acid to vanillin (Ad/Al)v, 3,5-Bd to total V phenols (3,5-Bd/V) and total p-hydroxy phenols to sum of vanillyl and syringyl (P/(S+V)) among lignin oxidation products increased with increasing particle density suggesting selective degradation of lignin among the different density fractions. Distinctively low OC/SSA loadings (<0.40mgm−2) were observed in the highest density fraction and the bulk sediments, indicating low preservation of OC in these sediments, consistent with the results of lignin decay parameters.


      PubDate: 2015-03-13T14:38:40Z
       
  • Unusually low TEX86 values in the transitional zone between Pearl River
           estuary and coastal South China Sea: Impact of changing archaeal community
           composition
    • Abstract: Publication date: Available online 11 March 2015
      Source:Chemical Geology
      Author(s): Jin-Xiang Wang , Yuli Wei , Peng Wang , Yiguo Hong , Chuanlun L. Zhang
      TEX86, calculated based on the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), is used worldwide for paleotemperature reconstruction in marine and lacustrine environments. Recently, however, increasing evidence showed that TEX86 could be affected by multiple environmental variables. In this study, TEX86 was calculated for core and polar GDGTs obtained from water column and surface sediments in the lower Pearl River and its estuary. Together with previously published core GDGT data from the coastal and open South China Sea (SCS), a comparison was made between TEX86-derived and satellite-based surface water temperature, which showed a TEX86-temperature minimum that is considerably lower than winter satellite temperature in the transitional zone between the Pearl River estuary and the coastal SCS. TEX86 showed significantly positive correlation with GDGT-2 and GDGT-3 in this transitional zone, indicating that they are the primary compounds for the TEX86 bias toward cooler temperature. Lipid and molecular DNA data both indicate that the variation in archaeal community composition rather than water depth or seasonality is likely the crucial factor causing the deviation of TEX86 in the transitional area. Our study has implications for studies in ancient continental margins where unusually low TEX86 temperatures may be observed in the geological record.


      PubDate: 2015-03-13T14:38:40Z
       
  • Iron availability as a dominant control on the primary composition and
           diagenetic overprint of organic-matter-rich rocks
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Nicolas Tribovillard , Ebraheem Hatem , Olivier Averbuch , Florent Barbecot , Viviane Bout-Roumazeilles , Alain Trentesaux
      Iron is known to stimulate surface ocean productivity, as well as intervene with bacterially-mediated processes of organic matter remineralization, during early diagenesis. In this paper, we examine the influence of iron supply on the geochemistry (trace metals, δ34S, organic matter) of sedimentary rocks deposited in a clastic-dominated marine ramp environment. To this end, we studied two Late Jurassic formations of the Boulonnais area (North-France). Both formations were deposited under quite similar conditions, but they differ in the reactive-iron supply they received. Only one of the two formations was affected by the particulate iron shuttle process. Our results indicate that 1)the iron shuttle may be recorded through concomitant enrichments in P, Mo, As and Sb; 2)a limited reactive-iron supply will allow the sulfurization of organic matter, even in a context of moderate productivity. Thus sulfurization can be a factor favoring a noticeable accumulation of organic matter: iron may thus be an important agent in the C cycle.


      PubDate: 2015-03-13T14:38:40Z
       
  • 40Ar/39Ar ages and residual volatile contents in degassed subaerial and
           subglacial glassy volcanic rocks from Iceland
    • Abstract: Publication date: Available online 11 March 2015
      Source:Chemical Geology
      Author(s): P.L. Clay , H. Busemann , S.C. Sherlock , T.L. Barry , S.P. Kelley , D.W. McGarvie
      Major volatile contents (H2O, CO2, F, Cl, and S) and 40Ar/39Ar ages have been determined in variably degassed rhyolite obsidians from Pleistocene-Holocene aged subaerial and subglacial eruption environments from the Torfajökull volcanic center and the monogenetic volcano at Prestahnúkur (Iceland). Icelandic subglacial rhyolites preserved residual H2O contents of 0.08 - 0.69 wt. %, non-detectable CO2, 840 - 1780 ppm F, 430 - 2000 ppm Cl and 6 - 45 ppm S. Most subglacial obsidians show degassed volatile signatures at the time of their eruption under ice. One eruption (Bláhnúkur, Torfajökull), showed H2O contents which exceed those expected for quenching at atmospheric pressures (up to 0.69 wt. % H2O) and are consistent with eruption at ~ 40 kbar pressure or equivalent to under ~ 450 m of ice. Altered and microcrystalline groundmass in some subglacial rhyolites yield variable volatile contents likely the result of micro-scale variability and the presence of alteration products. Two subaerial rhyolite obsidians gave low, volatile contents consistent with quenching at the surface. New 40Ar/39Ar ages were determined on the subglacially erupted rhyolites and provide broad constraints on the timing and frequency of glacio-volcanism during the Pleistocene. Three subglacial eruptions at Torfajökull yielded ages of 108 ± 22 ka (Bláhnúkur), 255 ± 20 ka (Hábarmur) and 236 ± 7 ka (Háskerðingur) which fit into a range of previously determined ages at Torfajökull of 67 - 384 ka. A new age of 132 ± 19 ka was determined for the monogenetic subglacial eruption at Prestahnúkur. Holocene aged subaerial volcanic glass produced no ages due large amounts of air-derived 40Ar (84-93%). Preliminary apparent 40Ar/39Ar ages from the limited feldspar phenocrysts available in these crystal-poor samples yield erroneous apparent plateau ages far older than eruption ages suggesting a homogenously distributed excess Ar component which can potentially be correlated with ice thickness. Disturbances to the 40Ar/39Ar geochronometer are correlated with a heterogeneous distribution of volatiles in glass and linked to hydration (e.g., Kaldaklofsjöll), mild alteration (e.g., Hábarmur) and alteration and/or recrystallization of glass (e.g., Bláhnúkur). Our observations indicate, however, that unaltered, subglacially erupted glasses that are partially to wholly degassed are promising materials for the application of the 40Ar/39Ar geochronometer. The precision to which we can date these young glassy samples currently limits the use of 40Ar/39Ar geochronometer as a tool for understanding the exact timing and duration of glaciovolcanism in the recent past.


      PubDate: 2015-03-13T14:38:40Z
       
  • Evaluating chemical equilibrium in metamorphic rocks using major element
           and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA
           
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Matthew P. Gatewood , Besim Dragovic , Harold H. Stowell , Ethan F. Baxter , David M. Hirsch , Rose Bloom
      A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume. Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.


      PubDate: 2015-03-13T14:38:40Z
       
  • Lead concentrations and isotope ratios in speleothems as proxies for
           atmospheric metal pollution since the industrial revolution
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Mohammed Allan , Nathalie Fagel , Maité Van Rampelbergh , James Baldini , Jean Riotte , Hai Cheng , R. Lawrence Edwards , David Gillikin , Yves Quinif , Sophie Verheyden
      Lead concentrations and isotope ratios from two speleothems from the Han-sur-Lesse cave in southern Belgium were measured in order to study the ability of speleothems to act as archives of atmospheric pollution. To address this aim we analyzed trace elemental Al and Pb compositions by LA-ICP-MS and ICP-MS as well as Pb isotopes by MC-ICP-MS. The results help to identify three intervals characterized by particularly high enrichment of Pb: from 1880 to 1905AD, from 1945 to 1965AD, and from 1975 to 1990AD. The speleothem record shows similar changes as the known historical atmospheric pollution level in Belgium. Lead isotope ratios discriminate between Pb sources and confirm that coal and gasoline combustion, combined with regional metallurgical activities, were the predominant Pb pollution sources in the stalagmites during the last 250years. This study opens possibilities to determine anthropogenic versus natural metal sources in well-dated speleothem archives.


      PubDate: 2015-03-13T14:38:40Z
       
  • In-situ iron isotope analysis of pyrites in ~3.7Ga sedimentary protoliths
           from the Isua supracrustal belt, southern West Greenland
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Kazumi Yoshiya , Yusuke Sawaki , Takafumi Hirata , Shigenori Maruyama , Tsuyoshi Komiya
      The timing of the emergence of life remains one of the principal unresolved questions in the Earth sciences. Putative relicts of microorganisms in the Eoarchean (ca. 3.6–3.85Ga) high-grade metamorphic terranes do not preserve morphological evidence for early life, but some relicts can be identified by their geochemical signatures created by metabolic processes. Among the oldest rocks of sedimentary origin (ca. 3.8Ga) occur in the Isua supracrustal belt (ISB), southern West Greenland; these have undergone metamorphism up to the amphibolite facies. Despite intense metamorphism, the carbon isotope compositions of graphite clots from the Isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process. Microbial dissimilatory iron reduction (DIR) is considered to be an early form of metabolism. The microbial DIR produced Fe2+ aq with a lower δ56Fe value from a precursor Fe3+-bearing iron mineral. However, δ56Fe values lower than −1‰ are not found in sedimentary rocks prior to about 2.9Ga. Here, we report in-situ iron isotope analysis of pyrites in sedimentary rocks from the ISB, using a near infrared-femtosecond-laser ablation-multicollector-ICP-MS (NIR-fs-LA-MC-ICP-MS). A large variation of δ56Fe values from −2.41 to +2.35‰, was documented from 190 points within pyrite grains from 11 rock specimens, including those interpreted to be banded iron-formations (BIFs), chert, amphibole-rich chert, quartz-rich clastic sedimentary rocks, mafic clastic sedimentary rocks, carbonate rocks and conglomerates. We found that the distribution of δ56Fe values depends on the lithology, whereas there is no correlation between their δ56Fe values and the metamorphic grade. The δ56Fe values of pyrites in BIFs range from +0.25 to +2.35‰, indicating partial oxidation in the deep ocean. Especially, the high δ56Fe values, up to +2.35‰, suggest that the BIF was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions. Pyrite grains in a conglomerate, carbonate rocks, mafic clastic sedimentary rocks, and amphibole-rich cherts show negative δ56Fe values around −1.5‰, down to −2.41‰, pointing to microbial DIR in the Eoarchean shallow sea. In addition, the relatively low δ56Fe values of pyrites in the shallow water sediments suggest anoxic, anoxygenic photoautotrophic iron oxidation in the photic zone.


      PubDate: 2015-03-13T14:38:40Z
       
  • Chemical and Pb isotope composition of olivine-hosted melt inclusions from
           the Hannuoba basalts, North China Craton: Implications for petrogenesis
           and mantle source
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Sheng-Ping Qian , Zhong-Yuan Ren , Le Zhang , Lu-Bing Hong , Jian-Qiang Liu
      Melt inclusions and their host olivine provide unique information about the nature, distribution and scale of mantle source heterogeneity. We present the first analyses of the chemical and Pb isotope compositions of melt inclusions and their host olivine from the Cenozoic Hannuoba basalts, North China Craton, which contains coexisting suites of alkali and tholeiitic basalts. There is limited variation in major and trace element composition, but significant Pb isotopic variation, in the tholeiitic samples. This contrasts with the substantial variation in major and trace elements, but limited Pb isotopic variation, in the alkali basalt samples. Based on the results of the major- and trace-element modeling, the compositional variation of the alkali basalts may be primarily attributed to garnet pyroxenite melting, with only a small input from garnet peridotite melts. The garnet–pyroxenite component involved in the genesis of the alkali basalts, of metasomatic origin, may have formed in the lithosphere. The wide range of isotopic composition of the tholeiitic basalts and remarkably limited variability of major and trace element composition is argued to be characteristic of the source region, which is a mixture of peridotite with small amounts of pyroxenite transformed from recycled ancient oceanic crust and sediment. The presence of recycled oceanic crust in the mantle source and the low–velocity anomaly observed beneath the Taihang Mountains, located 50km to the southeast of the Hannuoba region, supports a plume model for their origin.


      PubDate: 2015-03-13T14:38:40Z
       
  • Effects of melt percolation on iron isotopic variation in peridotites from
           Yangyuan, North China Craton
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Xin-Miao Zhao , Hong-Fu Zhang , Xiang-Kun Zhu , Bin Zhu , Hui-hui Cao
      The petrology, geochemistry and Fe isotopic composition of two suites of mantle xenoliths (magnesium-lherzolites and Fe-rich lherzolites) and their associated mineral separates from Yangyuan, in the Central Zone of the North China Craton, were measured in detail to investigate the effects of metasomatism on Fe isotope fractionation and mantle Fe isotopic heterogeneity. The magnesium-lherzolites (Mg#=89.8 to 90.9; TFeO=8.3 to 8.9wt.%) from Yangyuan are residual mantle lherzolites with no or only moderate metasomatism. They have a relatively limited Fe isotopic variation (δ57Fe=0.007 to 0.169‰) with an average of 0.083±0.044‰ (2SD/√n, n=8), similar to the range reported previously for typical mantle lherzolites, and they show no significant inter-mineral disequilibria. In contrast, the Fe-rich lherzolites examined in detail in this study are strongly metasomatized and are enriched in iron (Mg#=89.8 to 90.9; TFeO=11.2 to 16.8wt.%). These Fe-rich lherzolites exhibit extremely light Fe isotopic compositions, with δ57Fe ranging from −0.615 to −0.311‰, and disequilibrium Fe isotope fractionation occurs between most of their coexisting minerals (Δ57FeOpx–Ol =−0.616 to 0.522‰; Δ57FeCpx–Ol =−0.003 to 0.230‰; Δ57FeSp–Ol =0.191 to 0.522‰; Δ57FeSp–Opx =0.206 to 0.510‰; Δ57FeSp–Cpx =0.112 to 0.297‰; Δ57FePhl–Ol =0.097 to 0.450‰; and Δ57FeAmp–Ol =0.048 to 0.147‰). Overall, the large disequilibrium isotope fractionations in Fe-rich lherzolites likely results from the kinetic nature of the isotopic exchange between the wall-rock mantle and the percolating melt. This process results in the heterogeneous Fe isotopic composition of the lithospheric mantle beneath the Central Zone of the North China Craton.


      PubDate: 2015-03-13T14:38:40Z
       
  • The solubility of xenotime-(Y) and other HREE phosphates (DyPO4, ErPO4 and
           YbPO4) in aqueous solutions from 100 to 250°C and psat
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Alexander P. Gysi , Anthony E. Williams-Jones , Daniel Harlov
      Xenotime (YPO4) is a heavy rare earth element (HREE) phosphate, the principal host to economic HREE mineralization and a common accessory mineral in igneous, metamorphic and sedimentary rocks. Many occurrences of xenotime-(Y) are of hydrothermal origin, and therefore, determination of its solubility in aqueous fluids may help to evaluate the mobility of HREE in crustal fluids. We have measured the solubility of the HREE phosphate end-members YPO4, ErPO4, DyPO4 and YbPO4 in aqueous HClO4–H3PO4 solutions at temperatures from 100 to 250°C and saturated water vapor pressure (p sat) corresponding to the reaction: HREEPO4 =REE3+ +PO4 3− (Ks0). The logarithm of the solubility products (logKs0) determined for these end-members, can be evaluated as a function of temperature (in K) using the equation logKs0 =A+BT+C/T+Dlog(T). Reliable fits to our data yields the following coefficients: YPO4, A=−876.93, B=−0.25127, C=2.327e+4, D=342.82; DyPO4, A=−1150.2, B=−0.31778, C=3.132e+4, D=450.32; ErPO4, A=−555.90, B=−0.17933, C=1.373e+4, D=217.42; and YbPO4, A=−419.76, B=−0.15215, C=8.941e+3, D=165.27. A comparison of our solubility products to those calculated with the available enthalpy, entropy and heat capacity values from calorimetric determinations, and the corresponding properties of the aqueous species (Y3+, Dy3+, Er3+, Yb3+, PO4 3−), shows that our solubility products for YPO4 and ErPO4 are orders of magnitude different from the calculated solubility products. By contrast, the calculated solubility products for YbPO4 are in good agreement with the measured values for the range of temperature considered in our experiments. There are no data available for the heat capacity of DyPO4, and consequently our experimental results provide the first data that permit the solubility of this HREE phosphate to be calculated for elevated temperature. We have developed a method that allows our data to be extrapolated to 25°C and have reconciled the measured calorimetric and solubility data. The results are used to recommend standard thermodynamic properties for the HREE phosphates at 25°C and 1bar.


      PubDate: 2015-03-13T14:38:40Z
       
  • Formation of the Jurassic South China Large Granitic Province: Insights
           from the genesis of the Jiufeng pluton
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Hui-Qing Huang , Xian-Hua Li , Zheng-Xiang Li , Wu-Xian Li
      The magma source and thermal regime for the formation of Large Granitic Provinces (LGPs) have been the subjects of much debate. As mineralogical, geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution, the Jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the South China LGP. SIMS zircon U-Pb analyses of a biotite granite (158.9±1.3Ma), three two-mica granites (stage I: 159.7±1.2Ma, stage II: 158.6±1.2Ma and 156.8±1.2Ma) and a muscovite granodiorite (159.0±4.4Ma) of the Jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5.3m.y. Limited variations in whole-rock Nd isotope (εNd(t)=−12.6 to −9.5) and zircon Hf-O isotopes (εHf(t)=−12.0 to −6.3, δ18O=6.8‰−9.4‰) suggest that the Jiufeng pluton was mainly derived from melting of a common metasedimentary source, possibly with a minor basaltic contribution. We consider the geochemical variations of the Jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step-like transition from fluid-saturated to fluid-absent melting of the source. The muscovite granodiorite, with high Na2O (>3.80wt.%; K2O/Na2O=~1), is interpreted to have been produced by fluid-saturated melting at low temperature (~650°C). High P2O5 (0.09–0.17wt.%), zircon saturation temperature (TZr =769–816°C) and La/Yb ratios (8.4–55.8) of the stage II two-mica granite support its formation from high-temperature (>800°C) biotite-dehydration melting, whereas lower P2O5 (<0.02wt.%), TZr (685–742°C) and La/Yb (<3) of the stage I two-mica granite suggest its generation at lower temperature, likely by muscovite-dehydration melting. We propose that extensive emplacement of basaltic melts in the lower crust most likely drove the rapid increase of mid-crustal (~20km) temperature (~50°C/m.y.) and widespread crustal melting for the formation of the Jurassic South China LGP. Therefore, formation of the LGP signifies prominent crustal growth as well as crustal reworking in an intraplate setting and was likely a response to flat-slab delamination and foundering.


      PubDate: 2015-03-08T16:21:17Z
       
  • Integrated O–Sr–Nd isotope constraints on the evolution of
           four important Fe–Ti oxide ore-bearing mafic–ultramafic
           intrusions in the Emeishan large igneous province, SW China
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Song-Yue Yu , Xie-Yan Song , Edward M. Ripley , Chusi Li , Lie-Meng Chen , Yu-Wei She , Yan Luan
      In the Emeishan large igneous province, SW China, there are many layered mafic–ultramafic intrusions such as the Baima, Hongge, Panzhihua and Taihe intrusions that host world-class Fe–Ti oxide ore deposits. Despite numerous studies, the origin of these deposits still remains elusive. This includes the role of crustal contamination, especially addition of external CO2 from carbonate country rocks during contact metamorphism, in triggering Fe–Ti oxide crystallization from high-Mg basaltic magma. To address this important issue, we have carried out an integrated O–Sr–Nd isotope study of these ore-bearing intrusions and the country rocks. Our results show that in these intrusions clinopyroxene is much less susceptible to fluid–mineral oxygen isotope exchange than coexisting plagioclase and Fe–Ti oxides, which is similar to other intrusions worldwide (e.g., Taylor, 1967; Gregory and Taylor, 1981). Our calculations based on the least exchanged clinopyroxene oxygen isotope data show that the mean δ18O values for the parental magmas of these intrusions are Baima=5.7‰, Panzhihua=6.1‰, Taihe=5.9‰. The estimated mean δ18O value for the parental magma of the Upper and Middle Zones of the Hongge intrusion is 6.2‰, which is similar to those for the parental magmas of the other intrusions (Baima, Panzhihua and Taihe). By contrast, the estimated mean δ18O value for the parental magma of the Lower Zone of the Hongge intrusion is higher (6.9‰). This difference, together with higher initial 87Sr/86Sr ratios (0.7057 to 0.7076) and lower εNdt values (−2.82 to −0.07) for this zone, can be attributed to higher degrees of contamination with siliceous crustal materials in this zone than elsewhere in this intrusion. Comparison of O–Sr–Nd isotope compositions between the intrusions and country rocks reveals that bulk assimilation of carbonate country rocks is negligible in all of these intrusions. Mixing calculations using the O–Sr–Nd isotope data are consistent with variable degrees of contamination with siliceous crustal materials in the intrusions: Panzhihua, <5%, Baima and Taihe, <10%, the Middle and Upper Zone of the Hongge intrusion, <10%, the Lower Zone of the Hongge intrusion, <15%. These percentages are maximum values and may be reduced if contamination was selective in nature, involving Sr- and Nd-bearing fluids or partial melts. Based on the oxygen isotope results, an iterative calculation with a CO2/magma mass ratio=1/1000 for each increment reveals that the Panzhihua magma reacted with <1wt.% of CO2 released from the footwall during contact metamorphism. This amount is not sufficient to increase the oxidation state of the magma to the level that Fe–Ti oxides would crystallize alone from the magma. Therefore, we conclude that external CO2 did not play a major role in the formation of the Fe–Ti oxide ores in the Panzhihua intrusion.


      PubDate: 2015-03-08T16:21:17Z
       
  • A remarkable relationship of the stable carbon isotopic compositions of
           wood and cellulose in tree-rings of the tropical species Cariniana
           micrantha (Ducke) from Brazil
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): G.H. Schleser , D. Anhuf , G. Helle , H. Vos
      The stable isotopes of carbon were analysed in total wood and cellulose from the tree-rings of the tropical wood species Cariniana micrantha (Ducke). The aim was to examine the isotopic relationship between total wood and its cellulose over the last two and a half centuries. Although the correlation for the whole time period is very high (r=0.96) it is remarkable that different sub-periods deviate strongly from this close relationship. Consequently, a good correlation from the subset of a longer isotopic record cannot necessarily prove its validity for the whole record. The study indicates that changes of the carbon isotopes of cellulose and of total wood show sometimes during short sub-periods different isotope patterns presumably caused by different environmental effects. Thorough calculations indicate that strong variations within the isotopic record especially changes of the isotopic level along a chronology lead to high correlations between δ13Cwood and δ13Ccel. Contrary thereto subsections with low isotopic variability lead to low correlations. The results imply that long term trends provide similar patterns. Therefore, if long term trends are of interest such as e.g. in climate reconstruction then total wood can be analysed in favour of cellulose, thus saving a tremendous amount of work. However, if short term aspects from a longer record are of interest, cellulose and total wood may sometimes provide different information. In addition it is hypothesized that during intervals of low isotopic variability the proportions of the various wood components may change relative to each other, leading for certain time intervals to different isotope patterns.


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


      PubDate: 2015-03-08T16:21:17Z
       
  • Submarine groundwater discharge of rare earth elements to a tidally-mixed
           estuary in Southern Rhode Island
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Darren A. Chevis , Karen H. Johannesson , David J. Burdige , Jianwu Tang , S. Bradley Moran , Roger P. Kelly
      Rare earth element (REE) concentrations were analyzed in surface water and submarine groundwater within the Pettaquamscutt Estuary, located on the western edge of Narragansett Bay in Rhode Island. These water samples were collected along the salinity gradient of the estuary. Rare earth element concentrations in the majority of the groundwater samples are substantially higher than their concentrations in the surface waters. In particular, Nd concentrations in groundwater range from 0.43nmolkg−1 up to 198nmolkg−1 (mean±SD=42.1±87.2nmolkg−1), whereas Nd concentrations range between 259pmolkg−1 and 649pmolkg−1 (mean±SD=421±149pmolkg−1) in surface waters from the estuary, which is, on average, 100 fold lower than Nd in the groundwaters. Groundwater samples all exhibit broadly similar middle REE (MREE) enriched shale-normalized REE patterns, despite the wide variation in pH of these natural waters (4.87≤pH≤8.13). The similarity of the shale-normalized REE patterns across the observed pH range suggests that weathering of accessory minerals, such as apatite, and/or precipitation of LREE enriched secondary phosphate minerals controls groundwater REE concentrations and fractionation patterns. More specifically, geochemical mixing models suggest that the REE fractionation patterns of the surface waters may be controlled by REE phosphate mineral precipitation during the mixing of groundwater and stream water with incoming water from the Rhode Island Sound. The estimated SGD (Submarine Groundwater Discharge) of Nd to the Pettaquamscutt Estuary is 26±11mmolNdday−1, which is in reasonable agreement with the Nd flux of the primary surface water source to the estuary, the Gilbert Stuart Stream (i.e., 36mmolday−1), and of the same order of magnitude for a site in Florida.


      PubDate: 2015-03-05T13:03:23Z
       
  • The effect of pH on stable iron isotope exchange and fractionation between
           aqueous Fe(II) and goethite
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Thiruchelvi R. Reddy , Andrew J. Frierdich , Brian L. Beard , Clark M. Johnson
      Enriched Fe isotope tracer studies demonstrate that aqueous Fe(II) undergoes electron transfer and atom exchange with goethite. Such processes influence contaminant fate and trace-element mobility, and result in stable Fe isotope fractionation in both biological and abiological processes. To date, the majority of experimental studies of aqueous Fe(II) and Fe oxide interactions have been done at circumneutral pH. The effect of pH variations on the rate and extent of Fe isotope exchange between aqueous Fe(II) and iron oxide minerals, as well as the natural mass-dependent fractionation between these species, has not been adequately explored. Here, the three-isotope method (57Fe–56Fe–54Fe), using an enriched 57Fe tracer, was used to investigate the effect of pH (between 2.5 and 7.5) on the rate and extent of isotopic exchange. 56Fe/54Fe ratios were used to determine the natural, mass-dependent stable isotope fractionation, between aqueous Fe(II) and goethite. Three Fe(II) solutions differing in their initial 56Fe/54Fe ratios were used to approach isotopic equilibrium from multiple directions. The 57Fe-enriched tracer data indicate that the extent of isotopic exchange between Fe(II)aq and goethite was positively correlated with pH, where the least amount of exchange occurred at the lowest pH. Similarly, initial kinetic isotope fractionations were influenced by pH; at low pH, minimal kinetic isotope effects were observed relative to large effects at high pH, suggesting a relation between the extent of sorbed Fe(II) and kinetic isotope effects. Continued exchange over time at high pH, however, erases the initial kinetic isotope effects, and the system fundamentally reached isotopic equilibrium by the end of the experiment. Our results show that the interplay between kinetic and equilibrium effects may prevent confident extrapolation to infer equilibrium fractionation factors when only small amounts of Fe exchange occur.


      PubDate: 2015-03-05T13:03:23Z
       
  • The effect of Al on Si isotope fractionation investigated by silica
           precipitation experiments
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Marcus Oelze , Friedhelm von Blanckenburg , Julien Bouchez , Daniel Hoellen , Martin Dietzel
      Mass-dependent isotope fractionation occurring during precipitation of solids in low-temperature environments often depends on precipitation rate. Using a series of precipitation experiments in which continuous precipitation and dissolution of Si solids is forced by daily cyclic freezing (solid formation) and thawing (solid re-dissolution), we show this dependence. We conducted six Si precipitation experiments for about 120days with initial dissolved Si concentration of 1.6mmol/l Si, at pH values between 4.5 and 7, with additions of 0.1–1mM of dissolved aluminum (Al), and in the absence of Al. During all experiments increasing amounts of an X-ray amorphous silica-containing solid are formed. No Si isotope fractionation occurs during formation of almost pure Si solids, interpreted as an absence of Si isotope fractionation during polymerization of silicic acid. Si isotope fractionation occurs only in the high-Al concentration experiments, characterized by an enrichment of the light Si isotopes in the solids formed early. With ongoing duration of the experiments, a re-dissolution of these solids is indicated as the Si isotope value of the complementary solution shifts to lighter values and eventually reaches near-initial compositions. Hence, our high-Al experiments are characterized by a gradual shift from a regime that is dominated by unidirectional kinetic isotope fractionation with solids formed that are up to 5‰ lighter in their 30Si/28Si ratio than the corresponding solution, to one of steady-state between dissolution and precipitation with the 30Si/28Si ratio of the solid being almost identical to the solution (Δsolid‐solution ≈0). This suggests that the enrichment of light Si isotopes found in natural environments is caused exclusively by a unidirectional kinetic isotope effect during fast precipitation of solids, aided by co-precipitation with Al phases or other carrier phases (e.g. Fe(III)). By contrast, during slow precipitation, or in the absence of a carrier phase like Al, no Si isotope fractionation is expected and solids obtain the composition of the ambient fluid.


      PubDate: 2015-03-05T13:03:23Z
       
  • Origin of chemical and isotopic heterogeneity in a mafic, monogenetic
           volcanic field: A case study of the Lunar Crater Volcanic Field, Nevada
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): C. Rasoazanamparany , E. Widom , G.A. Valentine , E.I. Smith , J.A. Cortés , D. Kuentz , R. Johnsen
      Major and trace element geochemistry and Sr, Nd, Pb, Hf and Os isotope signatures of basaltic lavas and tephra from volcanic centers in the northern Lunar Crater Volcanic Field (LCVF), Nevada, provide insight into the nature of their mantle sources and the role of lithospheric contamination versus source-related enrichment in producing compositional variations in basaltic monogenetic volcanic fields. Three of the studied eruptive centers (Hi Desert and Mizpah, ~620–740ka; and Giggle Springs, <80ka) are located within ~500m of each other; the Marcath volcano (~35–38ka) and Easy Chair (140ka), two of the youngest eruptive centers in the field, are located ~6 and 12km southwest of these cones, respectively. Isotopic studies of the volcanic rocks show a limited range in 143Nd/144Nd and 176Hf/177Hf, but significant heterogeneity in 87Sr/86Sr, 206Pb/204Pb and 187Os/188Os. The older (>140ka) Hi Desert, Mizpah, proto-Easy Chair and several unnamed flows exhibit Nb–Ta enrichment, Rb, Cs and K depletion, and high 206Pb/204Pb but low 87Sr/86Sr. In contrast, the younger (≤140ka) Giggle Springs, Easy Chair and Marcath lavas have high Ba, Rb and Cs and lower 206Pb/204Pb and higher 87Sr/86Sr. The lavas produce a well-defined negative correlation between Sr and Pb isotopes, attributed to mixing of heterogeneous mantle sources. The geochemical and isotopic signatures of the older Hi Desert, Mizpah, proto-Easy Chair and unnamed lavas are consistent with derivation from a mantle source with a component of ancient recycled oceanic crust. In contrast, the relatively high Ba, Rb and Cs coupled with lower 206Pb/204Pb and higher 87Sr/86Sr of the younger Giggle Springs, Easy Chair and Marcath lavas are consistent with derivation from a similar, but fluid-enriched, mantle source. Mixing calculations indicate that incorporation of ~18% of 0.8Ga recycled oceanic crust into depleted mantle can explain the trace element and isotopic signatures of the older group end member. Subsequent addition to this source of minor (<1%) hydrous fluid derived from subducted oceanic crust could account for the chemical and isotopic compositions of the younger group end member. Variable degrees of mixing between these two mantle end members can generate the full range of isotopic compositions observed in the northern LCVF sample suite, as well as within single eruptions. Our data indicate that the mantle source region in the LCVF is characterized by chemical and isotopic heterogeneity that manifests itself over a very small spatial scale (<500m) and within the time frame of a single monogenetic eruption. Similar processes may explain the geochemical and isotopic heterogeneities observed in other mafic monogenetic volcanic fields, the evidence for which may be preferentially preserved where small degrees of melting and rapid source to surface transport prevail.


      PubDate: 2015-03-05T13:03:23Z
       
  • The TICE event: Perturbation of carbon–nitrogen cycles during the
           mid-Tournaisian (Early Carboniferous) greenhouse–icehouse transition
           
    • Abstract: Publication date: 24 April 2015
      Source:Chemical Geology, Volume 401
      Author(s): Le Yao , Wenkun Qie , Genming Luo , Jiangsi Liu , Thomas J. Algeo , Xiao Bai , Bo Yang , Xiangdong Wang
      Carbonate carbon (δ13Ccarb) and bulk nitrogen (δ15N) isotopic variation during the mid-Tournaisian was analyzed in the Malanbian and Long'an sections of South China. Their C-isotope profiles document a large positive excursion, herein termed the ‘mid-Tournaisian carbon isotope excursion’ (TICE), during the Siphonodella isosticha conodont Zone, although its magnitude differed between the two sections (>6‰ at Malanbian versus ~3‰ at Long'an). The TICE event coincided with sedimentologic and oxygen-isotopic evidence of climatic cooling and glaciation during the mid-Tournaisian. It was probably triggered by an increase in organic carbon burial rates linked to changes in global-ocean circulation. The study sections also document a large positive shift in δ15N, from 1.7‰ to 4.2‰ at Malanbian and from 1.5‰ to 3.8‰ at Long'an. The N-isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15N-enriched marine nitrate that lasted for the duration of the Late Paleozoic Ice Age. Its initiation coincided with TICE and thus may have been linked to ocean-circulation changes that resulted in intensified upwelling and an increase in water-column denitrification. The continuation of the N-isotope shift over millions of years may have been linked to glacio-eustatic fall and a long-term shift in the locus of denitrification from continental-shelf sediments to continent-margin oxygen-minimum zones. The TICE event thus marks the onset of sustained continental glaciation during the Late Paleozoic Ice Age.


      PubDate: 2015-03-05T13:03:23Z
       
  • Water in Hawaiian garnet pyroxenites: Implications for water heterogeneity
           in the mantle
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Michael Bizimis , Anne H. Peslier
      Mapping the compositional variability of the Earth's mantle is fundamental for understanding mantle dynamics, crustal recycling and melt generation. The geochemistry of intraplate oceanic basalts in particular points to a heterogeneous convecting mantle, often explained by variable proportions of fertile (pyroxenite, eclogite) and depleted (peridotitic) domains. A key parameter necessary to constrain the Earth's deep processes is water because it influences melting and affects the physical properties of the Earth's mantle. However, there are only a few direct water determinations on samples of the oceanic mantle. Here we report water concentrations by FTIR on garnet pyroxenite xenoliths from Salt Lake Crater vent, Oahu, Hawaii, in order to constrain the role of lithological variability in the distribution of water in the upper oceanic mantle. The clinopyroxenes have 260 to 576ppm H2O and the orthopyroxenes about half these amounts. Curiously, garnets have water concentrations below detection limits (<0.5ppm H2O). Calculated melts in equilibrium with the pyroxenes have, on average, 2.3±0.4wt.% H2O and 243±83 H2O/Ce. These H2O concentrations are similar to estimates for the H2O concentration of primary magmas of the rejuvenated Hawaiian stage volcanism, and 3 to 5 times higher than primary magmas of the shield stage. This supports earlier conclusions where the pyroxenites are interpreted as high pressure cumulates from alkali magmas similar to the rejuvenated stage magmas within the Pacific lithosphere. The reconstructed bulk pyroxenites have 211 to 467ppm H2O, similar to estimates for the source of Hawaiian magmas and two to four times higher than estimates for the depleted MORB source (~100ppm H2O). Despite their high water concentrations, however, the bulk pyroxenites have much lower H2O/Ce ratios than the MORB source (35–115 vs. 150–210, respectively) or Hawaiian magma sources (>160). The discrepancy between the low H2O/Ce ratios in the bulk pyroxenites and high H2O/Ce in the equilibrium melts is consistent with experimental data that predicts 4 to 5 times higher partition coefficient for Ce than H in these pyroxenes. Therefore, the process of high-pressure crystallization in the oceanic lithosphere will create pyroxene-rich lithologies which are paradoxically, both “wet” (i.e., high H2O concentrations) and “dry” (low H2O/Ce ratios) compared to the source of their parental melts. Phlogopite is present as a trace phase in these rocks (<0.4% modal) with relatively minor contribution on the bulk water contents. The coupled high H2O, low H2O/Ce ratios of the pyroxenites are similar to the inferred source of several Enriched Mantle (EM)-type Ocean Island Basalts (OIB), like the Samoa, Pitcairn, Society, and Kergulen hot spots, as well as the EM-1 type Koolau endmember of the Hawaiian magmas. We suggest that recycling of pyroxenite-bearing oceanic lithosphere can explain the relatively high H2O and low H2O/Ce ratios of some EM-type OIB. Our data suggests a link between lithological variability and heterogeneous water distribution in the upper mantle.
      Graphical abstract image

      PubDate: 2015-03-05T13:03:23Z
       
  • Geochemical fingerprinting and source discrimination of agricultural soils
           at continental scale
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Philippe Négrel , Martiya Sadeghi , Anna Ladenberger , Clemens Reimann , Manfred Birke
      2108 agricultural soil samples (Ap-horizon, 0–20cm) were collected in Europe (33 countries, area 5.6 million km2) as part of the recently completed GEMAS (GEochemical Mapping of Agricultural and grazing land Soil) soil mapping project. GEMAS soil data have been used to provide a general view of element origin and mobility with a main focus on source parent material (and source rocks) at the continental scale, either by reference to average crustal abundances or to normalized patterns of element mobility during weathering processes. The survey area covers a large territory with diverse types of soil parent materials, with distinct geological history and a wide range of climate zones, and landscapes. To normalize the chemical composition of European agricultural soil, mean values and standard deviation of the selected elements have been compared to model compositions of the upper continental crust (UCC) and mean European river suspended sediment. Some elements are enriched relative to the UCC (Al, P, Pb, Zr,) whereas others, such as Mg, Na and Sr are depleted. The concept of the UCC extended normalization pattern has been applied to selected elements. The mean values of Rb, K, Y, Ti, Al, Si, Zr, Ce and Fe are very similar to the values from the UCC model, even when standard deviations indicate slight enrichment or depletion. Zirconium has the best fit to the UCC model using both mean value and standard deviation. Lead and Cr are enriched in European soil when compared to the UCC model, but their standard deviation values span a large, particularly towards very low values, which can be interpreted as a lithological effect. GEMAS soil data have been normalized to Al and Na, taking into account the main lithologies of the UCC, in order to discriminate provenance sources. Additionally, sodium normalization highlights variations related to the soluble and insoluble behavior of some elements (e.g., K, Rb versus Ti, Al, Si, V, Y, Zr, Ba, and La, respectively), their reactivity (e.g, Fe, Mn, Zn) and association with carbonates (e.g., Ca and Sr). Maps of Europe showing the spatial distribution of normalized compositions and element ratios reveal difficulties with the use of classical element ratios because of the large lithological differences in compositions of soil parent material. The ratio maps and color composite images extracted from the GEMAS data can help to discriminate the main lithologies in Europe at the regional scale but need to be used with caution due to the complexity of superimposed processes responsible for the soil chemical composition.


      PubDate: 2015-03-05T13:03:23Z
       
  • Influence of natural organic matter on the bioavailability and
           preservation of organic phosphorus in lake sediments
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Yuanrong Zhu , Fengchang Wu , Zhongqi He , John P. Giesy , Weiying Feng , Yunsong Mu , Chenglian Feng , Xiaoli Zhao , Haiqing Liao , Zhi Tang
      Information about the bioavailability and sequestration of organic phosphorus (Po) in sediments is fundamental to understanding biogeochemical cycling of phosphorus (P) in eutrophic lakes. However, the processes governing preservation of Po in sediments are still poorly understood. Sequential extraction of Po by H2O (H2O-Po) and NaOH–EDTA (NaOH–EDTA Po), in combination with enzymatic hydrolysis/31P NMR, was applied to estimate the bioavailability of Po in sediments of Lake Tai (Ch: Taihu), China. Of H2O-Po and NaOH–EDTA Po, 45.5–89.4% and 30.4–71.3% respectively were hydrolyzed by phosphatase, and therefore considered to be biologically available. Of NaOH–EDTA Po, 28.7–69.6% could not be hydrolyzed by phosphatase; this portion was characterized by 31P NMR as monoester P and/or diester P. Simulation experiments of hydrolysis of model Po compounds in the presence of humic acids (HA), which were used as a model for natural organic matter (NOM), and metals, including Al, Ca, and Fe, have demonstrated that enzymatic hydrolysis of labile monoester P was weakly reduced by HA or metal ions. Condensed phosphate (e.g., pyrophosphate) and phytate-like P (e.g., inositol phosphates) were resistant to enzymatic hydrolysis in the presence of HA and/or metal ions, which indicated that they may be possibly preserved in sediments. These observations suggest that NOM in sediments can be a significant factor determining the bioavailability and preservation of Po in sediments. The presence of metals would enhance the effect of NOM on preservation of Po in sediments. Formation of Po–metal–HA or Po–metal complexes might be mechanisms responsible for these processes.
      Graphical abstract image

      PubDate: 2015-03-05T13:03:23Z
       
  • Magnesium isotope fractionation during shale weathering in the Shale Hills
           Critical Zone Observatory: Accumulation of light Mg isotopes in soils by
           clay mineral transformation
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Lin Ma , Fang-Zhen Teng , Lixin Jin , Shan Ke , Wei Yang , Hai-Ou Gu , Susan L. Brantley
      Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (−0.6‰ to −0.1‰) of stream and soil pore waters are about ~0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of +0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2+ concentrations but lower δ26Mg values (−1.1‰) in groundwater, ~1.5‰ lighter than the bedrock. δ26Mg values (+0.2‰ to +0.4‰) of soil samples on the planar hillslope are either similar or up to ~0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of +0.3‰ to +0.5‰) accumulated on the valley floor are ~0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir. Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier.


      PubDate: 2015-03-05T13:03:23Z
       
  • Heterogeneous growth of cadmium and cobalt carbonate phases at the
           101¯4 calcite surface
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Man Xu , Eugene S. Ilton , Mark H. Engelhard , Odeta Qafoku , Andrew R. Felmy , Kevin M. Rosso , Sebastien Kerisit
      The ability of surface precipitates to form heteroepitaxially is an important factor that controls the extent of heterogeneous growth. In this work, the growth of cadmium and cobalt carbonate phases on 10 1 ¯ 4 calcite surfaces is compared for a range of initial saturation states with respect to otavite (CdCO3) and sphaerocobaltite (CoCO3), two isostructural metal carbonates that exhibit different lattice misfits with respect to calcite (−4% and −15%, respectively, based on 10 1 ¯ 4 surface areas). Calcite single crystals were reacted in static conditions for 16h with CdCl2 and CoCl2 aqueous solutions with initial concentrations 0.3≤[Cd2+]0 ≤100μM and 25≤[Co2+]0 ≤200μM. The reacted crystals were imaged in situ with atomic force microscopy (AFM) and analyzed ex situ with X-ray photoelectron spectroscopy (XPS). AFM images of Cd-reacted crystals showed the formation of large islands elongated along the 42 1 ¯ direction, clear evidence of heteroepitaxial growth, whereas surface precipitates on Co-reacted crystals were small round islands. Deformation of calcite etch pits in both cases indicated the incorporation of Cd and Co at step edges. XPS analysis pointed to the formation of a Cd-rich (Ca,Cd)CO3 solid solution coating atop the calcite substrate. In contrast, XPS measurements of the Co-reacted crystals provided evidence for the formation of a mixed hydroxy-carbonate cobalt phase despite supersaturation with respect to CoCO3. The combined AFM and XPS results suggest that the lattice misfit between CoCO3 and CaCO3 is too large to allow for heteroepitaxial growth of a pure cobalt carbonate phase on calcite surfaces in aqueous solutions and at ambient conditions. The use of the satellite structure of the Co 2p 3/2 photoelectron line as a tool for determining the nature of cobalt surface precipitates is also discussed.


      PubDate: 2015-03-05T13:03:23Z
       
  • TIMS analysis of Sr and Nd isotopes in melt inclusions from Italian
           potassium-rich lavas using prototype 1013Ω amplifiers
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Janne M. Koornneef , Igor Nikogosian , Manfred J. van Bergen , Richard Smeets , Claudia Bouman , Gareth R. Davies
      Sr and Nd isotopes were determined using new thermal ionisation mass spectrometry (TIMS) techniques for a suite of 21 olivine-hosted (85–92mol% Fo) melt inclusions selected from potassic and ultra-potassic lavas from the Italian peninsula. Sr isotopes were measured using default 1011 Ω resistors, whereas Nd isotope compositions were determined using new 1013 Ω resistors mounted in the amplifiers' feedback loop. Compared to default 1011 Ω resistors, use of 1013 Ω resistors results in a 10-fold improvement of the signal-to-noise ratio and more precise data when analysing small ion beams (<20mV). A miniaturised Sr and Nd chemical separation procedure was developed to minimise total procedural blanks. Using the combined new chemical separation and TIMS analysis techniques samples as small as 2ng Sr and 30pg Nd were analysed successfully. 87Sr/86Sr ratios in the melt inclusions range from 0.70508 to 0.71543 and 143Nd/144Nd ratios range from 0.51175 to 0.51268. Significant differences in 87Sr/86Sr and 143Nd/144Nd were found between melt inclusions and host lavas indicating distinct evolution paths for the lava groundmasses and the primitive melts that were trapped in the olivine phenocrysts. In line with magmatic processes known to have affected Italian potassic volcanics, the observed differences between inclusions and host lavas can be attributed to (1) mixing and mingling of isotopically distinct magma batches, (2) assimilation of crustal material with different isotope compositions, and (3) early (incomplete) mixing of primitive melts derived from an isotopically heterogeneous mantle source. These data demonstrate the potential of the analysis of Sr and Nd isotope ratios in individual melt inclusions for detailed studies of magma mixing and evolution processes.


      PubDate: 2015-03-05T13:03:23Z
       
  • Editorial Board
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397




      PubDate: 2015-03-05T13:03:23Z
       
  • Variability of trace element uptake in marine reptile bones from three
           Triassic sites (S Poland): Influence of diagenetic processes on the host
           rock and significance of the applied methodology
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Monika Kowal-Linka , Klaus Peter Jochum
      Diagenetic alternation of apatite building vertebrate bioclasts is a complex process. It is influenced by numerous factors, but the impact of diagenetic processes affecting the host rock is commonly underestimated. The present study aims at identifying the causes of different concentration levels and distribution patterns of REE, and at examining the significance of the material chosen for normalisation. We measured REE and other trace element concentrations in three bone assemblages of Triassic marine reptiles using LA-ICP-MS, and in the host rocks using ICP-MS. Two groups of hard parts reveal clearly different trace element contents and distribution patterns. The remains from Gogolin show a distinct MREEN enrichment, which must have been caused by replacement involving numerous substitutions, that was possible due to a relatively high porosity of the host rocks, and the presence of pore water. The MREEN enrichment and low concentrations of Mg and Sr, combined with a relatively high content of U in these bones, indicate solutions derived from meteoric water as the source of the incorporated ions. The reductive dissolution of Fe oxyhydroxides that are abundant in the host rock matrix and that likely were an important source of REE, may have influenced and controlled the REE uptake. The significant HREEN enrichment over LREEN, as revealed in the rust-coloured bones from Raciborowice Górne, is likely connected with local acidification of the pore water, resulting from aqueous oxidation of pyrite, infilling the pore spaces in the bones. The material selected for normalisation of the chemical composition of apatite can have a significant impact on values of the normalised data used further for interpretations, particularly when vertebrate bioclasts from carbonate host rocks are involved. The carbonate rocks have element ratios that differ from those characterising the commonly used PAAS and NASC normalisation materials. In particular, Ce anomaly values can distinctly vary depending on the applied normalisation procedure. Vertebrate hard parts, that definitely spent more time in a host rock than in an animal body, must be rigidly handled as a constituent of a specific rock, never as a separate item.


      PubDate: 2015-03-05T13:03:23Z
       
  • Demystifying the interfacial aquatic geochemistry of thallium(I): New and
           old data reveal just a regular cation
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Katherine M. Coup , Peter J. Swedlund
      In aquatic systems the group 13 element Tl can be both highly mobile and highly toxic. Despite these properties there are inadequate studies regarding controls on thallium's solubility and significant discrepancies exist particularly relating to the affinity of Tl+ for iron oxide surfaces. It is unclear whether these discrepancies relate to other enigmatic aspects of the geochemistry of Tl+, which has both lithophilic and chalcophilic character. The solubility of many trace cations is limited by sorption to iron oxides and an important principle in aquatic geochemistry is that cation affinity for iron oxides is strongly related to the cation's hydrolysis constant (logKMOH). The Tl+ cation has a low logKMOH of 10−13.2 and is expected to sorb weakly, hence the high mobility of Tl+, but reported sorption constants for Tl+ sorption by ferrihydrite vary by up to 4 orders of magnitude. In this study we have measured Tl+ sorption to ferrihydrite over a wide range of conditions and also re-examined the few previous studies to provide a consistent understanding and description of Tl+ sorption using the Diffuse Layer Model. In terms of both the kinetics and thermodynamics of Tl+ sorption, it was found that Tl+ behaved exactly as expected for a cation with a logKMOH of 10−13.2. This means that in general Tl+ sorption will only be appreciable at alkaline pH and is consistent with the observed high mobility of Tl+ in aquatic environments.
      Graphical abstract image

      PubDate: 2015-03-05T13:03:23Z
       
  • Discriminating hydrothermal and terrigenous clays in the Okinawa Trough,
           East China Sea: Evidences from mineralogy and geochemistry
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Hebin Shao , Shouye Yang , Quan Wang , Yulong Guo
      The Okinawa Trough (OT) in the East Asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic–hydrothermal activities. In this study, the clays collected during IODP Expedition 331 to the middle OT (Iheya North Knoll) were analyzed for mineralogical and geochemical compositions. By comparing with the clays from the East China Sea shelf and surrounding rivers, we examine different clay origins. The hydrothermal field in the mid-OT is dominated by Mg-rich chlorite, while the recharge zone has clay mineral assemblages similar to the shelf and rivers, showing high content of illite, subordinate chlorite and kaolinite and scarce smectite. Compared to the terrigenous clays, the hydrothermal clays in the OT have high concentrations of Mg, Mn and Zr but low Fe, Na, K, Ca, Ba, Sr, P, Sc and Ti, while the hydrothermal clays in the mid-ocean ridge are relatively enriched in Fe and V and depleted in Al, Mg, Zr, Sc and Ti. Different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays, diagnostic of variable clay origins. We infer that the OT hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids. The remarkably different compositions of hydrothermal clays between the sediment-rich back arc basin like OT and the sediment-starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid–rock/sediment reactions under various geologic settings.


      PubDate: 2015-03-05T13:03:23Z
       
  • Combined oxygen-isotope and U-Pb zoning studies of titanite: New criteria
           for age preservation
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Chloë E. Bonamici , C. Mark Fanning , Reinhard Kozdon , John H. Fournelle , John W. Valley
      Titanite is an important U-Pb chronometer for dating geologic events, but its high-temperature applicability depends upon its retention of radiogenic lead (Pb). Experimental data predict similar rates of diffusion for lead (Pb) and oxygen (O) in titanite at granulite-facies metamorphic conditions (T=650-800°C). This study therefore investigates the utility of O-isotope zoning as an indicator for U-Pb zoning in natural titanite samples from the Carthage-Colton Mylonite Zone of the Adirondack Mountains, New York. Based on previous field, textural, and microanalytical work, there are four generations (types) of titanite in the study area, at least two of which preserve diffusion-related δ18O zoning. U-Th-Pb was analyzed by SIMS along traverses across three grains of type-2 titanite, which show well-developed diffusional δ18O zoning, and one representative grain from each of the other titanite generations. Type-2 and type-4 titanites show broadly core-to-rim decreasing 206Pb/238U zoning, consistent with Pb diffusion at higher temperatures, and uniform or even slightly increasing 206Pb/238U near grain rims, indicating subsequent recrystallization and/or new growth below the Pb blocking temperature. Type-2 and type-4 grain cores preserve ca. 1160Ma ages that correlate with the anorthosite-mangerite-charnockite-granite magmatic phase of the Grenville orogeny, whereas grain rims give ca. 1050Ma 206Pb/238U ages that coincide with the culminating Ottawan phase. The type-3 titanite grain was sampled from a vein and yields 206Pb/238U dates older than the syenite into which the vein was emplaced; accordingly, its 206Pb/238U dates are interpreted as indicating excess uncorrected common Pb. Type-2 grains with recrystallized or shear-eroded margins show truncated or reversed 206Pb/238U zoning but retain symmetrically decreasing δ18O zoning, consistent with grain margin modification following arrest of Pb diffusion but before arrest of O diffusion. It is concluded that O diffusion was slightly faster than Pb diffusion in Adirondack titanites at the conditions of (local) peak Ottawan metamorphism, making δ18O zoning a useful discriminator of closed-system age domains that did not suffer Pb loss. In addition, the small offset in the O and Pb partial retention zones constrains the timing and temperature of oblique-slip deformation along the Carthage-Colton Mylonite Zone: the details of porphyroclast microstructure and zoning data show that the oblique-slip shear zones were active at ca. 1050Ma, with deformation initiating near the peak of Ottawan metamorphism at ~700°C and continuing through the O blocking temperature at ~550°C.


      PubDate: 2015-03-05T13:03:23Z
       
  • Effect of dissolved organic matter composition on metal speciation in soil
           solutions
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Zong-Ling Ren , Marie Tella , Matthieu N. Bravin , Rob N.J. Comans , Jun Dai , Jean-Marie Garnier , Yann Sivry , Emmanuel Doelsch , Angela Straathof , Marc F. Benedetti
      Knowledge of the speciation of heavy metals and the role of dissolved organic matter (DOM) in soil solution is a key to understand metal mobility and ecotoxicity. In this study, soil column-Donnan membrane technique (SC-DMT) was used to measure metal speciation of Cd, Cu, Ni, Pb, and Zn in eighteen soil solutions, covering a wide range of metal sources and concentrations. DOM in these soil solutions was also fractionated into humic acid (HA), fulvic acid (FA), hydrophilic acid (Hy), and hydrophobic neutral organic matter (HON) by a rapid batch technique using DAX-8 resin. Our results show that in soil solution Pb and Cu are dominant in complex form, whereas Cd, Ni and Zn mainly exist as free ions; for the whole range of soil solutions, only 26.2% of DOM is humic substances and consists mainly of fulvic acid (FA). The metal speciation measured by SC-DMT was compared to the predicted ones obtained via the NICA-Donnan model using the measured FA concentrations. The free ion concentrations predicted by speciation modeling were in good agreement with the SC-DMT measurement. Moreover, we show that to make accurate modeling of metal speciation in soil solutions, the knowledge of DOM composition, especially FA fraction, is the crucial information, especially for Cu and Cd; like in previous studies the modeling of Pb speciation is not optimal and an update of Pb generic binding parameters is required to reduce model prediction uncertainties.


      PubDate: 2015-03-05T13:03:23Z
       
  • Garnet geochemistry records the action of metamorphic fluids in
           ultrahigh-pressure dioritic gneiss from the Sulu orogen
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Yi-Xiang Chen , Kun Zhou , Yong-Fei Zheng , Ren-Xu Chen , Zhaochu Hu
      A combined study of major and trace elements, water content and oxygen isotope composition was carried out for newly grown garnets in ultrahigh-pressure dioritic gneiss from the Sulu orogen. The results indicate the occurrence of a core–rim structure of garnet grains with growth of rims over cores via the mechanism of coupled dissolution–reprecipitation. This provides new insights into the action of geofluids on garnet growth during continental subduction-zone metamorphism. The garnet cores and rims show distinct geochemical compositions. The cores have high spessartine contents (4.2–19.4mol%), relatively low grossular contents (26.5–29.8mol%) and low δ18O values (5.0±0.2‰). The rims have relatively low spessartine contents (1.0–3.4mol%), high grossular contents (28.7–36.1mol%) and high δ18O values (6.1±0.3‰). Furthermore, the rims show much lower contents of Li and HREE, and considerably higher contents of Sc, V and Ga than the cores. In addition, the cores exhibit very low contents of structural hydroxyl in 32±10 to 48±20ppm H2O, whereas the rims show much higher contents of structural hydroxyl in 112±16ppm to 171±34ppm H2O. Although the boundary between core and rim is highly irregular in the garnet grains, the zoning in water contents, element and oxygen isotope compositions are all spatially correlated with each other. Taken all the observations together, it is suggested that the geochemical compositions of garnet cores and rims would be primarily dictated by two types of geofluids with distinctive compositions. Some garnet fragments, bounded by fractures, show the two types of domains with distinct water contents. This difference indicates insignificant exchange of structural hydroxyl by diffusion during growth of the rims over the cores. Mineral-pair O isotope thermometry yields consistent temperatures of ~550°C for the quartz-garnet rim and quartz-titanite, and titanite U–Pb dating yields a metamorphic age of 226±6Ma. Thus, the growth of garnet rims is linked to the action of geofluids (probably anatectic melts) during the exhumation of deeply subducted continental crust. Therefore, the compositions of garnet cores and rims are primarily controlled by the composition of attending fluids. As such, garnet provides a mineralogical record of the fluid action during subduction-zone metamorphism.


      PubDate: 2015-03-05T13:03:23Z
       
  • Seawater inundation of coastal floodplain sediments: Short-term changes in
           surface water and sediment geochemistry
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Vanessa N.L. Wong , Scott G. Johnston , Edward D. Burton , Phillip Hirst , Leigh A. Sullivan , Richard T. Bush , Mark Blackford
      Coastal floodplains are highly vulnerable to seawater inundation as a result of storm surge and sea-level rise due to their low elevation and proximity to the coastline. Intact soil cores from a levee, acid-sulfate soil scald and four backswamp sites on a coastal floodplain in eastern Australia were inundated with artificial seawater treatments (0%, 50% and 100%) for 14days to examine the short term consequences for surface water and floodplain sediment geochemistry. All sites displayed an initial decrease in surface water pH following inundation with 50% and 100% seawater. In addition, higher concentrations of trace metals (Al, Fe, Mn, Ni and Zn) were observed in most sites inundated with 50% or 100% seawater. This was generally attributed to competitive exchange and desorption of trace metals from sediments due to the higher ionic strength of the seawater solutions and upward diffusive flux of metals from the sediments to surface waters. At one backswamp site, reductive processes had established by day 7, which also resulted in elevated Fe2+ concentrations in the overlying surface waters. Transmission electron microscopy (TEM) identified the presence of poorly crystalline ferrihydrite and schwertmannite, and goethite and jarosite. These meta-stable Fe(III) minerals can act as a source of metals for desorption and can also be readily reduced and act as a source of Fe2+ to surface waters. Importantly, inundation with either 50% or 100% seawater resulted in a similar magnitude of acidity and trace metal mobilisation. The data suggest that an inundation event of ~0.2m depth with either 50% or 100% seawater could cause a pulse mobilisation of up to 64.8 and 9.1kgha−1 of Fe and Al, respectively — quantities of similar magnitude to previous estimates of annual drainage fluxes from similar backswamps. This study suggests that the short term inundation of coastal floodplain sediments by either brackish water or seawater will result in rapid declines in surface water quality as a result of increased liberation of acidity and trace metals.


      PubDate: 2015-03-05T13:03:23Z
       
  • Raman identification of Fe precipitates and evaluation of As fate during
           phase transformation in Tinto and Odiel River Basins
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Annika Parviainen , Pablo Cruz-Hernández , Rafael Pérez-López , José Miguel Nieto , José Manuel Delgado-López
      Newly-formed Fe terrace samples and corresponding water samples of Tinto and Odiel Rivers, as well as, ancient terrace samples representing conditions of ancient Tinto River, were collected and studied by optical microscopy, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, microprobe analysis, pseudo-total digestions, and by chemical analytics. The newly-formed terraces showed that currently schwertmannite is precipitating on the riverbed acting as a sink for arsenic. In a matter of months, this metastable phase transforms into goethite and, eventually, jarosite which are found in the deeper sediments below one centimeter. Due to long-term transformation, well-crystallized goethite and diagenetic hematite were the major phases in the ancient terraces. The microanalyses suggested that goethite retained slightly higher concentrations of As than hematite suggesting that As is mobilized in the transformation process. Additionally, this study shows that Raman spectroscopy is an efficient tool in the mineralogical characterization of Fe(III) oxides, hydroxides, and oxyhydroxysulfates at micro to millimeter scale in these types of samples. However, Raman spectra did not provide insights on the incorporation of As within the crystal lattice of schwertmannite, probably due to its low concentration.


      PubDate: 2015-03-05T13:03:23Z
       
  • Cleaning and pre-treatment procedures for biogenic and synthetic calcium
           carbonate powders for determination of elemental and boron isotopic
           compositions
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): M. Holcomb , T.M. DeCarlo , V. Schoepf , D. Dissard , K. Tanaka , M. McCulloch
      In preparing calcium carbonate samples for the measurement of various geochemical proxies, it is often necessary to remove contaminating phases while leaving the phase of interest altered as little as possible. Here we evaluate the effects of some common cleaning protocols (rinsing (H2O), bleach (~3% NaOCl), hydrogen peroxide (30%), sodium hydroxide (0.006–0.1M NaOH), and acid leaching (0.05N HNO3)) on the elemental (Li, B, Na, Mg, Sr, Ba, Pb, and U) and boron isotope composition of both biogenic and synthetic calcium carbonates formed in marine environments. In untreated samples, the presence of elevated concentrations of Na and Mg, the most abundant cations in seawater, can be reduced with minimal cleaning (e.g. rinsing). Cleaning protocols that cause partial dissolution are problematic, especially for samples that are compositionally heterogeneous because the remaining sample may be biased towards particular phases with distinctive elemental or isotopic compositions. We show that the use of either acid or unbuffered hydrogen peroxide can lead to partial dissolution which was associated with an increase in the U/Ca ratio of the remaining sample. Bleaching or rinsing with water did not result in significant sample dissolution, suggesting that these cleaning techniques may be safely used on heterogeneous samples. Cleaning treatments, other than those resulting in significant dissolution of heterogeneous samples, had no significant effect on δ11B, suggesting that boron isotopes are generally robust to the effects of sample pre-treatment.


      PubDate: 2015-03-05T13:03:23Z
       
  • Editorial Board
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398




      PubDate: 2015-03-05T13:03:23Z
       
  • Trace-level plutonium(IV) polymer stability and its transport in
           coarse-grained granites
    • Abstract: Publication date: 26 March 2015
      Source:Chemical Geology, Volume 398
      Author(s): Jinchuan Xie , Yu Wang , Jianfeng Lin , Mei Li , Jihong Zhang , Xiaohua Zhou , Yifeng He
      Colloid-associated actinides are widely studied in both lab-scale and field experiments. However, little is known about the fate and transport of Pu(IV) polymers (i.e., Pu intrinsic colloids) in environmental media. Different opinions on the mobilization of this Pu species were expressed in recent reviews. To fill this gap, we conducted studies on the stability of trace-level Pu(IV) polymers (~10−10 mol/L 239Pu) and on their transport characteristics in saturated coarse-grained granites. The results show that the percentage of polymerized Pu (P >1.5nm, i.e., the fraction of Pu(IV) polymer in the Pu suspensions aged at room temperature) declined within 15days and then slightly decreased to 21.0% at 135days. This reveals slow oxidative solubilization of Pu(IV) polymers. In contrast, the P >1.5nm in the suspensions aged at 95°C declined within 5h and then gradually increased from 5h to 50h. The average particle size of Pu(IV) polymers aged at these two temperature conditions overall decreased, as indicated by the continuous decline in P >450nm/P >1.5nm. Such a decrease in the size of the polymers was further confirmed by the observed reduction in the transport velocity of the polymers with respect to 3H2O (U Pu/U T) through the coarse-grained granites. According to the transport results, the fraction of mobile Pu (R Pu, i.e., the percent recovery of Pu) in the suspensions aged at room temperature followed a linear relationship with P >1.5nm. For the suspensions aged at 95°C, an exponential relationship between R Pu and P >1.5nm existed. These transport results strongly demonstrate that the polymers transported almost unretarded through the granite systems. The fact that the fraction of colloidal Pu (P >1.5nm =~98.5%) in the column effluents was significantly larger than that of Pu(V)aq (~1.6%) supported the finding of their unretarded transport characteristics. For the Pu suspensions aged at 95°C and room temperature, the log[Pu(IV)aq]total (i.e., the solubility of Pu(IV) hydroxide under neutral to alkaline conditions) were experimentally determined as −10.4±0.2 and −10.2±0.3, which are consistent with the reported solubility constant of logKs(14) o =−10.4±0.5. The findings in this study are helpful in understanding the fate and transport of Pu(IV) polymers and accurately assessing the environmental risks of the long-lived toxic 239Pu in the geosphere.


      PubDate: 2015-03-05T13:03:23Z
       
  • Pore-water squeezing from indurated shales
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Martin Mazurek , Takahiro Oyama , Paul Wersin , Peter Alt-Epping
      High-pressure mechanical squeezing was applied to sample pore waters from a sequence of highly indurated and overconsolidated sedimentary rocks in a drillcore from a deep borehole in NE Switzerland. The rocks are generally rich in clay minerals (28–71wt.%), with low water contents of 3.5–5.6wt.%, resulting in extremely low hydraulic conductivities of 10−14–10−13 m/s. First pore-water samples could generally be taken at 200MPa, and further aliquots were obtained at 300, 400 and 500MPa. Chemical and isotopic compositions of squeezed waters evolve with increasing pressure. Decreasing concentrations of Cl−, Br−, Na+ and K+ are explained by ion filtration due to the collapse of the pore space during squeezing. Increasing concentrations of Ca2+ and Mg2+ are considered to be a consequence of pressure-dependent solubilities of carbonate minerals in combination with sorption/desorption reactions. The pressure dependence was studied by model calculations considering equilibrium with carbonate minerals and the exchanger population on clay surfaces, and the trends observed in the experiments could be confirmed. The compositions of the squeezed waters were compared with results of independent methods, such as aqueous extraction and in-situ sampling of ground and pore waters. On this basis, it is concluded that the chemical and isotopic composition of pore water squeezed at the lowest pressure of 200MPa closely represents that of the in-situ pore water. The feasibility of sampling pore waters with water contents down to 3.5wt.% and possibly less opens new perspectives for studies targeted at palaeo-hydrogeological investigations using pore-water compositions in aquitards as geochemical archives.


      PubDate: 2015-03-05T13:03:23Z
       
  • Exploration of hydrothermal carbonate magnesium isotope signatures as
           tracers for continental fluid aquifers, Schwarzwald mining district, SW
           Germany
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Benjamin F. Walter , Adrian Immenhauser , Anna Geske , Gregor Markl
      The significance of magnesium isotope (δ26Mg) fractionation in the continental hydrothermal domain is poorly explored. Here, a detailed Mg isotope dataset from various aquifer host rocks and corresponding hydrothermal carbonate precipitates from the Schwarzwald mining area in SW Germany is documented and discussed. This study is motivated by the very considerable existing data set on hydrothermal mineralizations in the study area and the excellent regional control of sampling points. Based on structural, mineralogical and microthermometrical arguments, we here subdivide three clusters of veins: (i) Permian, (ii) Jurassic–Cretaceous and (iii) post-Cretaceous (Cenozoic). The focus is on clusters ii and iii and their corresponding, texturally older hydrothermal fluid inclusion-rich, coarse-grained, low-Mg calcite and dolomite–ankerite solid solutions and younger, oscillatory zoned low-Mg calcite and dolomite–ankerite solid solution crystals precipitated from fluids with temperatures between 50 and 350°C. In terms of their δ26Mg ratios, three characteristic groups of hydrothermal carbonates can be distinguished: (i) Jurassic–Cretaceous veins (δ26Mg=−3.38 to −0.82‰) in the central Schwarzwald precipitated from fluids containing a significant proportion of sedimentary cover-derived waters. They yield 26Mg-depleted Mg isotope signatures typical of sedimentary carbonate lithologies. (ii) Jurassic–Cretaceous vein carbonates in the southern Schwarzwald precipitated from mainly basement-derived fluids with 26Mg-enriched signatures (δ26Mg=−1.22 to +0.05‰). Supporting evidence for the origin of these fluids comes from Sr–C–Pb isotope systematics and trace element compositional variations of fahlore and sphalerite. The Mg isotope variations are controlled by differences in fluid source characteristics dominate over variations exerted by mineralogical differences (where low-Mg calcites are generally, but not in all cases, more depleted in 26Mg compared to dolomite–ankerite solid solutions). (iii) Post-Cretaceous (Cenozoic), Rhinegraben-related veins represent a complex tectonic juxtaposition of different aquifer lithologies. As expected, this phase spans the full range from silicate to carbonate Mg isotope signatures (+0.45 to −3.4‰). Magnesium isotope data are supported by textural analyses (optical and cathodoluminescence microscopy), electron microprobe, microthermometry and published radiogenic 87Sr/86Sr isotope systematics. Our data are not in agreement with a significant temperature-controlled Δ26Mgfluid-calcite and no obvious relation between fluid salinity and hydrothermal carbonate δ26Mg is found. The results of this study suggest that hydrothermal carbonate magnesium isotope ratios have a significant potential as tracer of hydrothermal fluid sources and corresponding aquifer lithologies.


      PubDate: 2015-03-05T13:03:23Z
       
  • REE patterns of microbial carbonate and cements from Sinemurian (Lower
           
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Giovanna Della Porta , Gregory E. Webb , Iain McDonald
      Marine microbialites serve as robust seawater rare earth element and yttrium (REE+Y) proxies through many intervals of Earth history, but questions remain about the partitioning of REEs into different coeval carbonate phases, potential syn-depositional contaminants and elemental redistribution during diagenesis. Microbial carbonates, cements and background sediments were analysed for stable isotopes (O, C) and trace element geochemistry in Sinemurian (Lower Jurassic) mid- to outer ramp siliceous sponge microbial mounds from the High Atlas, Morocco. Trace elements were analysed using laser ablation-inductively coupled plasma-mass spectrometry. Microbialites, non-luminescent radial/radiaxial fibrous (RF) cement and well-preserved brachiopods have stable isotope values similar to published Early Jurassic marine values. Luminescent blocky sparite (BS) cements have lighter stable isotope values consistent with burial diagenesis. Early marine RF cement has shale-normalised (subscript sn) REE+Y patterns with characteristics of oxygenated seawater, whereas cavity-occluding BS cement has high concentration bell-shaped (REE+Y)sn patterns, very unlike seawater. Allomicrite has relatively high REE concentrations with flatter (REE+Y)sn patterns. Microbialites include three subclasses distinguished on the basis of petrography and (REE+Y)sn patterns. Clotted peloidal microbialites (MC1) have (REE+Y)sn patterns broadly consistent with seawater, but with variable Ce anomalies and higher concentrations and slightly less LREE depletion relative to RF cements. Other clotted peloidal to leiolitic microbialites (MC2) have flatter (REE+Y)sn patterns and variable Ce anomalies, whereas leiolitic microbialites (MC3) have patterns similar to allomicrite. Hence, MC1 microbialites and early marine RF cements preserved seawater-like REE+Y patterns despite subsequent diagenesis, confirming that Early Jurassic marine REE distributions were similar to late Palaeozoic, Late Jurassic and Holocene distributions. Importantly, LREE enrichment in allomicrite and some microbialites (MC2, MC3) highlights the occurrence of LREE-enriched components that may represent marine particulate matter that preferentially scavenged LREEs from the water column prior to sedimentation. The Sinemurian siliceous sponge microbial mounds accumulated in well-oxygenated settings rather than on the edge of an oxygen minimum zone. Some of the high Ce contents in the microbialites may reflect redistribution of Ce during earliest diagenesis in suboxic pore fluids, or incorporation of LREE enriched particles or LREE uptake in the growing microbialite consistent with scavenging on organic ligands in the biofilm itself. This study demonstrates how various sedimentary sources and diagenetic processes can significantly affect otherwise robust marine REE patterns in microbial mounds within a relatively siliciclastic-free carbonate environment.


      PubDate: 2015-03-05T13:03:23Z
       
  • Ni isotope fractionation during sorption to ferrihydrite: Implications for
           Ni in banded iron formations
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Laura E. Wasylenki , Haleigh D. Howe , Lev J. Spivak-Birndorf , David L. Bish
      A major decline in marine methanogen populations during the Late Archaean Eon has been proposed as a driver for the Great Oxidation Event (GOE) at ~2.4Ga. Konhauser et al. (2009, Nature 458, 750–753) recently hypothesized that a dramatic decrease between 2.7 and 2.4Ga in the concentration of dissolved Ni, an essential nutrient for methanogens, may have been responsible for the decline. To test that hypothesis, Ni stable isotopes, a novel proxy for methanogen Ni limitation, will be applied to the Late Archaean–Paleoproterozoic rock record in the future, possibly through analysis of banded iron formations (BIFs). In order to facilitate interpretation of those data, we investigated the fractionation of Ni stable isotopes during sorption to synthetic ferrihydrite, which was likely the primary precipitating phase during BIF deposition, via bench-top experiments. The observed fractionations for adsorption and coprecipitation experiments were indistinguishable and averaged Δ60/58Nidissolved-sorbed =+0.35±0.10‰ (1 sd; light Ni sorbed). Parallel, linear trends in isotope compositions of dissolved and sorbed Ni as a function of fraction of Ni sorbed indicate an equilibrium isotope effect with rapid and continuous exchange between dissolved and sorbed Ni. Additionally, we observed fractionation of Δ60/58Nidissolved–sorbed =+0.23±0.07‰ (1 sd) in experiments in which ferrihydrite had been aged for up to 30days and had partially transformed to goethite and hematite; thus the fractionations in all of our experiments are the same within uncertainties. Although our simple system differs from Archaean seawater, our results suggest that ferrihydrite precipitating to form BIFs may always have been offset by a constant fractionation from coexisting dissolved Ni in seawater.


      PubDate: 2015-03-05T13:03:23Z
       
  • Evidence for initial excess 231Pa in mid-ocean ridge zircons
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Matthew Rioux , Samuel Bowring , Michael Cheadle , Barbara John
      A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived—and therefore most abundant—intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 =32.71ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U–Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325±0.0039Ma (n=9) and 11.920±0.011Ma (n=4), but distinctly older 207Pb/235U dates that vary from 12.330±0.048Ma to 12.140±0.044Ma and 12.03±0.24 to 12.40±0.27Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6±1.0 to 9.6±1.1 and 3.5±5.2 to 11.4±5.8. The data from the more precisely dated sample yields estimated DPa zircon/DU zircon from 2.2–3.8 and 5.6–9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPa zircon/DU zircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt, 2007, 2011). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.


      PubDate: 2015-03-05T13:03:23Z
       
  • Pyrite formation and mineral transformation pathways upon sulfidation of
           ferric hydroxides depend on mineral type and sulfide concentration
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Stefan Peiffer , Thilo Behrends , Katrin Hellige , Philip Larese-Casanova , Moli Wan , Kilian Pollok
      The reaction of ferric (hydr)oxides with dissolved sulfide does not lead to the instantaneous production of thermodynamically stable products but can induce a variety of mineral transformations including the formation of metastable intermediates. The importance of the various transformation pathways depends, among other factors, on the characteristics of the ferric (hydr)oxides but a mechanistic model which relates the mineralogy of the ferric (hydr)oxides to the type of reaction products and their evolution over time is still missing. Here, we investigate the kinetics of the reaction between dissolved sulfide (6.7–7.5mmolL−1) with ferrihydrite (Fh, 12mmolL−1), lepidocrocite (Lp, 26.6mmolL−1), and goethite (Gt, 22mmolL−1) in batch experiments at pH7 and room temperature. The time evolution of solution and solid phase composition was monitored over 2weeks while TEM, and Mössbauer spectroscopy were used to characterize the transformations of the solid phases. Dissolved sulfide was consumed within 2 (Fh, Lp) to 8h (Gt) with methanol extractable sulfur and HCl extractable Fe(II) (Fe(II)HCl) being the main products after this time. The mass balances of Fe and S indicated that a large fraction of the Fe(II)HCl in the reactions with Fh (46% of Fe(II)HCl) and Lp (36% of Fe(II)HCl) was solid-phase bound but not associated with sulfur. This excess Fe(II) exceeded the adsorption capacity of the solids and remained associated with the oxides. Over the time scale of days, the concentrations of MES and Fe(II)HCl decreased and this process was accompanied by the formation of secondary iron oxides and pyrite in all experiments. The pyrite yield after two weeks showed the same trend as the amounts of intermediately produced excess Fe(II): Fh (84% of initial S(−II))>Lp (50%)>Gt (13%). Besides the formation of pyrite, Fh transformed completely into thermodynamically more stable iron oxides such as hematite or magnetite. In contrast, formation of other iron oxides was only minor when Lp or Gt reacted with sulfide. We propose that the extent of pyrite and secondary iron mineral precipitation is controlled by the ratio between the competing formation rates of excess Fe(II) and surface bound FeS (FeSs) in the early stage of the reaction. Formation of excess Fe(II) is a prerequisite for rapid pyrite formation and induces secondary formation of iron oxides. The competition between excess Fe(II) and FeSs formation, in turn, is ruled by two factors: 1) the ratio between added sulfide and available surface area, and 2) the capability of the iron(hydr)oxide to conduct electrons from surface bound Fe(II) to bulk Fe(III) and to accommodate structural Fe(II). This capability is largest for Fh and explains the most pronounced excess Fe(II) production and, by this, the greatest pyrite yield in experiments with Fh. During the reaction with Gt, in contrast, formation of FeSs outcompetes the accumulation of excess Fe(II) and consequently the precipitation of pyrite is only minor. This conceptual model constrains conditions at which relatively fast pyrite formation within the time scale of days or weeks might be relevant in natural environments. Suitable conditions are expected in environments with low sulfide levels in which formation of reactive iron (hydr)oxides is stimulated by redox oscillations (e.g., wetlands, riparian soils, tidal flats).


      PubDate: 2015-03-05T13:03:23Z
       
  • Melting of siderite to 20GPa and thermodynamic properties of FeCO3-melt
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Nathan Kang , Max W. Schmidt , Stefano Poli , Ettore Franzolin , James A.D. Connolly
      The siderite (FeCO3) melting curve is determined through multi-anvil experiments at 6–20GPa, and 1300–1870°C. The experiments define a melting curve with a Clapeyron slope steepening from 45 to 18°C/GPa but without backbend at upper mantle conditions, i.e., siderite is denser than FeCO3-melt (FeCO3L). The melting curve fits T m =1037(44)+70.0(88)* P −1.43(37)* P 2 (valid from 5 to 20GPa) where pressure is in GPa and temperature in °C. Siderite melting is not stoichiometric, minor quench magnetite was always observed and is interpreted as the result of partial redox dissociation of FeCO3L leading to dissolved Fe3+ and CO2 in the carbonate melt. At pressures below ~6.8GPa, siderite does not melt but decomposes through an auto redox dissociation reaction to magnetite, a carbon polymorph and CO2. From the experimental determination of the pure siderite melting curve, we calculate thermodynamic properties of the FeCO3L end-member, which reproduce the siderite melting curve in P–T space better than 10°C. The metastable 1atm melting temperature is calculated to 1012°C. Siderite has the lowest melting temperature of the Ca–Mg–Fe carbonates, its melting curve may cross the mantle geotherm at transition zone pressures. The stability of siderite is not only dependent on pressure and temperature but also strongly on oxygen fugacity (fO2 ). Model calculations in P–T-fO2 space in a Fe–C–O2 system show that the siderite or FeCO3-melt maximum stability is always reached at conditions of the CCO buffer. Our experimental and thermodynamic data constitute a cornerstone to model carbonate melting in the deep Earth, necessary to understanding the deep carbon cycle.


      PubDate: 2015-03-05T13:03:23Z
       
  • Iron isotope fractionation during Fe(II) and Fe(III) adsorption on
           cyanobacteria
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Daniel S. Mulholland , Franck Poitrasson , Liudmila S. Shirokova , Aridane G. González , Oleg S. Pokrovsky , Geraldo R. Boaventura , Lucieth C. Vieira
      The present study aimed at testing the hypothesis that Fe adsorption and precipitation on bacterial phytoplankton likely cause significant isotopic fractionation with preferential adsorption of heavy isotopes on the cell surface. We measured the iron isotopic fractionation during the interaction of aqueous Fe with planktonic cyanobacteria (Gloeocapsa sp., Synehococcus sp., and Planthothrix sp.) in six independent experiments using two distinct Fe oxidation states (Fe(III) and Fe(II)) at pH3 and 6. Isotopic analyses demonstrated that the Fe adsorption on bacterial planktonic biomass yields a clear enrichment of heavy isotopes on the cell surfaces, producing isotopically light δ57Fe values in solutions. The adsorption experiments with Fe(II) in the initial solution yielded a Δ57Fecell-solution =2.66±0.14‰, whereas the adsorption experiments with Fe(III) in the initial solution yielded Δ57Fecell-solution =0.97±0.19‰. Because these data approached closed system equilibrium isotopic fractionation lines rather than Rayleigh curves, the most likely mechanism is a steady state isotopic fractionation, linked with short-term reversible Fe adsorption on cells. In agreement with X-ray Absorption Spectroscopy structural data obtained on the same adsorption experiment samples (González et al., 2014), the preferential enrichment of heavy Fe isotopes on the cell surfaces is attributed to the stronger covalent metal-ligand bonding (FeOC/P) of octahedrally coordinated Fe with phosphoryl or carboxyl groups on the cell walls when compared with the Fe aquacomplexes (OFeO) in solution. The larger isotopic fractionation factor in the experiments starting with Fe(II) results from the iron oxidation to Fe(III) which is itself accompanied by a strong isotopic effect. A natural case study of warm hydrothermal spring depositing Fe oxy(hydr)oxide with and without cyanobacterial biomass yielded an isotopic fractionation between the solid phase and aqueous solution (Δ57Fesolid-solution) of 0.62±0.16‰ (biotic) and 0.80±0.06‰ (abiotic). This natural case study therefore confirms experimental results, albeit of smaller magnitude. These findings suggest that Fe adsorption on cyanobacteria cell surfaces in nature should produce cell enrichments in heavier isotopes relative to the coexisting aqueous solution.


      PubDate: 2015-03-05T13:03:23Z
       
  • Accurate determination of elements in silicate glass by nanosecond and
           femtosecond laser ablation ICP-MS at high spatial resolution
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Zhen Li , Zhaochu Hu , Yongsheng Liu , Shan Gao , Ming Li , Keqing Zong , Haihong Chen , Shenghong Hu
      Despite the large number of successful applications of LA-ICP-MS, elemental fractionation remains the main limitation for many of its applications in the Earth sciences. This limitation is particularly notable for high spatial resolution analysis. Elemental fractionation and mass-load effect in silicate glasses NIST SRM 610 and GSE-1G were investigated by using 193nm ArF excimer nanosecond (ns) laser and 257nm femtosecond (fs) laser ablation systems coupled to inductively coupled plasma mass spectrometry. Contrary to those observed in ns-LA-ICP-MS, the most elemental fractionation at the small spot sizes of 16–24μm are lower than that at the large spot sizes of 44–60μm in fs-LA-ICP-MS. The significantly different fractionation behaviors of Li, Na, Si, K, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Cs and U between silicate glass materials NIST SRM 610 and GSE-1G observed in 193nm excimer LA-ICP-MS are eliminated by using 257nmfs-LA-ICP-MS at high spatial resolution. In addition, the mass load effect and matrix dependent mass load effect are also found to be reduced by using fs-LA-ICP-MS in comparison with ns-LA-ICP-MS. Except for Sb, Pb and Bi, the elemental fractionation is independent on the laser fluence chosen, which is irrespective of ns- or fs-LA-ICP-MS. In this study, a spot size of 24μm was used to test the capabilities of LA-ICP-MS analysis at high spatial resolution. The agreement between our data and the reference values is better than 10% for most of the elements in MPI-DING, USGS, and NIST glasses by using fs-LA-ICP-MS. For ns laser ablation analysis, the accuracy is highly dependent on the calibration strategies used (conventional external calibration method or 100% oxide normalization method) and the selected external reference materials (NIST SRM 610 or GSE-1G). The much less laser-induced elemental fractionation and matrix effect in fs-LA-ICP-MS in comparison with 193nm excimer LA-ICP-MS make it more suitable for the analysis of silicate materials at high spatial resolution.
      Graphical abstract image

      PubDate: 2015-03-05T13:03:23Z
       
  • Modeling plutonium sorption to kaolinite: Accounting for redox equilibria
           and the stability of surface species
    • Abstract: Publication date: 14 April 2015
      Source:Chemical Geology, Volume 400
      Author(s): Rémi Marsac , Nidhu L. Banik , Johannes Lützenkirchen , Razvan A. Buda , Jens V. Kratz , Christian M. Marquardt
      Plutonium with its particularly complex redox chemistry may be thermodynamically stable in the states +III to +VI depending on the redox conditions in the environment. Mineral surfaces can also affect Pu redox speciation. Therefore, the interpretation of Pu sorption data becomes particularly challenging, even for simplified laboratory experiments. The present study focuses on Pu sorption to kaolinite. Am(III), Th(IV), Np(V) and U(VI) literature sorption data are used as analogues for the corresponding Pu redox states to calibrate a simple surface complexation model, and the Nernst formalism is applied. Two independent pH–pe diagrams, one for the kaolinite surface and another for the aqueous phase, are constructed and superimposed. This allows visualization of the prevalent Pu redox state in both phases. The model suggests that the stability field of the most strongly adsorbing redox state is larger at the surface than in solution. Because Pu(V) weakly sorbs to kaolinite, it never prevails at the surface. Within the stability field of Pu(V) in 0.1M NaClO4 solution, Pu(VI) and Pu(IV) prevail at the kaolinite surface under oxidizing and slightly reducing conditions, respectively. By contrast, the Pu(IV)/Pu(III) boundary is hardly affected because both redox states strongly sorb to kaolinite, especially for pH>6. The present method is applied to literature data for Pu sorption to kaolinite. By estimating the pe from a Pu redox state analysis in solution, overall Pu uptake could be predicted. Generic equations are derived that are applicable to minerals and actinides other than kaolinite and Pu. The present study provides important progress in understanding Pu geochemistry, especially in the context of nuclear waste disposal where thermodynamic models are particularly necessary to predict Pu mobility.


      PubDate: 2015-03-05T13:03:23Z
       
  • The Mg/Ca–temperature relationship in brachiopod shells: Calibrating
           a potential palaeoseasonality proxy
    • Abstract: Publication date: 18 March 2015
      Source:Chemical Geology, Volume 397
      Author(s): Scott Butler , Trevor R. Bailey , Caroline H. Lear , Gordon B. Curry , Lesley Cherns , Iain McDonald
      Brachiopods are long-lived, long-ranging, extant organisms, of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell. Previous work has suggested that the incorporation of Mg into brachiopod calcite may be controlled by temperature (Brand et al., 2013). Here we build upon this work by using laser ablation sampling to define the intra-shell variations in two modern brachiopod species, Terebratulina retusa (Linnaeus, 1758) and Liothyrella neozelanica (Thomson, 1918). We studied three T. retusa shells collected live from the Firth of Lorne, Scotland, which witnessed annual temperature variations on the order of 7°C, in addition to four L. neozelanica shells, which were dredged from a water depth transect (168–1488m) off the north coast of New Zealand. The comparison of intra-shell Mg/Ca profiles with shell δ18O confirms a temperature control on brachiopod Mg/Ca and supports the use of brachiopod Mg/Ca as a palaeoseasonality indicator. Our preliminary temperature calibrations are Mg/Ca=1.76±0.27 e(0.16±0.03)T, R 2 =0.75, for T. retusa and Mg/Ca=0.49±1.27 e(0.2±0.11)T, R 2 =0.32, for L. neozelanica (errors are 95% confidence intervals).


      PubDate: 2015-02-07T18:04:02Z
       
 
 
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