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  Subjects -> EARTH SCIENCES (Total: 609 journals)
    - EARTH SCIENCES (448 journals)
    - GEOLOGY (67 journals)
    - GEOPHYSICS (28 journals)
    - HYDROLOGY (18 journals)
    - OCEANOGRAPHY (48 journals)

EARTH SCIENCES (448 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: 8)
Aeolian Research     Hybrid Journal   (Followers: 2)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 4)
AMBIO     Hybrid Journal   (Followers: 12)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 1)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 16)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 10)
Anthropocene Review     Hybrid Journal   (Followers: 1)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 7)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 9)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 5)
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: 10)
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: 17)
Bulletin of Volcanology     Hybrid Journal   (Followers: 13)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 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: 5)
Contemporary Trends in Geoscience     Open Access  
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: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 202)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 7)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 12)
Earth System Dynamics     Open Access   (Followers: 6)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 12)
Ecohydrology     Hybrid Journal   (Followers: 11)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  
Erwerbs-Obstbau     Hybrid Journal  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology     [SJR: 1.714]   [H-I: 111]
   [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2585 journals]
  • 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: Available online 22 January 2015
      Source:Chemical Geology
      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 hardparts 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 hardparts, that spent in a host rock definitely more time than in an animal body, must be rigidly handled as a constituent of a specific rock, never as a separate item.


      PubDate: 2015-01-22T20:20:05Z
       
  • Copper sorption by the edge surfaces of synthetic birnessite nanoparticles
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Jasquelin Peña , John R. Bargar , Garrison Sposito
      We investigated the sorption of Cu by δ-MnO2, an analog for natural birnessite (layer-type Mn oxide) that is characterized by randomly stacked and curled nanosheets, a low to moderate vacancy content, and variable amounts of layer and interlayer Mn3+. The synthetic δ-MnO2 used in this study had a Na:Mn molar ratio of 0.13, an average manganese oxidation number (AMON) of 3.85 after reaction, a specific surface area of 254m2 g−1 and a particle size of 2–4nm in the ab plane. The maximum surface excess (q max) value at pH6 estimated from sorption data of 0.72 (0.64–0.83, 95% confidence interval) molCumol−1 Mn far exceeded the nominal vacancy content for δ-MnO2 (ca. 6–11% molvacancymol−1 Mn), thus implicating multiple binding sites for Cu. The large values of q max and specific surface area of the mineral suggest a major role for surface sites at the particle edges relative to vacancy sites. The extended X-ray absorption fine structure (EXAFS) spectra from δ-MnO2 samples differ with respect to the EXAFS spectra for Cu(OH)2, CuO, and Cu3(CO3)2(OH)2 and Cu-sorbed by biogenic MnO2. The Cu K-edge EXAFS spectra show two second-shell peaks that can be modeled with Mn and Cu near-neighbors. Copper appears to bind dominantly at particle edges of δ-MnO2 as dimers or polynuclear surface species. This sorption mechanism is consistent with the moderate vacancy content of δ-MnO2 and explains the similarity in the EXAFS spectra from samples having surface loadings of 0.01 to 0.26molCumol−1 Mn. The strong proclivity of Cu to bind on the edge surfaces of nanoparticulate birnessite leads to very large surface excesses of Cu without the formation of a discreet precipitate, making the surface sites at the particle edges the dominant sorption site for Cu.


      PubDate: 2015-01-22T20:20:05Z
       
  • Uranium and molybdenum isotope systematics in modern euxinic basins: Case
           studies from the central Baltic Sea and the Kyllaren fjord (Norway)
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): J. Noordmann , S. Weyer , C. Montoya-Pino , O. Dellwig , N. Neubert , S. Eckert , M. Paetzel , M.E. Böttcher
      Recent investigations have revealed significant fractionation of 238U/235U between organic-rich sediments of anoxic marginal seas and seawater, indicating redox-dependent U isotope fractionation. This study explores the conditions controlling U isotope fractionation in selected modern anoxic basins (Baltic Sea: Landsort and Gotland Deeps and the Kyllaren fjord in Norway) and compares U with Mo isotope fractionation. Therefore, the concentrations and isotope compositions of dissolved U and Mo from the water column and organic-rich sediments from three currently anoxic basins have been analysed. The water column samples from the Kyllaren fjord display a moderate depletion of U and a strong depletion of Mo with increasing depth. These variations are associated with a decrease in δ238U and an increase in δ98Mo with depth, from −0.35 ‰ to −0.70 ‰ and from 2.4 ‰ to 2.6 ‰, respectively. From the U isotope composition of the deep euxinic water column of the Kyllaren fjord, a minimum value of Δ238Ured ≈0.7 ‰ for the U isotope fractionation during reduction is inferred. Due to the high sedimentation rate in the Kyllaren fjord, surface sediment samples are only moderately enriched in U and Mo (~4μg/g and 6μg/g to 37μg/g, respectively) and display δ238U and δ98Mo averages of −0.26 ‰±0.10 ‰ and 2.18 ‰±0.21 ‰, respectively. Water column samples from the Baltic Sea display a minor decrease in the U and Mo concentrations and only negligible U and Mo isotope fractionation compared to open seawater. The sediment layers from the Baltic Sea (Landsort Deep), which were deposited under mostly anoxic conditions, are moderately enriched in U (up to 8μg/g) and highly enriched in Mo (up to 222μg/g). However, U and Mo isotopic compositions are significantly lighter (with δ238U of −0.36 ‰±0.08 ‰ and δ98Mo of −0.04 ‰±0.11 ‰) than those of typical organic-rich sediments from anoxic basins. These findings reveal that compared to Mo, the U isotopic composition of sediments in restricted and strongly euxinic basins like the Kyllaren fjord is significantly more dependent on (1) the extent of U removal from the water column, which is lower than that of Mo, and on (2) the sedimentation rate (i.e., the fraction of authigenic U relative to detrital U in the sediment), which is also lower compared to that of Mo. In more open and only temporarily euxinic basins such as the investigated basins of the Baltic Sea, strong Mo is coupled with weak U isotope fractionation between water and sediment. These signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light Mo during frequently occurring flushing events with oxygen-rich water from the open sea. This implies that Mo and U isotope signatures of sediments only record paleo-water column redox conditions of restricted basins if the water column was permanently stratified.


      PubDate: 2015-01-22T20:20:05Z
       
  • The origin of solutes within the groundwaters of a high Andean aquifer
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Clinton Rissmann , Matthew Leybourne , Chris Benn , Bruce Christenson
      This paper investigates the origin of solutes within the groundwaters of the Monturaqui–Negrillar–Tilopozo (MNT) aquifer system within the high Andes of the Atacama Desert that discharges into the Salar de Atacama. Key questions include the relative significance of volcanic hydrothermal processes and evaporitic brine recycling over solute supply as well as the pathways of solute ingress to the MNT aquifer system. Groundwaters were analysed for elemental (major, minor and trace) and isotopic (δ18O/δ2H; δ13C–DIC; δ34S–SO4; 87Sr/86Sr) constituents to which various hydrochemical and multivariate statistical methods have been applied. Groundwaters are all classified as thermal and show increasing temperatures (27–35°C) and concentrations of HCO3 (4.4–10.4mmolL−1 dissolved inorganic carbon [DIC]) with increasing proximity to Volcano Socompa resulting from an increasing mass flux of steam and magmatic CO2 (pCO2 =0.016 to 0.10atm; δ13C–CO2 =−9.3 to −3.6‰ (V-PDB)) boiled off a deep hydrothermal reservoir. Superimposed upon this gradational and relatively smooth spatial increase in heat and mass flow is a sharp, structurally controlled, increase in TDS (826–3632mgl−1) and a concomitant change in δ34S–SO4 (+0.79 to 4.9‰ (V-CDT)) and 87Sr/86Sr values (0.707375–0.706859) associated with the inflow of evaporitic solutes. Evaporitic inputs are chemically and isotopically distinct from localised secondary hydrothermally derived solutes with major, minor and trace element data suggesting an origin within a highly oxidising, alkaline, evaporitic lake receiving dilute inflows enriched in volcanic/fumarolic sulfur mineralisation probably from volcanoes Socompa, Salín or Pular that delimit the eastern topographic extent of the aquifer system. The conceptual model presented in this paper proposes that basal leakage of evaporitic brines from active salar(s), within the high altiplano/volcanic arc, are actively entrained by sub-regional groundwater flow and conveyed to the MNT aquifer system where they mix with solutes derived from localised secondary hydrothermal gas–water–rock interaction. This work provides detail on the origin and processes controlling the solute composition of groundwater inflows to the Salar de Atacama within the volcanically active and hyperarid Atacama Desert and may be of significance to conceptual models of evaporitic brine evolution, recycling of evaporitic brines and hydrothermalism in arid regions.


      PubDate: 2015-01-22T20:20:05Z
       
  • High-temperature equilibrium isotope fractionation of non-traditional
           stable isotopes: Experiments, theory, and applications
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Edward D. Young , Craig E. Manning , Edwin A. Schauble , Anat Shahar , Catherine A. Macris , Codi Lazar , Michelle Jordan
      High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors.


      PubDate: 2015-01-19T01:34:47Z
       
  • Proposed sources of methane along the Dead Sea Transform
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): N. Avrahamov , F. Gelman , Y. Yechieli , Z. Aizenshtat , A. Nissenbaum , O. Sivan
      The concentrations and isotopic compositions of methane, higher alkanes, dissolved inorganic carbon (DIC) and CO2 were studied in fresh groundwater, brines and springs along the Dead Sea Transform (DST), from the Hula Valley in the north to the Dead Sea (DS) basin in the south. Although the occurrence of methane along the DST was documented before, this is the first time that comprehensive research was conducted on the methane provenance and the post-genetic reactions involved. The methane stable carbon isotopic composition (δ13CCH4) shows a distinct geographic pattern. In the northern part of the DST studied, in the Hula valley and Lake Kinneret, where recent marshy and lacustrine environments exist, the methane source seems to be related to intense shallow depth methanogenesis. This microbial methane shows low 13C values in the range of δ13CCH4 between −58‰ and −72‰ and high alkane ratios (C1/C2 +C3) between 100 and 1000. The isotope fractionation α CO2–CH4 of 1.065‰ suggests the domination of hydrogenotrophic methanogenesis rather than acetoclastic methanogenesis. In the waters of Hammat Gader and Tiberias hot spring, whose source is the Cretaceous aquifer, δ13CCH4 has high values of −28‰ and −56‰, respectively, with low alkane ratios of ~40 in both sites. The high δ2HCH4 values of Hammat Gader, in addition to the isotopic composition of the DIC and the difference between CO2 and CH4, reject the possibility of local microbially produced methane and suggest a sub-surface thermogenic source of an initial stage of thermal degradation of bituminous chalk of Senonian age. Similarly, methane in groundwater from the DS basin is also interpreted to be of thermogenic origin. In accordance with the local geology context in the DS basin of buried asphalt, and due to the low geothermal gradient in the region, it is speculated that methane is produced from slight heating of the asphalts. In general, it seems that there is a thermogenic methane contribution along the entire DST, which is locally masked by high microbial activity in shallow organic-rich environments, such as Lake Kinneret and the Hula basin.


      PubDate: 2015-01-19T01:34:47Z
       
  • Evaluating the removal of non-detrital matter from soils and sediment
           using uranium isotopes
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Ashley N. Martin , Anthony Dosseto , Leslie P.J. Kinsley
      The time elapsed since detrital minerals were reduced to <63μm by weathering can be constrained by applying the comminution dating method, which is based on the (234U/238U) activity ratio and surface area properties of the detrital minerals. In order to constrain an accurate age, the detrital minerals should be isolated and non-detrital matter present must be completely removed. Here we evaluate current sample pre-treatment procedures for removing non-detrital matter using uranium isotopes. The (234U/238U) activity ratio of the solid residue decreased stepwise throughout sequential extraction procedures, which is attributed to the removal of non-detrital matter. Despite the heterogeneity observed in the untreated samples, the final (234U/238U) activity ratio of solid residues from replicate experiments were within analytical error. This shows that the (234U/238U) activity ratio of the detrital minerals is consistent following removal of non-detrital matter. The addition of a complexing agent (sodium citrate) to extraction reagents decreased the readsorption of uranium, but did not affect the final (234U/238U) activity ratio. Mild HF/HCl etching experiments showed that the (234U/238U) activity ratio can be further decreased following sequential extraction. Particle-size distribution measurements revealed that the decrease in the (234U/238U) activity ratio is likely due to the dissolution of clay minerals. Mild HF/HCl etching of a rock standard also revealed a small amount of preferential leaching of 234U (<1%). The inferred comminution ages are generally beyond the limit of the technique (1000ka). By assuming an initial activity ratio of 0.95 to account for preferential leaching effects, the ages of samples following sequential extraction were within analytical error. Mild HF/HCl etching following sequential extraction results in older ages, which is attributed to the further removal of clay minerals. We recommend sequential extraction followed by mild HF/HCl etching as sample pre-treatment for comminution dating studies.


      PubDate: 2015-01-19T01:34:47Z
       
  • High precision Os isotopic measurement using N-TIMS: Quantification of
           various sources of error in 186Os/188Os measurements
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Rudra Chatterjee , John C. Lassiter
      The 190Pt–186Os system could be useful to study a number of different geologic processes. Over the last decade, this system has received considerable attention owing to its potential to detect core–mantle signatures in plume-derived lavas. Additionally, this system may also be useful as a proxy for melt depletion, examining the role of pyroxenites/sulfides in mantle melting and for dating of Platinum Group Element (PGE) based ores. The total natural variation of 186Os/188Os is limited (~150ppm in the mantle), which thus requires extremely high levels of accuracy and precision to discern natural variations. 186Os/188Os ratios were measured in laboratory standards and natural samples to examine and quantify the sources of error affecting high precision 186Os/188Os measurements and a set of refined analytical procedures to minimize and reduce these various sources of error were recommended. Inter and intra-run variations in oxygen isotopic composition used in oxide correction of OsO3 − peaks can produce systematic shifts in 186Os/188Os. Useful analysis time is wasted in measuring O isotopic composition line-by-line and it is sufficient to utilize pre- and post-run determination of O isotopic composition for performing oxide correction. Although the use of 189Os/188Os as a normalizing ratio for mass fractionation results in lower uncertainty in Os isotopes from oxide correction, this benefit is overwhelmed by the increased error propagation from mass fractionation. Instead, usage of the 192Os/188Os normalization generates the least propagated error due to mass fractionation and is recommended. Earlier studies have variably used different O isotopic compositions for oxide correction and different normalizations for mass fractionation correction, which can generate systematic shifts in the reduced data. Use of mutually consistent normalizing values for mass fractionation largely reduces the offsets between reduced data using different normalizing schemes, thus improving data comparison across laboratories. For the quantities of Os and beam intensities of 186Os utilized in previous 186Os/188Os studies (several 10's of ng Os, 80–250mV of 186Os with a 1011 Ω resistor), the largest source of analytical error is derived from Johnson noise on baseline integrations. Increasing the duration of baseline measurement significantly reduces the Johnson noise error, allowing to achieve maximum external precision for a given sample size. Several 186Os/188Os analyses reported in the literature seem to have been compromised by an interference, which may be WO3 −, organics or some other unidentified species that affects both the 186Os/188Os and the 184Os/188Os ratios. This makes it extremely critical to monitor and correct PtO2 − interferences, which could mask other potential interferences that directly affect 186Os/188Os measurements.


      PubDate: 2015-01-19T01:34:47Z
       
  • Seasonal and spatial variations in rare earth elements to identify
           inter-aquifer linkages and recharge processes in an Australian catchment
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Clément Duvert , Dioni I. Cendón , Matthias Raiber , Jean-Luc Seidel , Malcolm E. Cox
      With the aim of elucidating the seasonal behaviour of rare earth elements (REEs), surface and groundwaters were collected under dry and wet conditions in different hydrological units of the Teviot Brook catchment (Southeast Queensland, Australia). Sampled waters showed a large degree of variability in both REE abundance and normalised patterns. Overall REE abundance ranged over nearly three orders of magnitude, and was consistently lower in the sedimentary bedrock aquifer (18ppt<∑REE<477ppt) than in the other hydrological systems studied. Abundance was greater in springs draining rhyolitic rocks (∑REE=300 and 2054ppt) than in springs draining basalt ranges (∑REE=25 and 83ppt), yet was highly variable in the shallow alluvial groundwater (16ppt<∑REE<5294ppt) and, to a lesser extent, in streamwater (85ppt<∑REE<2198ppt). Generally, waters that interacted with different rock types had different REE patterns. In order to obtain an unbiased characterisation of REE patterns, the ratios between light and middle REEs (R(M/L)) and the ratios between middle and heavy REEs (R(H/M)) were calculated for each sample. The sedimentary bedrock aquifer waters had highly evolved patterns depleted in light REEs and enriched in middle and heavy REEs (0.17<R(M/L) <1.00 and −0.16<R(H/M) <0.93), whereas the springs draining intrusive and extrusive rocks had relatively flat patterns (0.20<R(M/L) <0.38 and −0.16<R(H/M) <0.09). Surface waters were generally enriched in middle REEs (median R(M/L) =0.35 and median R(H/M) =−0.04), and waters from the shallow alluvial aquifer had very diverse patterns with important spatial variations. Samples collected from the alluvium exhibited an increasing influence of the sedimentary bedrock from upgradient to downgradient; typically they showed flat patterns in the upstream section of the alluvium (median R(M/L) =0.21 and median R(H/M) =-0.06) gradually evolving towards patterns depleted in light REEs and enriched in middle and heavy REEs downgradient (median R(M/L) =0.48 and median R(H/M) =0.38). To document the seasonal variations in REE patterns, the difference in ratios between dry and wet sampling campaigns was determined for each repeated sampling location. Contributions from the sedimentary bedrock water to the alluvium during the wet season were identified at two locations (increase from R(H/M) =0.03 and 0.35 to R(H/M) =0.62 and 0.89). The effect of recharge through fractured igneous rocks was also observed in two boreholes intercepting the sedimentary bedrock, where the freshly recharged waters likely contributed to the deeper groundwater flow during the wet season (decrease from R(M/L) =0.81 and 0.56 to R(M/L) =0.46 and 0.17). Results from this study suggest that REEs may be usefully applied as indicators of recharge processes and inter-aquifer mixing. They also underline the importance of conducting seasonal sampling campaigns to capture possible short-term variations in REE patterns and abundance, which is essential to enable a better understanding of hydrological and hydrochemical processes in complex geological settings.


      PubDate: 2015-01-19T01:34:47Z
       
  • Copper isotope fractionation during adsorption onto kaolinite:
           Experimental approach and applications
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Dandan Li , Sheng-Ao Liu , Shuguang Li
      The adsorption of copper and other heavy metals onto clay minerals is an important process that controls the distribution of trace metals in natural environments. Copper isotopes are a potentially useful tool to track the source of contaminated metals in soils formed in natural systems, but Cu isotope fractionation during adsorption onto clay minerals, the major component in soils, has not been thoroughly studied. In this study, we carried out for the first time a series of experiments to investigate the isotope fractionation of Cu during adsorption onto kaolinite for a wide range of conditions, including the contact time (t=10–360min), temperature (T=1–50°C), initial Cu concentration of the starting solution (C0 =2–100μg/g), pH value (4.0–6.0) and ionic strength (NaNO3; I=0–0.1mol/L). Our results indicate that Cu isotopes are significantly fractionated with preferential adsorption of the light isotope (63Cu) onto the mineral surface. The fractionation factors (Δ65Cuadsorbed-solution =δ65Cuadsorbed −δ65Cusolution) weakly depend on the pH and temperature with a constant value of approximately −0.27‰ at C0 =20μg/g and in the absence of NaNO3. Addition of NaNO3 into the starting solution has a dramatic negative influence on the Δ65Cuadsorbed-solution values that range from −1.46‰ to −0.29‰. Such results are useful for interpreting Cu isotopic variations observed in sediments, soils and water from estuarine settings or industrial sewage pollution areas. The Δ65Cuadsorbed-solution values significantly increase with increasing initial Cu concentration of the starting solutions at C0 <30μg/g, but approach a stable value of −0.17±0.10‰ (2SD) when the kaolinite has reached its maximum adsorption capacity at C0 >30μg/g. The results imply that the isotopic compositions of the Cu adsorbed onto natural soils may vary greatly at relatively low Cu concentrations of the soil solutions. Furthermore, the pore waters after draining kaolinite-bearing rocks would become isotopically heavier due to the preferential adsorption of 63Cu onto kaolinite. Given that no redox change occurred in all experiments, we propose that the most likely mechanism responsible for such Cu isotope fractionation is the different adsorption capacities of isotopically different species in aqueous solutions and the formation of outer-sphere surface Cu(II) complexes. Our study represents one important step for future studies to use Cu isotopes to trace the source of metal contaminants in natural soils.


      PubDate: 2015-01-19T01:34:47Z
       
  • Boron isotope systematics of a fossil hydrothermal system from the Troodos
           ophiolite, Cyprus: Water–rock interactions in the oceanic crust and
           subseafloor ore deposits
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Kyoko Yamaoka , Seiya Matsukura , Tsuyoshi Ishikawa , Hodaka Kawahata
      We determined concentrations and isotopic compositions of boron in a complete section of the hydrothermally altered Cretaceous oceanic crust of the Troodos ophiolite. The boron content and δ11B value for each lithological section are: pillow lava (3.8–206.8μg/g, 63μg/g average; δ11B=+0.17‰ to +15.6‰, +8.1‰ average), sheeted dike complex (0.6–18.0μg/g, 4.0μg/g average; δ11B=+3.3‰ to +10.6‰, +6.0‰ average), and plutonic complex (0.3–8.4μg/g, 1.7μg/g average; δ11B=−1.7‰ to +18.5‰, +4.5‰ average). These boron contents are higher than the estimated original igneous values throughout the oceanic crust, indicating uptake of boron from seawater and hydrothermal fluid at temperatures ranging from ≤50°C to >300°C. Although our boron data for the Troodos ophiolite are generally consistent with those for the Oman ophiolite of similar age, the distinctly low δ11B values of the lower gabbro section in the Troodos ophiolite (<+3‰) suggest reaction with 11B-depleted fluid at a very small water/rock ratio. The boron content of the bulk oceanic crust (12.3μg/g) estimated for the Troodos ophiolite is relatively high as a result of strong boron enrichment in the pillow lava section, which underwent prolonged seafloor weathering. Despite these differences, the weighted average δ11B value of the bulk oceanic crust (+7.6‰) is similar to that of the Oman ophiolite (+7.9‰). We also analyzed the boron isotope geochemistry of a subseafloor hydrothermal stockwork sulfide deposit in the Troodos ophiolite to investigate its formation processes. In contrast to the normal upper oceanic crust, the δ11B values of the rocks below the ore body decrease with increasing depth and have large negative values (−6‰) in the highly altered uppermost dike section. These low δ11B values are coupled with high boron contents (2.5–17μg/g) and high and uniform 87Sr/86Sr ratios (0.7064 average), and are unlikely to have resulted from interactions with fluids at a small water/rock ratio. These characteristics are better explained by interaction of ore-forming hydrothermal fluids with oceanic crust that had previously been enriched in boron through hydrothermal alteration at low temperatures. These observations demonstrate that boron and boron isotopes are useful for quantitative evaluation of fluid-related processes with multiple stages, including petrogenesis of hydrothermal ore deposits.


      PubDate: 2015-01-19T01:34:47Z
       
  • Isotopic patterns of hydrothermal hydrocarbons emitted from Mediterranean
           volcanoes
    • Abstract: Publication date: Available online 9 January 2015
      Source:Chemical Geology
      Author(s): Jens Fiebig , Sven Hofmann , Franco Tassi , Walter D’Alessandro , Orlando Vaselli , Alan B. Woodland
      We have analyzed the carbon isotopic composition of CO2, methane, ethane, propane and n-butane, the hydrogen isotopic composition of methane as well as total concentrations of gas constituents contained in the Mediterranean volcanic-hydrothermal discharges of Nisyros (Greece), Vesuvio, La Solfatara, Ischia and Pantelleria (all Italy) to determine the origin of the hydrocarbons. Isotopic criteria conventionally used for hydrocarbon classification suggest thermogenic origins, except for Pantelleria, for which an abiogenic origin is indicated. These findings would imply that thermogenic sources can provide methane/(ethane+propane) concentration ratios as high as those usually observed for microbial hydrocarbons. However, additional consideration of gas concentration data challenges the suitability of conventional criteria for the classification of hydrocarbons emanating from hydrothermal environments. Methane seems to be in close equilibrium with co-occurring CO2, whereas its higher chain homologues are not. Therefore, it cannot be excluded that methane on the one hand and ethane, propane and n-butane on the other hand have distinct origins. The carbon isotopic composition of methane might be controlled by the carbon isotopic composition of co-occurring inorganic CO2 and by hydrothermal temperatures whereas the carbon isotopic composition of the higher n-alkanes could correspond to the maturity of organic matter and/or to the residence time of the gases in the source system.


      PubDate: 2015-01-13T06:28:05Z
       
  • Mercury in some arc crustal rocks and mantle peridotites and relevance to
           the moderately volatile element budget of the Earth
    • Abstract: Publication date: Available online 9 January 2015
      Source:Chemical Geology
      Author(s): Dante Canil , Peter W. Crockford , Riccardo Rossin , Kevin Telmer
      We measured Hg concentrations in 37 igneous rocks from an arc crustal section and in 30 mantle peridotites from ophiolite, orogenic massif and xenolith settings. Mercury is heterogeneously distributed in the igneous rocks and shows a ‘nugget effect’, suggesting it is concentrated in a trace phase, likely sulfide. The abundance of Hg in the crustal samples varies from 0.9 - 8ppb and correlates with S and Cu but no other element indicative of differentiation. The average of our data produces 2.9±2.6 Hg for the bulk crust, a factor of 10 lower than previous estimates. The mantle peridotites contained 0.2 - 5ppb Hg and a correlation of Hg with Al, Cu, S or loss on ignition (LOI) depending on sample type. Secondary uptake of Hg due to low-temperature alteration or mantle metasomatism is evident in the ophiolite and orogenic massif samples, respectively. The primitive upper mantle (PUM) contains 0.4 - 0.6ppb Hg based on the depletion/enrichment trends in the fresh xenolith samples that demonstrably retained primary Cu/S during emplacement. During mantle melting to produce the crust, Hg behaves as a mildly incompatible element (DHg residue/melt~0.1), not unlike Cu. For a chondritic abundance of 310ppb Hg, our estimate for Hg in the mantle requires this element has a similar depletion to Se, Te or S in the bulk silicate Earth.


      PubDate: 2015-01-13T06:28:05Z
       
  • Compositional and temperature effects of phosphoric acid fractionation on
           Δ47 analysis and implications for discrepant calibrations
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): William F. Defliese , Michael T. Hren , Kyger C. Lohmann
      An essential procedure to increase the analytical efficiency of Δ47 measurements requires raising the temperature of phosphoric acid digestion for carbonate materials. This temperature change introduces a fractionation offset in Δ47 that must be accounted for prior to calculation of temperatures of carbonate formation and to allow interlaboratory comparison of results. We measured the phosphoric acid fractionation factor relative to reaction at 25°C for calcite, aragonite, and dolomite across a temperature range from 25 to 90°C. Significantly, all three minerals behave similarly during phosphoric acid digestion, allowing for a single temperature dependent acid fractionation relationship: 1000 ln ∝ C O 2 Acid − Δ 47 = 0.022434 ± 0.001490 ∗ 10 6 T 2 − 0.2524 ± 0.0168 where α is the phosphoric acid fractionation factor, and T is in degrees Kelvin. Mineralogical or isotopic compositional effects on the fractionation factor were not observed, suggesting that this acid fractionation factor may be valid for all carbonate minerals. We also present inorganic temperature calibrations for both calcite and aragonite at low temperatures (5–70°C) and find them to agree with prior published data. Using the new acid fractionation factor, published Δ47–temperature calibrations are recalculated. This analysis confirms a statistically significant Δ47–temperature calibration difference between data analyzed at 25°C versus higher temperatures. The origin of the discrepancy remains unknown, but it appears that the acid fractionation factor is not the cause.


      PubDate: 2015-01-13T06:28:05Z
       
  • An evaluation of Mg/Ca, Sr/Ca, and Ba/Ca ratios as environmental proxies
           in aragonite bivalve shells
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): C. Poulain , D.P. Gillikin , J. Thébault , J.M. Munaron , M. Bohn , R. Robert , Y.-M. Paulet , A. Lorrain
      The influence of salinity and water chemistry on Mg/Ca, Sr/Ca, and Ba/Ca ratios in the aragonitic shells of the Manila clam was investigated. Clams were reared at constant temperature (20°C) under different controlled conditions of salinity, commonly encountered in their natural habitat. Clams were held in three tanks with a constant salinity of 35 for the first 35days, and then two tanks were changed to lower salinities (20 and 28) for the next 29days. Individual shell Mg/Ca, Sr/Ca, and Ba/Ca ratios were studied through time. Despite stable conditions (temperature, salinity, and Mg/Cawater) for clams reared at salinity 35 during the experiment, Mg/Ca shell ratios increased through the time. Moreover the salinity decrease at t35 slowed the increase of Mg/Ca shell ratios at salinity 28 and resulted in an Mg/Cashell decrease at salinity 20, despite similar Mg/Cawater ratios in the different salinity treatments. Microprobe analyses illustrate that Mg varies along contemporaneous growth lines. The variable shell Mg/Ca ratios suggest that incorporation of magnesium into shell carbonate is strongly regulated by the organism and not by environmental conditions. Interestingly, microprobe analyses illustrated that Mg was not associated with shell sulfur as other studies have suggested. Sr/Ca shell ratios of clams reared at salinity 35 and under constant conditions were also not as constant as expected if Sr/Ca ratios were an environmental proxy. There was an inverse correlation between shell Sr/Ca and salinity despite a slight positive correlation between salinity and Sr/Ca ratios of the water, indicating that Sr/Ca ratios do not reflect environmental conditions. A strong inverse correlation between salinity and Ba/Ca shell ratios (and a positive correlation between Ba/Ca shell and Ba/Ca water) was observed. Therefore, Ba/Ca shell ratios seem to be a promising proxy of high-resolution (1day) salinity variations in estuarine waters (via the relationship between Ba/Ca water and salinity). This study clearly illustrates that both Mg/Ca and Sr/Ca ratios in aragonite shells are not under environmental control and that Ba/Ca ratios are, with the later tracking high-resolution water Ba/Ca ratios and hence estuarine salinity variations.
      Graphical abstract image

      PubDate: 2015-01-13T06:28:05Z
       
  • A combined Y/Ho, high field strength element (HFSE) and Nd isotope
           perspective on basalt weathering, Deccan Traps, India
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): M.G. Babechuk , M. Widdowson , M. Murphy , B.S. Kamber
      High-precision high field strength element (HFSE: Zr, Hf, Nb, Ta, Th, U, W, Mo), Y/Ho, and Nd isotope chemostratigraphy of two contrasting Deccan Traps weathering profiles – an ancient, deeply weathered laterite, and a younger (Quaternary), more moderately weathered saprolite – are used to reconstruct different aspects of basalt weathering. Precision of the HFSE analyses is demonstrated through a report of the long-term concentrations and ratios determined in United States Geological Survey (USGS) and Geological Survey of Japan (GSJ) basalt rock standards (BHVO-1, BHVO-2, BIR-1, BCR-2, JB-2). The oxyanion-forming members (U, Mo, W) are the most mobile of the considered HFSE group. Extreme loss of W, far exceeding those of U and Mo during certain stages of basalt alteration, is first reported here. The ability to strongly fractionate Mo and W during weathering may contribute to solving the unresolved mass imbalance between the crustal and marine inventories of W. By contrast, Zr, Hf, Nb, and Ta preserve the ratios of the parent basalt in the profiles due to their limited mobility; these are therefore of great potential value in reconstructing basalt flow stratigraphy and correlating lava flows in weathered flood basalt provinces. Of the HFSE, Th is not a good choice as a conservative element because it is strongly susceptible to addition of aeolian dust; this is evidenced by strong excursions in Th/Nb that are correlatable with alkali element enrichment and less radiogenic 143Nd/144Nd ratios. The chemical fingerprints of dust were identified in a paleo-flow top of the saprolite profile, suggesting that dust accumulation occurred during periods of quiescence between basaltic eruptions. During protracted exposure and laterite development, the magnitude to which dust overprints the basalt chemistry increases substantially as evident from much less radiogenic Nd isotope ratios and higher Th/Nb ratios in the Bidar profile relative to the protolith basalt. Attempts at quantifying the magnitude of dust accumulation in the laterite based on Th enrichment indicate a mass fraction of greater than 0.5 when the dust is assumed to have the chemistry of average upper continental crust. Although mixing models between the basalt and assumed dust composition cannot unambiguously constrain the dust source, the Nd isotope mixture preserved in the laterite points to a relatively young crustal dust source (e.g., similar to loess in composition) rather than the Precambrian shield rocks in the vicinity of the Deccan Traps. The contrasting topologies of dust-derived Nd and dust-derived Th in the laterite appears to record both physical transport of dust (Th) through permeable laterite horizons as well as transport by chemical dissolution and precipitation (Nd) at an inferred paleo-water table and in deep saprolite zones. Yttrium and Ho fractionate substantially during all observed stages of weathering, with Y/Ho ratios ranging from 26.5 to 21.9 in the moderately weathered saprolite profile and from 30.2 to 14.7 in the laterite profile. The single strongly superchondritic Y/Ho ratio of 30.2 in the laterite is restricted to a sample at depth, and appears to fingerprint the deposition of REE derived from dissolution higher in the profile. Decrease in the Y/Ho ratio relative to the protolith basalt (24.4–24.7) in both profiles inversely correlates with chemical weathering indices, and suggests that Y/Ho ratios have significant potential as a silicate weathering proxy. Consequently, suspended vs. dissolved river loads may record the differing behaviour of these elements during weathering.


      PubDate: 2015-01-13T06:28:05Z
       
  • 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: 2015-01-09T01:54:32Z
       
  • Lead isotope variability in speleothems - a promising new proxy for
           hydrological change? First results from a stalagmite from western
           Germany
    • Abstract: Publication date: Available online 7 January 2015
      Source:Chemical Geology
      Author(s): Qichao Yang , Denis Scholz , Klaus Peter Jochum , Dirk L. Hoffmann , Brigitte Stoll , Ulrike Weis , Beate Schwager , Meinrat O. Andreae
      We studied an aragonitic section of ca. 6cm length of stalagmite HBSH-1 from the Hüttenbläserschachthöhle, western Germany. 230Th/U-dating revealed that this section grew between ~210 and ~190ka, with a relatively uniform growth rate of 3.1μm a-1. For the first time, we determined Pb isotope ratios (207Pb/206Pb and 208Pb/206Pb) in a stalagmite by laser ablation (LA)-ICP-MS. In addition, we analysed various trace element (Mg, Al, Si, P, Mn, Sr, Pb and Th) concentrations. The results indicate that Pb in HBSH-1 originated from two external sources: (i) the regional galena (PbS) mineralization with high Pb content as well as isotope ratios, and (ii) the carbonate fraction in the host rock with low Pb content as well as isotope ratios. Except for Sr, all trace elements show similar variations, probably driven by the amount of infiltration into the cave system. This is probably the result of the similar transport and incorporation mechanisms of these elements as colloids and/or particles. The Pb isotope ratios also show a strong correlation with trace element variability strongly suggesting that the Pb isotope variability in HBSH-1 has the potential to be used as a promising new proxy for hydrological change. Based on the observed variability, we identify two phases (from ~204.9 to ~201.5 and from ~198.4 to ~195.9ka) with enhanced water infiltration in the cave region. Between ~210.0 and ~204.9, ~201.5 and ~198.4 as well as ~195.9 and ~190.0ka less infiltration occurred.


      PubDate: 2015-01-09T01:54:32Z
       
  • Possible controls on Li, Na, and Mg incorporation into aragonite coral
           skeletons
    • Abstract: Publication date: Available online 7 January 2015
      Source:Chemical Geology
      Author(s): Claire Rollion-Bard , Dominique Blamart
      We present ion probe measurements of Li/Ca, Na/Ca and Mg/Ca ratios of three scleractinian corals (Lophelia pertusa, Desmophyllum cristagalli, Porites lutea). These ratios are systematically enriched in Rapid Accretion Deposits (RAD) compared to surrounding fibres, or Thickening Deposits (TD), and present huge variations that cannot be ascribed to any environmental parameters change. Moreover, these elemental ratios are positively correlated in the three coral species. We explore different mechanisms to explain these observations: (1) mixing between different carrier phases, (2) influence of specific ion pumps, (3) precipitation rate effects, (4) Rayleigh fractionation, and (5) pH change in the calcifying fluid. Of these possibilities, the most likely proposal seems to be kinetic effects that have influence on the number of defects (i.e. modification of the crystallographic structure), which is linked to the precipitation rate of the skeleton, and the partition coefficients, are the cause of the positive correlation between Li, Na, and Mg in the coral skeleton. Temperature has an indirect influence on the skeletal concentration of these elements through its effect on the skeletal growth rate.


      PubDate: 2015-01-09T01:54:32Z
       
  • Contrasting regimes of Cu, Zn and Pb transport in ore-forming hydrothermal
           fluids
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Richen Zhong , Joël Brugger , Yanjing Chen , Wenbo Li
      Sulfur and chlorine are the two most important ligands accounting for metal transport in the upper crust. In this study, four metal- and sulfur-saturated model fluids with varying salinities and redox states were simulated in the Fe-Cu-Pb-Zn-Au-S-C-H-O system, over a wide pressure-temperature (P-T) range (50–650°C, 0.8–5.0kbar), in order to compare the roles of chloride and bisulfide complexing for metal transport at the light of the latest available thermodynamic properties. The range in simulated Zn and Pb concentrations of the model fluids compares well with those of natural hydrothermal fluids, suggesting that the model can be used to evaluate hydrothermal ore-forming processes in Nature. The modeling reveals two different modes of Cu, Pb and Zn complexing in sulfur-saturated hydrothermal solutions. At lower temperature, chloride complexes are the predominant Cu, Pb and Zn species in sulfide-saturated systems, as expected from previous studies. However, hydrosulfide Cu, Pb and Zn complexes predominate at higher temperature. The predominance of bisulfide complexing for base metals at high temperature in sulfur-saturated systems is related to the prograde dissolution of pyrite and/or pyrrhotite, which results in a rapid increase in sulfur solubility. Metals transport as chloride or bisulfide complexes determines the modes of metal enrichment. In chloride-complexing dominated systems (e.g., Mississipi Valley Type deposits), low sulfide solubilities mean that the ore fluids cannot carry both reduced sulfur and metals, and ore precipitation is triggered when the ore fluid encounters reduced sulfur, e.g., via fluid mixing or via sulfate reduction. In contrast, in fluids where bisulfide complexing is predominant, cooling and desulfidation reactions are efficient mechanisms for base metal sulfide precipitation. Since both Au and base metals (Cu, Pb and Zn) are predicted to be transported as hydrosulfide complexes in high-temperature primary magmatic fluids in equilibrium with sulfide minerals, high-salinity is not a necessity for magmatic hydrothermal deposits such as porphyry- and skarn-style deposits.


      PubDate: 2015-01-09T01:54:32Z
       
  • Geochemical processes in a highly acidic pit lake of the Iberian Pyrite
           Belt (SW Spain)
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): C.R. Cánovas , S. Peiffer , F. Macías , M. Olías , J.M. Nieto
      Compared with pit lakes originated by coal mining, little is known about in-lake neutralization processes in pit lakes from sulfide ore mining in hard rock substrates, which are typically very deep and acidic and receive low carbon (C) inputs and groundwater flows. Physicochemical processes in water and sediments from a pit lake (San Telmo, 130m deep) in the Iberian Pyrite Belt were investigated. San Telmo is a meromictic and highly acidic (pH2.8) pit lake due to pH buffering by precipitation of Fe(III) minerals (schwertmannite and jarosite). The sediments have a low abundance of C (below 0.60%) and iron minerals (below 0.12%) compared to most coal-mining pit lakes. In San Telmo sediments, iron reduction and sulfide oxidation may be thermodynamically favored due to low pH values in pore waters (below 3.8) and the presence of reactive iron. Although schwertmannite is the main ferric mineral precipitating in the water column, mineralogical analyses reveal a low abundance of schwertmannite in the sediment. This may be due to the preferential use of this mineral in reduction reactions mediated by bacteria, together with a low replenishment rate of the schwertmannite pool in the sediment. The transformation of reactive iron (schwertmannite and jarosite) into goethite may limit sulfate reduction, promoting acidic conditions in the sediment. As long as the acid mine drainage continues to discharge into the lake, the pH buffering exerted by ferric minerals in the sediments will limit the neutralization of the pH by sulfate reduction.
      Graphical abstract image

      PubDate: 2015-01-09T01:54:32Z
       
  • Mineralogical control of selenium, tellurium and highly siderophile
           elements in the Earth’s mantle: Evidence from mineral separates of
           ultra-depleted mantle residues
    • Abstract: Publication date: 9 March 2015
      Source:Chemical Geology, Volume 396
      Author(s): Stephan König , Moritz Lissner , Jean-Pierre Lorand , Alessandro Bragagni , Ambre Luguet
      The budget of Se–Te and other highly siderophile elements (HSEs) in major silicate and oxide mineral separates (olivine, orthopyroxene, clinopyroxene, spinel), and intergranular fine components of an ultra-refractory harzburgite have been determined via isotope dilution hydride generation ICPMS after mineral separation, high-pressure acid digestion and chemical purification. In addition, x-ray computed micro-tomography (micro-CT) has been performed in order to constrain and illustrate the original location, distribution and size of sulfides and/or alloys inside their intact, bulk harzburgite matrix. The results show that the whole-rock Se–Te budget of mantle residues, that are devoid of base metal sulfides (BMS) after high degrees of partial melting (F ≈23%), is almost completely controlled by olivine-hosted micro-inclusions and intergranular fractions of platinum group minerals (PGMs). The scarcity and heterogeneous distribution of intergranular PGMs generally sized 3–5μm in the harzburgite are revealed by 3D micro-CT, while PGM inclusions in olivine are smaller than 2.5μm. The heterogeneous distribution of these microphases as well as the range of Se–Te contents in the bulk harzburgite complicate a quantification of individual contributions of separates to the whole-rock Se–Te budget. However, PGM micro-inclusions in olivine with residual-type CI chondrite-normalized HSE patterns and suprachondritic Se/Te ratios between 30 and 218 (Se/TeCI chondrite =9) host close to 100% of the bulk rock Se. Interstitial fine components up to 125μm grain size may contribute only minor amounts of Se, but up to 50% of Te to the bulk harzburgite and show subchondritic Se/Te ratios of ca. 4, resembling metasomatic PGM signatures. In dependence on increasing proportions of interstitial and metasomatic PGMs vs. olivine-hosted residual PGMs, the bulk rock Te contents may increase and the Se/Te ratios decrease. The systematics seen in mineral separates of one harzburgitic sample resembles the entire spectrum of Se–Te signatures of all suites of peridotites and host phases so far published. The results are consistent with a higher compatibility of Se in covalent monosulfides compared to Te that prefers sulfide melts. It is further consistent with the stabilization of residual PGMs after Mss exhaustion as well as with formation of metasomatic PGMs in a S-Cu-volatile-rich sulfide melt. Altogether this reconciles a higher incompatibility of Te over Se during partial mantle melting, subsequent removal of Te over Se with Cu-Ni-rich sulfide fractionation from high-degree partial mantle melts, and explains the similar Te but higher Se abundances in MORBs compared to peridotites that is globally observed.


      PubDate: 2015-01-09T01:54:32Z
       
  • Temperature-driven seasonal calcite growth and drip water trace element
           variations in a well-ventilated Texas cave: Implications for speleothem
           paleoclimate studies
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Richard C. Casteel , Jay L. Banner
      A two-year cave monitoring study at Westcave Preserve in central Texas provides insight into the controls on the rate of calcite growth and drip water Mg/Ca, Sr/Ca, and Ba/Ca variations. The cave is shallow and has a large ratio of its opening area to its volume, which results in year-round ventilation of the cave. Unlike larger and deeper caves in the region that ventilate seasonally, cave-air temperature and CO2 concentrations at Westcave are near atmospheric throughout the year and calcite growth is continuous. Changes in the rate of calcite growth positively correlate with seasonal temperature variations at all six drip sites studied (r2 =0.12–0.76; mean r2 =0.47). Average monthly surface air temperature is positively correlated with drip-water Sr/Ca at five of six drip sites studied (r2 =0.21–0.80; mean r2 =0.44), and Ba/Ca at all six sites (r2 =0.41–0.85; mean r2 =0.57); whereas this correspondence is only seen in one of six drip sites for Mg/Ca. Applying geochemical modeling of mineral-solution reactions to the Sr/Ca and Ba/Ca time series at Westcave indicates that the evolution of drip-water Sr/Ca and Ba/Ca can be accounted for by two mechanisms: (1) prior calcite precipitation and/or incongruent calcite dissolution (PCP/ICD), which dominate drip-water evolution at one site; and (2) a combination of PCP/ICD and water–rock interaction (WRI) at the other five drip sites. The results suggest a possible seasonality in the operation of the mechanisms of drip-water evolution, whereby PCP/ICD plays a larger role than WRI during the warmer months of the year. Understanding drip-water seasonal Sr/Ca and Ba/Ca variations has implications for paleoclimate studies using speleothems. It is important to first determine if seasonal geochemical variations in drip waters can be identified. One can then determine if these variations are preserved as geochemical laminae in speleothems, which may then provide seasonal temperature variations and thus seasonal age constraints for speleothems. Determining the proportional contributions of the mineral-solution reactions that drive drip-water trace element variations for different drip sites, as well as the extent to which trace element concentrations vary seasonally, will help inform speleothem sample selection and interpretation of geochemical data for paleoclimate study. Our results indicate that speleothems near the well-ventilated entrances of many larger and deeper caves may warrant further consideration for paleoclimate studies.


      PubDate: 2015-01-04T22:06:29Z
       
  • Sedimentary iron–phosphorus cycling under contrasting redox
           conditions in a eutrophic estuary
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Peter Kraal , Edward D. Burton , Andrew L. Rose , Benjamin D. Kocar , Robert S. Lockhart , Kliti Grice , Richard T. Bush , Eileen Tan , Samuel M. Webb
      Phosphorus (P) is often a limiting nutrient within freshwater and estuarine systems, thus excess inputs of P from anthropogenic activities (dominantly agriculture) can induce eutrophication in receiving water bodies. The sequestration of P within estuarine sediments is controlled by sorption and precipitation processes, which are impacted by local redox conditions and burial environment. Despite the global spread of oxygen depletion in coastal marine systems, P burial under reducing conditions remains poorly understood. We investigated P cycling in relation to iron (Fe) redox chemistry in sediments from the eutrophic Peel-Harvey Estuary in Western Australia, using a combination of porewater analysis, sequential chemical P extractions, synchrotron-based micro-scale X-ray fluorescence mapping and Fe K-edge X-ray absorption spectroscopy, and PO4 3− sorption experiments. The sediments represented redox regimes varying from strongly reducing, organic-rich sediments with little or no reactive Fe(III) (oxyhydr)oxides to oxygenated sediments that were enriched in reactive Fe(III) phases. Organic P and Fe-associated P were the main P burial phases, and the latter was quantitatively important even in sediments with an overall strongly reducing character. We attribute this to adsorption of P onto micro-scale Fe(III) oxyhydroxide enrichments and/or Fe-bearing clay minerals. The organic-rich sediments showed a strong decline in P contents with depth; P was released from organic matter and Fe phases but apatite precipitation was apparently inhibited in these sediments. Despite greater and stronger PO4 3− sorption capacity, the oxic sediments contained relatively little P due to a lack of the primary P source in marine sediments: organic matter. Our results provide detailed insight into P burial in dynamic estuarine sediments and show that micro-scale spectroscopic analyses greatly advance our understanding of P sequestration processes.


      PubDate: 2015-01-04T22:06:29Z
       
  • Boron isotope geochemistry of vent fluids from arc/back-arc seafloor
           hydrothermal systems in the western Pacific
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Kyoko Yamaoka , Ensong Hong , Tsuyoshi Ishikawa , Toshitaka Gamo , Hodaka Kawahata
      Boron concentrations and isotopic compositions were determined for vent fluids from 11 sites in arc/back-arc seafloor hydrothermal systems. This new dataset allowed a systematic comparison of boron behavior during water–rock interaction in different geological settings. In sediment-starved hydrothermal systems (e.g., Manus Basin, Izu-Bonin Arc, Mariana Trough), end-member fluids showed large variations in boron concentration (0.53–1.52mmol/kg) and δ11B value (13.5–29.7‰), reflecting the interaction of seawater with varied types of rock that occur in arc/back-arc settings. Based on the model calculations, the boron concentration and δ11B value of the fluid are better explained by an equilibration between seawater and rock than a mixing model. The Chlorine-depleted fluids from phase-separated hydrothermal systems (e.g., North Fiji Basin) had low boron concentrations (0.44–0.55mmol/kg) and high δ11B values (34.5–36.1‰). The small fractionations of boron and boron isotopes during phase separation suggest that these characteristics were acquired during water–rock interaction rather than phase separation and segregation processes. The fluids from sediment-hosted hydrothermal systems (e.g., Okinawa Trough) are characterized by high boron concentrations (3.9–4.8mmol/kg) and low δ11B values (2.5–2.9‰). These fluids also showed high Cs/B ratios, indicating interactions with terrigenous sediments. Model calculations demonstrate that the combined evaluation of δ11B values and Cs/B ratios are useful for a quantitative estimate of sedimentary contributions in seafloor hydrothermal systems.


      PubDate: 2015-01-04T22:06:29Z
       
  • A K-feldspar–liquid hygrometer specific to alkaline differentiated
           magmas
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Silvio Mollo , Matteo Masotta , Francesca Forni , Olivier Bachmann , Gianfilippo De Astis , Gordon Moore , Piergiorgio Scarlato
      We present a K-feldspar–liquid hygrometer specific to alkaline differentiated magmas that is calibrated through the regression analysis of sanidine and anorthoclase crystals coexisting with trachyte and phonolite melts. Partial-regression leverage plots were used to determine the minimum number of regression parameters that closely describe the variance of the dataset. The derived model was tested using K-feldspar–liquid pairs not included into the calibration dataset in order to address issues of systematic errors. When K-feldspar and plagioclase crystals coprecipitate from the same alkaline liquid under identical P-T-X-fO2-H2O conditions, the ability prediction of the new hygrometer is comparable to that of previous plagioclase–liquid models. To minimize the error of H2O estimate caused by the inadvertent use of disequilibrium data in natural samples, we have also calibrated a new test for equilibrium based on Or–Ab exchange between K-feldspar and coexisting melt. As an immediate application for both equilibrium and hygrometer models, we used as input data K-feldspar–liquid pairs from alkaline explosive eruptions at the Phlegrean Fields. The estimates of H2O dissolved in natural trachyte and phonolite magmas closely match those determined by melt inclusion analysis and H2O solubility modeling. This leads to the conclusion that our new models can significantly contribute to a better quantitative characterization of the H2O content in differentiated alkaline magmas feeding large-volume explosive eruptions.


      PubDate: 2015-01-04T22:06:29Z
       
  • Editorial Board
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392




      PubDate: 2015-01-04T22:06:29Z
       
  • Magnesium isotope composition of sabkha porewater and related (Sub-)Recent
           stoichiometric dolomites, Abu Dhabi (UAE)
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): A. Geske , S. Lokier , M. Dietzel , D.K. Richter , D. Buhl , A. Immenhauser
      Earliest marine diagenetic sabkha type dolomites are widespread in Earth's geological record. Potentially, these carbonates may act as archives of past seawater magnesium isotope (δ26Mg) ratios. At present, however, the fractionation of magnesium isotopes (Δ26Mg) between seawater – here evaporated marine porewater – and sabkha dolomite is not constrained. In order to explore Δ26Mgdol-Mg(porewater), we make use of actualistic sabkha type dolomite precipitation in the Gulf region (Trucial coast, United Arab Emirates). This paper documents and discusses the first detailed sabkha δ26Mg data set of Mg-bearing solids including stoichiometric dolomites (degree of ordering >0.9; mean δ26Mgdol =−0.79‰±0.41 2σ, n=17) and related marine pore waters. The presence of dolomite crystals with a broad range of ordering in shallow cores is documented using X-ray diffraction and scanning electron microscopy, but individual crystals are too small (<10μm) to be mechanically separated from their host sediment. Hence, a method was developed to chemically separate the most stoichiometric dolomite crystals from coexisting less stoichiometric dolomites and other Mg-bearing minerals and fluid phases by using disodium ethylenediaminetetracetic acid. Sabkha shallow ground water collected in trenches displays δ26Mg values of about −0.59‰, i.e. is only moderately enriched in 26Mg relative to the present-day seawater signature of −0.83‰. Conversely, the δ26Mg of evaporated porewater is enriched by +0.43‰ relative to that of δ26Mgseawater. Here we use the term “apparent” fractionation for the variable Δ26Mgdol-Mg(porewater) (+0.1 and −0.7‰) obtained that cannot be compared with experimentally deduced fractionation factors from controlled laboratory settings. Moreover, evaporated sabkha porewater differs, in terms of its isotope signature, from seawater. With regard to the current level of knowledge, the hypothesis that earliest diagenetic sabkha dolomites represent direct archives of secular changes of seawater δ26Mg values with time is not supported by our dataset. Our present understanding is that the magnesium isotope signature of sabkha dolomites is related to complex kinetics of precursor formation, dissolution/precipitation reactions including microbiological effects and involves variable Mg sources and sinks in a temporally and spatially variable microenvironment.


      PubDate: 2015-01-04T22:06:29Z
       
  • Predicting radioactive accessory mineral dissolution during chemical
           weathering: The radiation dose at the solubility threshold for
           epidote-group detrital grains from the Yangtze River delta, China
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Jason R. Price , Derek H.C. Wilton , Mike N. Tubrett , Jill S. Schneiderman , Xudong Fan , Katherine Peresolak
      The influence of heavy-ion irradiation on the solubility of natural epidote-group minerals has been investigated. The epidote-group are calc-silicate minerals that, if soluble during terrestrial chemical weathering, are capable of consuming atmospheric CO2 over geologic time. The experimental design was to calculate the α-particle dose for actinide-rich epidote-group grains from the deltas of rivers draining large watersheds. Large watersheds should yield a suite of epidote-group grains spanning the range of compositions and radiation damage commonly observed in nature. These grains would be resistant to dissolution during chemical weathering and survive into fluvial sediments. Therefore, the highest α-particle dose calculated for the suite of grains represents a minimum solubility threshold for the epidote-group minerals during chemical weathering. The α-particle dose is calculated from the 232Th, 238U, and 235U content of a grain, and the grain’s date. Actinide-rich epidote-group grains were isolated from Yangtze River and River Nile delta sediments, although only the grains from the Yangtze delta had 232Th/208Pb ratios sufficiently high to yield meaningful dates. A total of 28 Th-rich epidote-group grains were isolated from the Yangtze delta sediment, with 24 being classified as allanite and four being classified as REE-rich epidote. Isotopic ratio data were measured by laser ablation microprobe-inductively coupled plasma-mass spectrometry. Dates and REE patterns revealed at least 17 different sources of grains, with ThO2 contents ranging from 0.057 to 4.37 wt. %, and dates ranging from 32±2 to 3788±263Ma. The calculated maximum α-particle dose of the suite of Yangtze delta epidote-group mineral grains, and hence the minimum solubility threshold, is 7.1 × 1015 α-decay mg−1. Therefore, to predict the likelihood of an epidote-group mineral dissolving during chemical weathering at a specific study site the α-particle dose may be calculated from the date of the rock, even if estimated, and the radioactinide concentrations of the mineral. A calculated α-particle dose below ~7.1 × 1015 α-decay mg−1 likely reflects epidote-group mineral stability during chemical weathering. Calculation of the α-particle dose may be superior to microscopic identification of accessory (<2% by volume) mineral dissolution during chemical weathering. This is because accessory minerals occur in relatively low abundances, may occur as relatively small grains, be highly soluble, and/or be heterogeneously distributed in the bedrock and regolith. Furthermore, the solubility of radioactively damaged accessory minerals cannot be readily predicted or quantified by geochemical thermodynamic and/or kinetic principles. The α-particle dose at the solubility threshold of ~7.1 × 1015 α-decay mg−1 value is approximately double that reported for zircon. Therefore, relative to zircon, epidote-group minerals can withstand more radiation prior to becoming soluble. This finding is surprising considering the chemical durability of zircon. This substantial variance in solubility threshold radiation doses between the two minerals cannot be explained by the bond strengths of non-tetrahedral cations and structural oxygen which are lower in allanite than zircon.


      PubDate: 2015-01-04T22:06:29Z
       
  • A REE-in-garnet–clinopyroxene thermobarometer for eclogites,
           granulites and garnet peridotites
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Chenguang Sun , Yan Liang
      A REE-in-garnet–clinopyroxene thermobarometer for eclogites, granulites, and garnet peridotites has been developed on the basis of the temperature, pressure and mineral composition dependent partitioning of rare earth elements (REEs) between garnet and clinopyroxene. This new thermobarometer is derived from the garnet–clinopyroxene REE partitioning model of Sun and Liang (2014) that was calibrated against experimentally determined garnet-melt and clinopyroxene–melt partitioning data. It makes use of a group of trace elements that have similar geochemical behaviors at magmatic and subsolidus conditions, and allows one to invert temperature and pressure simultaneously using a least squares method. Application of the REE-in-garnet–clinopyroxene thermobarometer to REE partitioning data from laboratory experiments and field samples (quartz-bearing, graphite-bearing, and diamond-bearing granulites and eclogites; and well-equilibrated mantle eclogite xenoliths) published in the literature validates its reliability at both magmatic and subsolidus conditions. Application of the new thermobarometer to eclogites, garnet granulites and peridotites from various tectonic settings reveals an intriguing observation: temperatures derived from the REE-based thermobarometer are consistently higher than those derived from the widely used Fe–Mg thermometer of Krogh (1988) for samples that experienced cooling, but systematically lower than temperatures derived from the Fe–Mg thermometer for samples from thermally perturbed tectonic settings. The temperature discrepancies are likely due to the relative differences in diffusion rates between trivalent REEs and divalent Fe–Mg in garnet and clinopyroxene. Temperatures derived from the REE-based thermometer are closely related to closure temperatures for samples that experienced cooling, but are likely equilibrium or apparent re-equilibration temperatures at an early stage of heating for samples from thermally perturbed tectonic environments. The REE-in-garnet–clinopyroxene thermobarometer can shed new light on thermal histories of mafic and ultramafic rocks.
      Graphical abstract image

      PubDate: 2015-01-04T22:06:29Z
       
  • Unravelling sources of solutes in groundwater of an ancient landscape in
           NW Australia using stable Sr, H and O isotopes
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Shawan Dogramaci , Grzegorz Skrzypek
      The Precambrian meta-sedimentary fractured rock aquifers of the Hamersley Basin in northwest Australia are some of the oldest water-bearing formations on the planet and host enormous iron ore deposits. Groundwater is the only permanent source of water in the basin, therefore understanding the hydrological processes that effect water quality and quantity is a pre-requisite for sustainable water management. We used a combination of major dissolved ion concentrations, including Sr and Ca, in combination with δ2H, δ18O and δ87Sr in flood water and groundwater as tracers to constrain the processes affecting groundwater chemistry. The δ87Sr composition of groundwater in three major aquifer types ranges from 11.8‰ to 40.6‰ and reflects the mineralogy of altered Precambrian dolomite (15.1‰ to 55.4‰) rather than the host iron ore formations (22.5‰ to 46.5‰ >95% iron oxides) or highly radiogenic shale bands and clay minerals (200‰ to 2322.5‰). Groundwater in the terminal Fortescue Marsh wetland of the basin has a rather constant δ87Sr signature of 36.6±1.4‰ irrespective of variations in TDS, δ18O and Sr concentration. This groundwater is considered to be mature in a geochemical sense, representing the final stage of water evolution on a basin scale. Mixing calculations utilising δ87Sr and Ca/Sr data demonstrate contributions of salts from three major sources: on average >92% from precipitation, ~7% from carbonate rocks and <1% from rocks with highly radiogenic signatures (shales and clays). These results demonstrate groundwater evolution from a recharge area to discharge area at the regional scale, but more importantly that water quality in the terminal wetland is primarily driven by rainfall chemistry in floodwaters rather than water–rock interactions in the catchment.


      PubDate: 2015-01-04T22:06:29Z
       
  • The growth and concentration of uranium and titanium minerals in
           hydrocarbons of the Carbon Leader Reef, Witwatersrand Supergroup, South
           Africa
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): S. Fuchs , D. Schumann , A.E. Williams-Jones , H. Vali
      Uranium and gold-bearing pyrobitumen from the Carbon Leader Reef in the Witwatersrand Basin, South Africa, was investigated by high-resolution transmission electron microscopy. This study provides evidence for the in-situ growth of uraninite and anatase nanocrystals in the pyrobitumen, implying mobilization and concentration of uranium and titanium by formerly mobile liquid hydrocarbons. Individual nanocrystals of uraninite and anatase are pervasively distributed and locally isolated within the pyrobitumen matrix. Crystallization of uraninite and anatase led to the formation of complex nanocrystal aggregates by oriented attachment, in which anatase generally provided nuclei for the growth of uraninite. Single nanocrystals of curite occur locally in channel ways within masses of uraninite nanocrystals, consistent with later auto-oxidation of uraninite and limited release of water during hydrocarbon maturation. On the basis of evidence for the migration of liquid hydrocarbons in the Witwatersrand Basin and the presence of abundant uraninite and anatase nanoparticles in pyrobitumen, a new model is proposed for the transport and concentration of uranium (and titanium) in the Carbon Leader Reef. According to this model, liquid hydrocarbons that were circulating in the Witwatersrand Basin dissolved detrital U–Ti-bearing minerals and transported the uranium and titanium until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen. The latter process involved the release of volatiles and the destruction of bonds that may have held the uranium and titanium in solution, thereby inducing the growth of individual uraninite and anatase nanocrystals and the formation of complex nanocrystal aggregates within the pyrobitumen.


      PubDate: 2015-01-04T22:06:29Z
       
  • Experimental investigation of the S and S-isotope distribution between
           H2O–S±Cl fluids and basaltic melts during decompression
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Adrian Fiege , François Holtz , Harald Behrens , Charles W. Mandeville , Nobumichi Shimizu , Lars S. Crede , Jörg Göttlicher
      Decompression experiments (from 400 to 70MPa) were conducted to investigate sulfur (S) distribution and S-isotope fractionation between basaltic melts and coexisting fluids. Volatile-bearing [~3 to ~7wt.% water (H2O), ~300 to ~1200ppmS, 0 to ~3600ppm chlorine (Cl)] basaltic glasses were used as starting materials. The MgO content in the melt was either ~1wt.% (Mg-poor basalt) or ~10wt.% (alkali basalt) to investigate the possible role of compositional changes in basaltic systems on fluid-melt distribution of S and S-isotopes. The experiments were performed in internally heated pressure vessels (IHPV) at 1050°C to 1250°C, variable oxygen fugacities (fO2 ; ranging from log(fO2 /bar)~QFM to ~QFM+4; QFM=quartz–fayalite–magnetite buffer) and at a constant decompression rate (r) of 0.1MPa/s. The annealing time (tA ) at final pressure (p) and temperature (T) after decompression was varied from 0 to 5.5h to study the fluid–melt equilibration process. Sulfur and H2O contents in the melt decreased significantly during decompression, while the Cl contents remained almost constant. No changes in H2O and Cl content were observed with tA , while S concentrations decreased slightly with tA <2h; i.e., near-equilibrium fluid–melt conditions were reached within ~2h after decompression, even in experiments performed at the lowest T of 1050°C. Thus, fluid–melt partitioning coefficients of S (DS fl/m ) were determined from experiments with tA ≥2h. The MgO (~1 to ~10wt.%), H2O (~3 to ~7wt.%) and Cl contents (<0.4wt.%) in the melt have no significant effect on DS fl/m . Consistent with previous studies we found that DS fl/m decreased strongly with increasing fO2 ; e.g., at ~1200°C DS fl/m ≈180 at QFM+1 and DS fl/m ≈40 at QFM+4. A positive correlation was observed between DS fl/m and T in the range of 1150 to 1250°C at both oxidizing (QFM+4; DS fl/m =52±27 to 76±30) and intermediate (QFM+1.5; DS fl/m =94±20 to 209±80) redox conditions. Data compiled at 1050°C and relatively reducing conditions (~QFM; DS fl/m =58±18) indicate that the trends may be extrapolated to lower T, at least for intermediate to reducing conditions (~QFM+1.5 to ~QFM). The S-isotope composition in glasses of selected samples was measured by secondary ion mass spectrometry (SIMS). Gas–melt isotopic fractionation factors (αfl–m) were calculated via mass balance. At 1200°C an average αfl–m of 0.9981±0.0015 was determined for oxidizing conditions (~QFM+4), while an average αfl–m of 1.0025±0.0010 was found for fairly reducing conditions (~QFM+1). Furthermore, at lower T (1050°C) an average ...
      PubDate: 2015-01-04T22:06:29Z
       
  • Arsenic-bearing phases in South Andean volcanic ashes: Implications for As
           mobility in aquatic environments
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): G. Bia , L. Borgnino , D. Gaiero , M.G. García
      Three samples of volcanic ashes collected after eruptions of the volcanos Hudson in 1991, Chaitén in 2008 and Puyehue in 2011 were analyzed in order to define the solid speciation of arsenic and the dynamics of its release to the aqueous phase. The bulk chemical and mineralogical characterization of the samples was performed by ICP/OES, DRX, and SEM/EDS analyses. The chemical composition of the near surface region (first 2–10nm), along with the As and Fe solid speciation was performed by XPS. Batch experiments were conducted to evaluate the kinetics of the arsenic release under variable pH conditions. The integrated analysis of these data indicates that arsenic compounds are concentrated onto the ash surface in the form of As(III)–S and As(V)–O species. The As(III) species have been assigned to arsenian pyrite, while As(V)–O compounds have been assigned to adsorbed arsenate ions or Fe arsenate salts precipitated as thin coatings. Although the main As carrier in the studied volcanic ashes is Al-silicate glass, this phase is stable at the neutral pH that dominates the aqueous reservoirs of the area affected by ashfall. Thus, its contribution to the pool of dissolved arsenic is minor. Higher contributions are clearly associated with the more mobile As species that concentrate onto the surface of Al-silicate glass. This more available arsenic represents less than 6% of the total measured arsenic.


      PubDate: 2015-01-04T22:06:29Z
       
  • Editorial Board
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394




      PubDate: 2015-01-04T22:06:29Z
       
  • Oscillatory Sr isotopic signature in plagioclase megacrysts from the
           Damiao anorthosite complex, North China: Implication for petrogenesis of
           massif-type anorthosite
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Wei Terry Chen , Mei-Fu Zhou , Jian-Feng Gao , Tai-Ping Zhao
      Formation of Proterozoic massif-type anorthosites is known to be related to polybaric process involving early high-pressure crystallization of plagioclase and high-Al orthopyroxene megacrysts at mantle–crust boundary, followed by emplacement of plagioclase-dominated mushes to shallow-level crust. Therefore, the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites. The ~1.74-Ga Damiao complex, North China, comprises >90vol.% anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high-Al orthopyroxene megacrysts (HAOMs). The plagioclase megacrysts are generally euhedral to subhedral (mostly 1 to 30cm in diameter), and some of them contain very fine lamellae of orthoclase and undefined Fe–Ti-rich minerals (1–5μm). The HAOMs occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts. They contain abundant thin, regular lamellae of exsolved plagioclase (15–20vol.%), indicative of originally high-Al features (originally 6.0–7.3wt.% Al2O3). Based on the Al-in-orthopyroxene geobarometry, the HAOMs and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9.4–11.2kbar (33–36km), indicative of their crystallization at lower crust depths or crust–mantle boundary. In contrast, orthopyroxene grains from late differentiation phases such as oxide-apatite gabbronorites do not contain plagioclase lamellae, and have much lower Al2O3 contents (1.5 to 1.7wt.%), indicating final crystallization at <5kbar (<15km). Four plagioclase megacrysts analyzed in this study have slightly different compositions, but collectively they all display comparable, oscillatory variations of An values (43–55), Sr (1100–1800ppm), Ba (800–1400ppm), La (2 to <7ppm) and 87Sr/86Sr ratios (0.70283–0.70466) from center to rim. Although Sr content is also likely related to pressures, our work suggests that the oscillatory chemical and Sr isotopic signature of the plagioclase megacrysts was mainly controlled by the compositions of the magmas, which temporally changed in the deep magma chamber. A mixing-assimilation modeling, based on Sr contents and initial 87Sr/86Sr ratios of synchronous mantle-derived mafic dykes and ancient lower crustal xenoliths in North China, suggests that the parental magma was initially a depleted mantle-derived basaltic magma that assimilated with ~30% lower crustal materials (Al2O3 =15–24wt.%; Sr=800–2000ppm) or partial melts of the lower crust when ponding at the base of lower crust. We consider that an increasing of Al2O3 in the basaltic magma due to assimilation may have triggered saturation of plagioclase, because the oscillatory chemical and isotopic patterns are consistent from center to rim in all plagioclase megacrysts, indicative of initiation of the assimilation processes at the very beginning of plagioclase crystallization. Our study supports the model that the parental magmas were derived from partial melting of the depleted mantle combined with all-important crustal contamination in deep magma chambers or during rising of crystal mushes, and this may also account for variable isotopic signature for different phases in many anorthosite suites.


      PubDate: 2015-01-04T22:06:29Z
       
  • Perrhenate incorporation into binary mixed sodalites: The role of anion
           size and implications for technetium-99 sequestration
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Johnbull O. Dickson , James B. Harsh , Wayne W. Lukens , Eric M. Pierce
      Perrhenate (ReO4 −), as a TcO4 − analogue, was incorporated into mixed-anion sodalites from binary solutions containing ReO4 − and a competing anion Xn− (Cl−, CO3 2−, SO4 2−, MnO4 −, or WO4 2−). Our objective was to determine the extent of solid solution formation and the dependence of competing ion selectivity on ion size. Using equivalent aqueous concentrations of the anions (ReO4 −/Xn− molar ratio=1:1), we synthesized mixed-anion sodalites from zeolite and NaOH at 90°C for 96h. The resulting solids were characterized by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure (XANES) spectroscopy to determine crystal structure, chemical composition, morphology, and rhenium (Re) oxidation state. Rhenium in the solid phase occurred predominately as Re(VII)O4 − in the sodalites, which have a primitive cubic pattern in the space group P 4 ¯ 3 n . The refined unit-cell parameters of the mixed sodalites ranged from 8.88 to 9.15Å and showed a linear dependence on the size and mole fraction of the incorporated anion(s). The ReO4 − selectivity, represented by its distribution coefficient (Kd), increased in the following order: Cl− <NO3 − <MnO4 − and CO3 2− <SO4 2− <WO4 2− for the monovalent and divalent anions, respectively. The relationship between the ReO4 − distribution coefficient and competing anion size was nonlinear. When the difference in ionic radius (DIR) between ReO4 − and Xn− (n=1 or 2) was greater than ~12%, then ReO4 − incorporation into sodalite was insignificant. The results imply that anion size is the major factor that determines sodalite anion compositions. Given the similarity in chemical behavior and anion size, ReO4 − serves as a suitable analogue for TcO4 − under oxidizing conditions where both elements are expected to remain as oxyanions in the +7 oxidation state.
      Graphical abstract image

      PubDate: 2015-01-04T22:06:29Z
       
  • Cultural meromixis: Effects of road salt on the chemical stratification of
           an urban kettle lake
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Ryan J. Sibert , Carla M. Koretsky , Davina A. Wyman
      Saline runoff from application of road salt deicers has the potential to create persistent density gradients in urban freshwater lakes, resulting in significant changes to physical mixing and biogeochemical cycling. In this study, the seasonal influence of road salt influx on the geochemical cycling of a small, urban kettle lake is examined. Water column samples were collected approximately twice per month for fifteen months and analyzed for temperature, pH, conductivity, major ions, nutrients and redox-sensitive solutes. The lake water column was redox-stratified with high concentrations of dissolved Mn(II), Fe(II), ammonium, phosphate and, during some periods, sulfide and methane in the hypolimnion. Bottom water anoxia was persistent throughout the entire sampling period. Concentrations of sodium and chloride, which correlated strongly with conductivity, were over a hundred times greater compared with levels in nearby rural lakes. Conductivity, chloride and calculated density profiles indicated the presence of a pycnocline at ~9m depth that persisted throughout the sampling period. The chemical data demonstrate that road salt input has caused the lake to become meromictic, with a complete lack of fall and spring turnover.


      PubDate: 2015-01-04T22:06:29Z
       
  • Enhancing magnesite formation at low temperature and high CO2 pressure:
           The impact of seed crystals and minor components
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Andrew R. Felmy , Odeta Qafoku , Bruce W. Arey , Libor Kovarik , Jia Liu , Daniel Perea , Eugene S. Ilton
      The formation of magnesite was followed in aqueous solution containing initially added Mg(OH)2 equilibrated with supercritical carbon dioxide (90atm pressure, 50°C) in the presence of introduced magnesite particles and minor components, Co(II). As expected, the introduction of magnesite particles accelerated the formation of magnesite from solution. However, the formation rate of magnesite was even greater when small concentrations of Co(II) were introduced, indicating that the increased rate of magnesite formation in the presence of Co(II) was not solely due to the addition of a growth promoting surface. Detailed analysis of the magnesite particles by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atom probe tomography (APT) revealed that the originally added Co(II) was concentrated in the center but also present throughout the growing magnesite particles. Addition of the Co(II) in different chemical forms (i.e. as solid phase CoCO3 or Co(OH)2) could alter the growth rate of magnesite depending upon the addition of bicarbonate to the starting solution. Geochemical modeling calculations indicate that this difference is related to the thermodynamic stability of these different phases in the initial solutions. More broadly, these results indicate that the presence of even small concentrations of foreign ions that form carbonate compounds with a similar structure as magnesite can be incorporated into the magnesite lattice, accelerating the formation of anhydrous carbonates in natural environments.


      PubDate: 2015-01-04T22:06:29Z
       
  • The REE-composition and petrography of apatite in 2Ga Zaonega Formation,
           Russia: The environmental setting for phosphogenesis
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Lauri Joosu , Aivo Lepland , Kalle Kirsimäe , Alexander E. Romashkin , Nick M.W. Roberts , Adam P. Martin , Alenka E. Črne
      The first significant P-rich deposits appear in the global rock record during the Paleoproterozoic around 2Ga, however the specific triggers that led to apatite precipitation are still under debate. The ca. 2Ga Zaonega Formation, Karelia, Russia contains P-rich intervals in its upper part with abundantly occurring apatite. These apatites have been studied for their Rare Earth Element (REE) composition using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with an emphasis on the environmental condition during phosphogenesis. Petrographic observations by scanning electron microscopy (SEM) integrated with LA-ICP-MS results allow recognition of variously recrystallized apatite, and distinction of best-preserved diagenetic apatite which presumably records syn-depositional REE characteristics. Diagenetic apatite exhibits moderately negative Ce anomalies that indicate an at least partially oxygenated water column. A variable but typically positive Eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation. We conclude that phosphatic sediments in the Paleoproterozoic Zaonega Formation record phosphogenesis in a vent/seep influenced setting that experienced fluctuating redox conditions at the sediment–water interface.


      PubDate: 2015-01-04T22:06:29Z
       
  • Tracing historical trends of Hg in the Mississippi River using Hg
           concentrations and Hg isotopic compositions in a lake sediment core, Lake
           Whittington, Mississippi, USA
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): John E. Gray , Peter C. Van Metre , Michael J. Pribil , Arthur J. Horowitz
      Concentrations and isotopic compositions of mercury (Hg) in a sediment core collected from Lake Whittington, an oxbow lake on the Lower Mississippi River, were used to evaluate historical sources of Hg in the Mississippi River basin. Sediment Hg concentrations in the Lake Whittington core have a large 10–15y peak centered on the 1960s, with a maximum enrichment factor relative to Hg in the core of 4.8 in 1966. The Hg concentration profile indicates a different Hg source history than seen in most historical reconstructions of Hg loading. The timing of the peak is consistent with large releases of Hg from Oak Ridge National Laboratory (ORNL), primarily in the late 1950s and 1960s. Mercury was used in a lithium isotope separation process by ORNL and an estimated 128Mg (megagrams) of Hg was discharged to a local stream that flows into the Tennessee River and, eventually, the Mississippi River. Mass balance analyses of Hg concentrations and isotopic compositions in the Lake Whittington core fit a binary mixing model with a Hg-rich upstream source contributing about 70% of the Hg to Lake Whittington at the height of the Hg peak in 1966. This upstream Hg source is isotopically similar to Hg isotope compositions of stream sediment collected downstream near ORNL. It is estimated that about one-half of the Hg released from the ORNL potentially reached the Lower Mississippi River basin in the 1960s, suggesting considerable downstream transport of Hg. It is also possible that upstream urban and industrial sources contributed some proportion of Hg to Lake Whittington in the 1960s and 1970s.


      PubDate: 2015-01-04T22:06:29Z
       
  • Using 222Rn to identify and quantify groundwater inflows to the Mundo
           River (SE Spain)
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): L. Ortega , M. Manzano , E. Custodio , J. Hornero , J. Rodríguez-Arévalo
      Groundwater discharge to the Mundo River (SE, Spain) has been investigated from 2011 to 2013 by means of 222Rn activities in river water and groundwater. Starting nearby the river source, some 50km of river channel have been studied. The Mundo River is located in the water stressed region of the Segura River Basin. Identifying and quantifying groundwater discharge to rivers is essential for the Hydrological Plan of the Segura Basin Authority. Four main areas of groundwater discharge to the river have been identified by means of 222Rn. Moreover, groundwater fluxes have been quantified using radon activities and, when possible, have been validated with chloride mass balances. The uncertainty range (±2σ) of all water balances has also been assessed. Groundwater discharge (QGW ) values estimated by radon mass balances (RMB) and chloride mass balances (CMB) were similar in the river tracts and/or dates in which surface inputs from tributaries were null or negligible. This adds confidence to the QGW values estimated by RMB in the reaches were CMB could not be performed due to the existence of ungauged surface inputs, as is the case of the upper basin of the Mundo River, as well as to the applicability of the method to similar situations. Quantification of groundwater discharge allowed identifying Ayna zone as the main gaining reach of the studied area, with up to 29,553±8667m3 day−1 in year 2011. Overall, the total QGW estimated by means of RMB for the studied area was 8–16% of the total river flow. The results are coherent with the meteorological conditions of the study period (average rainfall around 450mm/y) and also with the undisturbed situation of the aquifers discharging to the Mundo River in the considered area.


      PubDate: 2015-01-04T22:06:29Z
       
  • Hydrogen and copper isotope analysis of turquoise by SIMS: calibration and
           matrix effects
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Guillaume Othmane , Sharon Hull , Mostafa Fayek , Olivier Rouxel , Majdi Lahd Geagea , T. Kurtis Kyser
      The hydrogen isotope system is used extensively to provide information on the genesis of minerals (e.g., source of fluids and mechanisms of precipitation). The copper isotopic system is less well understood, but has the potential to provide valuable insight on mineral precipitation, particularly supergene Cu-rich minerals. Here we present a rapid and precise method for measuring hydrogen and copper isotopes in semi-precious gem-quality turquoise (Cu(Al,Fe3+)6(PO4)4(OH)8 ·4H2O) by secondary ion mass spectrometry (SIMS). The suitability of standards for instrumental mass fractionation (IMF) calibration was assessed by external precision of SIMS measurements for each standard (1–4‰ for δDIMF and 0.1–0.4‰ for δ65CuIMF). IMF in turquoise was correlated with H and Fe contents for D/H measurements and Fe content for 65Cu/63Cu measurements. Based on these correlations, IMF can be corrected to enable δD and δ65Cu analyses by SIMS with accuracies of ±5‰ and ±0.5‰, respectively. The precision and accuracy of SIMS thus rivals those of other mass spectrometric methods for H and Cu isotopes and demonstrates the potential of SIMS applications in identifying gemstones provenance and understanding the genesis of turquoise deposits.


      PubDate: 2015-01-04T22:06:29Z
       
  • Precise Pb isotope ratio determination of picogram-size samples: A
           comparison between multiple Faraday collectors equipped with 1012Ω
           amplifiers and multiple ion counters
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Chiranjeeb Sarkar , D.G. Pearson , Larry M. Heaman , S.J. Woodland
      While typical ion-counting peak hopping methods using a single Daly detector or a single secondary electron multiplier have proven very effective in analyzing small Pb samples by TIMS, the measurements often require long acquisition times, typically of several hours. Here we compare and evaluate static multi-collector determination of Pb isotope ratios in samples containing between 100 and 1pg total Pb using Faraday detectors, equipped with 1012 Ω feedback resistors in the current amplifier system, versus a multiple ion counting (MIC) system, installed in a Triton Plus TIMS. Faraday cup measurements of pg size Pb samples using 1012 Ω amplifiers require precise measurement of long baselines for optimal repeatability (defined as internal precision or the precision of a single run) and intermediate precision (defined as the closeness of the replicated measurements over an extended period of time). In our system, using a 20min baseline before and after the measurement, we achieved a repeatability of 0.02–0.03% (2SE) and an intermediate precision of 0.05% (2SD) on 207Pb/206Pb and 208Pb/206Pb in as little as 1h for samples containing between 80 and 10pg Pb. This is equivalent to the performance obtained from a 4 to 5hour-long single SEM peak hopping analysis of larger (10.0–0.5ng) samples. For measurements in MIC mode, the main analytical uncertainty is the relative instability of the detector yields, which is 0.1–0.3% (2SD; following a 1hour detector stabilization period). Using a correction based on the average yield measured before and after sample analysis, we achieved a repeatability of 0.03%–0.06% (2SE) and an intermediate precision of 0.18%–0.23% (2SD) on 207Pb/206Pb for 80 to 10pg loads. For sample sizes between 10 and 1pg, which were only measured in MIC mode because the 204Pb intensity is below detection limit in Faraday mode, the main source of uncertainty is the variable loading blank contribution (5–15%). We conclude that a Faraday array equipped with 1012 Ω amplifiers performs very well for Pb sample load sizes down to 10pg, allowing much more rapid data acquisition than peak-hoping SEM measurements and producing data that is 4–5 times more precise than static MIC measurements. In contrast, the MIC system offers considerable promise for tracer Pb work in the sub-10pg Pb analyte range where data with a few % level of accuracy and precision are useful, providing that acceptable total procedural blanks (below 0.1pg) can be achieved.


      PubDate: 2015-01-04T22:06:29Z
       
  • The source of Mesozoic granitoids in South China: Integrated geochemical
           constraints from the Taoshan batholith in the Nanling Range
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Zi-Fu Zhao , Peng Gao , Yong-Fei Zheng
      A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic granitic intrusions from the Taoshan batholith in the Nanling Range, South China. The results not only place constraints on the origin of granitoids but also provide insights into the effect of melting temperature on granitoid compositions. LA-ICPMS zircon U–Pb dating yields weighted 206Pb/238U ages of 230±2Ma for the Caijiang intrusion, 167±2Ma for the Huangpi intrusion, 152±3Ma for the Daguzhai intrusion, and 146±3Ma for the Luobuli intrusion. Relict zircon cores with older Mesozoic to Paleozoic U–Pb ages are present in some samples. These Mesozoic granitoids are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.23. They exhibit arc-like trace element distribution patterns, with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion of HFSE (e.g., Nb, Ta, Ti). They show high whole-rock initial 87Sr/86Sr ratios of 0.7101 to 0.7156 and low εNd(t) values of −11.2 to −8.8 as well as negative zircon εHf(t) values of −14.3 to −4.8 and Paleoproterozoic Nd-Hf model ages. These results suggest their derivation from partial melting of ancient continental crust. Furthermore, they have high zircon δ18O values of 8.6 to 10.6‰ and calculated whole-rock δ18O values of 10.8 to 12.5‰. Along with their peraluminous features, high K2O/Na2O ratios, arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions, these Mesozoic granitoids are unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. There are significant differences in both major and trace elements between the Caijiang intrusion of Triassic age and the Huangpi, Daguzhai and Luobuli intrusions of Jurassic age. While the Triassic granites exhibit high (Fe2O3)T +MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but low Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with high zircon saturation temperatures, the Jurassic granites have low (Fe2O3)T +MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but high Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with low zircon saturation temperatures. These correlations indicate that melting temperature and thus extent of partial melting have played an important role in dictating the compositions of granitoids. Therefore, the geochemical compositions of S-type granitoids are dictated not only by the composition of source rocks but also by the temperature of partial melting.


      PubDate: 2015-01-04T22:06:29Z
       
  • Kinetics of CO2(g)–H2O(1) isotopic exchange, including mass 47
           isotopologues
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Matthieu Clog , Daniel Stolper , John M. Eiler
      The analysis of mass 47 isotopologues of CO2 (mainly 13C18O16O) is established as a constraint on sources and sinks of environmental CO2, complementary to δ 13C and δ 18O constraints, and forms the basis of the carbonate clumped isotope thermometer. This measurement is commonly reported using the Δ 47 value — a measure of the enrichment of doubly substituted CO2 relative to a stochastic isotopic distribution. Values of Δ 47 for thermodynamically equilibrated CO2 approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ≈0.9% at 25°C. While the thermodynamic properties of doubly substituted isotopologues of CO2 (and, similarly, carbonate species) are relatively well understood, there are few published constraints on their kinetics of isotopic exchange. This issue is relevant to understanding both natural processes (e.g., photosynthesis, respiration, air–sea or air–groundwater exchange, CO2 degassing from aqueous solutions, and possibly gas–sorbate exchange on cold planetary surfaces like Mars), and laboratory handling of CO2 samples for Δ 47 analysis (e.g., re-equilibration in the presence of liquid water, water ice or water adsorbed on glass or metal surfaces). We present the results of an experimental study of the kinetics of isotopic exchange, including changes in Δ 47 value, of CO2 exposed to liquid water between 5 and 37°C. Aliquots of CO2 gas were first heated to reach a nearly random distribution of its isotopologues and then exposed at low pressure for controlled periods of time to large excesses of liquid water in sealed glass containers. Containers were held at 5, 25 and 37°C and durations of exchange ranging from 5min to 7days. To avoid the formation of a boundary layer that might slow exchange, the tubes were vigorously shaken during the period of exchange. At the end of each experiment, reaction vessels were flash frozen in liquid nitrogen. CO2 gas was then recovered from the head space of the reaction vessel, purified and analyzed for its Δ 47, δ13C and δ18O by gas source isotope ratio mass spectrometry. Equilibrium was reached for both δ18O and Δ 47 after durations of a few hours to tens of hours. δ18O values at equilibrium were consistent with known fractionation factors for the CO2–H2O system. The evolution of δ18O and Δ 47 with experiment duration were consistent with first-order reactions, with rate constants equal to each other (within error), averaging 0.19h−1 at 5°C, 0.38h−1 at 25°C and 0.65h−1 at 37°C. We calculate an activation energy for this isotopic exchange reaction of 26.2kJ/mol. By comparison, Mills and Urey (1940) measured the rate of 18O exchange between CO2(aq) and water to have a rate of 11h−1 at 25°C and an activation energy of 71.7kJ/mol. Our finding of a slower rate and lower activation energy is consistent with the rate limiting step of our experiment being the CO2(g)–CO2(aq) exchange, even when samples are shaken during the partial equilibration. Our results broadly resemble those from the study of (Affek, 2013), though this prior study found a lower rate constant for Δ 47. We propose that the difference is due to analytical uncertainties and explore the theoretical consequences of unequal reaction rates between 12C18O16O and 13C18O16O with a forward model.


      PubDate: 2015-01-04T22:06:29Z
       
  • Sulfate: A time capsule for Earth's O2, O3, and H2O
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Huiming Bao
      The stable isotope composition of O2, O3, and H2O in the geological past conveys rich information on history of the Earth system. However, few compounds are known to record O2 or O3 isotope signals directly and reliably. Sulfate (SO4 2−), a non-labile oxyanion capable of forming weakly soluble minerals, plays important roles in global sulfur, carbon, and oxygen cycles. Here I review publications on the triple oxygen isotope composition of sulfate which has been recently expanded, demonstrating that in addition to H2O isotope signals, sulfate can carry isotope signatures from O2 and O3. I argue that sulfate, to this point, is the only compound from which direct atmospheric O2 and O3 signals from the distant past can be retrieved. If the current understanding of the Earth's surface oxygenation history holds, we expect to observe little to no measurable deviation from a “normal” triple oxygen isotope composition for sulfate throughout the Archean, but both positive and negative deviations since the early Proterozoic. The full potential of this unique proxy can be achieved by further study on isotope kinetics of sulfur redox cycling and by filling gaps in geological records.


      PubDate: 2015-01-04T22:06:29Z
       
  • The influence of terrigenous particulate material dissolution on ocean
           chemistry and global element cycles
    • Abstract: Publication date: 24 February 2015
      Source:Chemical Geology, Volume 395
      Author(s): Catherine Jeandel , Eric H. Oelkers
      Land to ocean transfer of material largely controls the chemical composition of seawater and the global element cycles. Overall this transfer is dominated by the riverine transport of particulate material to the oceans. A large number of isotopic tracers including 143Nd/144Nd, 87Sr/86Sr, 30Si/28Si, 56Fe/54Fe, and 232Th/230Th, demonstrate that a significant fraction of this particulate material dissolves in seawater after its arrival to the oceans. Laboratory experiments confirm that these particles dissolve readily in seawater; 0.5 to 10% of the Sr and Nd in riverine transported particulate material is found to dissolve in seawater over time scales ranging from weeks to months. Noting that the mass of most elements arriving to the oceans via particulates exceeds that of the elements arriving via dissolved transport by at least a factor of 50, it follows that 1) particulate material dissolution in the ocean may be the dominant mechanism contributing numerous elements to the oceans, and 2) estimates based on dissolved riverine transport alone may significantly underestimate the global element fluxes to the oceans. The role of particulate material dissolution in seawater may be most significant in the cycles of sparingly soluble elements, which are far more concentrated in particulate material than more soluble elements. As such, particulate material dissolution and transport likely play a major role in the availability of those elements limiting marine primary productivity. This effect will be most significant at the ocean margins, as a large fraction of the products of particulate material dissolution are re-precipitated locally via reversible scavenging. The major effect of particulate dissolution in the open-ocean will be on element isotope compositions. Moreover, as the transport of particulates to the oceans is far more sensitive to temperature and runoff than dissolved transport, the dissolution of particulate in seawater and subsequent reactions may provide a strong yet underappreciated link between continental weathering and climate.


      PubDate: 2015-01-04T22:06:29Z
       
  • An optimized method for stable isotope analysis of tree rings by
           extracting cellulose directly from cross-sectional laths
    • Abstract: Publication date: 30 January 2015
      Source:Chemical Geology, Volumes 393–394
      Author(s): Akira Kagawa , Masaki Sano , Takeshi Nakatsuka , Tsutomu Ikeda , Satoshi Kubo
      Stable isotopes in tree-ring α-cellulose are valued as environmental proxies and their use is steadily increasing; however, preparation of α-cellulose is a bottleneck in isotope analysis. Recent methodological breakthrough for extracting tree-ring α-cellulose directly from tree-ring cross-sectional laths drastically increased the throughput of tree-ring isotope data. In this paper, we evaluate our recently designed “cross-section” method. This method employs polytetrafluoroethylene (PTFE) cases, enabling direct extraction of α-cellulose from 1-mm thick tree-ring laths, in combination with fixation sheets to prevent disintegration of freeze-dried α-cellulose laths. Perforated PTFE cases are easily producible at an affordable cost. They are made of commonly available lab consumables in catalogs and do not require specially made PTFE parts. Freeze-dried α-cellulose laths preserved distinct anatomical structure, enabling precise separation at the tree-ring boundaries. Once separated from a lath, tree-ring α-cellulose can be weighed directly into silver or tin capsules for analysis. We checked chemical purity of α-cellulose prepared by the cross-section method from five tree species (larch, pine, spruce, beech, and oak). Residual lignin and hemicellulose contents were quantitatively assessed by Fourier transform infrared spectrometry and gas chromatography. The average chemical purity of α-cellulose laths from the five species was 94.5%, similar to the chemical purity of α-cellulose prepared with the standard Jayme-Wise method. Both oxygen and carbon isotope values of α-cellulose prepared by the cross-section method also closely matched those prepared by the standard method. We conclude that, by overhauling the method of α-cellulose preparation for tree-ring isotope analysis, we increased throughput of tree-ring oxygen and carbon isotope data without sacrificing sample purity.


      PubDate: 2014-12-09T06:07:49Z
       
  • Simultaneous determination of δ11B and B/Ca ratio in marine biogenic
           carbonates at nanogram level
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Karina Kaczmarek , Ingo Horn , Gernot Nehrke , Jelle Bijma
      In this study we introduce a new in situ technique which allows the determination of the boron isotopic composition and B/Ca ratios simultaneously at the nanogram level using a combination of optical emission spectroscopy and multiple ion counting MC ICP-MS with laser ablation. This technique offers a new application in the paleo-field of oceanography and climatology since small samples like e.g. single foraminiferal shells can be analyzed. The simultaneous determination of the boron isotopic composition and B/Ca ratios provides two independent proxies which allow the reconstruction of the full carbonate system. To test the new technique we performed measurements on the cultured, benthic foraminifer Amphistegina lessonii. Our results yielded an average boron isotopic composition δ11B=18.0±0.83‰ (SD) with an average internal precision of 0.52‰ (RSE). The boron concentration was 53±7μg/g (SD). These results agree with the range reported in the literature. The reconstructed mean pH value is in excellent agreement with the measured pH of the seawater in which the foraminifers grew. The analysis of a foraminifer consumed approximately 1200ng calcium carbonate containing ca. 0.06ng boron. Compared to bulk analytical methods, this new technique requires less material and reduces the time for sample preparation.


      PubDate: 2014-12-06T06:03:01Z
       
 
 
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