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  Subjects -> EARTH SCIENCES (Total: 603 journals)
    - EARTH SCIENCES (447 journals)
    - GEOLOGY (67 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (45 journals)

EARTH SCIENCES (447 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: 7)
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: 2)
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: 9)
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: 15)
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: 9)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
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 Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
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: 195)
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: 11)
Earth System Dynamics     Open Access   (Followers: 5)
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: 9)
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  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2575 journals]   [SJR: 1.714]   [H-I: 111]
  • 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: Available online 13 December 2014
      Source:Chemical Geology
      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 10–15 y 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 were 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: 2014-12-17T21:45:32Z
       
  • 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: 2014-12-13T06:22:53Z
       
  • 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: 2014-12-13T06:22:53Z
       
  • Geochemical fingerprinting and source discrimination of agricultural soils
           at continental scale
    • Abstract: Publication date: Available online 13 December 2014
      Source:Chemical Geology
      Author(s): Philippe Negrel , Martiya Sadeghi , Anna Ladenberger , Clemens Reimann , Manfred Birke
      2108 agricultural soil samples (Ap-horizon, 0–20 cm) 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 behaviour 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 colour 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: 2014-12-13T06:22:53Z
       
  • Proposed sources of methane along the Dead Sea Transform
    • Abstract: Publication date: Available online 13 December 2014
      Source:Chemical Geology
      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 depleted 13C values in the range of δ13CCH4 between -58‰to -72‰ and high alkane ratios (C1/C2 +C3) between 100 and 1000. The isotope fractionation α CO2-CH4 of 1.065‰ suggests 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 enriched values of -28‰ and -56‰, respectively, with low alkane ratios of ~40 in both sites. The enriched 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 microbial-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 the methane is produced there 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: 2014-12-13T06:22:53Z
       
  • The thermodynamic properties of bastnäsite-(Ce) and parisite-(Ce)
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Alexander P. Gysi , Anthony E. Williams-Jones
      The rare earth elements (REE) are critical metals that play a major role in emerging high technology and green industries. The light (L)REE occur dominantly in fluorocarbonate minerals and consequently information on the stability of these minerals is essential for a better understanding of the genesis of REE deposits and for the efficient processing of their ores. We have investigated the thermochemical properties of natural bastnäsite-(Ce) (Ce0.50La0.25Nd0.20Pr0.05CO3F) and parisite-(Ce) (CaCe0.95La0.60Nd0.35Pr0.10(CO3)3 F2) using differential scanning calorimetry at temperatures from 323K to 1022K and a pressure of 1bar employing heat ramping and isothermal methods. Crystal lattice parameters of the REE fluorocarbonate minerals and reaction products from the experiments were determined using X-ray diffraction. The measured isobaric heat capacity (Cp°) for bastnäsite-(Ce) can be described by the relationship 134.3–2.032×106 T-2 between 343.15 and 528.15K and for parisite-(Ce) by the relationship 398.8–1048T−0.5 –4.202×106 T−2 between 343.15 and 643.15K, where T is temperature in K. Bastnäsite-(Ce) decomposed irreversibly at >612K to form REE oxyfluorides and CO2. An endothermic peak at 824.2K yielded a heat of reaction of 245.2±2.5kJ/mol. Parisite-(Ce) decomposed irreversibly at >664K to form REE oxyfluorides, CaCO3 and CO2. An endothermic peak at 842.7K yielded a heat of reaction of 522.6±5.2kJ/mol. The enthalpy of formation at 298K and 1bar was retrieved from the decomposition enthalpies, yielding −1808.4±12.0kJ/mol and −4848.0±23.8kJ/mol for bastnäsite-(Ce) and parisite-(Ce), respectively. The measured molar volumes for bastnäsite-(Ce) and parisite-(Ce) are 42.91cm3/mol and 122.71cm3/mol, respectively. An estimation method, based on the dependence of entropy on volume, was used to retrieve the third law entropy (S°) at 298.15K, and together with the measured thermodynamic properties, permitted us to construct the first quantitative mineral–fluid stability diagrams involving bastnäsite-(Ce), parisite-(Ce), fluocerite-(Ce), calcite and fluorite at P–T–x conditions relevant for the study of natural Ca–REE–C–O–H–F systems. Further studies of the thermodynamic properties of REE-bearing minerals are urgently needed to better understand the genesis of REE ore deposits.


      PubDate: 2014-12-13T06:22:53Z
       
  • Redox-dependent fractionation of iron isotopes in suspensions of a
           groundwater-influenced soil
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Stephan Schuth , Julia Hurraß , Carsten Münker , Tim Mansfeldt
      Redox conditions control the release of iron (Fe) into soil pore waters. A fluctuating groundwater table in soils results in significant changes in redox conditions with both time and depth. The effects of short-term differences in redox conditions on the stable isotope inventory of dissolved Fe in such soils have not yet been studied. Bulk Fe isotope compositions of a Gleysol yielded δ57Fe values from +0.3‰ (humic topsoil, Ah horizon) to −0.2‰ (Fe-enriched subsoil, CrBg horizon). In microcosm experiments, soil suspensions of the Ah and CrBg horizons were subjected to controlled redox conditions ranging from high redox potential (EH) (>430mV, pH5.1 to 5.6), moderate EH (~330mV, pH4.9 to 5.9), to low EH (≤170mV, pH5.2 to 6.7). Membrane-filtered (0.45μm) solutions taken from the suspensions were analysed for their Fe concentrations (Fe2+, and total Fe: Fetot) and isotopic compositions. The microcosm experiments demonstrated that the ferrihydrite- and organic-rich Ah horizon is a highly dynamic and rapidly responding reservoir with respect to Fe mobilization and isotopic fractionation at low EH. Iron concentrations and isotope ratios of the solutions from the Ah horizon varied depending on EH with negative δ57Fe values (−0.4‰) and Fetot (~1.6mgL−1) at moderate EH, and even lower δ57Fe values (−1.1‰) but high Fetot (~7.8mgL−1) at low EH. At high EH, δ57Fe values slowly increased from +0.3‰ to +1.0‰ and Fetot decreased to ~0.2mgL−1 within six weeks. The goethite-rich CrBg horizon constitutes a stable redox-insensitive pool with very low amounts of mobilized Fe and a small degree of isotopic fractionation, even after exposure to low EH over several weeks. In a natural open system, removal of Fe from the dynamic Ah horizon will result in progressively higher soil δ57Fe values due to preferential release of 54Fe. Vertical movement of a low-δ57Fe solution from the topsoil may result with time in the formation of a subsoil with δ57Fe values that are lower than the topsoil after repeated low and high EH cycles. At high EH, Fe mobility in the Ah horizon is much lowered, but release of Fe with high δ57Fe values is in agreement with earlier studies for the formation of Fe pools (Fe oxides, colloids, organic complexes) with high δ57Fe signatures.


      PubDate: 2014-12-13T06:22:53Z
       
  • 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: 2014-12-13T06:22:53Z
       
  • 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: 2014-12-13T06:22:53Z
       
  • 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
       
  • 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: 2014-12-09T06:07:49Z
       
  • 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: 2014-12-09T06:07:49Z
       
  • 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: 2014-12-09T06:07:49Z
       
  • Editorial Board
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391




      PubDate: 2014-12-06T06:03:01Z
       
  • Volatile concentrations in olivine-hosted melt inclusions from the
           Columbia River flood basalts and associated lavas of the Oregon Plateau:
           Implications for magma genesis
    • Abstract: Publication date: 21 January 2015
      Source:Chemical Geology, Volume 392
      Author(s): Joan A. Cabato , Christopher J. Stefano , Samuel B. Mukasa
      The volatile budgets and elemental compositions of primary magmas and their sources can be estimated through study of olivine-hosted melt inclusions in basaltic lavas, thereby providing insights about melting processes in the mantle. We report data on melt inclusions in 58 host-olivine crystals from seven basalt samples from the Columbia River Basalt (CRB) group in eastern Oregon and Washington, which have been analyzed for their major-oxide, trace-element and volatile (H2O, S, F, and Cl) concentrations to constrain the origin of this large igneous province. A wide range in H2O concentrations has been discovered, with the highest amount of 4.2wt.% found in olivine grains from a lava flow in the Malheur Gorge area, which erupted several million years after the hypothesized initial impingement of the Yellowstone Hotspot against the lithosphere of the North American Plate. This H2O concentration and others in our Columbia River–Oregon Plateau sample suite are significantly higher than those observed in lavas from hotspots such as Hawaii (~0.8–0.9wt.%) or in Mid-Ocean Ridge Basalts (MORBs) (~0.2wt.%), and are comparable to those measured for the lavas in the Snake River Plain–Yellowstone volcanic corridor and in some island and continental arcs. Lavas considered to be of plume origin from the Picture Gorge basalts in the main-eruptive-stage of the CRB show H2O concentrations of up to 2.4 wt.%, also significantly higher than values published previously for any plume-related volcanism. The highest H2O concentrations have been found most often in the least differentiated melt inclusions, indicating that these volatiles are not the result of differentiation at shallow levels in the crust. The enrichment of Ba relative to Th in many of the melt inclusions may indicate that volatiles in the CRB-OPB large igneous province have a subduction zone origin, which underscores the importance of flux melting in the generation of at least some of Earth’s large igneous provinces.


      PubDate: 2014-12-06T06:03:01Z
       
  • 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: 2014-12-06T06:03:01Z
       
  • 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
       
  • 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: 2014-12-06T06:03:01Z
       
  • 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: 2014-12-01T05:49:15Z
       
  • 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: 2014-12-01T05:49:15Z
       
  • 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: 2014-12-01T05:49:15Z
       
  • Editorial Board
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390




      PubDate: 2014-12-01T05:49:15Z
       
  • The role of carbonate complexes and crystal habit on rare earth element
           uptake in low-temperature calcite in fractured crystalline rock
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Olga M. Maskenskaya , Henrik Drake , Frédéric A. Mathurin , Mats E. Åström
      This study focuses on rare earth element (REE) geochemistry of low-temperature calcite coatings occurring on the walls of fractures throughout the upper kilometer of crystalline rocks of the Baltic Shield. Fifty one calcite coatings were sampled from cores drilled with the triple-tube technique which successfully preserved the fragile calcite coatings on the fracture walls. The calcites, which based on geological and isotopic evidence were precipitated over the last 10millionyears, had highly variable ΣREE concentrations (0.61–2276ppm) that decreased weakly with the depth the calcite was sampled from. When normalized to shale (and host rock), the REE concentrations of habits with c-axis≈a-axes and the closely associated c-axis>a-axes, the most abundant crystal morphologies in the system, decreased strongly and smoothly across the series. In contrast, the REEs of habits with c-axis≫a-axes, identified only in fractures in the uppermost 260m of the bedrock, were flatter and occasionally expressed a weak middle REE enrichment. By using calcite–water partition coefficients derived for REEs in previous laboratory experiments, the La/Yb of the paleogroundwater from which the calcites precipitated was back-calculated and found to be overall similar (range 0.15–452) overlap to the corresponding ratio of the present groundwater (range: 2.1–36.4). In terms of REE/Ca, the values for the back-calculated paleogroundwater (La/Ca 9.9∗10−11–3.9∗10−7; Yb/Ca 1.5∗10−10–2.2∗10−7) were similar to those of LaCO3 + /Ca (4.5∗10−10–8.5∗10−7) and (YbCO3 + +Yb(CO3)2 −)/Ca (5.4∗10−11–1.8∗10−8), respectively, in the present groundwater. These patterns indicate that the LREE to HREE and REE to Ca ratios in the groundwater at the site are broadly similar to those existing when the calcites precipitated, and that carbonate complexes present in the paleogroundwater played a crucial role in sequestration and fractionation of REEs in calcite. The findings have implications for bedrock storage of high-level radioactive waste, which contains actinides for which the REEs can be used as natural analogues.


      PubDate: 2014-11-27T05:33:19Z
       
  • Solute sources and water mixing in a flashy mountainous stream (Pahsimeroi
           River, U.S. Rocky Mountains): Implications on chemical weathering rate and
           groundwater–surface water interaction
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Benjamin Hagedorn , Robert B. Whittier
      Identifying solute sources and mixing processes between various water types is challenging in geologically diverse, fractured rock settings, where various minerals contribute to solute loads and mixing between groundwater and surface water can occur at difficult-to-delineate point locations. In such regions, chemical indicators that allow constraining characteristic mineral fingerprints of drainage lithology or aquifer end-members are critical. This study assesses solute sources and water mixing in the Pahsimeroi River, a small stream with highly variable discharge draining two heavily deformed mountain ranges of the U.S. Rocky Mountains. Solute inputs to the main stream were estimated using an end-member mixing model with 87Sr/86Sr and Na/Sr ratios as tracers considering that these depict the compositional variability pertaining to the various carbonate and silicate lithologies of the basin. Our results show that the mean solute input from carbonate-dominated terrains decreases from 95.9% in the headwaters to about 36.4% in the lowlands while the corresponding inputs from volcanic and metamorphic sources increase from 3.63% to 51.0% and 0.45% to 12.6%, respectively. Data further indicate highly variable chemical weathering rates with highest values observed in the steeper uplands. Calculated CO2 consumption rates (mean value: 0.14Mmolkm−2 a−1) are lower than the reported continental average (0.21Mmolkm−2 a−1) possibly due to lower than average streamflow at the time of sampling and significant input of carbonate solute-enriched baseflow in the downstream sections where the basin shallows and decreases in width. The herein delineated gaining stream segments are consistent with those deduced from downstream seepage runs which suggests that groundwater sustains perennial flow in the agriculturally developed lowlands.


      PubDate: 2014-11-27T05:33:19Z
       
  • The intensity of chemical weathering: Geochemical constraints from marine
           detrital sediments of Triassic age in South China
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Ming-Yu Zhao , Yong-Fei Zheng
      A geochemical study of major-trace elements in detrital sediment and carbon–oxygen isotopes in carbonate was carried out for a marine stratigraphic profile of Early Triassic that is composed of argillaceous limestone and calcareous mudstone in the Lower Yangtze basin, South China. The results place constraints on the geochemical behaviors of various elements in the detrital sediment that was deposited in the residual Paleotethyan seawater. This leads to establishment of new geochemical proxies for chemical weathering of continental crust. In terms of the correlations between element concentrations and their variations in the profile, the elements are categorized into four groups with respect to the difference in their geochemical behaviors. The first group is composed of Al, Th, Sc, Be, In, Ga, K, Rb and Cs that are tightly correlated due to their immobility during chemical weathering. The second group is composed of Ca and Na that show opposite variation trends with Th and Sc, on account of their mobile behavior in the weathering profile. The third group is composed of high field strength elements such as Ti, Nb, Ta, Zr and Hf that are closely correlated with each other because they were primarily taken up by heavy minerals from sedimentary provenance. The fourth group is composed of redox sensitive elements such as Co, Cu, Fe, Mn and Ni that are correlated with S and thus mainly hosted by sulfides. Th, Sc, Ca and Na were not amenable to changes in sedimentary provenance, and thus are selected to establish the new proxies for chemical weathering. These are composed of logarithmic parameters such as log(Th/Ca), log(Sc/Ca), log[Th/(Na/5+Ca)] and log[Sc/(Na/5+Ca)]. They exhibit synchronous increases at the Permian–Triassic boundary, the middle Griesbachian and the early Smithian, indicating the enhancements of chemical weathering. High proxy values approaching the values for the extremely weathered product of granodiorite occurred in the middle to late Griesbachian and early Smithian, demonstrating the occurrences of extreme chemical weathering and very warm paleoclimate in those periods. These paleoclimatic changes are concordant with results from geochemical studies elsewhere in the world. Therefore, the intensity of chemical weathering can be indicated by the new geochemical proxies for the different properties of elements in marine detrital sediments.


      PubDate: 2014-11-27T05:33:19Z
       
  • Theoretical calculations of Cd isotope fractionation in hydrothermal
           fluids
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Junli Yang , Yongbing Li , Shanqi Liu , Huiquan Tian , Caiyun Chen , Jianming Liu , Yaolin Shi
      This study reports a systematic approach of quantum chemical calculations (density functional theory) of cadmium isotope reduced partition function ratios to understand the fractionation properties of Cd species in hydrothermal fluids, including cadmium hydrate, chloride, hydroxide, nitrate, hydrosulfide complexes. The optimized structures and the calculated vibrational frequencies of the selected Cd complexes by B3LYP method are consistent with the experimental data. The reduced partition function ratios 103ln (β 114–110) of the Cd complexes were calculated with varying temperatures from 0°C to 400°C or 500°C, which are from 0.680‰ to 2.098‰ at 50°C. The isotope fractionation between different Cd species seems to be small, and there exists a trend of heavy Cd isotope enrichment as cadmium hydroxides>cadmium nitrates>cadmium hydrates>cadmium chlorides>cadmium hydrosulfide, according to the calculated reduced partition function ratios 103ln (β 114–110) by this study. This suggests that Cd hydroxide complexes and hydrosulfide complexes may produce larger fractionation than others, which even reaches +1.641‰ between Cd(OH)Cl and CdHS+ at 25°C. As the major Cd-bearing species in hydrothermal conditions, CdOH(H2O)5 + and CdCl(H2O)5 +, the fractionation factor α is found to be +0.120‰ at 150°C. These theoretical calculations provide insight into Cd isotope behavior in solution conditions, especially in hydrothermal fluids.


      PubDate: 2014-11-23T05:16:21Z
       
  • Toward a quantitative model for the formation of gravitational magmatic
           sulfide deposits
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Youxue Zhang
      A preliminary quantitative model for the formation of magmatic sulfide deposits through gravitational sulfide droplet settling is developed. The model incorporates thermodynamic consideration of the oversaturation of sulfide liquid from a silicate melt, and the coupled growth kinetics and settling dynamics of sulfide droplets. The conditions for the sulfide droplets to have enough time to grow and settle to form a sulfide liquid layer at the bottom of a magma chamber are referred to as the necessary criteria for sulfide ore formation. Simulations are carried out for dry Etna basaltic melt because their melt viscosity and sulfur diffusivity have been measured so that the simulations are more realistic rather than parametric. Furthermore, the effects of empirical nucleation rate and condition, and of the variation of solubility, viscosity and diffusivity have been parametrically evaluated. The simulations show that for a given magma containing about 0.1 to 0.3wt.% sulfur, sulfide melt layer can form for realistic magma body size (with cooling time of 1000yr or more) and realistic solubility, viscosity and diffusivity. The ability of the sulfide melt to collect ore elements depends critically on the diffusivity of individual ore elements. When the diffusivity of an ore element is similar to or greater than that of sulfur, near equilibrium partitioning is reached. When the diffusivity of an ore element is much smaller than that of sulfur, which is often the case for trivalent and tetravalent ions, the concentration of the element in the sulfide melt would be far below the equilibrium concentration. It is hoped that the model will help both researchers and exploration geologists.


      PubDate: 2014-11-23T05:16:21Z
       
  • A Cr isotopic study of the Bon Accord NiO body in the Barberton greenstone
           belt, South Africa
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Marian Tredoux , Frederick Roelofse , Alexander Shukolyukov
      An unusual oxide body, enriched in Ni and the platinum-group elements, was found in the north-western part of the Barberton greenstone belt, South Africa, in the 1920s. With mineralogy and geochemistry which are extremely anomalous, and do not conform to that of any other Ni-rich crustal rocks, it has been interpreted as either an Archaean paleo-meteorite or of terrestrial deep-mantle origin. Cr isotopic ratios were used to evaluate the merits of these two possibilities. Isotopic analyses returned values for ε(53) of 0.00 epsilon units, within a 2s error, relative to a terrestrial standard. We interpret this information, together with trace element data from the surrounding peridotites, as pointing to a terrestrial origin for the body, and conclude that the current dataset is not inconsistent with a deep-mantle origin, as a remnant of core formation, with subsequent transport to and emplacement in the lithosphere by a mantle plume.


      PubDate: 2014-11-23T05:16:21Z
       
  • Nitrogen isotopic compositions and concentrations in MARID xenoliths
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Sandeep Banerjee , T. Kurtis Kyser , Roger Mitchell
      Nitrogen isotopic compositions and concentrations in micas from kimberlite-hosted MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths from the Kimberley area of the Kaapvaal Craton, South Africa, were measured using continuous-flow isotope-ratio mass spectrometry and Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy data suggest that the nitrogen in MARID phlogopite is from ammonium in the mica structure. The replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites. The δ15N values of micas from MARID xenoliths range from −11 to +9‰, and nitrogen contents range between 113 and 272ppm. The 15N enrichment of most samples relative to the average upper mantle δ15N value of −5‰ suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region. The presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub-continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle.


      PubDate: 2014-11-23T05:16:21Z
       
  • Paleoproterozoic formation age for the Siberian cratonic mantle: Hf and Nd
           isotope data on refractory peridotite xenoliths from the Udachnaya
           kimberlite
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Luc S. Doucet , Dmitri A. Ionov , Alexander V. Golovin
      The formation age of the Siberian cratonic mantle is not well established as yet. Re–Os data on various mantle-derived materials have shown that it contains Archaean components, but the reported Re-depletion ages show a broad variation range (3.4 to 1Ga) and are commonly ≤2Ga for peridotite xenoliths. We report Hf and Nd isotope data for cpx and garnet separated from nine refractory spinel and garnet peridotite xenoliths from the Udachnaya kimberlite. The cpx from low-opx spinel harzburgites show extremely high εHf values, from +607 to +2084, which testify to long-term evolution of these rocks with high Lu/Hf ratios, consistent with their residual origin and near absence of post-melting enrichments in the Lu–Hf system. Such high εHf values are unusual for cpx from other cratonic peridotites and are higher than those reported for depleted cpx in off-cratonic peridotites. The clinopyroxenes from low-opx spinel harzburgites yield Hf model ages from 1.9 to 1.7Ga while the cpx from high-opx spinel harzburgites yield Hf model ages from 3.0 to 1.9Ga. When plotted together, they define a Lu/Hf isochron with an age of 1.80Ga, which we consider as a robust estimate of the formation age (melt extraction event) because it is obtained on residual rocks that show no evidence for HREE and Hf enrichments and because the model ages for three out of four individual samples are similar to each other. The cpx have high εNd of +94 to +123, which are inconsistent with their low Sm/NdPM of <1 and yield no meaningful age estimates. The consistently high, positive εNd in these cpxs can be interpreted in terms of long-term evolution of refractory peridotites with high Sm/Nd, followed by relatively recent LREE enrichments. We infer that a significant part of the lithospheric mantle in the central Siberian craton may have been formed during a major event (or a series of events) at around 1.8Ga. Older ages reported for the central Siberian craton may refer to less common materials from cratonic or other domains formed in the Archaean that were later incorporated into the cratonic roots. The transition from the “Archaean” to “modern” tectonic regimes in Siberia and possibly elsewhere may have taken place at 1.8–1.9Ga rather than at ~2.5Ga, i.e. in the second half of the Paleoproterozoic rather than at the Archaean–Proterozoic boundary, at which time the asthenospheric mantle became generally too cold to experience high-degree melting on a large scale. The ~1.8Ga formation age of the Siberian cratonic mantle is coeval with that for a major part of the ancient continental crust in the central Siberian craton. The temporal crust–mantle links may be explained either by the generation of the initial source materials for continental crust in the same melting event that formed the residual peridotites or, alternatively, by subduction and melting of pre-existing proto-lithosphere destabilized by a major mantle upwelling that formed the residual mantle.


      PubDate: 2014-11-23T05:16:21Z
       
  • An improved protocol for 87Sr/86Sr by laser ablation multi-collector
           inductively coupled plasma mass spectrometry using oxide reduction and a
           customised plasma interface
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): J. Lewis , C.D. Coath , A.W.G. Pike
      Strontium isotope analysis has become one of the most effective means to characterise mobility and migration of humans and animals in the past. Analysis of strontium isotopes in bulk dental enamel by dissolution and chemical separation is precise, reliable and accurate. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offer high spatial, and therefore temporal, resolution sampling. However, the application of LA-MC-ICP-MS to strontium isotopes in bioapatites has been limited due to the presence of isobaric interferences. In particular, 40Ca31P16O+ (CaPO+) on 87Sr+ has been identified as causing a consistent positive offset in 87Sr/86Sr by LA-MC-ICP-MS, relative to dissolution methods, of approximately 500 to 1500ppm. It has been demonstrated that tuning the mass spectrometer for reduced oxide production can reduce the size of this effect but not eliminate it completely. Here we present a revised protocol using a customised plasma interface through which helium gas is introduced. We find that using the oxide reduction and the customised plasma interface protocols can further reduce the offset such that we can achieve accuracy to within typical analytical precisions (±50ppm, 2σ) without any need for further mathematical corrections. We test the methodology by ‘blind’ comparisons between LA-MC-ICP-MS and micro-drill/dissolution thermal ionisation mass spectrometry on two archaeological teeth. Both techniques are in excellent agreement.


      PubDate: 2014-11-19T04:57:32Z
       
  • Effects of CeO2 nanoparticles on microbial metabolism
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Shota Masaki , Hiroyuki Shiotsu , Toshihiko Ohnuki , Fuminori Sakamoto , Satoshi Utsunomiya
      To understand the effects of nanoparticles on microorganisms, we experimentally investigated the effects of CeO2 nanoparticles (CeNPs) on yeast (S. cerevisiae) focusing on microbial metabolites and intracellular proteins. The yeast were harvested from a yeast extract peptone dextrose medium containing 0, 10, 100, and 250ppm of CeNPs and incubated for 120h in 1mM NaCl solution at three different pH values: 3, 5, and 7. The yeast released organic matter, P, K, and Mg into the NaCl solution at all pH values, even without CeNPs. Distinct differences were detected in the released organic species and intracellular proteins after exposure to CeNPs. High-performance liquid chromatography revealed that various organic species released from the yeast were expressed or suppressed after exposure to CeNPs. Although cytotoxicity was not caused by CeNPs, the results of the peptide mass fingerprint analysis of the intracellular protein revealed that Eno2p, a glycolysis enzyme, was expressed after exposure to CeNPs. These results suggest that nanoparticles have the potential to alter microbial metabolism, leading to changes in the compositions of the released substances in the surrounding environment.


      PubDate: 2014-11-19T04:57:32Z
       
  • Rare earth elements as reactive tracers of biogeochemical weathering in
           forested rhyolitic terrain
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Angélica Vázquez-Ortega , Julia Perdrial , Adrian Harpold , Xavier Zapata-Ríos , Craig Rasmussen , Jennifer McIntosh , Marcel Schaap , Jon D. Pelletier , Paul D. Brooks , Mary Kay Amistadi , Jon Chorover
      Rare earth elements (REEs) were evaluated as potential tracers of biogeochemical weathering at pedon, hillslope, and catchment scales in the Jemez River Basin Critical Zone Observatory (JRB-CZO), Valles Caldera National Preserve, NM, USA. We investigated time series of REE patterns in precipitation, soil pore water, groundwater, and stream water, and related these data to REE composition of soil, rock and atmospheric dust. REE signatures in stream waters are dynamic, reflecting processes that occur along hydrologic flowpaths during transport to the stream, including organic matter complexation, primary and secondary mineral weathering, water/soil/bedrock interaction, and atmospheric deposition. Strong compositional similarities for the REE between soil waters and stream waters during the initial snowmelt are consistent with shallow subsurface flows to streams. Most (bio)chemical denudation of REE occurred during the snowmelt-derived dissolved organic carbon (DOC) pulse, during which time apparent colloidal mobilization of REE occurred in association with Fe and Al (oxy)hydroxides. The REE and DOC concentrations in stream waters were positively correlated (R2 =0.80, p<0.0001) during snowmelt, suggesting REE complexation and mobilization in association with organic ligands during the period of shallow subsurface flow. Positive Eu-anomalies occur in the soil matrix ([Eu/Eu⁎]RT range from 1.79 to 2.52), soil solutions ([Eu/Eu⁎]RT range from 1.89 to 5.98), and stream waters ([Eu/Eu⁎]RT range from 1.01 to 2.27) with respect to the host lithologies—effects attributable to both eolian deposition and preferential feldspar dissolution. Cerium anomalies in soil solids and porewaters indicate seasonally dynamic translocation and oxidative accumulation in subsurface soil horizons (surface horizons: [Ce/Ce⁎]RT range from 0.70 to 1.1; subsurface horizons: [Ce/Ce⁎]RT range from 0.95 to 1.29), consistent with prior research reporting Ce(IV) co-precipitation with Fe- and Mn-oxide minerals.


      PubDate: 2014-11-19T04:57:32Z
       
  • Determining in situ pH values of pressurised fluids using stable carbon
           isotope techniques
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): A. Myrttinen , V. Becker , B. Mayer , R. van Geldern , J.A.C. Barth
      Accurate determination of pH values is fundamentally important for understanding and predicting geochemical processes in surface and subsurface environments. However, conducting in situ pH measurements of fluids from geological formations exposed to high CO2 pressures and elevated temperatures is technically and logistically challenging. Here we introduce a proxy-method for determining in situ pH values based on stable carbon equilibrium isotope fractionation (103lnα13C) between dissolved inorganic carbon (DIC) and headspace CO2, which is valid for a DIC system composed of a mixture of H2CO3 ⁎ (CO2(aq) +H2CO3) and HCO3 −. The method is based on determination of DIC speciation and δ13C measurements of DIC and headspace CO2 on de-pressurised fluid samples in the laboratory. Since 103lnα13CDIC–CO2 values vary predictably dependent upon temperature and pH, DIC speciation and carbon isotope ratio measurements on two carbon species (CO2 and DIC) enable the determination of in-situ pH values if temperatures are known. This approach was verified in a series of laboratory experiments with water samples equilibrated with gaseous CO2 at 55bars for various temperatures and time periods. Concentration and carbon isotope measurements upon opening of the reaction vessels yielded 103lnα13CDIC–CO2(g) values between −1.3±0.1 and +1.4±0.2‰ corresponding to reconstructed in situ pH values ranging from 5.5 to 7.4 at temperatures between 20 and 120°C. Measurements with a pH-metre immediately after opening the reaction vessels were on average 1.4pH units higher than those determined by the proxy method due to CO2 outgassing that increased the measured pH. This CO2 degassing effect does not impact the here presented carbon isotope-based pH reconstruction method, which constitutes a viable alternative to direct pH measurements.


      PubDate: 2014-11-19T04:57:32Z
       
  • Silica hydrate preserved with δ18O-rich quartz in high-temperature
           hydrothermal quartz in the high sulfidation copper-gold deposit at El
           Indio, Chile
    • Abstract: Publication date: Available online 16 November 2014
      Source:Chemical Geology
      Author(s): Dominique Tanner , Richard W. Henley , John A. Mavrogenes , Peter Holden , Terrence P. Mernagh
      Quartz microcrystals from the El Indio Au-Ag-Cu deposit (Chile) preserve a rare glimpse into the high-temperature evolution of silica. Here, we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates ‘opal’ and moganite. We propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase. Evidence for sequential dehydration to from silica hydrate to quartz (silica hydrate⌫opal⌫moganite⌫quartz) is provided by SHRIMP 18O microanalytical data that show oscillatory isotopic zoning from 3.6 to 16.2‰ δ18O (±0.5‰) coupled with K and Al variations. We estimate that the precursor silica hydrate deposited between ~480-680°C and contained 32-63wt% H2O. Silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation, a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the El Indio copper-gold deposit. Modern understanding of ore-forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions. The discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids. Quartz matured from silica hydrate would record ‘pseudo-primary’ fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while δ18O data may bias fluid provenance interpretation within sub-volcanic systems.
      Graphical abstract image

      PubDate: 2014-11-19T04:57:32Z
       
  • The influence of pH on barite nucleation and growth
    • Abstract: Publication date: 6 January 2015
      Source:Chemical Geology, Volume 391
      Author(s): Cristina Ruiz-Agudo , Christine V. Putnis , Encarnación Ruiz-Agudo , Andrew Putnis
      Nanoscale Atomic Force Microscopy (AFM) experiments show that barite (BaSO4) growth is influenced by the pH of the growth solution. AFM observations provide evidence that growth and nucleation rates measured along the [100] crystallographic direction on the initial layer grown on barite (001) natural surfaces increase at both high and low pH of the growth solutions. At alkaline pH, growth is arrested in the second and successive layers, possibly as a result of the structure distortion resulting from incorporation of foreign ions (OH− and/or CO3 2−). Macroscopic nucleation experiments also show that with increasing pH, the induction times, the precipitation rate and the interfacial tension are all reduced, consistent with nanoscale observations. Smaller particle size at high pH provides further evidence for enhanced barium sulfate nucleation in alkali solutions. This enhancement in growth as well as in nucleation of barite at high pH could be explained by taking into account the effect of hydroxyl ions on hydration shells of aqueous Ba2+ and SO4 2− in solution and on the barite surface. The energetic interaction between water molecules and the barite building units is affected by the presence of OH− ions in solution. The frequency of water exchange around Ba2+ and SO4 2− could increase due to the effect of OH− ions on the structure of water and consequently promote nucleation and growth. Increased growth at low pH can be attributed to increased Ba2+ activity with respect to SO4 2−.


      PubDate: 2014-11-19T04:57:32Z
       
  • 238U–230Th dating of chevkinite in high-silica rhyolites from La
           Primavera and Yellowstone calderas
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Jorge A. Vazquez , Noel O. Velasco , Axel K. Schmitt , Heather A. Bleick , Mark E. Stelten
      Application of 238U–230Th disequilibrium dating of accessory minerals with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical evolution of magma within the framework of absolute time. Chevkinite, a Th-rich accessory mineral that occurs in peralkaline and metaluminous rhyolites, may be particularly useful as a chronometer of crystallization and differentiation because its composition may reflect the chemical changes of its host melt. Ion microprobe 238U–230Th dating of single chevkinite microphenocrysts from pre- and post-caldera La Primavera, Mexico, rhyolites yields model crystallization ages that are within 10's of k.y. of their corresponding K–Ar ages of ca. 125ka to 85ka, while chevkinite microphenocrysts from a post-caldera Yellowstone, USA, rhyolite yield a range of ages from ca. 110ka to 250ka, which is indistinguishable from the age distribution of coexisting zircon. Internal chevkinite–zircon isochrons from La Primavera yield Pleistocene ages with ~5% precision due to the nearly two order difference in Th/U between both minerals. Coupling chevkinite 238U–230Th ages and compositional analyses reveals a secular trend of Th/U and rare earth elements recorded in Yellowstone rhyolite, likely reflecting progressive compositional evolution of host magma. The relatively short timescale between chevkinite–zircon crystallization and eruption suggests that crystal-poor rhyolites at La Primavera were erupted shortly after differentiation and/or reheating. These results indicate that 238U–230Th dating of chevkinite via ion microprobe analysis may be used to date crystallization and chemical evolution of silicic magmas.


      PubDate: 2014-11-15T04:38:19Z
       
  • Sensitivity of carbonate weathering to soil CO2 production by biological
           activity along a temperate climate transect
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Damien Calmels , Jérôme Gaillardet , Louis François
      We investigated the controls on carbonate weathering in a well-drained pure carbonate area subject to strong environmental gradients, the Jura Mountains, Western Europe. The water chemistry of sampled springs and resurgences is dominated by Ca2+ (87 to 96Eq% of the cationic charge) and HCO3 − (90 to 97Eq% of the anionic charge), reflecting the overwhelming imprint of calcium carbonate dissolution by atmospheric/biogenic CO2. Ca2+ concentration, which directly gives access to the amount of calcium carbonate dissolved per unit of water runoff, shows a gradual two-fold decrease (from 3000 to 1400μmol/L) along the elevation gradient (from 300 to 1200m). After discussing the possible influence of each environmental parameter on the observed water chemistry gradient, a decreasing soil pCO2 (the main source of acidity) with increasing altitude appears as the most likely explanation. As no spatial and temporal record of soil pCO2 are available for the Jura Mountains, we performed soil pCO2 modeling using the ecological and hydrological ASPECTS model that allows reconstructing carbon and water exchange fluxes between the vegetation, soil and atmosphere. Modeling results suggest that soil pCO2 decreases with altitude in response to both the change in vegetation species from deciduous-dominated forest in the lowlands to evergreen-dominated forest above 800m (responsible for 65% of the variation) and the change in climate and soil properties (responsible for 35% of the variation). Carbonate weathering would thus be strongly sensitive to the type of vegetation, which drives both temporal and spatial variations of soil carbon and water budgets. Based on field observations, we show that carbonate weathering rates are 20–30% higher under deciduous vegetation cover than under conifers (at a given water runoff value), in agreement with modeling results. Chemical denudation rates of carbonate in the Jura Mountains vary from 152 to 375t/km2/yr, corresponding to 60–150mm/ka of carbonate being removed. Carbonate weathering within the 10,000km2 of the study area accounts for an atmospheric CO2 consumption of 0.3 TgC/yr, showing that carbonate rocks have an enhanced capacity of atmospheric CO2 neutralization at least transiently. This study demonstrates that carbonate weathering is sensitive to the ecosystem dynamics, a conclusion that might be much more general, and suggests that carbonate weathering and associated CO2 consumption fluxes quickly react to any global change or land use modification.


      PubDate: 2014-11-15T04:38:19Z
       
  • Dissolution rates of actinolite and chlorite from a whole-rock
           experimental study of metabasalt dissolution from 2≤pH≤12 at
           25°C
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): T. Critelli , L. Marini , J. Schott , V. Mavromatis , C. Apollaro , T. Rinder , R. De Rosa , E.H. Oelkers
      The dissolution rates of the minerals actinolite and chlorite were determined from metabasalt element release rates measured at 25°C and 2<pH<12 in mixed flow reactors. At pH 2.0 and 3.2, chlorite rates are 3 and 5 times faster, respectively, than corresponding actinolite rates, whereas the Si release rates from metabasalt are intermediate between chlorite and actinolite rates. In contrast, at pH 7.2 and 12.0, chlorite, actinolite and the metabasalt release Si at the same rates within analytical uncertainties. At pH 6.3, it was only possible to obtain the chlorite dissolution rate; at this pH the measured chlorite dissolution rate is 10−11.86 mol/m2/s. Mineral dissolution rates obtained in this study are within the range of corresponding values reported in the literature. This observation suggests that the dissolution rates of major-constituent minerals in a multi-phase rock are not affected by the presence of the other minerals. This conclusion validates the common assumption that the dissolution rate of an individual mineral is equal to that of the same mineral in a dissolving multi-mineralogic rock, at least for major constituents.


      PubDate: 2014-11-15T04:38:19Z
       
  • Enhanced heterotrophic denitrification in clay media: The role of mineral
           electron donors
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Marc Parmentier , Patrick Ollivier , Catherine Joulian , Achim Albrecht , Jebril Hadi , Jean-Marc Greneche , Hélène Pauwels
      Denitrification is expected to occur at and near the interface with clayey formations dedicated to radioactive waste disposal, due to the nitrate content in some waste canisters and the high probability of introducing denitrifiers during the operational phase. Nitrate reduction to nitrous-oxide gas by pure-strain Pseudomonas mandelii, in the presence or absence of sterilized Callovian–Oxfordian (COx) clay rock, was studied over a period of ~41.5months (1267days) by means of batch experiments. A culture medium with a similar porewater chemistry to that of COx rocks was used, supplemented with acetate and nitrate. Bacterial growth was monitored by genomic-DNA and narG-gene quantification. Nitrite accumulated in solution concomitantly with a decrease in nitrate content and the weak generation of nitrous oxide, but denitrification rates drastically decreased over the study period. Acetate was both oxidized to inorganic carbon and incorporated into biomass. The presence of solid COx particles significantly affected the geochemical reactions and particularly caused an enhanced nitrate reduction, a higher bacterial growth and the precipitation of calcium carbonate. Moreover, in the presence of COx a delay of several weeks was observed before the accumulation of nitrite in solution, leading to an imbalance between nitrate consumption and the production of nitrite, nitrous oxide and ammonium. Chemical oxidation of clay along with nitrite reduction to dinitrogen is expected to occur, explaining both the delay in nitrite accumulation and the apparent imbalance in nitrogen species. Although the electron donor of the COx oxidation was not identified, several hypotheses may be advanced, and we provide new insight into the biogeochemical and geochemical processes that may occur concomitantly at the excavation damaged interface of the clayey host rock after closure, resaturation and release of waste components in a nuclear waste repository. Because of denitrification, the oxidative impact of nitrate released from waste will diminish. However, nitrites produced through denitrification and diffusing into the rock will likely play in redox reaction via their abiotic reactivity which may occur within the compact clay formation despite a lack of space for bacterial activity.


      PubDate: 2014-11-15T04:38:19Z
       
  • Selenium isotopes trace anoxic and ferruginous seawater conditions in the
           Early Cambrian
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Hanjie Wen , Jean Carignan , Xuelei Chu , Haifeng Fan , Christophe Cloquet , Jing Huang , Yuxu Zhang , Huajin Chang
      Selenium (Se) isotopes can yield substantial isotopic fractionation (up to 20‰) confirmed by experiments and field investigations, depending on various biotic or abiotic redox transformations. Therefore, it is expected that redox changes in the ancient oceans would induce significant isotopic fractionation, and the Se isotopic signatures recorded in old sedimentary rocks might provide new insight into how the redox state of the ancient ocean has evolved. However, previous studies have shown that Se is slightly enriched in the lighter isotope relative to the bulk earth values in most deposited conditions (oxic, anoxic, and even sulfidic). Here, our results reveal that ferruginous conditions can result in excessive accumulation of Se in sediments with an elevated Se/S ratio and significant isotope fractionation (about 6‰), which leads us to propose that Se isotopes are an appropriate geochemical proxy to trace unique oceanic conditions over times. Accordingly, Se isotopic variations measured in three Early Cambrian formations in southern China suggest that anoxic waters with ferruginous conditions must have been present in early Cambrian ocean along the eastern margin of the Yangtze platform, and oceanic circulation was stepwise reorganized. This may have triggered biological diversification from the Ediacaran to the Early Cambrian.


      PubDate: 2014-11-15T04:38:19Z
       
  • U(VI) and Sr(II) batch sorption and diffusion kinetics into mesoporous
           silica (MCM-41)
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): David M. Singer , Hua Guo , James A. Davis
      Important reactive phenomena that affect the transport and fate of a radioactive material such as uranium (U) and strontium (Sr) in the environment occur at the mineral–water interface, particularly in mesoporous materials which are ubiquitous in surface and near-surface environments, and typically dominate the reactive surface area of geologic media. Ion sorption and physical bonding forces (including electrostatic forces) can be significantly modified within these confined pore spaces, leading to preferential enrichment of trace elements in mesopores. Pore space confinement may also lead to kinetic restraints on thermodynamically favorable sorption/desorption, precipitation/dissolution, and redox reactions, due to slow migration of metals out of mesopores, chemical gradients within the pore space, or steric constraints for inward migration of larger molecules. Using a combination of benchtop sorption experiments, TEM, and synchrotron-based X-ray absorption spectroscopy, U(VI) and Sr(II) uptake on mesoporous silica (MCM-41) with a 4.67nm pore diameter was measured in batch conditions at pH4.0 and 9.8 as a function of time and metal speciation. Uptake of U was determined for U-hydrolysis, U–CO3, and U–CO3–Ca aqueous species. This suite of techniques enabled determination of the rate of metal sorption and precipitation in the pore spaces, and identification of the reaction products. Our results indicate that Sr and U (at less than 10μM total U) rapidly diffuse into MCM-41. U at a higher concentration than 10μM also rapidly diffuses in, but the higher pore volume U concentration eventually leads to polymerization and precipitation of nano-U-bearing phases. The steady state U sorption maximum after 48h of exposure to MCM-41 prior to precipitation was dependent on the size and charge of the dominant U species in solution, where the trend is: UO2(OH)3 − >UO2 +2 >UO2(CO3)3 −4 ≈CaUO2(CO3)3 −2. Precipitation of a U-bearing phase within the silica pore spaces occurred only after a threshold time point and indicated that U uptake was both thermodynamically and kinetically controlled. Initial diffusion and adsorption were controlled by aqueous speciation and precipitation was controlled by the buildup of sorption species that subsequently created a bottleneck effect near pore openings. Acidic solutions wee more efficient at extracting U than carbonate solutions once the U has diffused into the mesopore region, and this may explain frequent observations of this behavior in extractions of natural sediments. A nitric acid wash was not completely effective at desorbing U and Sr from the mesoporous silica, where there was 5% (initial pH4.0) and 20% (pH9.8) remaining even after 500h of exposure to the nitric acid wash. The results show that U and Sr diffusion and sorption into mesopores result in a recalcitrant pool of ions that are sequestered in deep internal pore spaces, and this may have significant impacts on attempts to clean up contaminated soils and sediments.
      Graphical abstract image

      PubDate: 2014-11-15T04:38:19Z
       
  • Effects of U–Th-rich grain boundary phases on apatite helium ages
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Kendra E. Murray , Devon A. Orme , Peter W. Reiners
      Single-grain apatite (U–Th–Sm)/He ages (He ages) from non-detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion, such as grain size, radiation damage, parent zonation, fracturing, or intracrystalline inclusions. We present observations and model age bias effects of an additional source of apatite He age dispersion observed in some cases: U–Th-rich grain boundary phases (GBPs) precipitated on apatite crystal surfaces. Apatite grains from several samples with dispersed apatite He ages are coated or partially coated by reddish-orange GBPs rich in Fe, U, and Th. These GBPs are heterogeneous in thickness, grain coverage, and composition, and have effective U concentrations ([eU]) up to ~1000ppm. These phases may have large effects on the bulk [eU] and 4He compositions of apatite grains and can produce significant He age biases. The direction and magnitude of this age bias is primarily a function of four factors: 1) the host grain's size, 2) the GBPs' thickness and [eU] relative to the host grain, 3) the timing of GBP formation relative to the cooling age of the host grain, and 4) whether the GBPs are preserved and analyzed with the grain during He dating. Some of the most severe effects occur when GBPs formed before or near the time of the apatite cooling age but were lost just prior to analysis, for example during mineral separation. In this case, GBPs of commonly observed thicknesses (1–10μm) with [eU] 2–10 times that of the host apatite grain implant enough 4He into typical-sized apatite grains to produce hundreds of percent positive age biases. In contrast, when U–Th-rich GBPs are preserved and analyzed with apatite grains, the resulting He ages can be negatively biased. Though the heterogeneity, variable preservation, and ambiguous formation ages of GBPs preclude a general, quantitative, and practical solution to this problem, our model demonstrates that observed age dispersions in some samples are consistent with the effects of U–Th-rich grain boundary phases and illustrates the conditions under which, and by how much, they will bias He ages.


      PubDate: 2014-11-15T04:38:19Z
       
  • Assessing redox zones and seawater intrusion in a coastal aquifer in South
           Korea using hydrogeological, chemical and isotopic approaches
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Duk-Min Kim , Seong-Taek Yun , Man Jae Kwon , Bernhard Mayer , Kyoung-Ho Kim
      A shallow (<25m), coastal alluvial groundwater system underneath a paddy field in the Yangyang area of South Korea was investigated to examine the occurrence of redox processes. The aquifer is affected by seawater intrusion, and is characterized by a highly reducing environment facilitated by high organic matter in the sediments. Hydrochemical data with δ34S and δ18O of sulfate were examined for depth-specific groundwater from two multilevel samplers that were installed at seaward (YY2) and landward (YY1) locations. Shallow groundwater showed distinct patterns of redox zoning. Evidence of significant bacterial sulfate reduction (BSR) was observed throughout the nearly entire depths of the two boreholes, while at the depths of active seawater intrusion in YY2, conditions suitable for methanogenesis were never reached. Thus, at YY2 the deep zone of intense BSR was overlain by a zone in which methanogenesis occurred in a low-sulfate environment. In contrast, concurrent BSR and methanogenesis in YY1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive BSR. Considerable BSR in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of δ34Ssulfate up to 99.9‰. The inferred sulfur isotopic enrichment factor (ε) for BSR in the lower part of YY2 was −12.3‰, while ε at YY1 was much higher (−45.9‰). In addition, the observed trends of δ18Osulfate at YY1 indicated significant oxygen isotope exchange of sulfate-oxygen with ambient water, likely because of lower cell-specific rates of BSR and higher sulfur isotope fractionation as indicated by the δ34S. In contrast, there was little evidence of oxygen isotope exchange between water and SO4 2− at YY2. This study indicates that in coastal aquifers with sulfate-reducing activity, δ34S and δ18O of sulfate can reveal zones of active seawater intrusion and of trapped seawater. This study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic-rich coastal environment.


      PubDate: 2014-11-15T04:38:19Z
       
  • Accurate analysis of shallowly implanted solar wind ions by SIMS backside
           depth profiling
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Veronika S. Heber , Kevin D. McKeegan , Donald S. Burnett , Jean Duprat , Yunbin Guan , Amy J.G. Jurewicz , Chad T. Olinger , Stephen P. Smith
      A method to quantitatively determine the fluences of shallowly-implanted solar wind ions returned to Earth by the Genesis Discovery mission is described. Through backside depth-profiling, we recover nearly complete depth profiles of implanted solar wind for several nonvolatile elements, including Mg, Al, Ca, Cr, and, to a lesser extent, Na, in silicon targets that collected bulk solar wind and solar wind from specific velocity regimes. We also determine the fluences of the volatile elements C, N, and O in silicon targets that collected bulk solar wind. By the use of appropriately calibrated ion implanted standards, fluences as low as 2×1010 atoms cm−2 can be determined with precision and accuracy typically in the few percent range. Specific approaches to sample preparation, sputtering artifacts during depth profiling by secondary ion mass spectrometry, and quantification including the production of ion implant standards are discussed.


      PubDate: 2014-11-10T04:06:55Z
       
  • Zn isotope fractionation during interaction with phototrophic biofilm
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Aude Coutaud , Merlin Meheut , Jérôme Viers , Jean-Luc Rols , Oleg S. Pokrovsky
      This study addresses Zn isotopic fractionation during its sorption and excretion with a mature phototrophic biofilm using both a batch reactor and an open drip flow reactor (DFR). Short-term batch adsorption experiments demonstrated an enrichment in heavier isotopes larger than that induced by biofilm's Zn uptake, in general agreement with previous results for aquatic micro-organisms. Presumably, the adsorption of heavy isotopes was controlled by the formation of strong covalent bonds with the surface moieties rather than the kinetics of diffusion through the boundary layer to the biofilm surface. The rapid physico-chemical passive desorption was driven by the exchange of H+/Zn2+ ions with a highly negative isotopic enrichment factor (e.g. ε66Znexcretion of −1.4±0.35‰). The Zn efflux dominated by diffusion due to the osmotic biofilm–solution gradient favored lighter isotope release (e.g. ε66Znexcretion of −0.40±0.07‰ for wet biofilm in the DFR). The passive physico-chemical processes of Zn diffusion and H+/Zn2+ ion exchange are capable of inducing greater isotopic separation between solution and biomass compared to the active (intracellular) uptake. Regardless of the nature of aquatic microorganisms, the biomass enrichment with heavy isotopes during adsorption and uptake and preferential release of light isotopes seem to be general characteristics of aquatic biota.
      Graphical abstract image

      PubDate: 2014-11-07T03:50:48Z
       
  • Characterization of cores from an in-situ recovery mined uranium deposit
           in Wyoming: Implications for post-mining restoration
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): G. WoldeGabriel , H. Boukhalfa , S.D. Ware , M. Cheshire , P. Reimus , J. Heikoop , S.D. Conradson , O. Batuk , G. Havrilla , B. House , A. Simmons , J. Clay , A. Basu , J.N. Christensen , S.T. Brown , D.J. DePaolo
      In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7m of core of the formerly the ISR-mined Smith Ranch–Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (>1wt.%) and clay (>6–17wt.%) contents yielded higher 234U/238U activity ratios (1.0–1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374–11,534ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234U/238U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining. EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U–O distances of ~2.05Å were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from UO at 1.79Å, which indicates that U is partially oxidized. There is no apparent U–S or U–Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite. Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of their limited access to fluid flow, these areas might not contribute significantly to post-mining U release or attenuation. Areas with pyrite that are accessible to fluids seem to be more reactive and could act as reductants and facilitate U reduction and accumulation, limiting its migration.


      PubDate: 2014-11-02T03:26:06Z
       
  • Sr/Ca differences within and among three Tridacnidae species from the
           South China Sea: Implication for paleoclimate reconstruction
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Hong Yan , Da Shao , Yuhong Wang , Liguang Sun
      In this study, eight Tridacnidae specimens of three different species (Tridacna gigas, Tridacna derasa and Hippopus hippopus) were collected from Xisha Islands of South China Sea. The Sr/Ca ratio, which has been widely used as sea surface temperature (SST) proxy in coral samples, was determined on these specimens using ICP-OES to examine their potential for paleoclimate reconstruction. While within-species differences were found to be insignificant, the mean Sr/Ca ratios of the three species (2.24±0.1, 1.77±0.1 and 1.91±0.14mmol/mol for modern T. gigas, T. derasa and H. hippopus specimens, respectively) are statistically different. Species identification, therefore, is necessary before using Tridacnidae Sr/Ca ratios as paleoclimate recorders. While the relationship between local SST and Sr/Ca ratios of H. hippopus was unclear, the Sr/Ca profiles of modern T. gigas and T. derasa specimens are significantly correlated to local SST, implying that they can be used as SST proxies. The range of Sr/Ca ratios measured in this study is comparable to the Tridacnidae Sr/Ca measurements obtained in previous studies from other areas, but there are also substantial differences in detail, indicating that calibration for regional differences is also needed before paleoclimate reconstruction is attempted. In addition, although the mean Sr/Ca values of T. gigas, T. derasa and coral samples in Xisha Islands were significantly different, their Sr/Ca-SST calibration slopes were comparable, pointing to a similar thermodynamic fractionation and highlighting the potential of the combination of Tridacnidae and coral samples for paleo-SST seasonality reconstruction.


      PubDate: 2014-10-30T03:15:29Z
       
  • Adhesion of Pseudomonas putida onto kaolinite at different growth phases
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Huayong Wu , Wenli Chen , Xingmin Rong , Peng Cai , Ke Dai , Qiaoyun Huang
      Bacterial adhesion to minerals in soils and sediments is of fundamental importance in mineral weathering and formation, soil aggregate stability, organic matter degradation and the fate of pollutants. Bacterial surface properties are considered to govern adhesion, and these properties likely change as a function of bacterial growth phase. However, the effect of growth stage on bacterial adhesion to clay minerals remains unclear. This work examined the influence of growth phase on the adhesion of Pseudomonas putida to kaolinite-coated coverslips. Fluorescence microscopy, together with a bacterial viability stain, was used to directly quantify surface cell density and viability of adhered P. putida. In situ attenuated total reflectance Fourier transform infrared spectroscopy was applied to yield molecular information about the characteristics of the bacteria, and the adsorption and desorption kinetics. Stationary-phase cells exhibited a higher adsorption density on kaolinite surfaces than mid-exponential-phase cells under static deposition conditions. Compared with the mid-exponential-phase cells, the stationary-phase cells displayed higher saturation coverage, and we fitted the results using a pseudo-first-order kinetics equation. The greater extent of adhesion of the stationary-phase cells was probably due to their smaller cell size and less negative surface charges compared with the cells from other growth stages, which resulted in deeper secondary energy minima and lower energy barriers for adhesion. The results from this study suggest that growth phase may strongly influence cell mobility and biofilm formation in aqueous geochemical environments.


      PubDate: 2014-10-26T02:53:07Z
       
  • High-precision Mg isotope analyses of low-Mg rocks by MC-ICP-MS
    • Abstract: Publication date: 18 December 2014
      Source:Chemical Geology, Volume 390
      Author(s): Yajun An , Fei Wu , Yuanxin Xiang , Xiaoyun Nan , Xun Yu , Jinhui Yang , Huimin Yu , Liewen Xie , Fang Huang
      We present a method for precise measurement of Mg isotope ratios for low-Mg rock samples (where MgO <1wt.%) by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The efficiency of Mg purification is significantly improved by using a newly calibrated HNO3 +HF step to remove undesired matrix elements (such as Ti, Al, Fe, and K) in low-Mg samples. We also establish that increasing the amount of Mg loaded to the chromatographic column minimized blank effects of organics leached from cation resin. All parameters that could affect the accuracy and precision of Mg isotope analyses were rigorously examined by two independent laboratories in Beijing and Hefei. The δ26Mg of mono-elemental Mg standard CAM-1 measured in the two laboratories were −2.597±0.042‰ (2σ, n=49) and −2.598±0.039‰ (2σ, n=79), respectively; in house standard IGGMg1 were −1.742±0.041‰ (2σ, n=53) and −1.749±0.049‰ (2σ, n=72), respectively. The average δ26Mg over ten months of two synthetic standards, made by doping IGGMg1 and IGGMg2 with matrix elements, agrees well with their recommended values, within error. The robustness of our method was further assessed by replicated analyses of sixteen rock standards with MgO contents from 0.28wt.% to 49.4wt.%. The δ26Mg of USGS rhyolite standards RGM-1 and RGM-2 are −0.188±0.031‰ (2σ, n=35) and −0.182±0.041‰ (2σ, n=72), respectively; granite standard GA is −0.165±0.038‰ (2σ, n=57), G-2 is −0.129±0.045‰ (2σ, n=34), GS-N is −0.204±0.059‰ (2σ, n=33), GSP-2 is −0.042±0.020‰ (2σ, n=15), and GSR-1 is −0.234±0.016‰ (2σ, n=17). Based on repeated analyses of standards, the long-term external precision of our method is better than ±0.05‰ for δ26Mg. This precision allows us to distinguish the fractionation of Mg isotopes in low-Mg granites and rhyolites as well as that between mantle minerals.


      PubDate: 2014-10-26T02:53:07Z
       
 
 
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