for Journals by Title or ISSN
for Articles by Keywords
help
  Subjects -> EARTH SCIENCES (Total: 579 journals)
    - EARTH SCIENCES (427 journals)
    - GEOLOGY (64 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (17 journals)
    - OCEANOGRAPHY (44 journals)

EARTH SCIENCES (427 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (1 follower)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (2 followers)
Acta Geophysica     Hybrid Journal   (7 followers)
Acta Geotechnica     Hybrid Journal   (8 followers)
Acta Meteorologica Sinica     Hybrid Journal   (2 followers)
Advances in High Energy Physics     Open Access   (12 followers)
Advances In Physics     Hybrid Journal   (6 followers)
Aeolian Research     Hybrid Journal   (1 follower)
African Journal of Aquatic Science     Hybrid Journal   (13 followers)
Algological Studies     Full-text available via subscription   (2 followers)
Alpine Botany     Hybrid Journal   (2 followers)
AMBIO     Hybrid Journal   (13 followers)
Anales del Instituto de la Patagonia     Open Access   (2 followers)
Andean geology     Open Access   (5 followers)
Annales Henri Poincaré     Hybrid Journal   (2 followers)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (1 follower)
Annals of Geophysics     Full-text available via subscription   (9 followers)
Annals of GIS     Hybrid Journal   (14 followers)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (9 followers)
Applied Clay Science     Hybrid Journal   (2 followers)
Applied Geochemistry     Hybrid Journal   (6 followers)
Applied Geomatics     Hybrid Journal   (6 followers)
Applied Geophysics     Hybrid Journal   (6 followers)
Applied Ocean Research     Hybrid Journal   (6 followers)
Applied Petrochemical Research     Open Access   (3 followers)
Applied Remote Sensing Journal     Open Access   (6 followers)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (19 followers)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (6 followers)
Artificial Satellites     Open Access   (11 followers)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (2 followers)
Asian Journal of Earth Sciences     Open Access   (16 followers)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (3 followers)
Atmosphere-Ocean     Full-text available via subscription   (3 followers)
Atmospheric and Climate Sciences     Open Access   (14 followers)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (11 followers)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (7 followers)
Bragantia     Open Access   (2 followers)
Bulletin of Earthquake Engineering     Hybrid Journal   (9 followers)
Bulletin of Geosciences     Open Access   (7 followers)
Bulletin of Marine Science     Full-text available via subscription   (12 followers)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (2 followers)
Bulletin of the Seismological Society of America     Full-text available via subscription   (15 followers)
Bulletin of Volcanology     Hybrid Journal   (11 followers)
Canadian Journal of Plant Science     Full-text available via subscription   (10 followers)
Canadian Mineralogist     Full-text available via subscription   (1 follower)
Canadian Water Resources Journal     Hybrid Journal   (17 followers)
Carbonates and Evaporites     Hybrid Journal   (3 followers)
CATENA     Hybrid Journal   (3 followers)
Central European Journal of Geosciences     Hybrid Journal   (5 followers)
Central European Journal of Physics     Hybrid Journal   (2 followers)
Chemical Geology     Hybrid Journal   (9 followers)
Chemie der Erde - Geochemistry     Hybrid Journal   (3 followers)
Chinese Geographical Science     Hybrid Journal   (3 followers)
Chinese Journal of Geochemistry     Hybrid Journal   (2 followers)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (3 followers)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (8 followers)
Coastal Management     Hybrid Journal   (17 followers)
Comptes Rendus Geoscience     Full-text available via subscription   (5 followers)
Computational Geosciences     Hybrid Journal   (10 followers)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (2 followers)
Computers and Geotechnics     Hybrid Journal   (5 followers)
Continental Journal of Earth Sciences     Open Access   (1 follower)
Continental Shelf Research     Hybrid Journal   (6 followers)
Contributions to Mineralogy and Petrology     Hybrid Journal   (7 followers)
Contributions to Plasma Physics     Hybrid Journal   (2 followers)
Coral Reefs     Hybrid Journal   (17 followers)
Cretaceous Research     Hybrid Journal   (3 followers)
Cybergeo : European Journal of Geography     Open Access   (3 followers)
Developments in Geotectonics     Full-text available via subscription   (1 follower)
Developments in Quaternary Science     Full-text available via subscription   (3 followers)
Développement durable et territoires     Open Access   (2 followers)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (2 followers)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (3 followers)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (105 followers)
Earth Interactions     Full-text available via subscription   (7 followers)
Earth Science Research     Open Access   (5 followers)
Earth Surface Processes and Landforms     Hybrid Journal   (10 followers)
Earth System Dynamics     Open Access   (4 followers)
Earth System Dynamics Discussions     Open Access   (3 followers)
Earth's Future     Open Access   (1 follower)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (7 followers)
Earthquake Spectra     Full-text available via subscription   (11 followers)
Ecohydrology     Hybrid Journal   (11 followers)
Ecological Questions     Open Access   (5 followers)
Electromagnetics     Hybrid Journal   (2 followers)
Energy Efficiency     Hybrid Journal   (9 followers)
Energy Exploration & Exploitation     Full-text available via subscription   (2 followers)
Environmental Earth Sciences     Hybrid Journal   (10 followers)
Environmental Geology     Hybrid Journal   (10 followers)
Environmental Geosciences     Full-text available via subscription   (4 followers)
Erwerbs-Obstbau     Hybrid Journal  
Estuaries and Coasts     Hybrid Journal   (1 follower)
Estuarine, Coastal and Shelf Science     Hybrid Journal   (15 followers)
Estudios Geográficos     Open Access  
European Journal of Mineralogy     Full-text available via subscription   (6 followers)

        1 2 3 4 5 | Last

Chemical Geology    [11 followers]  Follow    
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2556 journals]   [SJR: 1.714]   [H-I: 111]
  • Simultaneous precipitation of magnesite and lizardite from hydrothermal
           alteration of olivine under high-carbonate alkalinity
    • Abstract: Publication date: Available online 21 January 2014
      Source:Chemical Geology
      Author(s): Romain Lafay , German Montes-Hernandez , Emilie Janots , Rodica Chiriac , Nathaniel Findling , François Toche
      The present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in Earth systems (e.g. functioning of hydrothermal fields) or in engineered systems (e.g. ex-situ and in-situ mineral sequestration of CO2). For this case, specific experimental conditions were examined (200°C, saturated vapor pressure≈16bar, solution/solid weight ratio=15, olivine grain size<30μm and high-carbonate alkalinity≈1M NaHCO3). Under these conditions, competitive precipitation of magnesite and serpentine (preferentially lizardite type) were clearly determined by using conventional analytic tools (XRD, FESEM, FTIR and TGA); excluding the fate of the iron initially contained in olivine, the alteration reaction for olivine under high-carbonate alkalinity can be expressed as follows: 2 M g 2 Si O 4 + 2 H 2 O + HC O 3 − → MgC O 3 + M g 3 S i 2 O 5 OH 4 + O H − This reaction mechanism implied a dissolution process, releasing Mg and Si ions into solution until supersaturation of solution with respect to magnesite and/or serpentine. The released iron contained in the olivine has not implied any precipitation of iron oxides or (oxy)hydroxides; in fact, the released iron was partially oxidized (about 50%) via a simple reduction of water (2Fe 2+ +2H 2 O →2Fe 3+ + H 2 +2OH −). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). This latter was clearly determined by FESEM/EDS chemical analysis on the single magnesite crystals. The nucleation and epitaxial growth processes at the olivine-fluid interfaces cannot be excluded in our investigated system. The experimental kinetic data fitted by using a kinetic pseudo-second-order model have revealed a retarding process of serpentine formation with respect to magnesite (about three times slower); in fact, the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution. We assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given CO2 pressure is imposed in the system. On the other hand, the reactivity of serpentinized olivine (chrysotile+brucite+small amount of residual olivine) and high-purity chrysotile at the same above investigated conditions; and the olivine serpentinization in initial acid pH≈0.66 are also reported as complementary information in this study. These novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate; this could contribute to a better understanding of fluid-rock interactions in natural active hydrothermal fields on Earth.


      PubDate: 2014-01-24T00:06:00Z
       
  • Water depth affecting thaumarchaeol production in lake qinghai,
           northeastern qinghai-tibetan plateau: Implications for paleo lake levels
           and paleoclimate
    • Abstract: Publication date: Available online 23 January 2014
      Source:Chemical Geology
      Author(s): Huanye Wang , Hailiang Dong , Chuanlun L. Zhang , Hongchen Jiang , Meixun Zhao , Zhonghui Liu , Zhongping Lai , Weiguo Liu
      Archaeal glycerol dialkyl glycerol tetraethers (GDGTs) are increasingly popular and versatile tool for palaeolimnology studies, but their applications in paleohydrology are scarce, especially for thaumarchaeol which is specific for the newly proposed phylum Thaumarchaeota. After investigating our published GDGT data of Lake Qinghai, we found that both the concentration of thaumarchaeol and the relative abundance of thaumarchaeol to total archaeal GDGTs (%thaum) in core-top sediments increased significantly with increasing water depth (R=0.88 and 0.95, respectively), with lower concentrations of 5±5ng/g in shallow areas (water depth<5m) and higher concentrations of 121±65ng/g in deep areas (water depth>10m). This is likely because that the producers of thaumarchaeol, Thaumarchaeota, prefer living in the relative deeper zone in lacustrine systems, where probably both competition of ammonium (the substrate) from other microbes and light intensity are low. Therefore, we proposed that thaumarchaeol was mainly produced in situ and changes in %thaum might reflect water-depth variations in this closed-basin lake. The application of %thaum as a water-depth indicator in a Holocene sediment sequence of core QH-2011 provided a high-resolution relative lake-level history of Lake Qinghai which resembles that inferred from the δ13Corg value obtained in the same core. This supports the use of %thaum as an indicator of lake water depth in paleohydrology studies, especially for medium lakes. Moreover, the records of the two independent proxies in core QH-2011 confirmed a shallow Lake Qinghai in the early Holocene and a late-Holocene highstand, highlighting the importance of local temperature (and evaporation loss) in controlling effective moisture in the arid/semi-arid region.


      PubDate: 2014-01-24T00:06:00Z
       
  • Collaborative microbial Fe-redox cycling by pelagic floc bacteria across
           wide ranging oxygenated aquatic systems
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): A.V.C. Elliott , J.M. Plach , I.G. Droppo , L.A. Warren
      Fe(III)-reducing bacteria (IRB) and Fe(II)-oxidizing bacteria (IOB) significantly impact the transformations and geochemical cycling of Fe. As these bacteria are thought to be differentially segregated to specific environments reflecting oxygen and pH restrictions on their respective metabolisms, their impact on Fe biogeochemistry in pelagic environments has not been well investigated. Here we report the discovery of cooperative Fe-redox cycling bacterial consortia within flocs found in circumneutral freshwaters with a wide range of oxygenated conditions (O2 Sat. =1–103%). Favorable low-oxygen microscale conditions are engineered through consortial aggregate formation, enabling both IRB and IOB metabolisms within floc and thus the macroscale expansion of aero-intolerant IRB and IOB activity into presumed inhospitable oxic waters. Floc-associated IOB and IRB identified here (16S rDNA) differ from those commonly identified within sediment/groundwater (IRB) and seep (IOB) habitats, indicating a likely wider habitat distribution and genetic diversity of Fe-bacteria than currently considered. Further, both in situ floc communities and experimentally enriched floc consortial communities constitute aero-intolerant IRB and microaerobic IOB together with oxygen-consuming organotrophic species (i.e. aerobes). These results identify a collaborative Fe-cycling strategy through aggregate formation and cooperation with aerobic species that substantively extends the potential environmental range of IRB-IOB impact on Fe geochemical cycling. Supporting this hypothesis, experimental microcosm IRB-IOB-aerobe consortial aggregates generated unusual assemblages of co-occurring reduced and oxidized Fe minerals, not predicted by treatment oxygen levels, but also observed in environmental pelagic aggregate samples from systems with similar O2 levels. These findings have large implications for the use of Fe minerals as geochemical proxies to constrain the chemistry and infer the history of atmospheric O2 on early Earth, and question the utility of theoretical thermodynamic constraints alone as a guide to investigating and interpreting microbe–geosphere interactions.


      PubDate: 2014-01-24T00:06:00Z
       
  • Optimizing pre-eruptive temperature estimates in thermally and chemically
           zoned magma chambers
    • Abstract: Publication date: Available online 21 January 2014
      Source:Chemical Geology
      Author(s): Silvio Mollo , Matteo Masotta
      We present a method to minimize the error of temperature estimate when multiple discrete populations of glass and clinopyroxene occur in a single heterogeneous eruptive unit. As test data we have used ~1100 clinopyroxene-melt pairs from isothermal and thermal gradient experiments. These latter are characterized by the crystallization of multiple modes of clinopyroxene as frequently documented for chemically and thermally zoned magma chambers. Equilibrium clinopyroxene-melt pairs are identified through the difference between predicted and measured components in clinopyroxene. The use of these equilibrium compositions as input data for one of the most recent clinopyroxene-based thermometers demonstrates that the error of temperature estimate is minimized and approaches to the calibration error of the thermometric model. To emphasize the paramount importance of this method for predicting the crystallization temperature of heterogeneous magmas, we have tested for equilibrium ~480 and ~150 clinopyroxene-melt pairs from the explosive eruptions of the Sabatini Volcanic District (Latium Region, Central Italy) and the Campi Flegrei Volcanic Field (Campanian Region, Southern Italy), respectively. These eruptions were fed by zoned magma chambers, as indicated by the occurrence of multiple modes of clinopyroxene in the eruptive units. Results from calculations demonstrate that clinopyroxene-melt pairs in equilibrium at the time of eruption are effectively captured by our method and, consequently, the error of temperature estimate is significantly reduced.


      PubDate: 2014-01-24T00:06:00Z
       
  • Hydrogen isotope fractionation in lipid biosynthesis by the piezophilic
           bacterium Moritella japonica DSK1
    • Abstract: Publication date: 28 February 2014
      Source:Chemical Geology, Volume 367
      Author(s): Jiasong Fang , Chao Li , Li Zhang , Tara Davis , Chiaki Kato , Douglas H. Bartlett
      The δD of fatty acids is emerging as an important marine biogeochemical proxy, but the microbiological and environmental factors controlling the variations of δD of the lipids are not fully constrained. We report here the first measurement of D/H ratios of fatty acids in a piezophilic bacterium and show that hydrostatic pressure and the lipid biosynthetic pathway probably exerts dominant control over the δD of fatty acids. Piezophilic bacterium Moritella japonica DSK1 was grown at a pressure of 30MPa with glucose as substrate. Fatty acids in DSK1 showed vastly varied δD, ranging from +44.4 to −171‰. Short-chain fatty acids (SCFA), which are synthesized by the fatty acid synthase (FAS) pathway, had positive δD (average +3‰), whereas long-chain polyunsaturated fatty acid (LC-PUFA) synthesized via the polyketide pathway exhibited much depleted δD (−171‰). Our results suggest that the lipid biosynthetic pathways can exert first-order control on the hydrogen isotope signature of bacterial membrane lipids under elevated pressure. Our findings have important implications in marine biogeochemistry. D-depleted fatty acids in marine sediments and in the water column may be derived from piezophilic bacterial reworking and resynthesis of organic matter at high pressure condition. Thus, caution must be exercised in the interpretation of hydrogen isotope signatures of lipids in, e.g., deducing sources of organic matter and tracing microbial biogeochemical processes in the deep ocean and the deep biosphere.
      Graphical abstract image

      PubDate: 2014-01-24T00:06:00Z
       
  • The amphoteric behavior of water in silicate melts from the point of view
           of their ionic-polymeric constitution
    • Abstract: Publication date: 28 February 2014
      Source:Chemical Geology, Volume 367
      Author(s): Roberto Moretti , Charles Le Losq , Daniel R. Neuville
      Dissociation of water into protons and hydroxyl ions is a fundamental feature of aqueous solutions. Although it exerts a profound influence on properties of magmas, this autoprotolysis reaction has been hitherto neglected for water dissolved in silicate melts. As made here with an acid–base model, in fact one has to deal with molecular water (H2Omol) and two kinds of hydroxyl groups, bonded or not to network-forming cations (OH and OH−, respectively) in hydrous silicate melts. By mixing cations and anions on distinct sublattices and quantifying the disproportionation of water dissolved in silicate melts into its ionic products, H+ and OH−, we reconcile spectroscopic determinations of water speciation, and highlight the main compositional features involving chemical exchanges between H2Omol, TOH and MOH groups (T and M being a former and a modifier, respectively). In particular, water addition to depolymerized systems, such as basalts, determine a relative predominance of OH− with respect to silicic systems, such that the increase of water concentration tends to immediately limit depolymerization rather than enhance it. This opens new perspectives to the understanding of the chemical control of hydrous magmas and their physical properties, as well as the attainment of saturation in hydrous minerals such as amphiboles or micas, particularly in depolymerized melts.


      PubDate: 2014-01-19T23:51:07Z
       
  • Noble gas concentrations in fluid inclusions as tracer for the origin of
           coarse-crystalline cryogenic cave carbonates
    • Abstract: Publication date: Available online 17 January 2014
      Source:Chemical Geology
      Author(s): Tobias Kluge , Thomas Marx , Werner Aeschbach-Hertig , Christoph Spötl , Detlev K. Richter
      The chemically inert behaviour of noble gases, their well-known solubility in water and the long-term constancy of atmospheric noble gas mixing ratios make them a unique tool for paleoclimate studies of groundwater. This concept has recently been extended to fluid inclusions in speleothems. Here we use noble gas concentrations of fluid inclusions contained in calcite crystals to constrain the formation history of coarse crystalline cryogenic cave calcites from Heilenbecker Cave (Germany). Cryogenic cave carbonates form under conditions related to the freezing of cave pools and are a new archive for paleo-permafrost timing and extent. The cryogenic origin of these carbonates is typically deduced from their crystal habit and the carbon and oxygen isotope ratios, but this can be ambiguous in some cases. A geochemical method that provides clear-cut proof of the cryogenic origin of individual crystals has been lacking so far. We examined the formation process of cryogenic cave calcites using water and noble gases extracted from inclusions in these crystals. Based on their δ13C and δ18O values, these calcite crystals could have also formed as 'normal', i.e. non-cryogenic, speleothems. Noble gas concentrations derived from fluid inclusions in these cryogenic calcites, however, deviate significantly from commonly observed concentrations in atmospheric air, surface water, groundwater, and stalagmites. These concentrations cannot be explained simply by a solubility component nor by the addition of various amounts of excess-air to a solubility component. The data, however, are consistent with the scenario of a slowly freezing water body lacking exchange with the cave atmosphere. The gradual freezing process leads to a partitioning of the noble gases between ice and remaining water and thereby to a pronounced over-abundance of heavy noble gases in the liquid phase recorded in the fluid inclusions of the cryogenic calcite crystals.


      PubDate: 2014-01-19T23:51:07Z
       
  • Aluminum-in-olivine thermometry of primitive basalts: Evidence of an
           anomalously hot mantle source for large igneous provinces
    • Abstract: Publication date: Available online 16 January 2014
      Source:Chemical Geology
      Author(s): L.A. Coogan , A.D. Saunders , R.N. Wilson
      The maximum crystallization temperature of mantle-derived melts provides constraints on the minimum temperature of their mantle source regions. This temperature can be estimated using phenocryst-based thermometry on primitive lavas that contain olivine and spinel phenocrysts using the Al-in-olivine thermometer. Application of an updated version of this thermometer to primitive basalts gives precise estimates of olivine-spinel crystallization temperatures in different geodynamic settings. These are minimum temperatures for melt extraction from the mantle due to both adiabatic cooling and cooling prior to co-saturation in these phases. Application of this thermometer to primitive MORB gives co-saturation temperatures of up to 1270°C (and a maximum olivine forsterite (Fo) content of Fo91.3) consistent with previous constraints (e.g. from the volume of melt produced at mid-ocean ridges). Substantially higher crystallization temperatures are recorded by Tertiary (~60Ma) picrites from Baffin Island (up to 1408°C; Fo91.3) and East Greenland (up to 1354°C; Fo90.1), Cretaceous (86Ma) picrites from Madagascar (up to 1486°C; Fo91.8) and Cretaceous (88Ma) komatiites from Gorgona (up to 1435°C; Fo91.7). Unlike olivine-melt based thermometers these temperature estimates are not significantly influenced by uncertainty in melt H2O content or fO2. These results indicate that the mantle source region of large igneous provinces is substantially hotter than the ambient upper mantle, supporting the standard thermal plume model.


      PubDate: 2014-01-19T23:51:07Z
       
  • Noble-gas signature of magmas from a heterogeneous mantle wedge: The case
           of Stromboli volcano (Aeolian Islands, Italy)
    • Abstract: Publication date: Available online 15 January 2014
      Source:Chemical Geology
      Author(s): M. Martelli , A.L. Rizzo , A. Renzulli , F. Ridolfi , I. Arienzo , A. Rosciglione
      We report on the variation of the elemental (He, Ne, and Ar) and isotopic (He and Ar) compositions of olivine- and clinopyroxene-hosted fluid inclusions hosted in lavas, pyroclastics, and cumulate xenoliths from the last 60 ka of the eruptive history of Stromboli volcano, Italy. Samples belong to (i) the present-day activity as represented by shoshonitic (SHO) basalts, i.e., pumices with low porphyritic (LP) content and high porphyritic (HP) scoriae; (ii) the subalkaline versus alkaline products erupted at Stromboli during its subaerial history among the extreme magmatic series, i.e., calc-alkaline (CA) and potassic (KS); and (iii) the only known ultramafic cumulates (San Bartolo wehrlite xenoliths, SBX) outcropping in the island, carried to the surface by recent (ca. 2 ka) basaltic lava. To interpret the results better, we also investigated trace elements in the clinopyroxenes of wehrlite xenoliths and the Sr and Nd isotopes of all of the products in which the 3He/4He ratio was measured. Wehrlite xenoliths are igneous cumulates crystallized at mantle depth that mostly consist of clinopyroxene and olivine crystals with limited compositional variation. The texture, mineral chemistry, pattern of trace elements and Sr-Nd-isotope ratios (in clinopyroxene) suggest that these cumulates were produced by the early crystallization of a primitive basaltic magma with CA or HKCA-SHO affinity. The gas contents measured in the mafic crystals decrease from the wehrlite xenoliths through LP pumice, CA and KS lavas and, finally, to the HP scoria. This observation is consistent with crystallization and fluid entrapment from mantle depths to progressively shallower crustal levels. The low gas content extracted from the HP crystals did not allow measurement of their 3He/4He ratios. Most of the investigated crystals exhibit a 3He/4He ratio in the range of 4.0–4.9 Ra, with only the KS mafic minerals exhibiting lower 3He/4He values (≤3.5 Ra). The low values of He-isotope ratios relative to those of the most-uncontaminated mantle source of the Aeolian lavas (i.e., 3He/4He ~ 7 Ra at Alicudi) and in common volcanic arcs, suggest that the Stromboli mantle wedge is more contaminated by sediments and aqueous fluids derived by the active subduction of the Ionian slab. We also hypothesize that a process of mantle He loss that occurred during the mantle metasomatism contributed to the decrease of 3He/4He. The low 3He/4He values of the KS minerals with respect to the other Stromboli magmatic series are consistent with the higher Sr- and lower Nd-isotope ratios measured in the same samples and are attributed to source heterogeneity. Finally, data for the 3He/4He ratios from mafic minerals were compared with those of currently emitted hydrothermal fluids, which are regularly sampled for volcano surveillance. The maximum 3He/4He ratio found in the hydrothermal fluids matched the maximum ratio measured in the LP fluid inclusions (i.e., 4.6 Ra) and thus corresponds to the upper limit that should be expected for surface gases during or before high-intensity eruptive events in which a deep gas component is released from the magma. These data also represent a reference for Stromboli volcanic surveillance.


      PubDate: 2014-01-16T00:06:54Z
       
  • Melting and metasomatism in the lithospheric mantle of NE Spain:
           Geochemical and SrNd isotopic characteristics
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): Gumer Galán , Valentí Oliveras
      The study of major and trace element compositions of whole rocks and minerals, along with SrNd isotopes on separated clinopyroxene crystals, from mantle xenoliths in Neogene–Quaternary basaltic rocks, serves to evaluate the processes which have affected the subcontinental lithospheric mantle in the Catalan Volcanic Zone of NE Spain, and to compare it to other mantle domains in Europe. These xenoliths are mainly anhydrous spinel lherzolites and harzburgites, with rare olivine websterites. Concentrations of Al2O3, CaO, TiO2 and mildly incompatible trace elements decrease gradually from lherzolites to harzburgites. Light rare earth elements (LREE) and other incompatible trace elements display scatter vs. depletion indexes and usually increase from depleted lherzolites to much enriched harzburgites, indicating that metasomatism affected mainly the latter. Clinopyroxene rare earth element (REE) patterns show slight LREE depletion in most lherzolites and LREE–middle rare earth element (MREE) enrichment in harzburgites. The exceptions are a few clinopyroxene patterns from lherzolites with significant LREE–MREE depletion or LREE enrichment and u-shapes. The SrNd isotopic compositions of clinopyroxene also show a gradual variation from depleted MORB mantle (DMM) lherzolites to very enriched mantle (EM) harzburgites (87Sr/86Sr: 0.702486–0.709772; 143Nd/144Nd: 0.513359–0.512411). Most of these compositions match those for off-craton xenoliths. They are also similar to compositions observed in xenoliths of anhydrous spinel lherzolites and harzburgites from the neigbouring subcontinental lithospheric mantle of SE Spain and the southern part of the Massif Central, France. Fractional melting estimates from a Primitive Mantle source indicate that melt extraction was under 12% and 26% for lherzolites and harzburgites respectively. Melting took place in the spinel lherzolite field except for a few lherzolites that may have started to melt in the presence of garnet and continued in the spinel field. The formation of DMM lherzolites as refertilisation products, via percolation and reaction of N-MORB type basalts with refractory and ancient isotopically enriched harzburgites, cannot be ruled out. However, there is no definitive compositional evidence of such a process. It is suggested that harzburgites would have undergone multi-stage metasomatism. An earlier episode would have been caused by subduction-related hydrous fluids or melts related to the Variscan orogeny or an earlier orogenic episode. This is deduced from the time integrated low 143Nd/144Nd and very enriched 87Sr/86Sr values of harzburgite clinopyroxene. A later metasomatic episode is mainly cryptic and would have been related to the percolation of alkaline silicate melts and carbonatite derivatives. It would have affected lherzolites and especially harzburgites, causing opposite isotopic trends in the former compared to the latter. The SrNd isotopic compositions of the metasomatic agents would be close to that of a websterite xenolith. The age of this second metasomatism is also uncertain, since magmatic alkaline episodes have been registered since the Permo-Triassic periods.


      PubDate: 2014-01-16T00:06:54Z
       
  • Provenance versus weathering control on the composition of tropical river
           mud (southern Africa)
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): Eduardo Garzanti , Marta Padoan , Massimo Setti , Alberto López-Galindo , Igor M. Villa
      This study presents an integrated mineralogical–geochemical database on fine-grained sediments transported by all major rivers of southern Africa, including the Zambezi, Okavango, Limpopo, Olifants, Orange and Kunene. Clay mineralogy, bulk geochemistry, Sr and Nd isotopic signatures of river mud, considered a proxy of suspended load, are used to investigate the influence of source-rock lithology and weathering intensity on the composition of clay and silt produced in subequatorial to subtropical latitudes. Depletion in mobile alkali and alkaline-earth metals, minor in arid Namibia, is strong in the Okavango, Kwando and Upper Zambezi catchments, where recycling is also extensive. Element removal is most significant for Na, and to a lesser extent for Sr. Depletion in K, Ca and other elements, negligible in Namibia, is moderate elsewhere. The most widespread clay minerals are smectite, dominant in muds derived from Karoo or Etendeka flood basalts, or illite and chlorite, dominant in muds derived from metasedimentary rocks of the Damara Orogen or Zimbabwe Craton. Kaolinite represents 30–40% of clay minerals only in Okavango and Upper Zambezi sediments sourced in humid subequatorial Angola and Zambia. After subtracting the effects of recycling and of local accumulation of authigenic carbonates in soils, the regional distribution of clay minerals and chemical indices consistently reflect weathering intensity primarily controlled by climate. Bulk geochemistry identifies most clearly volcaniclastic sediments and mafic sources in general, but cannot discriminate the other sources of detritus in detail. Instead, Sr and Nd isotopic fingerprints are insensitive to weathering, and thus mirror faithfully the tectonic structure of the southern African continent. Isotopic tools thus represent a much firmer basis than bulk geochemistry or clay mineralogy in the provenance study of mudrocks.


      PubDate: 2014-01-16T00:06:54Z
       
  • Multi-proxy study of soil organic matter dynamics in permafrost peat
           deposits reveal vulnerability to climate change in the European Russian
           Arctic
    • Abstract: Publication date: Available online 13 January 2014
      Source:Chemical Geology
      Author(s): Joyanto Routh , Gustaf Hugelius , Peter Kuhry , Timothy Filley , Päivi Kaislahti Tillman , Marina Becher , Patrick Crill
      Soil organic carbon (SOC) in permafrost terrain is vulnerable to climate change. Perennially frozen peat deposits store large amounts of SOC, but we know little about its chemical composition and lability. We used plant macrofossil and biomarker analyses to reconstruct the Holocene paleovegetation and paleoenvironmental changes in two peat plateau profiles from the European Russian Arctic. Peat plateaus are the main stores of permafrost soil C in the region, but during most of the Holocene peats developed as permafrost-free rich fens with woody vegetation, sedges and mosses. Around 2200cal BP, permafrost aggraded at the site resulting in frost heave and a drastic reduction in peat accumulation under the drier uplifted surface conditions. The permafrost dynamics (aggradation, frost-heave and thaw) ushered changes in plant assemblages and carbon accumulation, and consequently in the biomarker trends too. Detailed biomarker analyses indicate abundant neutral lipids, which follow the general pattern: n-alkanols>sterols≥ n-alkanes≥triterpenols. The lignin monomers are not as abundant as the lipids and increase with depth. The selected aliphatic and phenolic compounds are source specific, and they have different degrees of lability, which is useful for tracing the impact of permafrost dynamics (peat accumulation and/or decay associated with thawing). However, common interpretation of biomarker patterns, and perceived hydrological and climate changes, must be applied carefully in permafrost regions. The increased proportion (selective preservation) of n-alkanes and lignin is a robust indicator of cumulative decomposition trajectories, which is mirrored by functional compounds (e.g. n-alkanol, triterpenol, and sterol concentrations) showing opposite trends. The distribution of these compounds follows first order decay kinetics, and concurs with the downcore diagenetic changes. In particular, some of the biomarker ratios (e.g. stanol/sterol and higher plant alkane index) seem promising for tracing SOC decomposition despite changes in botanical imprint, and sites spanning across different soil types and locations. Carbon accumulation rate calculated at these sites varies from 18.1 to 31.1 gC m-2 yr-1, and it is evident selective preservation, molecular complexity of organic compounds, and freezing conditions enhance the long-term stability of SOC. Further, our results suggest permafrost dynamics strongly impact the more undecomposed SOC that could be rapidly remobilized through ongoing thermokarst expansion.


      PubDate: 2014-01-16T00:06:54Z
       
  • Reactive transport modelling of geologic CO2 sequestration in saline
           aquifers: The influence of pure CO2 and of mixtures of CO2 with CH4 on the
           sealing capacity of cap rock at 37°C and 100bar
    • Abstract: Publication date: Available online 13 January 2014
      Source:Chemical Geology
      Author(s): S. Mohd Amin , D.J. Weiss , M.J. Blunt
      The costs of CO2 separation for carbon capture and storage can be reduced through capturing less pure CO2. The presence of impurities such as methane (CH4) in the CO2 gas stream, however, affects the geochemical and geophysical processes in the subsurface. The dissolved CO2 in the brine decreases the pH which dissolves minerals such as calcite and albite. The dissolution of these minerals increases the amount of Ca2+ and Na+ in the brine. The presence of these ions leads the precipitation of the secondary solid carbonates calcite and dawsonite. To test this process, we developed a kinetic batch and a one-dimensional reactive transport model using PHREEQC 2.15.0, to predict mineral alteration induced in the cap rock by penetration of brine containing dissolved CO2 from the underlying aquifer over a period of 10,000years. The chemical composition of the Nordland shale formation water that overlies the Utsira sand in the Sleipner field was used as a model case in this study. The model was run for pure CO2 and for mixtures with CH4 (1-4 (w/w)%) in the injected gas stream at a temperature of 37°C and at a pressure of 100bar. The simulations suggest that a mixture of CO2and CH4 suppresses an anticipated increase in the porosity of the cap rock. Thus, our results suggest that injection of a CO2-CH4 mixture inhibits cap rock dissolution and helps maintain the sealing capacity of the cap rock, while reducing separation costs.


      PubDate: 2014-01-16T00:06:54Z
       
  • High-resolution stable water isotopes as tracers of thaw unconformities in
           permafrost: A case study from western Arctic Canada
    • Abstract: Publication date: Available online 15 January 2014
      Source:Chemical Geology
      Author(s): Denis Lacelle , Marielle Fontaine , Alex P. Forest , Steve Kokelj
      The knowledge of past permafrost conditions is of importance to assess the potential magnitude of changes that periglacial environments may experience as a result of climate warming or disturbance. To assess if past thaw unconformities may be preserved from an isotopic and geochemical discontinuity within permafrost, this study investigates the distribution of ground ice, stable water isotopes and major cations in two permafrost cores collected in a hummocky terrain site near Inuvik, Northwest Territories, Canada; a site where the evolution of the active layer during a recent period of permafrost degradation and subsequent aggradation was documented. Based on the high-resolution isotope geochemistry profiles, closed-system Rayleigh-type ionic segregation and isotope fractionation occurred during thermally-induced water migration into shallow permafrost and its freezing along a negative soil temperature gradient. Due to thermally-induced water migration into permafrost, δ18O may not always be able to identify thaw unconformities; however the calculation of the 18O enrichment factors between ice and water (ε18Oi-w) may be used to determine position of thaw unconformities in permafrost, if thaw events are followed by permafrost aggradation. The approach of using ε18Oi-w provides additional information regarding past permafrost conditions that can complement change in cryostructures observed along natural exposures.


      PubDate: 2014-01-16T00:06:54Z
       
  • The effect of oxygen fugacity, melt composition, temperature and pressure
           on the oxidation state of cerium in silicate melts
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): A.D. Burnham , A.J. Berry
      Ce L III-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from −10 to +11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500°C), and pressures (1atm and 1GPa). The oxidation state ratio of Ce, Ce4+/∑Ce (where ∑Ce=Ce3++Ce4+), was determined from the spectra allowing the relationships between Ce4+/∑Ce and fO2, temperature, pressure and melt composition to be determined. Ce4+/∑Ce varied systematically with fO2 from 0 to ~0.8 over the range of conditions studied. Ce4+ is stabilised relative to Ce3+ by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4+). Ce4+ in an Fe2+-bearing melt is not preserved on cooling to a glass due to the reaction Ce4++Fe2+ =Ce3++Fe3+. Ce4+/∑Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.


      PubDate: 2014-01-16T00:06:54Z
       
  • Zircon radiation damage ages
    • Abstract: Publication date: 28 February 2014
      Source:Chemical Geology, Volume 367
      Author(s): R.T. Pidgeon
      The suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s. In the original technique radiation damage was determined by X-ray diffraction however, this is not suitable for the small sample analysis needed in modern geochronology. It is now possible to measure radiation damage and U and Th contents on micron-sized areas on the polished surface of zircon grains using Raman spectroscopy and SIMS mass spectrometry. This opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re-examine the technique through its application to zircons from two granitoids from the Archean Yilgarn Craton and ancient detrital zircons from the Jack Hills in Western Australia. The three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence. The 420±110Ma radiation damage age for the Darling Range granite is coincident with 500–400Ma biotite Rb–Sr ages in the same region of the Yilgarn Craton. Likewise a tonalite from the Youanmi Terrane in the Craton with a 1650±150Ma zircon radiation damage age, lies within the domain of a 1600Ma event recorded by biotite Rb–Sr ages. The Jack Hills zircons have a 1120±130Ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite Ar–Ar ages of about 1140Ma. The positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events.


      PubDate: 2014-01-16T00:06:54Z
       
  • Quantifying the kinetics of olivine dissolution in partially closed and
           closed batch reactor systems
    • Abstract: Publication date: 28 February 2014
      Source:Chemical Geology, Volume 367
      Author(s): Raul E. Martinez , Sebastian Weber , Kurt Bucher
      This study quantified the rates of olivine dissolution under constant air injection, in partially closed batch reactors and in control closed batch reactors (in the absence of air flow), in the presence of 4g of olivine, and 0, 0.001 and 0.01M NaNO3. Experiments were performed for 40,440 and 36,048min in partially closed and closed reactors respectively. Under constant air injection, the solution pH ranged from 6.8 and 7.3 for all conditions at the start of the experiments, to 7.5 and 7.8 at 40,440min. In the absence of air flow, an alkaline pH of 9.6 to 9.3 was observed for 36,048min. This pH difference is attributed to the constant buffering the solution through the formation of dissolved carbonate species (H2CO3/HCO3 −). The alkaline pH, measured in the presence of olivine, implied a mechanism of pH control caused by the presence of Si(OH)4 species from the mineral surface. This effect was suggested by a drop to near neutral pH of the same aliquots, upon filtration of the olivine through a N2 degassed filter. A decrease in total alkalinity was measured for experiments under constant air injection, as a function of increasing NaNO3, suggesting a mechanism of Na+/CO3 2− interaction is involved in the enhancement of olivine dissolution rates through proton/Mg exchange. In the absence of air flow, the total alkalinity reached a maximum of 0.25mM, compared to 0.9mM with air flow at 0.01M NaNO3. Magnesium concentration increased in the presence of NaNO3 and permanently dissolving CO2 in partially closed batch reactors. This strongly suggests a key role of Na+ (coupled to CO2 dissolution) for enhancing olivine dissolution. Mg/Si ratios for all conditions showed incongruent olivine dissolution, suggesting a combination of two dissolution mechanisms at alkaline and near neutral pH values, in closed and partially closed batch reactors. The rate of olivine dissolution (ROL) was calculated as a function of mineral surface area and forsterite equilibrium solution parameters. In the presence of constant air injection, highest log ROL values range from −11 to −10.7 at the acidic to near neutral conditions (pH=6.8 to 7.2). Slower dissolution rates, with log ROL values of −11.5 to −11.4 were determined for experiments in the absence of air flow at alkaline pH values (9.6 to 9.8). Constant air injection (i.e. constant dissolution of CO2 at atmospheric pressure) in the presence of Na+ has proven effective for enhancing olivine dissolution.
      Graphical abstract image

      PubDate: 2014-01-16T00:06:54Z
       
  • CO2-water-mineral reactions during CO2 leakage: Geochemical and isotopic
           monitoring of a CO2 injection field test
    • Abstract: Publication date: Available online 13 January 2014
      Source:Chemical Geology
      Author(s): Pauline Humez , Philippe Négrel , Vincent Lagneau , Julie Lions , Wolfram Kloppmann , Frédérick Gal , Romain Millot , Catherine Guerrot , Christine Flehoc , David Widory , Jean-François Girard
      A shallow CO2 injection experiment was performed at the CO2FieldLab site near Svelvik, Norway in Sept. 2011. The gas was injected through a 45° inclined well to a target depth of 20m. Its aim was to test various geophysical and geochemical monitoring tools for integrated near-surface leakage monitoring. A total of 1.67 tons of CO2 was injected over a period of six days. Groundwater level was at 60cm depth and a complex salinity stratification with brackish water overlying fresh- and saltwater was observed at the beginning of the experiment. Four water sampling boreholes were located at each corner of a square configuration of 8m with the nominal injection at the centre. From baseline conditions to the post-CO2 injection phase, pH, temperature and electrical conductivity of the groundwater were measured, collecting water samples at three different depths (5, 10 and 15m) for analyses of major, minor, and trace elements (Ca, Na, SO4, Cl, Mg, Al, Ba, Mn, Ni, Co, B, Li), and isotopes (δ11B, δ7Li, δ34SSO4, δ18OSO4, 87Sr/86Sr, δ18OH2O, δ2HH2O). Significant changes in chemical and isotope signatures of water over the duration of the experiment indicated two processes: 1) Binary mixing with seawater and rainwater as the possible end-members due to a near-seashore location, and 2) CO2-water-rock interactions enhanced by dissolved CO2 causing acid conditions and favouring rock dissolution. The relative contribution of those processes was quantified by a mass balance model and equilibrium calculations. This comprehensive geochemical and isotope approach allowed discriminating reactive mechanisms from non-reactive (mixing) processes associated with CO2 leaks, within an aquifer with a strong conductivity stratification and heterogeneous mineralogy and this approach seems promising for investigating CO2 leaks at field scale.


      PubDate: 2014-01-16T00:06:54Z
       
  • The geochemistry of Tl and its isotopes during magmatic and hydrothermal
           processes: The peralkaline Ilimaussaq complex, southwest Greenland
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): Kai Hettmann , Michael A.W. Marks , Katharina Kreissig , Thomas Zack , Thomas Wenzel , Mark Rehkämper , Dorrit E. Jacob , Gregor Markl
      We use thallium (Tl) concentrations, K/Rb, K/Tl and Rb/Tl ratios and Tl isotopes in minerals from the alkaline to peralkaline Ilimaussaq complex (South Greenland) to trace magmatic differentiation, crustal assimilation, magmatic degassing, ore precipitation and hydrothermal metasomatism. Closed-system magmatic differentiation is marked by a coherent decrease of K/Tl- and K/Rb-ratios, whereas crustal assimilation results in a strong Tl-enrichment, causing low K/Tl-ratios compared to K/Rb-ratios. Thallium isotopes show only slight changes during orthomagmatic differentiation and the assimilation of crustal material cannot be traced, since the isotopic composition of the average crust is within the range of the mantle and mantle-derived rocks. Magmatic degassing, however, increases Rb/Tl-ratios and changes the isotopic composition of Tl. The released fluids are enriched in Tl, characterized by high 205Tl/203Tl ratios and can precipitate Tl-rich sulfide and silicate minerals as indicated by some late-magmatic hydrothermal veins, which contain a conspicuous assemblage of Tl–Fe–Cu-sulfides (thalcusite, djerfisherite, chalcothallite). The oxidative alteration of these assemblages at high pH results in small-scale redistribution of Tl. Thallium released by this process is entrained into late-stage Tl-enriched astrophyllite.


      PubDate: 2014-01-12T00:05:59Z
       
  • Editorial Board
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364




      PubDate: 2014-01-12T00:05:59Z
       
  • Editorial Board
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363




      PubDate: 2014-01-12T00:05:59Z
       
  • Rheology and the Fe3+–chlorine reaction in basaltic melts
    • Abstract: Publication date: 14 February 2014
      Source:Chemical Geology, Volume 366
      Author(s): Sharon L. Webb , Bramley J. Murton , Andrew J. Wheeler
      The viscosity of mid-ocean ridge basalt (MORB) as a function of halogen content was determined in the high viscosity range (109–1012 Pas) using the micropenetration technique. In this study we show that, in the presence of iron, the addition of chlorine reduces the viscosity of a metaluminous melt (basalt); and that the addition of chlorine increases the viscosity of the analog iron-free haplo-basalt. Previous studies of the effects of chlorine and fluorine on the viscosity of peralkaline and metaluminous melts has shown that while fluorine decreases the viscosity of such melts, chlorine acts to increase their viscosity. The major effect of the addition of chlorine to the present basalt melt is the conversion of a network-forming Fe3+ to a network-modifying Fe2+ for each added Cl− ion, resulting in an increase in Fe2+/Fetot from 0.126 to 0.432 upon the addition of 1.41mol% Cl2O−1, in air at a constant log10 ƒO2 of −0.7. It would appear that it is this increase in Fe2+ induced by the addition of chlorine, rather than the structural position of the chlorine atom in the melt which causes the reduction in viscosity. Of greater importance is that such an increase in Fe2+ would, in general, be interpreted as a 3 order of magnitude decrease in ƒO2, instead of the addition of chlorine.


      PubDate: 2014-01-12T00:05:59Z
       
  • A detailed geochemical investigation of post-nuclear detonation trinitite
           glass at high spatial resolution: Delineating anthropogenic vs. natural
           components
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Jeremy J. Bellucci , Antonio Simonetti , Elizabeth C. Koeman , Christine Wallace , Peter C. Burns
      This study documents, for the first time, the combined abundances of major and trace elements at high spatial resolution for trinitite glass, which was produced during the first atomic weapon test (Trinity site; New Mexico, USA). The results indicate that the chemical composition of trinitite is largely dependent on the precursor mineral phases found in the arkosic sand at the Trinity site. The chemical compositions of trinitite were evaluated using principal component analysis, which indicates that trends may be attributed to mixing between an anthropogenic component and phases (both major and minor) within the arkosic sand. The resolvable anthropogenic component in trinitite is made up of metals including Al, Co, Cr, Cu, Fe, Ga, Mg, Mn, Nb, Pb, Ta, and Ti. Uranium in trinitite appears to have two sources: natural U-bearing phases and the tamper used in the device. The concentrations of volatile anthropogenic metals (Co, Cr, Cu, and Pb) are enriched in samples that originate from >74m away from ground zero. This increase may be related to a temperature-controlled fractionation, and implies that the (lower temperature) peripheral zone of a blast site is the optimal area to sample volatile metals for nuclear forensic analysis.


      PubDate: 2014-01-08T00:06:05Z
       
  • Trace element budgets and (re-)distribution during subduction-zone
           ultrahigh pressure metamorphism: Evidence from Western Tianshan, China
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Yuanyuan Xiao , Yaoling Niu , Huaikun Li , Huichu Wang , Xiaoming Liu , Jon Davidson
      We have conducted an LA-ICP-MS in situ trace element study of garnet, epidote group minerals, phengitic muscovite and paragonite in rocks of basaltic and sedimentary protolith from an ultrahigh pressure metamorphic belt along Western Tianshan, China. The data are used to evaluate the capacity of these minerals for hosting incompatible elements in response to subduction-zone metamorphism (SZM). The results confirm existing studies in that the presence and stability of these minerals largely control the geochemical behaviors of elements during SZM. We found that redistribution of rare earth elements (REEs), Th and U into newly-formed minerals during progressive SZM precludes the release of these elements from the down-going ocean crust, which contradicts the common perception in models of slab-dehydration and flux-melting. This suggests that additional processes, such as the involvement of supercritical fluids or hydrous melts formed at depth are required to supply these elements to the mantle wedge for arc magmatism. In addition, the ready release of large ion lithophile elements (LILEs) by different minerals, and the high immobility of REEs in rocks of basaltic protolith indicate that the contribution of altered ocean crust after SZM may not be responsible for the correlated Sr–Nd (Hf) isotope systematics observed in oceanic basalts. That is, subducted ocean crust that has gone through SZM cannot be the major source material for ocean island basalts.


      PubDate: 2014-01-08T00:06:05Z
       
  • Halogens (F, Cl and Br) at Oldoinyo Lengai volcano (Tanzania): Effects of
           magmatic differentiation, silicate–natrocarbonatite melt separation
           and surface alteration of natrocarbonatite
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Martin F. Mangler , Michael A.W. Marks , Anatoly N. Zaitzev , G. Nelson Eby , Gregor Markl
      Halogen abundances of natrocarbonatites and their alteration products from Oldoinyo Lengai (Tanzania) are compared to those of associated silicate rocks and to various calcite carbonatites from eleven occurrences worldwide. Fresh natrocarbonatites are extremely enriched in F (up to 3.6wt.%), Cl (up to 5.9wt.%) and Br (up to 100μg/g). During meteoric alteration, however, the major Cl- and Br-bearing minerals (mainly sylvite and gregoryite) are easily dissolved, leaving behind residual materials with relatively low Cl (≤1000μg/g) and Br (≤15μg/g) contents. The comparatively low Cl/Br ratios suggest preferential leaching of Cl relative to Br during alteration. At the same time, F is passively enriched as fluorite remains relatively stable during alteration. In the associated silicate rocks at Oldoinyo Lengai concentrations of all three halogens increase from primitive olivine melilitite to evolved combeite–wollastonite nephelinite, demonstrating their incompatible behavior during magmatic differentiation. Relatively constant Cl/Br ratios, but strongly decreasing F/Cl ratios, in these samples are explained by moderately incompatible behavior for F compared to pronounced and very similar incompatibilities for Cl and Br. Our data further imply that during silicate–natrocarbonatite melt separation all three halogens show a strong affinity for the natrocarbonatite. F and Cl are equally enriched in the natrocarbonatite while Cl and Br are slightly fractionated from each other with a preferential partitioning of Cl relative to Br into the natrocabonatite melt.


      PubDate: 2014-01-03T23:26:48Z
       
  • XAS study of iron speciation in soils and waters from a boreal catchment
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Anneli Sundman , Torbjörn Karlsson , Hjalmar Laudon , Per Persson
      Iron (Fe) is a key element, strongly influencing the biogeochemistry of soils, sediments and waters, but the knowledge about the variety of Fe species present in these systems is still limited. In this work we have used X-ray absorption spectroscopy (XAS) to study the speciation of Fe in soils and waters from a boreal catchment in northern Sweden. The aim was to better understand the controls of Fe speciation across different, but adjacent landscape elements including soil, soil solution, groundwater and stream water draining catchments with contrasting land characteristics. Our results showed that all samples contained mixtures of Fe(II) and Fe(III). The soils consisted of Fe phyllosilicates, Fe (hydr)oxides and Fe complexed by natural organic matter (NOM). All aqueous samples contained Fe(II)– and Fe(III)–NOM complexes, often in combination with Fe(III) (hydr)oxides that were associated with NOM. The variation in contribution from Fe–NOM and Fe (hydr)oxides was controlled by pH and total concentrations of NOM. The XAS spectra suggested formation of mononuclear Fe–NOM complexes consisting of chelate ring structures, but it could not be determined whether they originated solely from Fe(III)– or from a mixture of Fe(II)/Fe(III)–NOM complexes. Our collective results showed that the Fe speciation was highly variable across the different landscape elements and streams. This variation was manifested both in the distribution between mononuclear Fe–NOM complexes and Fe (hydr)oxides associated with NOM and between Fe(II) and Fe(III). These results highlight the complexity of Fe speciation in natural environmental systems and thus the challenges in interpreting Fe reactivity.


      PubDate: 2013-12-31T01:25:03Z
       
  • Monazite to the rescue: U–Th–Pb dating of the intrusive
           history of the composite Karkonosze pluton, Bohemian Massif
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Monika A. Kusiak , Ian S. Williams , Daniel J. Dunkley , Patrík Konečny , Ewa Słaby , Hervé Martin
      The large (~700km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290Ma, with more recent results converging to ~320–300Ma. Dating of zircon and monazite from samples of a variety of major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313±3 and 311±4Ma, and monazite ages are 312±2, 313±3 and 311±3Ma. Monazite from one hybrid granitoid has an age of 314±3Ma, and zircon from another an age of 314±4Ma. Zircon from a composite dyke has an age of 311±6Ma. The monazite U–Pb age of an equigranular granite, at 318±6Ma, is consistent with geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncertainties, is not conclusive. Zircon from one microgranular enclave is anomalously young, 302±4Ma. Evidence is mounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within a short time interval between 314±4 and 311±3Ma. Given the uncertainties, emplacement of these units could have been effectively simultaneous. The larger difference between the ages from the equigranular granite and microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted as long as 15Ma.


      PubDate: 2013-12-31T01:25:03Z
       
  • Integrated modeling of biogeochemical reactions and associated isotope
           fractionations at batch scale: A tool to monitor enhanced
           biodenitrification applications
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Paula Rodríguez-Escales , Boris M. van Breukelen , Georgina Vidal-Gavilan , Albert Soler , Albert Folch
      Enhanced in-situ biodenitrification (EIB) is a potential technology for remediating nitrate-polluted groundwater. EIB aims to create optimal biodenitrification conditions through the addition of carbon sources, enabling the autochthonous microbial community to degrade nitrate via different redox pathways. Biogeochemical numerical models are useful tools for predicting and designing such biodenitrification applications. Compound-specific stable isotope analysis (CSIA) is another valuable method for determining the degree of nitrate transformation. Therefore, incorporating isotope fractionation in biogeochemical models combines the two tools and is a key step in the development of reactive transport models of EIB under field conditions. In this work, we developed such an integrated model using the Phreeqc code and calibrated the model with batch scale experimental data using either ethanol or glucose as external carbon sources. The model included the following: microbiological processes —exogenous and endogenous nitrate respiration coupled to microbial growth and decay; geochemical processes —precipitation or dissolution of calcite; and isotopic fractionation —δ15N-NO3 −, δ18O-NO3 −, and δ13C-DIC, incorporating the full δ13C isotope geochemistry involved in EIB. The modeled results fit well with the hydrochemical and isotopic experimental data. The model also incorporated nitrite accumulation observed during the glucose experiment. The biogeochemical model indicates that, depending on the added carbon source, calcite precipitates (using ethanol) or dissolves (using glucose). In both cases, changes in hydraulic conductivity can be induced for actual and long-term EIB applications. The incorporation of isotope fractionation in the model better enables to account for other natural attenuation processes, such as dilution and dispersion, in EIB applications at field scale. Both calibrated enrichment factors (+8‰ for ethanol and +17‰ for glucose) suggest that an inverse fractionation effect occurred (in which the heavy isotope reacts faster than the light isotope) during their oxidation.


      PubDate: 2013-12-31T01:25:03Z
       
  • A reactive-transport model for examining tectonic and climatic controls on
           chemical weathering and atmospheric CO2 consumption in granitic regolith
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): Darcy D. Li , Andrew D. Jacobson , David J. McInerney
      We developed a 1D reactive-transport model for examining how tectonic and climatic parameters, namely uplift rate, water seepage velocity, and temperature, control chemical weathering and atmospheric CO2 consumption rates during regolith development. The model consists of mass-conservation equations describing how mineral and solute concentrations change temporally and spatially (vertically) during weathering of granite containing 30% plagioclase (An20) and up to 3% accessory calcite. The equations are coupled by volumetric weathering rates, which depend on mineral abundances, intrinsic dissolution rate constants, and surface areas as well as the departure of pore water from thermodynamic equilibrium. We numerically solve a non-steady-state, non-dimensional version of the model. By eliminating the need to prescribe regolith thickness, non-dimensionalization introduces two key scaling parameters that drive variations in the model results: the rock residence time (τr , the time for rock to travel upward through the model domain) and the water residence time (τw , the time for water to travel downward through the model domain). We use the model to examine fundamental properties of chemical weathering, especially reaction front propagation, with the primary aim of characterizing weathering regimes and identifying factors that maximize the dissolution of plagioclase because only silicate weathering regulates atmospheric CO2 levels over geological timescales. Weathering regimes are commonly classified as transport (or supply)-limited versus weathering (or kinetically)-limited, but as outgrowths of geomorphology, these terms primarily refer to the role of τr in regulating regolith thickness. Our findings suggest that the paradigm should be expanded to include kinetic controls determined by τw . Both regimes can experience far-from- and near-equilibrium mineral dissolution. However, transport-limited regimes generally have lower τw /τr ratios compared to kinetically-limited regimes. Under transport limitation, thick to thin reaction fronts propagate downward indefinitely yielding extensive mineral depletion, deeply weathered regolith, and high ratios of silicate-to-carbonate weathering. Under kinetic limitation, thin to non-existent reaction fronts evolve to steady-state yielding minimal mineral depletion, no regolith development, and low ratios of silicate-to-carbonate weathering. Kinetically-limited regimes typifying tectonically active mountain ranges have low long-term atmospheric CO2 consumption rates. Transport-limited regimes typifying tectonically stable cratons also have low long-term atmospheric CO2 consumption rates, but because much of the Earth's surface is transport-limited, we infer that such environments have the greatest impact on the global atmospheric CO2 consumption flux. The model output leads us to hypothesize that the maximum contribution should occur for extensive regions of silicate bedrock consolidated in warm, wet climates because the combination of tectonic stability, high temperatures, and rapid seepage velocities accelerates reaction front propagation, facilitates calcite depletion, and sustains deep regolith development over long timescales. Our findings point to the importance of internal climate feedbacks for stabilizing long-term atmospheric CO2 levels.


      PubDate: 2013-12-31T01:25:03Z
       
  • Insights into subduction zone sulfur recycling from isotopic analysis of
           eclogite-hosted sulfides
    • Abstract: Publication date: 4 February 2014
      Source:Chemical Geology, Volume 365
      Author(s): K.A. Evans , A.G. Tomkins , J. Cliff , M.L. Fiorentini
      Subduction of sulfur in ocean crust makes a significant but poorly understood contribution to the global sulfur cycle. Part of the uncertainty arises from a lack of knowledge about the metamorphic changes that affect subducted sulfur-bearing minerals, and the ultimate source of sulfur that is subducted to depth. Sulfur δ34S varies both as a function of the original sulfur source, and as a consequence of processes subsequent to sulfide crystallisation such as devolatilisation, redox reactions, and fluid loss. To investigate sulfur liberation during subduction, secondary ion mass spectroscopy (SIMS) was used to measure δ34S in grains of pyrite, chalcopyrite and pyrrhotite in eclogites from the Zermatt–Saas zone in the Western Alps, and the Pouébo terrane of New Caledonia. Trace element mapping on selected sulfide grains was also performed. Sulfides in these rocks are generally associated with greenschist retrogression assemblages, but also occur as inclusions in garnet, associated with glaucophane and omphacite, and as polysulfide grains with typical magmatic combinations of minerals. δ34S varies significantly within individual pyrite grains, with striking correlations, in some cases, between Co zoning and changes in δ34SVCDT. δ34SVCDT is, in many cases, greater than 13‰, consistent with derivation from seawater-derived sulfate. The dataset suggests that sulfur isotopes in pyrite experienced little or no post-crystallisation re-equilibration, that pyrite grew under open system conditions with heterogeneous fluid flow on a thin section scale, and that sulfide growth involved sulfur addition. Prograde subduction processes most likely involved sulfur loss. Sulfide growth occurred in some samples at the very earliest stages of exhumation. Therefore these sulfides provide useful information on the fluids present in slabs at great depths.


      PubDate: 2013-12-31T01:25:03Z
       
  • Constraining the internal variability of the stable isotopes of carbon and
           nitrogen within mantle diamonds
    • Abstract: Publication date: Available online 25 December 2013
      Source:Chemical Geology
      Author(s): S. Mikhail , A.B. Verchovsky , D. Howell , M.T. Hutchison , R. Southworth , A.R. Thomson , P. Warburton , A. Jones , H.J. Milledge
      The carbon and nitrogen isotope values of mantle xenoliths and xenocrysts are used to trace the cycling of volatiles in the deep Earth, for example, to place empirical constraints on the origin of diamond-forming carbon in the mantle. The global database for diamond shows the δ13C value of peridotitic diamonds is very narrow, typically around −5‰, whereas eclogitic diamonds can show positive and very negative δ13C values resembling crustal carbonates and crustal organic carbon (<−40 to>+2‰); commonly interpreted to reflect a relationship between eclogitic diamond formation and subduction zone planet tectonics. Curiously, diamonds from both paragenesis can show positive (crust-like) and negative (mantle-like) δ15N values (from<−40 to>+20‰). Most of these data are derived from single stage combustion mass spectrometry gas sourced mass spectrometry, which produces simplistic datasets. By fragmenting single diamonds or using in-situ ion-beam techniques it is known that single diamonds can show large-scale heterogeneity for their carbon isotope values and nitrogen abundances, sometimes as large as entire populations of diamond across a few hundred micrometres. What is less well known is the scale of nitrogen isotope heterogeneity within single diamonds, and if the nitrogen isotope heterogeneity of single diamonds can provide an insight into why diamonds that show very restricted δ13C values show a much large range of δ15N values. To investigate the scale, and to determine the origin of the nitrogen isotope heterogeneity (source vs. fractionation during diamond-formation) shown for populations of mantle diamonds we have determined multiple δ13C-δ15N values and nitrogen abundances from 14 monocrystalline (MCD) and 25 polycrystalline diamonds (PCD) using step-wise oxidation gas sourced mass spectrometry. These data show the heterogeneity shown for carbon and nitrogen isotope values from single diamond samples presented here are typically <5‰ and <8‰ respectively, both of which are comparable to the standard deviation for the mean mantle δ13C and δ15N values (±3 and ±4‰). However, there are samples that show much larger heterogeneities for δ13C and δ15N values (≤23‰ and ≤33‰ respectively), which cannot be generated by equilibrium stable isotope fractionation during, or prior to diamond-formation. These data suggest that isotopic heterogeneity may be present within the diamond-forming fluid on sub-mm scale, or that these diamonds formed during multiple diamond-formation events from isotopically distinct sources. From these 39 samples there are only 5 PCD that show a larger range of carbon isotopes relative to nitrogen isotopes, but of these 5 samples only 2 show a range of δ13C values outside of analytical uncertainty. The remaining 34 samples show a greater isotopic heterogeneity for δ15N relative to δ13C values. The samples with the largest carbon and nitrogen isotopic heterogeneity are also samples with low-bulk δ13C values (<−10‰), whilst there is no relationship between the ranges of nitrogen isotope values for a given sample and the corresponding bulk δ15N value or nitrogen content. These data show δ15N values recorded in mantle diamonds are relatively heterogeneous, and can show both mantle-like (negative) and crustal (sedimentary)-like (positive) δ15N values within the same sample. We conclude that the large range of nitrogen isotope values seen within individual diamonds means the observation of negative, mantle-like, nitrogen in mantle diamonds acquired by single-stage bulk combustion isotope ratio mass spectrometry cannot be used as a conclusive indicator for a mantle origin for the entirety of the diamond-forming carbon, and vice versa. Also, the behaviour of 15 N/14 N is not coupled with the behaviour of 14 N/12C during diamond-formation. Instead, it appears that diamond-forming fluids can have positive and negative δ15N values, irrespective of their δ13C value(s). These data suggest subduction induced nitrogen isotope heterogeneity may not coupled with subduction induced carbon isotope heterogeneity in diamond-forming fluids.


      PubDate: 2013-12-27T08:51:52Z
       
  • Zinc isotope fractionation during surface adsorption and intracellular
           incorporation by bacteria
    • Abstract: Publication date: Available online 25 December 2013
      Source:Chemical Geology
      Author(s): Fotios Christos A. Kafantaris , David M. Borrok
      Zinc (Zn) isotopes are fractionated during biogeochemical processing by microorganisms. Uncertainties remain, however, regarding the roles of cell surface adsorption and speciation of aqueous Zn on the extents of isotopic fractionation. In this study, we conducted bacterial surface adsorption and intracellular incorporation experiments using Zn and representative Gram-positive (B. subtilis) and Gram-negative (P. mendocina, E. coli) bacterial species, as well as a natural bacterial consortium derived from soil. Under conditions of high Zn:bacteria ratio, surface complexes preferentially incorporated the heavier isotopes of Zn, resulting in an average Δ66Znadsorbed-solution of +0.46‰ (αadsorbed-solution ≈ 1.00046). Adsorption experiments conducted under conditions of low Zn:bacteria ratio appear to have been complicated by the presence of dissolved organic exudates that competed with surface functional group sites for Zn. We were able to empirically model this process to show that very small amounts of Zn-organic complexes with fractionation factors in the range of α =1.002 to 1.003 could account for the observed δ66Zn of the experimental solutions. For the intracellular incorporation experiments, the presence of 0.2 and 2mg/L of Zn (as Zn-Citrate) resulted in a Δ66Znincorporated-solution ranging from −0.2‰ to +0.5‰, depending upon the bacterial species and the growth phase. The addition of 0.2 and 2mg/L Zn2+ to the growth medium appeared to create a metal stress response (or at least a change in metal processing) in P. mendocina that resulted in a positive Δ66Znincorporated-solution of up to +2.04‰. Our study suggests that Zn isotopes have the potential to be used to elucidate metal-binding pathways associated with microorganisms in natural systems, but that the interpretation of these effects is likely complicated by factors such as competing surface interactions and differences in bacterial species and metal speciation.


      PubDate: 2013-12-27T08:51:52Z
       
  • Nanoparticulate pyrite and other nanoparticles are a widespread component
           of hydrothermal vent black smoker emissions
    • Abstract: Publication date: Available online 22 December 2013
      Source:Chemical Geology
      Author(s): Amy Gartman , Alyssa J. Findlay , George W. Luther III
      The presence of nanoparticulate pyrite is reported in hydrothermal emissions from Rainbow, TAG and Snakepit on the Mid- Atlantic Ridge (MAR). When coupled with previously collected data from East Pacific Rise 9 °N (EPR) and Lau Basin, these data demonstrate that pyrite nanoparticles are a widespread component of black smoker emissions from hydrothermal vents and are found in significant concentrations at a fast spreading mid-ocean Ridge (EPR), a back- arc basin (Lau Basin), and a slow spreading mid- ocean Ridge (MAR). The maximum percentage of filtered iron emitted as nanoparticulate pyrite was found to be as high as 25%, 10%, and 5%, respectively. As a widespread component of hydrothermal vent emissions, these nanoparticles may be an important source of iron to the world’s oceans. Metals such as Cu and Zn are detected in pyrite- containing aggregates at all sites, and chalcopyrite was a component of nanoparticle aggregates at MAR. Iron containing silicate nanoparticles are also identified, and indicate that nanoparticles other than sulfides should also be considered when determining transport implications of hydrothermal vent emissions. The varied morphologies and the presence of different minerals within these nanoparticles provide insight into their formation and stability.


      PubDate: 2013-12-22T23:53:35Z
       
  • Effect of co-solutes on the products and solubility of uranium(VI)
           precipitated with phosphate
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Vrajesh S. Mehta , Fabien Maillot , Zheming Wang , Jeffrey G. Catalano , Daniel E. Giammar
      Uranyl phosphate solids are often found with uranium ores, and their low solubility makes them promising target phases for in situ remediation of uranium-contaminated subsurface environments. The products and solubility of uranium(VI) precipitated with phosphate can be affected by the pH, dissolved inorganic carbon (DIC) concentration, and co-solute composition (e.g. Na+/Ca2+) of the groundwater. Batch experiments were performed to study the effect of these parameters on the products and extent of uranium precipitation induced by phosphate addition. In the absence of co-solute cations, chernikovite [H3O(UO2)(PO4)·3H2O] precipitated despite uranyl orthophosphate [(UO2)3(PO4)2·4H2O] being thermodynamically more favorable under certain conditions. As determined using powder X-ray diffraction, electron microscopy, and laser induced fluorescence spectroscopy, the presence of Na+ or Ca2+ as a co-solute led to the precipitation of sodium autunite ([Na2(UO2)2(PO4)2] and autunite [Ca(UO2)2(PO4)2]), which are structurally similar to chernikovite. In the presence of sodium, the dissolved U(VI) concentrations were generally in agreement with equilibrium predictions of sodium autunite solubility. However, in the calcium-containing systems, the observed concentrations were below the predicted solubility of autunite, suggesting the possibility of uranium adsorption to or incorporation in a calcium phosphate precipitate in addition to the precipitation of autunite.
      Graphical abstract image

      PubDate: 2013-12-22T23:53:35Z
       
  • Nanoscale characterisation of crystal zoning
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Kate Saunders , Ben Buse , Matt R. Kilburn , Stuart Kearns , Jon Blundy
      Advances in analytical techniques are fundamental to the enhanced understandings of many geological processes. Zoned volcanic crystals have been analysed by low (5) kV field emission gun electron probe micro-analyser (FEG-EPMA) and NanoSIMS to obtain sub-micrometre chemical profiles and compared to time-of-flight SIMS (TOF-SIMS) and high (15–20) kV EPMA profiles. Plagioclase and orthopyroxene crystals have been analysed by FEG-EPMA, at accelerating voltages of 5kV providing a spatial resolution (step size) of ≤350nm (the resolution of the lowest energy X-ray) for orthopyroxene crystals using a 30nm beam and ca. 750nm for plagioclase crystals which at low voltages are unstable and require a 500nm defocused beam. Step sizes are comparable in size to interaction volumes. Analytical protocols are detailed that permit quantitative major and minor element compositions to be acquired at similar precision and accuracy as traditional EPMA analyses at 15–20kV. NanoSIMS analysis of the same crystals provides a greater spatial resolution of up to 200nm and allows the measurement of Li also. The NanoSIMS profiles, however, cannot currently be quantified. The ability to analyse crystals at sub-micrometre scales is demonstrated by the good agreement between NanoSIMS, FEG-EPMA, conventional EPMA and TOF-SIMS data. FEG-EPMA, NanoSIMS and TOF-SIMS techniques have broad applications within the earth sciences. In petrologic studies for example, these methods have the ability to analyse small crystals in experimental charges and provide chemical profiles of crystal zoning at a spatial resolution of ca. 200–300nm. Such profiles are important in crystal forensics and diffusion chronometry studies. The implications for the latter application are that timescales of volcanic processes that occur in the days–years immediately prior to the eruption can now be studied.


      PubDate: 2013-12-22T23:53:35Z
       
  • Unmasking xenolithic eclogites: Progressive metasomatism of a key Roberts
           Victor sample
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Jin-Xiang Huang , William L. Griffin , Yoann Gréau , Norman J. Pearson , Suzanne Y. O'Reilly , John Cliff , Laure Martin
      Most eclogite xenoliths from the Roberts Victor kimberlite (South Africa) have been metasomatized by melts/fluids in the carbonatitic–kimberlitic spectrum, and these can be used to define a progressive metasomatic process. Several stages of this progressive metasomatism have been recognized within one sample (RV07-17), using petrography, major- and trace-element compositions, and Sr- and O-isotope data. Four zones in RV07-17 are distinguished in terms of the compositions of their garnets. From Zone 1 to Zone 4, the microstructure becomes less equilibrated; secondary minerals and fluid inclusions become abundant; the pyrope content of the garnets increases; and clinopyroxene shows progressive enrichment in MgO. The garnets of Zone 1 have flat REE patterns from Lu to Sm, with a strong depletion in the LREE. Toward Zone 4, the relative abundance of the MREE of the garnets drops significantly, giving smoother patterns. A large relict clinopyroxene grain in Zone 1 shows a strong depletion in the LREE, but the LREE/MREE of the recrystallized clinopyroxene increases from Zone 1 to Zone 4. From Zone 1 to 4, 87Sr/86Sr in clinopyroxene increases along with Sr content and δ18O of the garnet decreases from 8.2 to 5.7‰ as the MgO content increases. Trace-element data imply that the fluids that metasomatized RV07-17 were closer to carbonatitic than kimberlitic in composition. Aside from the one relict clinopyroxene, all original compositional information in this xenolith has been swept away, making it difficult to define the protolith. To know the origin of the xenolithic eclogites, and to use them as evidence for different geodynamic/tectonic scenarios, the least metasomatized samples must be studied; unfortunately these make up a tiny proportion of the widely studied xenolith population at Roberts Victor.


      PubDate: 2013-12-22T23:53:35Z
       
  • Which minerals control the Nd–Hf–Sr–Pb isotopic
           compositions of river sediments?
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Marion Garçon , Catherine Chauvel , Christian France-Lanord , Mara Limonta , Eduardo Garzanti
      River sediments naturally sample and average large areas of eroded continental crust. They are ideal targets not only for provenance studies based on isotopic compositions, but also can be used to establish average continental crust isotopic values. In large fluvial systems, however, mineral sorting processes significantly modify the mineralogy, and thus the geochemistry of the transported sediments. We still do not know, in any quantitative way, to what extent mineral sorting affects and fractionates the isotopic compositions of river sediments. Here, we focus on this issue and try to decipher the role of each mineral species in the bulk isotopic compositions of bedloads and suspended loads sampled at the outflow of the Ganga River that drains the Himalayan mountain range. We analyzed Nd, Hf, Sr, and Pb isotopic compositions as well as trace element contents of a large number of pure mineral fractions (K-feldspar, plagioclase, muscovite, biotite, magnetite, zircon, titanite, apatite, monazite/allanite, amphibole, epidote, garnet, carbonate and clay) separated from bedload and bank sediments. We combine these data with mineral proportions typical of the Ganga sediments to perform Monte Carlo simulations that quantify the contributions of individual mineral species to the Nd, Hf, Sr, and Pb isotopic budgets of bedloads and suspended loads. The isotopic systematics of river sediments are buffered by very few minerals. Despite their extremely low proportions in sediments, zircon and monazite/allanite control Hf and Nd isotopes, respectively. Feldspars, epidote, and carbonate buffer the Sr isotopic budget while clay, feldspars, and heavy minerals dominate Pb isotopes. Hafnium, Sr, and Pb isotopic differences between bedloads and suspended loads are well explained by their different mineral compositions. This confirms that Hf, Sr and Pb isotopic compositions of sediments are strongly biased by mineral sorting processes during fluvial transport; hence they do not always constitute good proxies for provenance studies. In addition, we anticipate that fractionation of the isotopic systems continues at the river/ocean interface to deliver sediments to the deep ocean that are not necessarily similar to their crustal precursors, creating a systematic bias between the compositions of crustal sources and oceanic sediments.


      PubDate: 2013-12-22T23:53:35Z
       
  • Speciation of dissolved chromium and the mechanisms controlling its
           concentration in natural water
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Sulistyo Saputro , Kazuhisa Yoshimura , Shiro Matsuoka , Kô Takehara , Narsito , Jun Aizawa , Yoshika Tennichi
      The circulation of chromium at naturally occurring concentration levels (i.e., 1μgdm−3 or lower) was examined using solid-phase spectrophotometry and inductively coupled plasma-mass spectrophotometry (ICP-MS) to determine the Cr(VI) and Cr(Total) concentrations, respectively. Natural water and stream sediments were collected from areas with various types of geologic features, such as metamorphic rocks, volcanic rocks and limestone in Japan and Indonesia. Cr(VI) was predominant in weakly alkaline natural waters, and the Cr(III) concentration was less than 1μgdm−3, which was considerably lower than that expected based on the solubility of Cr(OH)3. The dissolution of chromium in natural water was described by the leaching of Cr(VI) from Cr(III)-containing minerals under oxic conditions. The Cr(VI) concentration in a solution that was in contact with chromite, FeCr2O4, linearly increased with the reaction time. The results indicated that under oxic conditions at P O2 =0.21atm, the leaching rate of Cr(VI) was pseudo zero-order at a fixed pH and was higher in alkaline than in acidic solutions. The removal of Cr from natural waters was due to the reduction of Cr(VI) to Cr(III) by organic matter, such as humic substances, as well as to the adsorption of Cr(III) onto suspended matter and river sediments. The reduction of Cr(VI) to Cr(III) followed the equation −d[Cr(VI)]/dt=[H+] a [Cr(VI)][DOC] with a =0 at pH<4 and a =−1 at pH>4.5. The removal of Cr(III) by river sediments and/or suspended particulate matter was rapid and the adsorbability of Cr(III) was dependent on pH, which suggests that the active species were Cr(OH)2 + and Cr(OH)3. Schematic models of the circulation of dissolved chromium in natural water were proposed under acidic and alkaline conditions.


      PubDate: 2013-12-22T23:53:35Z
       
  • Deep-sea limestone block as a source of 14C-depleted dissolved inorganic
           carbon at the Palau Trench
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Tatsuya Tsuboi , Hideki Wada , Toshio Nakamura , Tran Thuy Hang , Hiroyuki Matsuzaki , Naho Otsuji , Kantaro Fujioka , Kazumasa Oguri , Hiroshi Kitazato
      The submersible Shinkai 6500 located a large limestone block on the landward slope of the Palau Trench at ca. 6500m depth, which is significantly deeper than the lysocline. Herein, we document the deep-sea carbon flux from the limestone block to seawater. The Sr isotope age of this limestone block indicates that it formed in the middle and late Miocene at ca. 13–7Ma. Accelerator mass spectrometry measurements of the 14C of dissolved inorganic carbon (DIC) extracted from seawater collected around the limestone block yield values as low as −295‰. Based on isotope mass-balance calculations, we have estimated the flux of limestone-derived carbonate ions to deep seawater. The calculated contribution ratio of limestone-derived carbonate ions is up to 6.7% of the bottom water DIC concentration. The annual DIC flux from the limestone block is estimated to be 2.1 (±1.4)×108 mol/y or 2.5 (±1.7)×109 gC/y. These data demonstrate a major apparent carbon flux into the deep-sea environment and are the first example of carbonate dissolution below the lysocline.


      PubDate: 2013-12-18T21:30:03Z
       
  • Magnesium isotope geochemistry in the Han River, South Korea
    • Abstract: Publication date: 22 January 2014
      Source:Chemical Geology, Volume 364
      Author(s): Sin-Woo Lee , Jong-Sik Ryu , Kwang-Sik Lee
      Magnesium (Mg) isotopes can be a proxy for directly constraining the sources of riverine Mg, but the dominant controls on riverine Mg isotope ratios are still uncertain. Here, we report Mg isotope ratios for river waters, experimental leachates and digestions, bulk rocks, and fertilizers in the Han River (HR), South Korea. The HR is composed of two lithologically distinct tributaries: the North Han River (NHR) that flows over only silicate rocks, and the South Han River (SHR) that flows over silicate and carbonate rocks. The lithological differences between the NHR and SHR are reflected in major ion, 87Sr/86Sr, and δ26Mg geochemistry. In particular, the NHR has lower major ion concentrations but higher 87Sr/86Sr ratios and δ26Mg values than the SHR. Simple mass balances and mixing equations indicate that if the riverine δ26Mg values in the HR system are mainly controlled by conservative mixing between silicate and carbonate weathering, the average carbonate end-member δ26Mg value should be unlikely lower than what are measured in this study. Although multiple process-related fractionations occur in the HR system, the enrichment of 24Mg in river waters relative to silicate rocks they drain could be mostly controlled by either fractionation or mixing between isotopically distinct reservoirs, such as minerals or fractions (labile and structural Mg), during dissolution, while the little depletion of 24Mg in the SHR waters relative to carbonate rocks they drain could be likely due to the input of groundwater with lower δ26Mg value rather than fractionation. However, it is difficult to identify the contribution of anthropogenic inputs to riverine δ26Mg because their effects are little. This study suggests that the potential of Mg isotopes for constraining Mg sources in a lithologically varied river basin can be enhanced with a better understanding of process-related fractionation.


      PubDate: 2013-12-18T21:30:03Z
       
  • The biogeochemistry and bioremediation of uranium and other priority
           radionuclides
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Laura Newsome , Katherine Morris , Jonathan R. Lloyd
      Microbial metabolism has the potential to alter the solubility of a broad range of priority radionuclides, including uranium, other actinides and fission products. Of notable interest has been the biostimulation of anaerobic microbial communities to remove redox-sensitive radionuclides such as uranium U(VI) from contaminated groundwaters at nuclear sites. Particularly promising are bioreduction processes, whereby bacteria enzymatically reduce aqueous U(VI) to insoluble U(IV) coupled to oxidation of an organic electron donor; and uranium phosphate biomineralisation, in which bacterial phosphatase activity cleaves organophosphates, liberating inorganic phosphate that precipitates with aqueous U(VI) as uranyl phosphate minerals. Here we review the mechanisms of uranium bioreduction and phosphate biomineralisation and their suitability to facilitate long-term precipitation of uranium from groundwater, with particular focus on in situ trials at the US Department of Energy field sites. Redox interactions of other priority radionuclides (technetium, neptunium, plutonium, americium, iodine, strontium and caesium) are also reviewed.


      PubDate: 2013-12-03T00:00:03Z
       
  • In-situ Fe isotope ratio determination in Fe–Ti oxides and sulfides
           from drilled gabbros and basalt from the IODP Hole 1256D in the eastern
           equatorial Pacific
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Wanja Dziony , Ingo Horn , Dominique Lattard , Jürgen Koepke , Grit Steinhoefel , Jan A. Schuessler , François Holtz
      In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe–Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (>300°C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower δ56Fe values by 0.2‰–0.5‰ as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of <0.1‰ (2σ level) at micrometer-scale. We have found significant variations of the δ56FeIRMM-014 values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of δ56Femagnetite in 1256D rocks ranges from −0.12 to +0.64‰, and of δ56Feilmenite from −0.77 to +0.01‰. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive δ56Fe values between +0.29 and +0.56‰, whereas pyrite in the other samples has generally negative δ56Fe values from −1.10 to −0.59‰. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite–ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including β-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the δ56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite–ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.


      PubDate: 2013-11-29T00:05:09Z
       
  • Guidelines for reporting zircon Hf isotopic data by LA-MC-ICPMS and
           potential pitfalls in the interpretation of these data
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Christopher M. Fisher , Jeffery D. Vervoort , John M. Hanchar
      High quality analytical data are essential for the development of sound scientific interpretations. To ensure the quality of the data published in Chemical Geology, a new type of contribution has been introduced - the "Invited Technical Paper". When a particular technical issue needing discussion or clarification is identified, specialists in the technique will be invited to share their expertise. As is true for all articles published in the journal, these contributions will be subject to peer review before publication. The first topic to be treated concerns in situ Hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry. This is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world. However, if insufficient care is taken during analysis and data processing, inaccurate results can be obtained, notably because of the existence of very large isobaric interferences on the isotope of interest. To address this issue, we have asked Christopher Fisher, Jeffery Vervoort and John Hanchar to provide a set of guidelines that can be adopted to assure that reliable Hf isotopic data are obtained by this technique. Over the past decade, the Hf isotope composition of zircon, as determined in situ by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS), has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool. There has been no uniformity, however, in the reporting of in situ Hf isotopic data and, quite often, reviewers and readers of papers are not provided with sufficient information to assess data quality. The goal of this invited contribution is to provide the non-specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ Hf isotopic data from zircon, including details of how the challenging large isobaric interference corrections have been made. In addition, we discuss a number of potential pitfalls vis-à-vis the assignment of the incorrect age to the measured Hf isotope composition. Non-specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others.


      PubDate: 2013-11-29T00:05:09Z
       
  • Environmental implication of nitrogen isotopic composition in ornithogenic
           sediments from the Ross Sea region, East Antarctica: Δ15N as a new
           proxy for avian influence
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Yaguang Nie , Xiaodong Liu , Tao Wen , Liguang Sun , Steven D. Emslie
      We analyzed δ15N in both acid-treated and untreated sediment profiles from McMurdo Sound of the Ross Sea region, East Antarctica that were influenced by penguin guano. The difference between treated and untreated δ15N (Δ15N) was significant in three profiles which were heavily impacted by guano, and minor in two profiles with less guano influence. We determined that the total nitrogen in the sediments is primarily derived from penguin guano and algae, and used an N-species test to explain the variation of Δ15N in two profiles. It was found that post-depositional decomposition and ammonia volatilization, which have important roles in the cold and arid environment of East Antarctica, would render an elevated δ15N through kinetic isotopic fractionation in the inorganic nitrogen from guano. N-species analysis revealed that the percentage of inorganic nitrogen in total nitrogen, indicative of the degree of guano influence, is the key factor controlling Δ15N in the sediments. This hypothesis successfully explained the nitrogen isotopic composition in the remaining three sediment profiles. We conclude that the parameter Δ15N, rather than traditionally used untreated δ15N, can be taken as an effective proxy for the strength of avian influence on ornithogenic sediments in East Antarctica.


      PubDate: 2013-11-29T00:05:09Z
       
  • Quantifying chemical weathering intensity and trace element release from
           two contrasting basalt profiles, Deccan Traps, India
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): M.G. Babechuk , M. Widdowson , B.S. Kamber
      Weathering profiles developed on basalt substrate contain information relevant to climate, atmospheric composition and evolution, nutrient release into the hydrosphere, and understanding Martian regolith. In this study, the chemical compositions of two profiles developed on Deccan Trap basalt are examined. One is sub-Recent and has only progressed to a moderate degree of alteration (Chhindwara profile), whereas the other is ancient (Paleocene) and the degree of alteration is extreme (Bidar laterite). In an attempt to better quantify the chemical changes during incipient to intermediate weathering of mafic substrates, a new index is proposed: the mafic index of alteration (MIA). Similar to the chemical index of alteration (CIA), the MIA quantifies the net loss of the mobile major elements (Ca, Mg, Na, K±Fe) relative to the immobile major elements (Al±Fe). The redox-dependent weathering behaviour of Fe is factored into two separate arrangements of the MIA that apply to oxidative [MIA(O)] or reduced [MIA(R)] weathering. The MIA can be visualised in a variety of ternary diagrams in the Al–Fe–Mg–Ca–Na–K system. To chemically quantify the stages of advanced to extreme weathering, at which the MIA and CIA are ineffective, the SiO2 to (Al2O3 +Fe2O3) mass ratio, based on the established Si–Al–Fe (SAF) ‘laterite’ ternary diagram, is used; we propose that this ratio be referred to as the ‘index of lateritisation’ (IOL). Major element chemical variations, as expressed by weathering indices, are used to relate the extent of weathering with the behaviour of trace elements (alkali, alkaline earth, rare earth, and Nb) in the profiles. During the early stages of basalt weathering, the mobile trace elements (Sr, Be, Li) are anti-correlated with the chemical weathering indices and thus released during these stages. By contrast, the monovalent elements (K, Rb, Cs, Tl), excluding Na and Li, appear to be associated with the pedogenetic clay minerals. Of these elements, those with the most similar ionic radii are closely related in their weathering behaviour. Fractionation of the REE (Sm/Nd, Eu/Eu*, Ce/Ce*) is evident during weathering of the basalt. The loss of Eu is linked with that of Sr, Ca, and Na and thus associated with plagioclase dissolution during the stages of incipient to intermediate weathering. The fractionation of Sm/Nd suggests that basaltic weathering products may not always preserve their parent rock ratio and, consequently, their Nd isotope composition over time. Finally, weathering in the sub-Recent profile is shown to have progressed across two lava flows, whose morphology initially controlled the extent of weathering. Certain compositional variations in the original flows (e.g., immobile element ratios) are preserved through the effects of chemical weathering and have the potential to influence mass balance calculations across the entire profile.


      PubDate: 2013-11-24T22:44:07Z
       
  • Red bed and basement sourced fluids recorded in hydrothermal
           Mn–Fe–As veins, Sailauf (Germany): A LA-ICPMS fluid inclusion
           study
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Tobias Fusswinkel , Thomas Wagner , Thomas Wenzel , Markus Wälle , Joachim Lorenz
      The hydrothermal Mn–Fe–As vein mineralization at Sailauf (Germany) hosts a complex sequence of oxide and carbonate minerals that record a protracted fluid history. The mineralization is related to a major unconformity that separates Permian (Zechstein) sedimentary rocks from underlying Variscan crystalline basement. The hydrothermal veins contain two principal mineralization stages, which are Mn oxides associated with calcite and hematite associated with Mn-calcite. The fluid evolution of the hydrothermal system has been reconstructed from fluid inclusion petrography, microthermometry, and LA-ICPMS microanalysis, coupled with stable isotope geochemistry of carbonate and oxide minerals. The fluid inclusions are high salinity sodic–calcic brines and the bulk fluid properties show no major differences between the Mn oxide and the hematite stage. LA-ICPMS analysis of major and trace elements demonstrates that the mineralization formed from chemically distinct fluid pulses characterized by variations in their K, Li, B, Pb and Zn concentrations. The fluid that precipitated the Mn oxide stage has anomalous Pb/Zn and Li/B ratios, which closely resemble fluids found in fracture fillings in red beds of the Permian Rotliegend basin. By contrast, the fluids associated with the hematite stage have Pb/Zn and Li/B ratios typical of crustal fluids that were derived from interaction with crystalline basement. Both fluids possess characteristic element ratios including Cl/Br, but variable absolute concentrations of most metals. This suggests that both fluids were modified by mixing with a common metal-depleted brine that had a similar Cl/Br ratio, most likely a formation water from the overlying Zechstein sedimentary rocks. The Sailauf mineralization provides insight into the protracted post-Variscan fluid evolution at the basement–cover interface. The compositionally anomalous fluid that precipitated the Mn oxides is comparable to brines derived from interaction with red beds and likely represents the ore fluid of Kupferschiefer-type sediment-hosted Cu deposits. Conversely, the fluid that deposited the hematite mineralization resembles fluids that typically form basement- and sediment-hosted Pb–Zn deposits.


      PubDate: 2013-11-24T22:44:07Z
       
  • The nano-scale anatomy of a complex carbon-lined microtube in volcanic
           glass from the ~92Ma Troodos Ophiolite, Cyprus
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): D. Wacey , N. McLoughlin , M. Saunders , C. Kong
      Microtubular alteration textures in the glassy rims of pillow basalts and hyaloclastite may provide evidence of a sub-seafloor biosphere that has persisted since perhaps 3.5Ga. Reports of organic carbonaceous linings within such microtubes constitute one of the key lines of evidence for a biogenic origin. However, such linings have until now been studied mainly by low spatial resolution surface analysis techniques, resulting in a limited understanding of their origin. Here we analyze a large, complex microtube (Tubulohyalichnus annularis isp.) from the ~92Ma Troodos Ophiolite using focused ion beam milling combined with transmission electron microscopy (FIB-TEM), and focused ion beam serial sectioning combined with scanning electron microscopy (3D-FIB-SEM). These analyses reveal a distinct 50–200nm thick carbonaceous lining within the microtube. The lining is almost continuous and closely replicates the outer annulated morphology of the terminal end of the microtube. A spiral sub-structure previously seen using optical microscopy is also shown to be carbonaceous, comprising sheets that in places span the entire width of the microtube. Elemental mapping plus electron energy loss spectroscopy (EELS) shows that the carbon is organic and co-occurs with nitrogen. EELS and selected area electron diffraction (SAED) reveal that a clay mineral infills the remainder of the microtube, and this has a chemistry and structure consistent with the Fe-rich smectite, nontronite. Anti-correlation between carbon and the elements (Al, Ca, Fe, Mg, O, Si) in the infilling clay, plus the morphology of the carbon, show that the carbon lining is a discrete structure that predates clay mineral growth, thereby excluding the later absorption of organics onto the clay. This new data permits the rejection of a hydrothermal or metamorphic fluid derived origin for the carbonaceous linings to T. annularis isp., plus an origin from recent post-obduction groundwater hosted microorganisms. Our favored hypothesis is that the carbonaceous lining and sheet-like sub-structure were formed on the Cretaceous sub-seafloor by microbial activity in the volcanic glass. It is not yet possible however, to ascertain if the organic material comes from the putative microbe(s) that constructed the T. annularis isp. trace fossil, or from microbes that inhabited the pre-existing microtube. Further analysis of young in-situ oceanic crustal samples using these ultra-high-resolution FIB-TEM and 3D-FIB-SEM techniques may answer this critical outstanding question.


      PubDate: 2013-11-24T22:44:07Z
       
  • Characterisation of Andosols from Laacher See tephra by wet-chemical and
           spectroscopic techniques (FTIR, 27Al-, 29Si-NMR)
    • Abstract: Publication date: 10 January 2014
      Source:Chemical Geology, Volume 363
      Author(s): Thilo Rennert , Karin Eusterhues , Syuntaro Hiradate , Hergen Breitzke , Gerd Buntkowsky , Kai U. Totsche , Tim Mansfeldt
      At 12,900aBP, the eruption of the Laacher See volcano generated a new parent material for Holocene soil formation in parts of Western Germany. Weathering of these ashes commonly includes the formation of poorly crystalline minerals such as allophane, imogolite and ferrihydrite. Detection of these minerals in soil is difficult, yet an important task, because they may govern soil functions and processes, e.g., stabilisation of organic matter and nutrient availability. Therefore, we characterised three forested Andosols by a combination of wet-chemical and spectroscopic techniques including infrared and (27Al, 29Si) nuclear magnetic resonance (NMR) spectroscopy together with X-ray diffractometry. Deconvoluting the 29Si-NMR spectra revealed that 1.6 to 10.4% of total Si was present as allophanic compounds, which coincided with the amounts of oxalate-extractable Si. Since extraction methods are not completely selective, we observed a slight overestimation of allophanic Si estimated from oxalate extraction. Although the sites under study are located close to each other in similar relief positions and with similar vegetation, the combination of our results revealed varying amounts of loess in the parent materials and varying weathering intensity. High weathering intensities correlate with the amounts of allophane.


      PubDate: 2013-11-24T22:44:07Z
       
  • Metal release from limestones at high partial-pressures of CO2
    • Abstract: Publication date: Available online 6 November 2013
      Source:Chemical Geology
      Author(s): Assaf Wunsch , Alexis K. Navarre-Sitchler , Joel Moore , John E. McCray
      CO2 leakage from underground CO2 sequestration and storage poses potential risks to degradation of water quality in shallow aquifers. Increased CO2 concentrations can result in decreased pH and lead to subsequent metal release from mineral dissolution or desorption from mineral surfaces. Dissolution of carbonate minerals present in aquifer sediments or rocks will buffer pH and are generally thought to reduce the potential risk of metal release in the event of a CO2 leak. As a result, much of the research on geochemical impacts of CO2 leakage has focused on siliciclastic aquifers with little to no carbonate minerals present. However, carbonate minerals contain trace amounts of metals in their crystal structure that will be released into solution with dissolution and may pose a risk to drinking water quality. Here, we perform laboratory water-rock experiments to analyze the potential for metal release due to carbonate mineral dissolution in limestone aquifers. Rock samples from three limestone aquifers were dissolved in batch reactors with varying partial-pressures of CO2 (from 0.01 to 1 bar) in the headspace. As CO2 dissolved into the fluid and decreased the pH, the carbonate minerals dissolved and released metals into solution. The concentrations of calcium, magnesium, strontium, barium, thallium, uranium, and cobalt increased but remained below any regulatory limits. The concentrations of arsenic and nickel increased and exceeded primary drinking water standards set by the USEPA and the State of California, respectively. Potential sources of metals in the rocks were determined through detailed sample characterization using sequential extractions, laser ablation inductively coupled mass spectrometry, and high resolution mineralogical mapping with QEMSCAN. We found that calcite dissolution released more metals to solution than pyrite dissolution or metal desorption from mineral surfaces in these experiments. Geochemical models based on the experimental data were used to evaluate the relative importance of calcite versus pyrite dissolution over a 30-year time frame. Under both oxic and sub-oxic conditions, calcite dissolution is the dominant source of metals to solution immediately after exposure to CO2. Pyrite dissolution becomes the dominant source at later times as the fluid reaches equilibrium with respect to calcite. For all model scenarios, the cumulative contribution of metals to solution was dominated by calcite dissolution. Results from this study suggest that the pH-buffering benefit of carbonate mineral dissolution in the event of a CO2 leak may be offset by the potentially negative effect of trace metal release from the crystal structure. This study highlights the need for detailed sample characterization at individual sites to identify sources of metals when assessing the potential risk of CO2 leakage into shallow aquifers.


      PubDate: 2013-11-09T00:07:37Z
       
  • Using perovskite to determine the pre-shallow level contamination magma
           characteristics of kimberlite
    • Abstract: Publication date: Available online 7 November 2013
      Source:Chemical Geology
      Author(s): Chiranjeeb Sarkar , Craig D. Storey , Chris J. Hawkesworth
      It remains difficult to obtain reliable geochemical signatures of uncontaminated kimberlite magma from bulk rock studies due to the combined effects of crustal assimilation and element mobility during post-emplacement alteration processes. Groundmass perovskite (CaTiO3), a typical accessory phase, from Orapa (Botswana) and Wesselton (South Africa) kimberlites has been used to evaluate the isotope and trace element composition of the pre-contamination magmas and the effects of shallow level contamination. In-situ trace element signatures of Orapa and Wesselton perovskite grains are broadly similar and unaffected by crustal contamination. Single grain 87Sr/86Sr isotope ratios of perovskite from Orapa (0.7030-0.7036) are less scattered than bulk rock analyses (0.7063-0.7156), which are variably affected by contamination and late stage alteration. Initial 87Sr/86Sr isotope ratios of perovskite (0.7044-0.7049) from Wesselton overlap with published whole rock studies on fresh hypabyssal kimberlites (0.7042-0.7047). The limited intra-kimberlite variation in Sr isotope ratios recorded by the perovskite are unlikely to be due to crustal contamination as the calculated liquid compositions in equilibrium with the perovskite analysed typically have > 1500ppm Sr, and most common crustal lithologies underlying these kimberlites have relatively low Sr contents and are not highly radiogenic. Calculated pre-shallow level contamination magma compositions for Orapa and Wesselton have significantly fractionated LREE and highly variable non-smooth trace element patterns. Initial Sr and Nd isotope ratios of both kimberlites fall on the mantle Nd-Sr array with enriched Sr and slightly depleted Nd signatures, similar to Group I kimberlites. Overall, the trace element and isotopic composition of Orapa and Wesselton kimberlites are similar to the reported Group I kimberlites from southern Africa, which are derived by very low degrees of partial melting from a LREE depleted metasomatised sub-continental lithospheric mantle (SCLM) source.


      PubDate: 2013-11-09T00:07:37Z
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2014