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  Subjects -> EARTH SCIENCES (Total: 589 journals)
    - EARTH SCIENCES (436 journals)
    - GEOLOGY (66 journals)
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EARTH SCIENCES (436 journals)                  1 2 3 4 5 | Last

Acta Geodaetica et Geophysica     Hybrid Journal   (Followers: 1)
Acta Geodaetica et Geophysica Hungarica     Full-text available via subscription   (Followers: 2)
Acta Geophysica     Hybrid Journal   (Followers: 7)
Acta Geotechnica     Hybrid Journal   (Followers: 8)
Acta Meteorologica Sinica     Hybrid Journal   (Followers: 2)
Advances in High Energy Physics     Open Access   (Followers: 12)
Advances In Physics     Hybrid Journal   (Followers: 7)
Aeolian Research     Hybrid Journal   (Followers: 1)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 3)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 2)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 1)
Annals of Geophysics     Full-text available via subscription   (Followers: 9)
Annals of GIS     Hybrid Journal   (Followers: 15)
Annals of Glaciology     Full-text available via subscription  
Annual Review of Marine Science     Full-text available via subscription   (Followers: 9)
Anthropocene Review     Hybrid Journal   (Followers: 1)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 6)
Applied Geomatics     Hybrid Journal   (Followers: 7)
Applied Geophysics     Hybrid Journal   (Followers: 6)
Applied Ocean Research     Hybrid Journal   (Followers: 6)
Applied Petrochemical Research     Open Access   (Followers: 3)
Applied Remote Sensing Journal     Open Access   (Followers: 9)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 7)
Artificial Satellites     Open Access   (Followers: 14)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 17)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 4)
Atmospheric and Climate Sciences     Open Access   (Followers: 14)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 11)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 8)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 9)
Bulletin of Geosciences     Open Access   (Followers: 8)
Bulletin of Marine Science     Full-text available via subscription   (Followers: 13)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 16)
Bulletin of Volcanology     Hybrid Journal   (Followers: 11)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 3)
CATENA     Hybrid Journal   (Followers: 3)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 5)
Central European Journal of Physics     Hybrid Journal   (Followers: 1)
Chemical Geology     Hybrid Journal   (Followers: 9)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 3)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 2)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 9)
Coastal Management     Hybrid Journal   (Followers: 18)
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 5)
Computational Geosciences     Hybrid Journal   (Followers: 10)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 5)
Contemporary Trends in Geoscience     Open Access  
Continental Journal of Earth Sciences     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 7)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 4)
Cybergeo : European Journal of Geography     Open Access   (Followers: 4)
Developments in Geotectonics     Full-text available via subscription   (Followers: 2)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 3)
Développement durable et territoires     Open Access   (Followers: 2)
Diatom Research     Hybrid Journal  
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamics of Atmospheres and Oceans     Hybrid Journal   (Followers: 3)
E3S Web of Conferences     Open Access  
Earth and Planetary Science Letters     Hybrid Journal   (Followers: 160)
Earth Interactions     Full-text available via subscription   (Followers: 8)
Earth Science Research     Open Access   (Followers: 6)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 1)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 11)
Earth System Dynamics     Open Access   (Followers: 4)
Earth System Dynamics Discussions     Open Access   (Followers: 3)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 1)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Spectra     Full-text available via subscription   (Followers: 11)
Ecohydrology     Hybrid Journal   (Followers: 11)
Ecological Questions     Open Access   (Followers: 6)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 11)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 10)
Environmental Geology     Hybrid Journal   (Followers: 11)
Environmental Geosciences     Full-text available via subscription   (Followers: 4)
Environmental Geotechnics     Open Access  

        1 2 3 4 5 | Last

Journal Cover Chemical Geology
   Journal TOC RSS feeds Export to Zotero [11 followers]  Follow    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
     ISSN (Print) 0009-2541
     Published by Elsevier Homepage  [2566 journals]   [SJR: 1.714]   [H-I: 111]
  • Veins in the combusted metamorphic rocks, Israel; Weathering or a
           retrograde event?
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Yehoshua Kolodny , Avihu Burg , Yona I. Geller , Ludwik Halicz , Yevgeni Zakon
      For about four decades the Hatrurim Formation (Israel, the Palestinian Authority) and its equivalent in Jordan were considered to have formed between 16 and 3Ma ago by combustion of bituminous chalks, dehydration and decarbonation, resulting in two stages: a prograde country rock of High-T, Low-P (Portland Cement-like) assemblages, and a retrograde, closely time-related stage, of concrete-like vein fillings of rehydration and recarbonation. We now dated carbonate (calcite, aragonite, vaterite) containing veins in both spurrite marbles and gehlenite metamorphic rocks by 230Th–234U dating. The ages of most veins range between 250ka and 30ka, with only a minority being older than 450ka (the method limit). δ18O of the vein carbonates indicates water-δ18O as low as −14‰ (SMOW), much lighter than present-day rain in the area. These values are in accord with previous information on the area's paleo-rain derived from speleothem analysis and from fossil aquifers. Both δ13C (−18‰ to −5‰) and 87Sr/86Sr (0.70786–0.70811) ratios indicate interaction between rain-derived circulating solutions and country metamorphic rock-affected water. Thus vein formation is closer to a weathering process rather than to retrograde metamorphism. Textural relationships between vein-filling minerals suggest the possibility that they represent reactivation and replacement of a pre-existing phase by younger minerals. The veins, some of them enriched in Cr and Fe bearing minerals, indicate short-distance metal mobilization in the weathering process, as opposed to a large scale nearly closed system behavior in the prograde stage. The occurrence of high-pH waters in the Hatrurim outcrop of north Jordan suggests vein formation in an alkaline environment.
      Graphical abstract image

      PubDate: 2014-09-21T00:37:27Z
       
  • Revisiting the 142Nd deficits in the 1.48Ga Khariar alkaline rocks, India
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Antoine S.G. Roth , Erik E. Scherer , Colin Maden , Klaus Mezger , Bernard Bourdon
      The 146Sm–142Nd system plays a central role in tracing the silicate differentiation of the Earth prior to 4.1Ga. After this time, given its initial abundance, the 146Sm can be considered to be extinct. Upadhyay et al. (2009) reported unexpected negative 142Nd anomalies in 1.48Ga rocks of the Khariar nepheline syenite complex (India) and inferred that an early enriched, low-Sm/Nd reservoir must have contributed to the mantle source rocks of the Khariar complex. As 146Sm had been effectively extinct for about 2.6billionyears before the crystallisation of the Khariar samples, this Nd signature should have remained isolated from the convective mantle for at least that long. It was thus suggested that the source rock of Khariar samples had been sequestered in the lithospheric root of the Indian craton. Using a different chemical separation method, and a different Thermal Ionization Mass Spectrometry (TIMS) analysis protocol, the present study attempted to replicate these negative 142Nd anomalies, but none were found. To determine which data set is correct, we investigated three possible sources of bias between them: imperfect cancellation of Faraday collector efficiencies during multidynamic TIMS analysis, rapid sample fractionation between the sequential measurement of 146Nd/144Nd and 142Nd/144Nd, and non-exponential law behaviour resulting from so-called “domain mixing.” Incomplete cancellation of collector efficiencies was found unlikely to cause resolvable biases at the estimated level of variation among collector efficiencies. Even in the case of highly variable efficiency and resolvable biases, there is no reason to suspect that they would reproducibly affect only four rocks out of 10 analysed by Upadhyay et al. (2009). Although domain mixing may explain apparent “reverse” fractionation trends observed in some TIMS analyses, it cannot be the cause of the apparent negative anomalies in the study of Upadhyay et al. (2009). It was determined that rapid mass fractionation during the course of a multidynamic TIMS analysis can bias all measured Nd ratios. After applying an approximate correction for this effect, only one rock from Upadhyay et al. (2009) retained an apparent negative 142Nd anomaly. This, in conjunction with our new, anomaly-free data set measured at fractionation rates too low to cause bias, leads to the conclusion that the anomalies reported by Upadhyay et al. (2009) are a subtle and reproducible analytical artefact. The absence of negative 142Nd anomalies in these rocks relaxes the need for a mechanism (other than crust formation) that can isolate a Nd reservoir from the convective mantle for billions of years.


      PubDate: 2014-09-21T00:37:27Z
       
  • Uranium mobilization by fluids associated with Ca–Na metasomatism: A
           P–T–t record of fluid–rock interactions during
           Pan-African metamorphism (Western Zambian Copperbelt)
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Aurélien Eglinger , Alexandre Tarantola , Cyril Durand , Clément Ferraina , Olivier Vanderhaeghe , Anne-Sylvie André-Mayer , Jean-Louis Paquette , Etienne Deloule
      A dozen uranium occurrences are known in the Western Zambian Copperbelt (Domes region, Zambia). The host-rocks, Neoproterozoic kyanite±talc micaschists, have been interpreted as meta-evaporites that formed part of an epicontinental sequence affected by upper amphibolite metamorphism during the Pan-African orogeny. Ca–Na metasomatism, expressed by the presence of apatite, epidote and scapolite, is regionally extensive in the Katanga and the pre-Katanga rocks in the Domes region. The present study focuses on the Solwezi dome in the Western Zambian Copperbelt. Two distinct uranium mineralization events that took place here mark the transition from crustal-thickening by tectonic accretion to exhumation of the metamorphic host rocks. U–Pb dating of uraninite and Th–Pb dating of monazite show that the first U-mineralization event took place between ~550Ma and ~530Ma. The event is therefore contemporaneous with crustal thickening and, on the basis of fluid inclusion analyses, can be related to circulation of hypersaline Ca-rich metamorphic fluids trapped at minimum P–T conditions of 460MPa and 500°C. On a regional scale, the event is related to an epidote–apatite metasomatic assemblage. The second mineralization event, dated at between ~510 and ~500Ma by Th–Pb dating of monazite, is related to high-salinity Ca–Na-rich metamorphic fluids and occurred during exhumation of the high-grade metamorphic rocks. These fluids were trapped at the ductile/brittle transition at 150 to 260MPa and 200 to 300°C. A regional-scale scapolitization is attributed to these late fluid circulations. These fluid–rock interactions, described at the regional scale, are also characterized at the mineral scale. Alteration reactions, assisted by CaCl2-rich fluids, indicate that monazite and allanite hosted by the pre-Katanga rocks potentially acted as U sources for these Pan-African synmetamorphic uranium mineralizations.


      PubDate: 2014-09-21T00:37:27Z
       
  • New aspects of sulfur biogeochemistry during ore deposition from δ34S
           of elemental sulfur and organic sulfur from the Here's Your Chance
           Pb/Zn/Ag deposit
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Alex I. Holman , Kliti Grice , Paul F. Greenwood , Michael E. Böttcher , John L. Walshe , Katy A. Evans
      Sulfur isotope studies of base metal sulfide deposits have mostly focussed on sulfide minerals, but elemental sulfur and organic sulfur are also potentially significant components of the sulfur cycle during ore deposition. The δ 34S of elemental sulfur and organic sulfur isolated from the Paleoproterozoic Here's Your Chance (HYC) Pb/Zn/Ag deposit (McArthur Basin, northern Australia) were measured to be between +5 and +8‰, approximately 6 to 7‰ heavier than the median values of first-generation HYC sulfides. Elemental sulfur and organic sulfur are thought to have been formed contemporaneously with the first generation of metal sulfides. The δ 34S of organic sulfur showed an increasing trend along the path of the mineralising fluid, as sulfate was progressively 34S-enriched due to Rayleigh distillation. The δ 34S data support a model in which bacterial sulfate reduction produced dissolved sulfide with δ 34S of 0 to +5‰. The subsequent oxidation of sulfide produced reactive sulfur species such as polysulfide ions, which were then incorporated into organic matter.


      PubDate: 2014-09-21T00:37:27Z
       
  • Redistribution of Pb during transformation of monohydrocalcite to
           aragonite
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Takashi Munemoto , Keisuke Fukushi , Yoshiki Kanzaki , Takashi Murakami
      Monohydrocalcite was transformed to aragonite in the presence of Pb in solution to understand the Pb redistribution mechanisms and kinetics during the transformation. Monohydrocalcite (Ca0.98Mg0.02CO3·1.00H2O) was synthesized, and along with Pb stock solution, added to a buffer solution; batch sorption experiments were carried out (i) at different initial-Pb concentrations ranging from 1 to 100μM (1–30 and 50–100μM, referred to as low and high initial-Pb concentrations, respectively) for 24h and (ii) at different reaction time for 1–24h at 1 and 100μM of the initial Pb concentrations. The CO3 2− concentration (24.4meq/L), pH (9.50) and temperature (25°C) were kept constant within experimental errors throughout the experiments. The transformation of monohydrocalcite was completed in 24h, probably in 15h, at the low initial-Pb concentrations but not even in 24h at the high initial-Pb concentrations, suggesting that the aqueous Pb concentration affects the rates of dissolution of monohydrocalcite and/or nucleation and crystal growth of aragonite, and thus, the transformation rate. Aragonite was shortly formed as monohydrocalcite was dissolved and almost all amount of Ca dissolved from monohydrocalcite formed aragonite. ~90–96% of the initial Pb was adsorbed onto monohydrocalcite before the transformation started, and >95% of the initial Pb was distributed in solid phases at any elapsed time of the reaction and at any initial-Pb concentrations, indicating that the Pb redistribution rate was as fast as the transformation rate of monohydrocalcite. Pb carbonate (possibly hydrocerussite) was formed at the high initial-Pb concentrations but not at the low initial-Pb concentrations. The precipitation rate of the Pb carbonate is slower than the transformation rate of monohydrocalcite. The Pb sorption on aragonite occurred dominantly by incorporation. After the transformation, accordingly, almost all Pb was incorporated in aragonite at the low initial-Pb concentrations, and in aragonite and Pb carbonate at the high initial-Pb concentrations. The incorporation of Pb in aragonite is well explained by the formation of (Ca,Pb)CO3 solid-solution. The Pb redistribution during transformation of monohydrocalcite to aragonite revealed by the present study will give a deeper understanding of trace element transport at the Earth's surface when the transport is accompanied by mineral transformation.
      Graphical abstract image

      PubDate: 2014-09-21T00:37:27Z
       
  • Microbial dissolution and reduction of uranyl crystals by Shewanella
           oneidensis MR-1
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Yu Yang , Shuao Wang , Thomas E. Albrecht-Schmitt
      Dissimilatory metal-reducing bacteria (DMRB) can harvest energy for growth and activities by respiring metals, but it is so far unknown whether DMRB can acquire crystalline-phase actinides. In the present study, we used Shewanella oneidensis MR-1 to investigate microbially-mediated dissolution and reduction of U(VI) in three uranyl(VI) borate and boronate crystals (i.e., UO2(CH3BO2)(H2O) (UCBO); UO2B2O4 (UBO); and Na[(UO2)B6O10(OH)]·2H2O (NaUBO)). Comparison of the dissolved U(VI) concentrations between samples with and without bacteria indicates that MR-1 facilitated dissolution of UCBO and UBO. Based on the assumption that only dissolved U(VI) was reduced, U(VI) reduction was substantially underestimated for UCBO and NaUBO, indicating that MR-1 directly reduced crystalline U(VI) in these two compounds. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis implied that interactions occurred between microbial ligands and the residual particles of uranyl compounds. We found that S. oneidensis MR-1 can mediate the dissolution and reduction of crystalline U(VI) through facilitated dissolution and consequent reduction of crystals or direct reduction of U(VI) in crystals. These results help evaluate the environmental fate of solid-phase U(VI), critical for predicting U transport and remediating U-contaminated sites.


      PubDate: 2014-09-21T00:37:27Z
       
  • The curved 14C vs. δ13C relationship in dissolved inorganic carbon: A
           useful tool for groundwater age- and geochemical interpretations
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Liang-Feng Han , L. Niel Plummer , Pradeep Aggarwal
      Determination of the 14C content of dissolved inorganic carbon (DIC) is useful for dating of groundwater. However, in addition to radioactive decay, the 14C content in DIC (14CDIC) can be affected by many geochemical and physical processes and numerous models have been proposed to refine radiocarbon ages of DIC in groundwater systems. Changes in the δ13C content of DIC (δ13CDIC) often can be used to deduce the processes that affect the carbon isotopic composition of DIC and the 14C value during the chemical evolution of groundwater. This paper shows that a curved relationship of 14CDIC vs. δ13CDIC will be observed for groundwater systems if (1) the change in δ13C value in DIC is caused by a first-order or pseudo-first-order process, e.g. isotopic exchange between DIC and solid carbonate, (2) the reaction/process progresses with the ageing of the groundwater, i.e. with decay of 14C in DIC, and (3) the magnitude of the rate of change in δ13C of DIC is comparable with that of 14C decay. In this paper, we use a lumped parameter method to derive a model based on the curved relationship between 14CDIC and δ13CDIC. The derived model, if used for isotopic exchange between DIC and solid carbonate, is identical to that derived by Gonfiantini and Zuppi (2003). The curved relationship of 14CDIC vs. δ13CDIC can be applied to interpret the age of the DIC in groundwater. Results of age calculations using the method discussed in this paper are compared with those obtained by using other methods that calculate the age of DIC based on adjusted initial radiocarbon values for individual samples. This paper shows that in addition to groundwater age interpretation, the lumped parameter method presented here also provides a useful tool for geochemical interpretations, e.g. estimation of apparent rates of geochemical reactions and revealing the complexity of the geochemical environment.


      PubDate: 2014-09-21T00:37:27Z
       
  • Evidence for the control of the geochemistry of Amazonian floodplain
           sediments by stratification of suspended sediments in the Amazon
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Martin Roddaz , Jérôme Viers , Patricia Moreira-Turcq , Camille Blondel , Francis Sondag , Jean-Loup Guyot , Luciane Moreira
      Holocene and historical Amazonian floodplain deposits collected from two cores of the “Varzea do Curuai” flooded area (Brazil) were analysed for major and trace element geochemistry as well as Nd–Sr isotopic compositions (21 samples). The TA11 and TA14 cores (110cm and 270cm in depth, respectively) were collected at different locations in the Varzea, near a channel inlet connecting the Amazon River to the Varzea and at the centre of the Varzea, respectively. The two cores represent records of sedimentation on different time-scales, with TA11 covering the last 100years and TA14 extending back to 5600yearscal BP. Although the sediments are generally coarser in TA11 than in TA14, the major and trace element concentrations, Cr/Th and Th/Sc and Eu anomalies and Nd–Sr isotopic compositions in both cores fail to show any clear variations with depth. However, there are chemical differences between the two analysed cores. The TA14 sediments have higher Al/Si and Chemical Index of Alteration (CIA) values than those of TA11. The TA14 sediments are enriched in Th, U, Y, Nb, REE, Cs, Rb, V and Ni but show slightly depleted MgO, CaO and Sr and more strongly depleted Na2O, Zr and Hf compared to TA11. In addition, the Nd–Sr isotopic compositions of the TA11 sediment core are on the whole similar to the Solimões suspended particulate matter (SPM), whereas TA14 has a similar Nd–Sr isotopic composition compared with the SPM of the Amazon River at Obidos. These differences are best explained by chemical stratification of the SPM of the Amazon River. During flooding of the Amazon River, coarser grained particulates supplied by the Solimões River are deposited in the deepest environments near the channel inlet, as recorded in the TA11 sediment core. By contrast, finer grained suspended sediments derived from the Madeira River are transported into the shallower environments of the Varzea system and deposited as a result of flow expansion and loss of carrying power, as recorded in the TA14 sediment core. Using the Nd isotopic compositions and concentrations of each sediment core, we calculate between ~20 and ~85% of the sediment input to the TA14 core site is derived from the Madeira River, while between 52% and 100% of the sediment input to the TA11 core sample is supplied by the Solimões River. Similar calculation using the Sr isotopic compositions and concentrations of TA11 and TA14 sediments gave similar results with between ~32% and ~49% of TA14 sediments deriving from the Solimões River while between ~70% and ~100% of TA11 sediments originate from the Solimões River. Finally, our findings lead us to question whether particular Amazon deposits that do not integrate the entire range of SPM present in the river are representative of the overall sediment load provided today and in the past by global rivers.


      PubDate: 2014-09-21T00:37:27Z
       
  • Hydrothermal carbonate chimneys from a continental rift (Afar Rift):
           Mineralogy, geochemistry, and mode of formation
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): V.M. Dekov , N.M. Egueh , G.D. Kamenov , G. Bayon , S.V. Lalonde , M. Schmidt , V. Liebetrau , F. Munnik , Y. Fouquet , M. Tanimizu , M.O. Awaleh , I. Guirreh , B. Le Gall
      Carbonate chimney-like deposits up to 60m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting (Lake Abhé, Afar Rift, Djibouti). The chimneys formed sub-aqueously in the lake water body at a higher water level than observed today. Alternating calcite and low-Mg calcite+silica concentric layers compose the chimney structures. Mineralogical and geochemical investigations of the chimneys, lake water, and hot spring (hydrothermal) fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water. In contrast to the hot spring fluid, lake water is enriched in HREE and possesses a pronounced positive Ce anomaly, features that are preserved in the carbonate chimney layers. Mixing calculations based on Sr-isotope and concentration data indicate a hydrothermal fluid contribution of ~45% in the chimney interior, which decreases to ~4% in the external chimney layer. Sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks, whereas the lake's Sr budget is dominated by riverine input. Considering the fluid mixing ratios calculated by Sr-data, the measured C and O isotope compositions indicate that chimney carbonates precipitated at temperatures between 14°C (internal part) and 22°C (external part) with δ13C-carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric CO2. The low-Mg calcite layers, including the outermost layer, have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements, ΣREE, and Sr and Ca isotope compositions. Ca-isotope data reveal that internal chimney layers formed by non-equilibrium calcite precipitation with a predominantly hydrothermal Ca source. The external low-Mg calcite layer received Ca contributions from both hydrothermal fluid and lake water, with the latter being the dominant Ca source. Highly positive δ44/40Ca of lake water likely reflects non-equilibrium Ca-carbonate precipitation during lake water evaporation with resulting 44Ca enrichment of residual lake water. The strong degree of 44Ca enrichment may point towards multiple lake drying and Ca-reservoir depletion events. Coupled C–O–Ca-isotope data of the sampled carbonate chimney suggest late-stage (low-temperature) hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low-Mg calcite precipitation. U–Th age dating suggests that the chimneys formed no earlier than 0.82kyrBP (0.28±0.54).


      PubDate: 2014-09-21T00:37:27Z
       
  • Diagenesis of speleothems and its effect on the accuracy of 230Th/U-ages
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Denis Scholz , Jan Tolzmann , Dirk L. Hoffmann , Klaus P. Jochum , Christoph Spötl , Dana F.C. Riechelmann
      Speleothems can be precisely dated by U-series disequilibrium methods. One basic assumption of the 230Th/U-dating method is that the system remains closed and U and Th isotopes are neither lost nor added after deposition. For stalagmites, this requirement is usually fulfilled due to their protected environment in caves. However, undersaturated drip water may lead to diagenesis of speleothem CaCO3 and altered 230Th/U-ages. Here we present a detailed petrographic investigation using thin sections and 230Th/U-ages of a stalagmite from the Riesenberghöhle, northern Germany, which grew during Marine Isotope Stages 5e, c and a. The occurrence of a mosaic fabric in large parts of the stalagmite in combination with several age inversions is a clear indication for diagenetic alteration. We discuss the potential diagenetic processes and quantitatively model their influence on the 230Th/U ages. Post-depositional U loss leads to increasing (230Th/238U) activity ratios and, thus, apparently older ages. In contrast, post-depositional addition of U results in decreased (230Th/238U) activity ratios and apparently younger ages. Comparison with our modelling results suggests that diagenesis occurred early after deposition. Initial and post-depositional detrital contamination probably played a minor role for the observed age inversions.


      PubDate: 2014-09-21T00:37:27Z
       
  • Determination of in situ dissolved inorganic carbon concentration and
           alkalinity for marine sedimentary porewater
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Justine Sauvage , Arthur J. Spivack , Richard W. Murray , Steven D'Hondt
      Dissolved inorganic carbon (DIC) concentration and total alkalinity in marine sediment vary with biological activity, mineral diagenesis and past bottom ocean water composition. Reliable interpretation of this data is often compromised due to precipitation of calcium carbonate (CaCO3) during sediment recovery, processing and sample storage. Here we present and test a method that corrects for this precipitation and consequently allows quantification of in situ carbonate system chemistry. Our method relies on the over-determination of the dissolved carbonate system by (i) measuring DIC, alkalinity and calcium, and (ii) explicitly assuming CaCO3 saturation in the sediment. We experimentally tested this method using data from Integrated Ocean Drilling Program (IODP) Site U1368 in the South Pacific Gyre. Our results show that we can accurately reproduce in situ aqueous carbonate system chemistry if DIC, alkalinity and calcium concentration are measured simultaneously. At Site U1368, the correction for sampling associated precipitation is equivalent to 4.5 and 8.9% of the measured DIC and alkalinity, respectively. The method is well suited for any sediment porewater that is saturated with respect to calcium carbonate; consequently, it is applicable for approximately 50% of the global oceanic seafloor.


      PubDate: 2014-09-21T00:37:27Z
       
  • Constraining causes of fish mass mortality using ultra-high-resolution
           biomarker measurement
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Abby Othman Wilson , John Parnell , Adrian J. Boyce , Stephen A. Bowden
      Lamina by lamina measurement of biomarkers at a sub-millimetre resolution within the Achanarras Limestone Member has helped to resolve the changing environmental conditions associated with a fish mass mortality horizon. An anomalous proportion of C30 sterane (24-n-propylcholestane) marks the beginning of the horizon and likely corresponds to an influx of marine water. This appears to have been short lived and was likely analogous to a modern day storm tide. The subsequent laminae record an increased incidence of water column stratification and hypoxic bottom waters in the form of an elevated gammacerane index. The mass mortality horizon studied was from an upper interval of the Achanarras Limestone Member with a fossil fish assemblage comprising mostly Dipterus, an early Dipnoan (lungfish). However, lower intervals of the Achanarras Limestone Member have greater assemblage diversity, including species associated with marine conditions such as Coccosteus, and evidence higher proportions of C30 sterane indicating better connection to the marine environment. Therefore, it appears that ingressing seawater in and of itself was not responsible for creating a stressed environment. Rather, disconnection of the lake from marine waters stranded fish in a lake, that when perturbed by storm tides, killed en masse by exposing fish to hypoxic conditions in a similar way to modern water bodies affected by storm tides generated during hurricanes.


      PubDate: 2014-09-21T00:37:27Z
       
  • Key factors to understand in-situ behavior of Cs in
           Callovo–Oxfordian clay-rock (France)
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Z. Chen , G. Montavon , S. Ribet , Z. Guo , J.C. Robinet , K. David , C. Tournassat , B. Grambow , C. Landesman
      Understanding the behavior of 137Cs and 135Cs in soils and geological formations is of considerable interest in the context of nuclear accidents and nuclear waste repositories. Although the clay fraction is known to be responsible for sorption, there are still unanswered questions raised by the literature data concerning (i) the reversibility of the sorption process(es), (ii) the validity of the additivity rule (the overall distribution coefficient (Kd) for a radionuclide on a mixture of minerals is predicted from the distribution coefficients measured on individual minerals) and (iii) the validity of model transposition from dispersed systems to consolidated/intact systems. Because of these uncertainties, the validity of sorption models at equilibrium under in-situ conditions and for very long-term interaction is still pending. These different issues are studied in the present work for the Callovo–Oxfordian (COx) clay-rich rock Formation, which is under investigation in France as a geological barrier for a long-term nuclear waste repository. The work is based on sorption data measured on thirteen samples of different mineralogy taken from five different boreholes at several depths within the COx sedimentary layer. To our knowledge, it is the most extended Cs sorption dataset that has been published for a single clay formation in term of (i) sample locations (and thus natural variability), (ii) sorption conditions (powder dispersed in suspension, compacted powders and intact samples) and (iii) equilibration time (from one week to five years). Moreover, for the first time ever, radioactive Cs sorption results were compared to the natural distribution of non-radioactive Cs isotopes between pore water and the solid phase. The experimental system appeared to be in chemical equilibrium as much as can be expected for an ion-exchange reaction. More particularly, no kinetically-controlled process leading to partial Cs irreversibility was observed, in contrary to what was found in the literature for soils. This difference in behavior may be related to the difference in the illite studied, i.e. a soil-type illite which would be more altered than a sedimentary formation-type illite. No decrease in site capacity was observed between dispersed and intact/compacted states. A model based on exchange reactions with cations interacting with illite (frayed edge, type-II and planar sites) and mixed layer illite–smectite (I/S) (planar sites) using parameters published in the literature enabled the Kd variation to be described as a function of Cs concentration, the mineralogy of the samples, the change in water composition and the temperature (22–80°C). Our study clearly demonstrates that no frayed edge sites should be considered on the illite fraction of I/S, thus emphasizing the difference of sorption properties between an I/S mixed layer mineral and a corresponding mechanical mix of illite and smectite minerals. The robustness of the model was confirmed by data analysis describing the behavior of naturally-occurring Cs in the formation thereby demonstrating the effectiveness of the Cs sorption processes in a very long-time period prospective. Lastly, the model was used to predict the sorption of trace concentrations of Cs in the COx Formation on the time-scale relevant for nuclear waste disposal performance assessment. As expected, the retention was significant with Kd values ranging from 100 to 2000L/kg whatever the conditions that were probed and a simulation covering a period of over 105 years could show that the COx Formation is an efficient barrier to prevent Cs transport from the storage facility to the surrounding environment.
      Graphical abstract image

      PubDate: 2014-09-08T23:16:02Z
       
  • Non-equilibrium fractionation of stable carbon isotopes in chemosynthetic
           mussels
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): K. Nedoncelle , N. Le Bris , M. de Rafélis , N. Labourdette , F. Lartaud
      Chemosynthetic bivalves from deep-sea hydrothermal vents exploit the energy derived from chemical compounds, such as methane, sulfide or hydrogen, using symbiotic bacteria that are able to fix inorganic carbon. Available chemical resources in their habitat vary widely at various scales, from the vent field scale to the micro-habitat scale. Parallel to this environmental heterogeneity, Bathymodiolus species are considered to be flexible in their energy acquisition pathways. The goal of this study was to determine whether the isotopic compositions archived in the shells of hydrothermal vent mussels could trace chemical energy sources and their variability over spatial and temporal scales. Two different species (Bathymodiolus azoricus and Bathymodiolus thermophilus) inhabiting three vent fields with contrasted geochemical features on the Mid Atlantic Ridge (MAR; Rainbow and Menez Gwen) and the East Pacific Rise (EPR; 9°47′N), were considered for carbon isotopes and growth rate variation along the shell length. The study revealed that 13C fractionation between shells and seawater is higher than expected from calcite–bicarbonate equilibrium fractionation, suggesting a significant influence of the chemosynthetic pathway on the shell composition. Furthermore, significant differences in δ13Cshell fractionation with respect to seawater are observed between sites and habitats of the two MAR vent fields, suggesting that different chemosynthetic pathway (e.g. methanotrophic and thiotrophic) could lead to variable enrichments of the shell in 13C. Mussels supposed to rely more largely on methanotrophy (at Rainbow where free sulfide is unavailable) display a lower δ13Cshell values than mussels relying also on sulfide-oxidizing symbiosis (at Menez Gwen). Variability in δ13Cshell between habitats, or between individuals within the same assemblage, could thus reflect differences in the symbiosis activity at a micro-habitat scale. These isotopic signatures could provide useful information on the relationships between micro-habitat properties, symbiont activity and shell mineralization.


      PubDate: 2014-09-08T23:16:02Z
       
  • Formation of H2, CH4 and N-species during low-temperature experimental
           alteration of ultramafic rocks
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): I. Okland , S. Huang , I.H. Thorseth , R.B. Pedersen
      Hydrogen, methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities. It is well known that production of H2 through serpentinization of the primary Fe(II)-containing minerals, olivine and pyroxene, in ultramafic rocks and reduction of CO2 to CH4 are processes that occur at higher temperatures. Knowledge is, however, limited about these reactions under low-temperature conditions, and how they are affected by Fe(II)-containing secondary minerals, such as brucite and serpentine, which are commonly found in these environments. In this experimental study, we explored the formation of H2, CH4 and NH4 during low-temperature (25°C) reactions between deionised water and 1) unaltered, 2) medium altered and 3) highly altered dunites, over a period of 99days. Hydrogen was detected in all three experiments and the concentration increased over time. The O2 concentration decreased over time, but O2 was still present at significant levels when H2 started to form. This suggests that the H2 was formed locally at the surface of mineral grains rather than in the hypoxic solution. The highly altered dunite gave the highest and the unaltered dunite the lowest H2 concentrations, and while the O2 concentration decreased fast with the medium altered dunite, significant amounts of O2 still remained with the unaltered dunite at the end of the experiments. Both the medium and highly altered dunite gave O2 concentrations close to or below the detection limit during the last part of the experiment. Factors that likely affected the H2 production are Fe-content, mineralogy, dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite. Methane was also detected and increased with time in all experiments. The results indicate that CH4 formation also occurred locally on the surface of minerals. Detection of both NH4 and NO3 in the aqueous solutions indicates that previously absorbed N-species were leached out during the experiment. Increasing NH4 and decreasing NO3 concentrations over time suggest NO3 reduction, likely catalysed by mineral surfaces. The results imply that low-temperature water–rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy-yielding metabolic processes by subsurface microorganisms. Such reactions could play an important role in sustaining subsurface microbial communities, particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow-spreading mid-ocean ridges and possibly also in other rocks and sediments. The results suggest that brucite and possibly also serpentine may be more important sources of Fe(II) and thus H2 formation than olivine in low-temperature ultramafic systems. Formation of H2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of H2 formation and the subsurface habitat for aerobic H2-oxidizing microorganisms.


      PubDate: 2014-09-08T23:16:02Z
       
  • Iron–clay interactions: Detailed study of the mineralogical
           transformation of claystone with emphasis on the formation of iron-rich
           T–O phyllosilicates in a step-by-step cooling experiment from
           90°C to 40°C
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Isabella Pignatelli , Franck Bourdelle , Danièle Bartier , Régine Mosser-Ruck , Laurent Truche , Enrico Mugnaioli , Nicolas Michau
      Iron–minerals–water interactions are of primary importance in the contexts of underground structure engineering (e.g. reactive barriers or deep geological storage) and for the understanding of secondary alteration processes in primitive meteorites. To improve our understanding of these systems, we determine the mineralogical transformations induced by the association of iron and silicates during a cooling through an experimental simulation of iron–clay interactions with a step-by-step procedure in the range of 90°C to 40°C. The run products and solutions are well characterised, by means of different techniques (X-ray diffraction, scanning and transmission electron microscopy, manocalcimetry, inductively coupled plasma optical emission spectrometry and ion chromatography), and the thermodynamic data concerning Fe-bearing phyllosilicates are well-tested comparing the modelling and experimental results. Therefore, the main mineralogical modifications observed include the remarkable formation of cronstedtite and greenalite, as well as the formation of magnetite at all temperatures, along with a significant dissolution of quartz, mixed-layer illite–smectite clays, illite (affecting more than 70% of each mineralogical phase) and a partial alteration of chlorite, kaolinite and dolomite. The experimental results confirm the reaction path predicted by thermodynamic modelling, i.e. the formation of iron-rich T–O phyllosilicates (cronstedtite and greenalite) and magnetite at the expense of metal iron and silicates. Both the experimental and thermodynamic results presented in this study provide important constraints to well predict the impact of nuclear waste canister corrosion in a claystone media and to better understand secondary alteration processes, which could also affect the mineralogical and chemical composition of primitive meteorites.


      PubDate: 2014-09-08T23:16:02Z
       
  • Production of branched tetraether lipids in Tibetan hot springs: A
           possible linkage to nitrite reduction by thermotolerant or thermophilic
           bacteria?
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Fuyan Li , Chuanlun L. Zhang , Shang Wang , Yufei Chen , Chijun Sun , Hailiang Dong , Wenjun Li , Martin G. Klotz , Brian P. Hedlund
      Branched glycerol dialkyl glycerol tetraethers (bGDGTs) are produced by bacteria and originally identified from soils and peat bogs; recently, however, in situ production of bGDGTs in hot springs has been reported. Consequently, we designed this study to evaluate the linkage between the distribution of bGDGTs, their biological sources and inferred metabolic processes based on the distribution and abundance of bGDGTs, pertinent water chemistry, the nirS gene (possibly in Beta- and Gamma-proteobacteria) and available 16S rRNA (tag) gene pyrosequencing data from 37 Tibetan hot springs. The absolute and relative concentrations of intact polar and core bGDGTs suggest that bGDGTs are predominantly produced in situ in these hot springs. Redundancy analyses revealed correlations between the distribution of bGDGTs and concentrations of ammonium, nitrite, and nitrate and the abundance of nirS gene, which are better reflected in the core bGDGT fraction than in the respective intact polar bGDGT fraction. Reanalysis of published bacterial 16S rRNA gene sequences showed that residence of members of the bacterial phyla Proteobacteria and Bacteroidetes correlated positively with a new methylation index (R(III+II)/I) of bGDGTs. Some representatives of these taxa examined in this study are capable of nirS-encoded nitrite reduction, suggesting that bGDGT-synthesizing bacteria might be affiliated with these two phyla in Tibetan hot springs.


      PubDate: 2014-09-04T23:06:07Z
       
  • Platinum-group element geochemistry of komatiite-derived 3.1Ga
           ultramafic–mafic rocks and chromitites from the Nuggihalli
           greenstone belt, Western Dharwar craton (India)
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Ria Mukherjee , Sisir K. Mondal , Hong Zhong , Zhong-Jie Bai , Vysetti Balaram , G.R. Ravindra Kumar
      The 3.1Ga Nuggihalli greenstone belt in the Western Dharwar craton (southern India) comprises a sill-like layered ultramafic–mafic igneous complex with associated metasedimentary and metavolcanic (komatiitic to komatiitic basalt) schists that are enclosed by the tonalite–trondhjemite granodiorite suite of rocks (TTG). The sill-like layered complex is represented by a succession of chromitite-bearing serpentinite (after dunite) and peridotite, anorthosite, pyroxenite, and gabbro hosting magnetite bands. Extensive bulk-rock trace element and platinum-group element (PGE) geochemical study of the plutonic sill-like layered complex and the metavolcanic schists, suggest immobility of most trace elements (except La and Cu) and the PGEs, despite greenschist facies metamorphism and hydrothermal alteration experienced by the rocks. Their immobile nature is understood from good correlation of the trace elements and PGE with MgO and Cr. Other than chromitites and serpentinites all plutonic rocks show PPGE (Pd, Pt, Rh) enriched primitive-mantle normalized PGE patterns (Pd/IrN =3.9–81.1) that are suggestive of fractionation of IPGEs (Ir, Os, Ru) by the early crystallizing chromite mineral, and the incompatible nature of PPGEs in the same. The chromitites show high PGE abundances (∑PGE=96–296ppb), especially IPGEs (∑IPGE=63–223ppb), due to the presence of inclusions of IPGE-bearing minerals. In the primitive-mantle normalized PGE plot the chromitites show an IPGE enriched pattern. The PPGE enriched pattern (Pd/IrN =7.7–26) of the komatiitic to komatiitic basalt schists in a primitive-mantle normalized PGE plot indicates retention of IPGEs in the mantle or IPGE-bearing alloy saturation in the melt, while incompatible behavior of the PPGEs implies the sulfide undersaturated nature of the mantle source. The PGE pattern of the metavolcanic schists resembles the pattern of early Archean (3.5Ga) Barberton komatiites (Pd/IrN Barberton =1–40.7; Pd/IrN Nuggihalli =6.3–21.3), which corroborates our previous results based on REE study, and also resembles the pattern of komatiites from the 2.9Ga Sandstone greenstone belt in the Youanmi Terrane of Western Australia (Pd/IrN Sandstone =6). The metavolcanic schists exhibit the typical PGE depleted character observed in early Archean komatiites (∑PGEschist =0.4–27.2ppb; ∑PGEBarberton =15.0–20.8ppb; ∑PGEYouanmi Terrane, Western Australia =4.2–7.0ppb) which is explained to be a result of progressive mixing of late veneer matter in the Earth's mantle with time. Pt fractionation in the Nuggihalli metavolcanic schists and in early or late Archean komatiites indicates Pt alloy dispersal in the lower mantle during crystallization of the primary magma ocean and a consequent formation of Pt-enriched and Pt-depleted isolated upper mantle domains that did not homogenize and mix away by 2.7Ga. In the plutonic layered sequence, pyroxenite represents a change from sulfide-undersaturation to sulfide-saturation. The pyroxenite represents a break in trend from the negative correlation of Pt and Pd with MgO displayed by the serpentinites and peridotites due to incompatible behavior of the PPGEs during lava differentiation, to the positive pattern displayed by the gabbro and metavolcanic schists due to attainment of sulfide saturation. Sulfide-saturation was probably triggered by fractional crystallization of olivine, chromite and pyroxenes. Chondrite-normalized REE patterns and a plot of incompatible elements negate the role of crustal contamination of the parental komatiitic magma. In addition, the absence of ambient sulfidic sediments rules out assimilation of crustal sulfur in the Nuggihalli rocks. The immiscible sulfides segregated from the Al-depleted komatiitic parental magma concentrating the PGEs during crystallization of the pyroxenes that accumulated to form pyroxenite.


      PubDate: 2014-09-04T23:06:07Z
       
  • Hydrogen enhances the stability of glutamic acid in hydrothermal
           environments
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Namhey Lee , Dionysis I. Foustoukos , Dimitri A. Sverjensky , Robert M. Hazen , George D. Cody
      The potential for chemical evolution of complex organic molecules such as peptides in hydrothermal environments requires the persistence of the component amino acids under such conditions. Here, we show experimentally that the redox state (activity of H2) of the aqueous fluids plays a key role in the stability of glutamic acid during hydrothermal processes. The results demonstrate that highly reducing redox conditions imposed by elevated concentrations of dissolved H2 suppresses the oxidative decomposition of glutamic acid at elevated temperatures. Our experimental data support proposals that amino acids may persist, albeit metastably, under geochemically relevant hydrothermal conditions. The reduced nature of deep-sea vent fluids might have been a critical parameter in sustaining the needed ingredients for the origin of life on the early Earth, and may currently play a role in facilitating the persistence of biomolecules supporting heterotrophic microbial communities in modern near-seafloor hydrothermal environments.


      PubDate: 2014-09-04T23:06:07Z
       
  • Subduction-related enrichment of the Neapolitan volcanoes (Southern Italy)
           mantle source: New constraints on the characteristics of the slab-derived
           components
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): F.C. Mazzeo , M. D'Antonio , I. Arienzo , M. Aulinas , V. Di Renzo , D. Gimeno
      The Neapolitan volcanic area (Southern Italy), which includes the Phlegrean Volcanic District and the Somma–Vesuvius complex, has been the site of intense Plio-Quaternary magmatic activity and has produced volcanic rocks with a subduction-related geochemical and isotopic signature. High-Mg, K-basaltic lithic lava fragments dispersed within hydromagmatic tuff of the Solchiaro eruption (Procida Island) provide constraints on the nature and role of both the mantle source prior to enrichment and the subduction-related components. The geochemical data (Nb/Yb, Nb/Y, Zr/Hf) indicate a pre-enrichment source similar to that of enriched MORB mantle. In order to constrain the characteristics of subducted slab-derived components added to this mantle sector, new geochemical and Sr–Nd-isotopic data have been acquired on meta-sediments and pillow lavas from Timpa delle Murge ophiolites. These represent fragments of Tethyan oceanic crust (basalts and sediments) obducted during the Apennine orogeny, and may be similar to sediments subducted during the closure of the Tethys Ocean. Based on trace element compositions (e.g., Th/Nd, Nb/Th, Yb/Th and Ba/Th) and Nd-isotopic ratio, we hypothesize the addition of several distinct subducted slab-derived components to the mantle wedge: partial melts from shales and limestones, and aqueous fluids from shales, but the most important contribution is provided by melts from pelitic sediments. Also, trace elements and Sr–Nd-isotopic ratios seem to rule out a significant role for altered oceanic crust. Modeling based on variations of trace elements and isotopic ratios indicates that the pre-subduction mantle source of the Phlegrean Volcanic District and Somma–Vesuvius was enriched by 2–4% of subducted slab-derived components. This enrichment event might have stabilized amphibole and/or phlogopite in the mantle source. 6% degree of partial melting of a phlogopite-bearing enriched source, occurring initially in the garnet stability field and then in the spinel stability field can generate a melt with trace elements and Sr–Nd-isotopic features matching those of high-Mg, K-basalts of Procida Island. Furthermore, 2% partial melting of the same enriched source can reproduce the trace elements and isotopic features of the most primitive magmas of Somma–Vesuvius, subsequently modified by assimilation of continental crust during fractional crystallization processes at mid-lower depth. Combined trace element and Sr–Nd isotope modeling constrains the age of the enrichment event to 45Ma ago, suggesting that the Plio-Quaternary magmatism of the Neapolitan area is post-orogenic, and related to the subduction of oceanic crust belonging to the Tethys Ocean.


      PubDate: 2014-09-04T23:06:07Z
       
  • Boron isotopic geochemistry of the McMurdo Dry Valley lakes, Antarctica
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Deborah Leslie , W. Berry Lyons , Nathaniel Warner , Avner Vengosh , John Olesik , Kathleen Welch , Kelly Deuerling
      The geochemistry of boron was investigated in the ice-covered lakes and glacier meltwater streams within Taylor and Wright Valley of the McMurdo Dry Valleys (MCM), Antarctica, in order to achieve a greater understanding of the source of boron to these aquatic systems and how in-lake processes control boron concentration. Selected lake depths (surface and bottom water) and streams were analyzed for boron geochemistry. Boron stable isotope values in these waters span the range of +12.3‰ to +51.4‰, which corresponds to the variations from glacier meltwater streams to the hypolimnion of a highly evaporated hypersaline lake. The data demonstrate that the major sources of B to the aquatic system are via terrestrial chemical weathering of aluminosilicates within the stream channels, and a marine source, either currently being introduced by marine-derived aerosols or in the form of ancient seawater. Lakes Fryxell, Hoare, and upper waters of Lake Joyce, which experience more terrestrial influence of aluminosilicate chemical weathering via glacial meltwater streams, display a mixture of these two major sources, while the source of B in the bottom waters of Lake Joyce appears to be primarily of marine origin. Lakes Bonney and Vanda and the Blood Falls brine have a marine-like source whose δ11B values have become more positive by mineral precipitation and/or adsorption. Don Juan Pond displays a terrestrial aluminosilicate influence of a marine-like source. These hypersaline lake waters from Antarctica are similar in δ11B to other hypersaline lake waters globally, suggesting that similar processes control their B geochemistry.


      PubDate: 2014-09-04T23:06:07Z
       
  • Biosorption mechanisms of Cu(II) by extracellular polymeric substances
           from Bacillus subtilis
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Linchuan Fang , Shanshan Yang , Qiaoyun Huang , Aifang Xue , Peng Cai
      Biosorption mechanisms of Cu(II) by extracellular polymeric substances (EPS) from Bacillus subtilis were investigated using a combination of batch experiments, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, isothermal titration calorimetry (ITC), and X-ray absorption fine structure (XAFS) spectroscopy. A three discrete site non-electrostatic model fit the potentiometric titration data best, with the pK a values of 4.12±0.12, 6.60±0.06, and 9.09±0.03, and site concentrations of 4.81±0.62×10−3, 2.16±0.14×10−3, and 2.87±0.21×10−3 mol per gram dry mass of EPS, respectively. The ATR–FTIR results confirmed the presence of the functional groups with above pK a values within the EPS molecules, and indicated that Cu(II) binding onto the EPS involves either phosphoryl or carboxyl sites, or both. The calculated enthalpies and entropies of Cu(II) adsorption onto EPS suggest that Cu(II) binds with anionic oxygen-bearing ligands and forms inner-sphere complexes with the EPS functional groups. The XAFS results were consistent with inner-sphere binding of Cu(II) by carboxyl sites with 2.03 C atoms at distance of 2.96Å in the second shell, which further suggests that the carboxyl groups are the dominant sites for Cu(II) adsorption by EPS at pH5.0, and that a five-membered chelate ring structure is the most likely binding environment for Cu(II) bound to EPS. The molecular binding mechanisms obtained in this study will add fundamental knowledge of understanding the fate of heavy metals in natural environments.


      PubDate: 2014-09-04T23:06:07Z
       
  • Silicon isotopes and trace elements in chert record early Archean basin
           evolution
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Sonja Geilert , Pieter Z. Vroon , Manfred J. van Bergen
      Silicon isotopes of chemical sediments have received growing attention, given their applicability in the search for properties of ancient seawater. An important target is the reconstruction of secular changes in surface temperature of the Precambrian Earth, but interpretations are problematic since controls of the isotopic signals are potentially manifold. Here we report the existence of significant silicon isotope variability in chemically precipitated chert layers covering a continuous stratigraphic section across the ~3.42Ga Buck Reef Chert (BRC) in the Barberton Greenstone Belt (South Africa). Black chert bands are variably but consistently higher in δ30Si (up to ca. 0.9‰) than co-existing translucent counterparts at the same stratigraphic level, which is interpreted as a primary feature acquired during deposition upon interaction between submarine discharging hydrothermal water and a stratified water body. Averaged δ30Si values range from +0.3‰ (maximum +0.5‰) for shallow-marine sediments in the lower part of the section to −0.5‰ (minimum −1.3‰) for deeper water deposits in the upper part when results from translucent and black cherts are taken together. An average shift of ca. 0.8‰ (maximum 1.8‰) thus accompanied the formation of chemical cherts in a single Archean basin. Cherts at the base of the section represent silicified felsic volcanics and volcaniclastics and have exclusively positive δ30Si values between +0.1‰ and +0.7‰. Rare-earth-elements-and-yttrium patterns confirm the marine origin of the cherts and support the presumed basin evolution. From the combined evidence we infer that the δ30Si variations in the BRC reflect changes in the predominant origin of the silica, with terrigenous input supplying positive δ30Si to shallow waters and seafloor hydrothermal sources negative δ30Si to deeper levels. Our findings demonstrate the viability of silicon isotopes in cherts for reconstructing the evolution of ancient marine basins.


      PubDate: 2014-09-04T23:06:07Z
       
  • Comparative geochemistry of rhenium in oxidized arc magmas and MORB and
           rhenium partitioning during magmatic differentiation
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Yuan Li
      The geochemical behavior of Re in oxidized arc magmas (Eastern Manus basin) and MORB has been reexamined. In the early differentiation stage of oxidized arc magmas, Yb, Cu, Au, Ag, and Re have a bulk partition coefficient (D) between the crystallized mineral assemblage (olivine, clinopyroxene, and plagioclase) and the residual melt on the order of 0.33=DYb ~DAu >DRe >DAg >DCu, while in the late stage they have a bulk D between the crystallized mineral assemblage (clinopyroxene, plagioclase, magnetite, and monosulfide solid solution — MSS) and the residual melt on the order DRe >DCu >DAu ≥1>DYb =0.29>DAg. In oxidized arc magmas, Cu, Au, and Ag are primarily controlled by crystalline MSS, whereas Re is primarily controlled by magnetite and it has a MSS/silicate melt partition coefficient significantly lower than 20. Unlike oxidized arc magmas, most MORB are saturated with sulfide liquid. Based on the positive correlation of log (DCu /DRe) and ∆FMQ regardless of fS2, log (DCu /DRe)=0.997ΔFMQ+0.181 (R2 =0.99) (D=partition coefficient between sulfide liquid and silicate melt; FMQ=fayalite–magnetite–quartz oxygen buffer), the partition coefficient of Re between sulfide liquid and silicate melt for MORB is estimated to be in the range of 600–10,000 at fO2 of FMQ to FMQ−1, increasing with decreasing fO2. These partitioning data constrain that during MORB differentiation Re may be as compatible as Cu or Ag at relatively oxidized conditions (FMQ−0.5< fO2 <FMQ) or as Au at relatively reduced conditions (FMQ−1< fO2 <FMQ−0.5), and its concentration in MORB maybe expected to decrease with decreasing MgO in a similar manner as Cu, Ag, or Au. Yb is an incompatible element during MORB differentiation. Consequently, during MORB differentiation Yb/Re ratio may be expected to increase with decreasing MgO. However, the previously observed nearly constant Yb/Re ratio in a suite of MORB indicates similar incompatibility of Yb and Re and thus a minor role of sulfide liquid in controlling the geochemical behavior of Re during MORB differentiation. More work is required to resolve this paradox.


      PubDate: 2014-09-04T23:06:07Z
       
  • Trace metal distribution in the Atlantis II Deep (Red Sea) sediments
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Tea E. Laurila , Mark D. Hannington , Sven Petersen , Dieter Garbe-Schönberg
      The Atlantis II Deep is one of the only locations on the modern seafloor where active formation of a brine pool-type stratiform ore deposit can be studied. The presence of the brine pool causes retention of the hydrothermally released metals within the brine covered area, resulting in the accumulation of 90Mt of low-grade metalliferous sediment (2.06% Zn, 0.46% Cu, 41g/t Ag, and 0.5g/t Au; Guney et al., 1988). Almost all metals are derived from hydrothermal input, but some are also derived from seawater (e.g., Mo), pelagic phytoplankton (Ni) and detrital input (Cr). The hydrothermal fluid that is vented into the pool is rich in metals but relatively low in reduced sulfur compared to open ocean black smokers. Metals are deposited as sulfides from the cooling hydrothermal fluid but also by adsorption onto non-sulfidic “surface-active” particles (Si–Fe-OOH) in the brine pool. An unexpected increase in the Cu/Zn ratio of the sediments with distance from the vent source(s) may reflect pulses of higher-temperature venting and increased Cu fluxes to the brine pool, which are recorded as higher Cu/Zn ratios in the distal sediments or, alternatively, more efficient adsorption of Cu to Fe-OOH particles in the distal brine. During early diagenesis (a few thousand years) metals that are loosely bound to surface-active particles in the sediment apparently react with H2S to form sulfides. Proximal to the inferred vents, the ambient pore water is highly concentrated in trace metals such as Cd, Ag and Hg that are incorporated in diagenetic sulfides, including chalcopyrite and sphalerite. At greater distance from the vents, trace metals such as Mo, As, and Ga are taken up by framboidal pyrite. High concentrations of Au (up to 3ppm) are found in both proximal and distal metalliferous sediments, indicating that both primary deposition with sulfides and adsorption by diagenetic pyrite are important depositional processes. Some of the inferred pathways for metal precipitation in the Atlantis II Deep sediments, especially adsorption onto surface-active particles and subsequent incorporation in sulfides during diagenesis, may have been important unrecognized processes for metal accumulation in ancient stratiform ore deposits thought to have formed in brine pools.
      Graphical abstract image

      PubDate: 2014-09-04T23:06:07Z
       
  • Novel heating/cooling stage designed for fluid inclusion microthermometry
           of large stalagmite sections
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Yves Krüger , Beat Hiltbrunner , Andres Luder , Dominik Fleitmann , Martin Frenz
      Liquid–vapour homogenisation temperatures of fluid inclusions in stalagmites are used for quantitative temperature reconstructions in paleoclimate research. Specifically for this application, we have developed a novel heating/cooling stage that can be operated with large stalagmite sections of up to 17×35mm2 to simplify and improve the chronological reconstruction of paleotemperature time-series. The stage is designed for use of an oil immersion objective and a high-NA condenser front lens to obtain high-resolution images for bubble radius measurements. The temperature accuracy of the stage is better than ±0.1°C with a precision (reproducibility) of ±0.02°C.


      PubDate: 2014-09-04T23:06:07Z
       
  • Formation of the Troodos Ophiolite at a triple junction: Evidence from
           trace elements in volcanic glass
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): M. Regelous , K.M. Haase , S. Freund , M. Keith , C.G. Weinzierl , C. Beier , P.A. Brandl , T. Endres , H. Schmidt
      Fresh volcanic glasses from the extrusive section of the Troodos Ophiolite in Akaki Canyon are tholeiitic and basaltic to dacitic in composition. Compared to normal MORB they have extremely low fractionation corrected Na8, Fe8 and Ti8 and are enriched in fluid-mobile trace elements, including U, Ba, Rb, Sr and Pb, relative to non-fluid mobile elements of similar incompatibility. Trace element compositions of Akaki lavas define an array extending between ‘back-arc lava’-like compositions, and the field defined by Troodos boninites from the upper part of the lava sequence. Troodos lavas were derived from a mantle source that underwent early melt depletion, and later enrichment by both fluids and small degree melts. These processes can explain the unusual negative correlation of Pb/Ce with Zr/Nb and Ba/Nb in Troodos extrusives. Although some Troodos lavas are similar in composition to lavas from back-arc spreading centres, the boninites from the upper parts of the lava pile do not appear to have exact compositional equivalents among lavas from fore-arcs, back-arcs or other tectonic settings where similar rocktypes have been recovered. We suggest that the geochemical evolution inferred for the mantle source of Troodos lavas, together with geological evidence is most consistent with an origin for the Troodos Ophiolite at a spreading centre close to a ridge–trench–trench, or ridge–trench–transform triple junction, where highly depleted, subduction-modified, fluid-enriched mantle wedge material was able to upwell and decompress to shallow depths in a ‘fore-arc’ location. In such a tectonic setting, arc volcanism is captured by the spreading centre, explaining the lack of evidence for subaerial arc magmatism in Troodos. Rapid lateral migration of the triple junction could account for the similar ages of other Tethyan supra-subduction zone ophiolites.


      PubDate: 2014-09-04T23:06:07Z
       
  • Mantle dynamics and secular variations beneath the East African Rift:
           Insights from peridotite xenoliths (Mega, Ethiopia)
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Gianluca Bianchini , Julia G. Bryce , Janne Blichert-Toft , Luigi Beccaluva , Claudio Natali
      Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break-up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to a manifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate. We present new Hf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (εHf up to +1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/Hf ratio and ultimately leading to extremely high 176Hf/177Hf. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and 176Hf/177Hf indicates apparent ingrowth at ca. 1.9Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths from the same site indicating melt extraction between 2.4 and 2.8Ga (Reisberg et al., 2004; Chem. Geol. 208, 119–140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by pre-existing tectonic structures, far from the influence of plumes originating deep in the convecting mantle.


      PubDate: 2014-09-04T23:06:07Z
       
  • Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence
           from HP/UHP metasedimentary rocks, Italian Alps
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Jennie Cook-Kollars , Gray E. Bebout , Nathan C. Collins , Samuel Angiboust , Philippe Agard
      Metamorphosed Jurassic oceanic sediment in the Italian Alps experienced peak P–T conditions (1.5–3.0GPa; 330–550°C) similar to those experienced by sediments subducting through forearcs in most modern subduction zones. Integrated field, petrologic, and geochemical study of the devolatilization history of these rocks provides evidence regarding the extents of loss and mobility of oxidized and reduced C during subduction of sediments to depths beneath volcanic fronts, thus constraining models of the release of initially subducted C into the atmosphere via arc volcanism or contributing to the C budget of the deeper mantle. In this suite, occurrences of lawsonite and grossular-rich garnet in higher grade calcschists indicate some decarbonation reaction along a P–T gradient of ~8°C/km. Across grade, carbonates largely retain δ13CVPDB values typical of marine carbonates (mostly ranging from −1.5 to +1.5‰), with some shifts to lower values in low-carbonate samples possibly in part reflecting decarbonation but likely largely due to isotopic exchange with abundant reduced C. Carbonaceous matter in some carbonate-poor samples shows increase in δ13C at higher grades, from values of −25 to −22‰ typical of marine organic matter, to values as high as −10‰, consistent with the expected effects of devolatilization involving release of varying fractions of the original reduced C into fluids as CH4. In more carbonate-rich samples, shifts in the carbonaceous matter to higher values, by up to 15‰, reflect varying degrees of exchange with the carbonate in these samples. Carbonate across grade has δ18OVSMOW of +17 to +22‰ significantly lower than values typical of marine carbonates (the latter +28 to +30‰), likely in part reflecting exchange with silicate phases. Such shifts in the more carbonate-rich samples cannot be explained by closed-system exchange with silicates and require exchange with an external reservoir. The observed extents of decarbonation in these carbonate-bearing sediments subducted to depths of up to 90–100km are consistent with limited open-system behavior, hence intermediate between models of Rayleigh-like CO2 loss and recent models supporting open-system flushing of carbonate-rich rocks by H2O-rich fluid from underlying subducting oceanic lithosphere. Based on a separate study of devolatilization history in the metapelites intercalated with the metacarbonates, and consideration of the O isotope compositions of the two lithologies, the metapelites appear to be sources for the H2O-rich fluid that drove decarbonation reactions and shifted the δ18O values of the carbonates to their present values. Calculated decarbonation histories of the calcschists, using the Perple_X software, predict significant release of CO2 related to the breakdown of CO2-bearing phases (dolomite, aragonite) and the growth of Ca-rich silicate phases (lawsonite, grossular-rich garnet). This decarbonation occurred at very low XCO2 (<0.01), for the P–T range considered here, perhaps attained during pulses of infiltration by H2O-rich fluids from the nearby metapelites. These results demonstrate that a large fraction of the initially subducted oxidized and reduced C in pelitic–carbonate sediment can be retained during forearc metamorphism and thus be available for loss beneath arcs or addition to the deeper mantle. In forearcs, the C inventory in these sediments experience minimal isotopic shift due to devolatilization, and isotopic reequilibration of the oxidized and reduced reservoirs during heating likely results in little net change in the bulk δ13C of the subducting section. Further attention should be paid to understanding the extent of C release, and possible isotope fractionation, during decarbonation, partial melting, and carbonate dissolution at the 80–100km depths over which subducting slabs experience greater heating by exposure to the convecting mantle wedge.


      PubDate: 2014-09-04T23:06:07Z
       
  • Determination of boron isotope compositions of geological materials by
           laser ablation MC-ICP-MS using newly designed high sensitivity skimmer and
           sample cones
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Lin Lin , Zhaochu Hu , Lu Yang , Wen Zhang , Yongsheng Liu , Shan Gao , Shenghong Hu
      The effects of addition of nitrogen gas with use of three different combinations of sample and skimmer cones on the performance of LA-MC-ICP-MS for in situ B isotope ratio measurements were investigated in detail. Compared to the standard arrangement (H skimmer cone+standard sample cone), sensitivities of B isotopes were improved by a factor of 2.4 and 3.8 with use of X skimmer cone+standard sampler cone and the X skimmer cone+Jet sample cone, respectively. The best within-run precision (2s) of 11B/10B was approximately 50ppm for B4 reference material (B content=31,400μg/g) with use of X skimmer cone+Jet sample cone. It was found that different cone combinations had significant effects on the instrument mass bias, but had little effect on the precisions of B isotope ratios. Addition of nitrogen (2–4ml/min) in the central channel gas did not improve the sensitivity of B. However, significant wider ion axial distribution profile and more stable mass-bias for B isotope ratios were evident. Under selected optimum conditions, good agreements between the measured δ11B values and the reference values were obtained for the international reference minerals (B4, Dravite, Elbaite, IMMRB1 and Schorl), the NIST SRM 610–612 synthetic soda-lime glasses and the MPI-DING glasses (GOR-128-G, GOR-132-G, and StHs6/80). It was found that for low-B containing (11ppm and 31ppm) materials, the precisions (2s) of 11B/10B can be improved by a factor of 2–10 by using the newly designed X-skimmer cone and Jet sample cone, compared to that of using the standard cones. The international reference mineral Danburite (δ11B of −14.92±6.77 2SD, n=32) showed a significant non-uniformity of B isotope ratio distribution. Our first boron isotopic results from standards GSD-1G (δ11B: 11.92±1.03, n=30), GSE-1G (δ11B: 1.63±0.32, n=30), GP-4 (δ11B: −5.85±1.01, n=40) and DD-1 (δ11B: −13.21±0.56, n=35) showed these materials were fairly homogeneous in boron isotope ratios, making them as good candidates as reference materials. The proposed LA-MC-ICPMS method is suitable for the direct determination of B isotope ratio in a variety of geological materials.


      PubDate: 2014-09-04T23:06:07Z
       
  • The gypsum–anhydrite paradox revisited
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): M. Ossorio , A.E.S. Van Driessche , P. Pérez , J.M. García-Ruiz
      Despite much experimentation precipitation of anhydrite from solution in conditions similar to those occurring in sedimentary environments has not yet been reproduced in the laboratory. To resolve this long-standing contradiction we have monitored the precipitation and stability behaviors of calcium sulfate during experiments lasting up to two years. Calcium sulfate was precipitated from solution between 40 and 120°C at three different salinities and the formed solid phase was sampled at different time intervals (from 2min up to 2yr). We found that below 80°C gypsum is the sole primary phase and in the range of 80 to 120°C gypsum and bassanite are the primary phases. The stability of the latter increased with increasing salinity. As expected, we did not observe primary anhydrite precipitation, but over time phase transition occurred and anhydrite eventually appeared at temperatures >80°C. We show that intrinsic thermodynamic and kinetic properties severely constrain the precipitation of anhydrite (compared to gypsum and bassanite), and consequently, a considerable amount of time (e.g. >2yr at 60°C) is needed for anhydrite to form. Even so, at a geological time-scale, anhydrite can be considered as a pseudo-primary phase thus resolving the long-standing paradox of our inability to directly precipitate anhydrite in the laboratory at temperatures below 120°C and the abundant presence of anhydrite in evaporitic environments. Our results also show that at low water activity, bassanite becomes an important phase, which could be relevant to explain its presence on the surface of Mars.


      PubDate: 2014-09-04T23:06:07Z
       
  • Ferrihydrite precipitation in groundwater-fed river systems (Nete and
           Demer river basins, Belgium): Insights from a combined
           Fe-Zn-Sr-Nd-Pb-isotope study
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): V.M. Dekov , E. Vanlierde , K. Billström , C.-D. Garbe-Schönberg , D.J. Weiss , G. Gatto Rotondo , K. Van Meel , E. Kuzmann , D. Fortin , L. Darchuk , R. Van Grieken
      Two groundwater-fed river systems (Nete and Demer, Belgium) carry red suspended material that settles on the river bed forming red sediments. The local aquifer that feeds these river systems is a glauconite-rich sand, which provides most of the dissolved Fe to the rivers. The solid component of these systems, i.e., the red suspended material and sediments, has a simple mineralogy (predominantly ferrihydrite), but shows a complex geochemistry pointing out the different processes contributing to the river chemistry: (1) the red sediments have higher transition metal (excluding Cu) and detrital element (e.g., Si, Al, K, Rb, etc.) concentrations than the red suspended matter because of their longer residence time in the river and higher contribution of the background (aquifer) component, respectively; (2) the red suspended material and sediments have inherited their rare earth element (REE) patterns from the aquifer; (3) the origin of Sr present in the red suspended matter and red sediments is predominantly marine (i.e., Quaternary calcareous rocks), but a small amount is geogenic (i.e., from detrital rocks); (4) Pb in both solids originates mostly from anthropogenic and geogenic sources; (5) all of the anthropogenic Pb in the red suspended material and sediments is hosted by the ferrihydrite; (6) Nd budget of the red riverine samples is controlled by the geogenic source and shows little anthropogenic component; (7) the significant Fe- and Zn-isotope fractionations are in line with the previous studies. Their fractionation patterns do not correlate, suggesting that the processes controlling the isotope geochemistry of Fe and Zn are different: oxidation/reduction most likely governs the Fe-isotope fractionation, whereas adsorption/desorption or admixing of anthropogenic sources controls the isotope fractionation of Zn.


      PubDate: 2014-09-04T23:06:07Z
       
  • The role of bacterial extracellular polymeric substances in
           geomicrobiology
    • Abstract: Publication date: 29 October 2014
      Source:Chemical Geology, Volume 386
      Author(s): Janette Tourney , Bryne T. Ngwenya
      The last two decades have seen geomicrobiology evolve into a broad field encompassing a wide range of environmentally significant processes such as proton and metal adsorption onto cell surfaces, and the effect of microbes on mineralisation, redox cycling, and contaminant transport. Within this sphere, research groups have conducted studies using bacteria, fungi, diatoms and sludges, all of which can play a part in geochemical processes. Here, we review research on the role played by microbial extracellular polymeric substances (EPS), focussing on bacterial and cyanobacterial EPS. We conclude by outlining future research directions in order to investigate unresolved questions in the field. The effect of EPS on metal adsorption is complex; whereas some studies report an increase in metal adsorption when cells contain EPS, some report no differences; yet others report a decrease. These discrepancies may reflect differences in molecular and functional group composition of the EPS. EPS provides a template for adsorption of metal cations to which carbonate ions are attracted to induce local mineral supersaturation. This may be behind observed changes in both crystal polymorphism favouring formation of less stable polymorphs, and crystal morphology where the presence of EPS promotes the formation of rounded, smoothed crystals or spheroids. The role of EPS in mineral dissolution, bioleaching and corrosion is equivocal. EPS alone appears to have little effect on mineral dissolution, bioleaching or biocorrosion. Instead, it appears that EPS increases rates of mineral weathering and leaching by forming complexes with metallic ions released by the mineral surface, which may themselves catalyse bioleaching in the case of sulphides. EPS in biofilms forms effective barriers to the transport of particulate phases, and exerts important controls on the transport and deposition of natural and engineered nanoparticles. Much less is known about the role of EPS in the cycling of redox-sensitive trace metals. Intuitively, and as observed in microbial mats with sulphate reducing bacteria, EPS is a labile electron donor for microbial metabolism that could affect metal biogeochemistry but this remains to be conclusively demonstrated in laboratory experiments. Hence, in this and other areas, further studies are required to develop a mechanistic basis for including the effects of EPS in biogeochemical models.


      PubDate: 2014-09-04T23:06:07Z
       
  • Characterising the nickel isotopic composition of organic-rich marine
           sediments
    • Abstract: Publication date: 10 November 2014
      Source:Chemical Geology, Volume 387
      Author(s): Sarah J. Porter , David Selby , Vyllinniskii Cameron
      New Ni stable isotope data (δ60Ni) determined by double-spike MC-ICP-MS for two geologically distinct suites of organic-rich marine sediments from the Sinemurian–Pliensbachian (S–P) Global Stratotype Section and Point (GSSP; Robin Hood's Bay, UK) and the Devonian–Mississippian Exshaw Formation (West Canada Sedimentary Basin) is presented herein. These sediments yield δ60Ni values of between 0.2‰ and 2.5‰, and predominantly have Ni isotopic compositions that are heavier than those of abiotic terrestrial and extraterrestrial samples (0.15‰ and 0.27‰), and in some cases present-day seawater (1.44‰) and dissolved Ni from riverine input (0.80‰). In addition, the observed degree of isotopic fractionation in the marine sediments is far greater than that of these other sample matrices. However, a strong similarity is exhibited between the δ60Ni values of the organic-rich sediments studied here and those of ferromanganese crusts (0.9–2.5‰), suggesting that factors ubiquitous to the marine environment are likely to play a key role in the heightened level of isotopic fractionation in these sample matrices. A lack of correlation between the Ni stable isotope compositions of the organic-rich sediments and Ni abundance suggests that isotopic fractionation in these sediments is not controlled by incorporation or enrichment of Ni during sediment accumulation. Further, no relationship is observed between δ60Ni and TOC concentrations or bottom–water redox conditions, indicating that the organic carbon reservoir and levels of oxygenation at the sediment–water interface do not exert a primary control on Ni isotope fractionation in marine sediments. Following examination of these relationships, it is therefore more likely that the heavy Ni isotope compositions of marine sediments are controlled by the weathering environment and the dominant sources of dissolved Ni into the global ocean reservoir.


      PubDate: 2014-09-04T23:06:07Z
       
  • Time integrated variation of sources of fluids and seepage dynamics
           archived in authigenic carbonates from Gulf of Mexico Gas Hydrate Seafloor
           Observatory
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Dong Feng , Daniel Birgel , Jörn Peckmann , Harry H. Roberts , Samantha B. Joye , Roger Sassen , Xiao-Lei Liu , Kai-Uwe Hinrichs , Duofu Chen
      Authigenic carbonate rocks recovered from the Gulf of Mexico Gas Hydrate Seafloor Observatory in Mississippi Canyon block 118 (MC118) at approximately 900m water depth were studied using mineralogical, bulk geochemical, and lipid biomarker analyses. Carbonate rocks occurred as fractured blocks and nodular masses incorporated in carbonate breccias. The carbonates were comprised mainly of high-Mg-calcite and aragonite. The stable carbon isotope composition (δ13C) of authigenic carbonate varied from −29.8‰ to −18.1‰ vs. V-PDB, suggesting a complex mixture of various carbon sources, including dissolved marine inorganic carbon (DIC), oil, as well as methane. Oxygen isotopes (δ18O) varied from +3.4‰ to +5.8‰. The observed 18O-enrichment in relation to calculated equilibrium values in the carbonates probably reflects decomposition of gas hydrates. The most abundant lipid biomarkers in the carbonates were isoprenoidal glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominated by GDGT-2 and GDGT-3, which are typically indicators of anaerobic methane oxidizing archaea (ANMEs). Mono- and bicyclic biphytanes (derived after ether cleavage of GDGT-2 and GDGT-3) showed strong 13C-depletion, which is characteristic for ANMEs. Interestingly, large differences between the δ13C values of the archaeal diether archaeol and acyclic biphytane on the one hand and monocyclic biphytane on the other hand suggest the presence of archaea other than ANMEs. Archaeol and GDGT-0 (containing two acyclic biphytane moieties) are commonly assigned to various methanogenic archaea. Where methane seepage activity is intermediate or low within acoustic wipeout zones at the MC118 gas hydrate site nowadays, microbial communities must have coped with changing conditions as well as longer-term fluctuations in oil and gas seepage or the temporary cessation of hydrocarbon flux in the past. The change from methane seepage to oil seepage or vice versa in addition to flux variability apparently favors the establishment of complex prokaryotic communities dominated by archaea. In addition to anaerobic oxidation of methane, local production of methane is apparently prominent at the study site based on the occurrence of biomarkers of methanogens in the authigenic carbonate. This finding adds to the ongoing multidisciplinary effort to better constrain the environment at the MC118 observatory site and to determine the locally dominant biogeochemical processes.


      PubDate: 2014-08-17T21:04:42Z
       
  • An osmium-based method for assessing the source of dissolved rhenium and
           molybdenum to Archean seawater
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Brian Kendall
      Authigenic enrichments of Re and Mo in 2.7–2.5Ga organic-rich mudrocks (ORM) indicate that dissolved Re and Mo were present in late Archean seawater. However, it can be difficult to determine if the dissolved Re and Mo were sourced from oxidative crustal weathering or magmatic/hydrothermal/extraterrestrial (MHE) inputs. Comparison of Re, Mo, and 192Os (non-radiogenic Os isotope) concentrations, and initial 187Os/188Os isotope compositions from Re–Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater 187Os/188Os reflects the relative contributions of radiogenic crustal versus unradiogenic MHE Os inputs to seawater. During the Phanerozoic, transient increases in magmatic/hydrothermal activity, marked by unradiogenic 187Os/188Os excursions and higher 192Os concentrations in ORM, are accompanied in some cases by higher Re concentrations. These unradiogenic Os sources could thus have played a greater role in the Archean marine Re (and possibly Mo) budget given that the riverine Re (and Mo) flux at that time was likely smaller than the Phanerozoic flux. Hence, a positive correlation between Re, Mo, and 192Os concentrations in Archean ORM that yields unradiogenic initial 187Os/188Os may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O2 of mafic-ultramafic rocks and crustal sulfide minerals with low Re/Os ratios is an alternative explanation, but the two scenarios cannot be readily distinguished on the basis of initial 187Os/188Os. By contrast, an initial 187Os/188Os from Archean ORM that is statistically higher than the MHE baseline of 0.10–0.11 points to oxidative mobilization of Re, Os, and Mo from crustal sulfide minerals. Rhenium and Mo concentrations that are negatively correlated with 192Os concentrations in Archean ORM yielding unradiogenic initial 187Os/188Os suggests that some Re and Mo were derived from oxidative weathering. Such negative correlations may reflect an increased marine Os sink (adsorption to oxide minerals) during a mild increase in local environmental O2 levels. These concepts may help trace the onset of photosynthetic O2 production and accumulation along Archean ocean margins.


      PubDate: 2014-08-17T21:04:42Z
       
  • The isotopic fingerprint of Fe cycling in an equatorial
           soil–plant–water system: The Nsimi watershed, South Cameroon
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Alisson Akerman , Franck Poitrasson , Priscia Oliva , Stéphane Audry , Jonathan Prunier , Jean-Jacques Braun
      Following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile, the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20% of the studied watershed surface (from Nsimi, South Cameroon). This study considers the soil–plant–water continuum in order to better understand the Fe elemental and isotopic transfer out of the ecosystem. Within the swamp system, the iron isotope compositions of gleysol samples (δ57FeIRMM-14 ~+0.6‰) are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope (δ57FeIRMM-14 =+0.1‰). This enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes. Pedogenic transformations (i.e., gleyzation, organic complexation of metal and leaching) are responsible for the reductimorphic features observed in ferralitic horizons (i.e., incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions). The organic carbon-rich waters of the swamp system are prone to redox processes and strong metal chelation. The dissolved iron (i.e., fraction <0.22μm) of the Mengong stream shows positive δ57Fe signatures, with a downstream enrichment in heavy isotopes, from +0.511±0.266‰ to +1.076±0.240‰. The binding of iron (FeIII) with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction. On the contrary, plant leaves are significantly enriched in light Fe (δ57Fe of −0.665±0.035 and −1.119±0.080‰) relative to (i) the litter compartment (−0.166±0.078 to −0.262±0.013‰ for δ57Fe) and (ii) the most superficial soils. Iron isotopic compositions in plants and litter vary as a function of both plant species and season. Hence, the differences in Fe isotopic compositions between the various studied compartments suggest that Fe isotopes can be used (i) to study elemental transfers during soil pedogenesis in tropical environment and (ii) to better appraise and constrain iron biogeochemical cycle between surface horizon of soils, surface waters and the vegetation.


      PubDate: 2014-08-14T20:40:41Z
       
  • Using stable Mg isotopes to distinguish dolomite formation mechanisms: A
           case study from the Peru Margin
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Vasileios Mavromatis , Patrick Meister , Eric H. Oelkers
      The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~6m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~230mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid δ26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. −0.8‰, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8±0.2‰ within the sediments located below 100mbsf. Type I dolomites have a δ26Mg of −3.5‰, and exhibit apparent dolomite-pore fluid fractionation factors of about −2.6‰ consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have δ26Mg values ranging from −2.5 to −3.0‰ and are up to −3.6‰ lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~200mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~200mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.


      PubDate: 2014-08-14T20:40:41Z
       
  • Gold concentrations in metamorphic fluids: A LA-ICPMS study of fluid
           inclusions from the Alpine orogenic belt
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): K. Rauchenstein-Martinek , T. Wagner , M. Wälle , C.A. Heinrich
      Vein and shear-zone hosted gold deposits in orogenic terrains of Archean to Phanerozoic age are formed from dominantly metamorphic fluids. It is debated, however, whether normal crustal source rocks are adequate to generate economic deposits, or whether selectively gold-enriched sources such as mafic to ultramafic lavas, black shales or an input of magmatic fluid make a decisive difference between the formation of high-grade deposits and barren vein systems. As an essential baseline study in this debate, we have analyzed the metal and sulfur content of fluid inclusions in barren veins across a prograde sequence from greenschist to amphibolite facies in the Central Alpine metamorphic belt. Comparison of the analyzed fluid compositions with thermodynamic solubility calculations shows that the fluids record metamorphic dehydration, decarbonation and desulfidation of the continental crust. However, gold with concentrations of 0.003 to 0.03ppm is increasingly undersaturated in the highest-temperature aqueous–carbonic fluids, which otherwise resemble those forming major gold deposits elsewhere. Our results show that the regional-metamorphic fluids could carry 10–1000 times higher gold concentrations, implying that pre-enriched source rocks or hydrous magmas may play an essential role in generating highly gold-endowed orogenic belts.


      PubDate: 2014-08-14T20:40:41Z
       
  • Measurements of HFC-134a and HCFC-22 in groundwater and unsaturated-zone
           air: Implications for HFCs and HCFCs as dating tracers
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Karl B. Haase , Eurybiades Busenberg , L. Niel Plummer , Gerolamo Casile , Ward E. Sanford
      A new analytical method using gas chromatography with an atomic emission detector (GC–AED) was developed for measurement of ambient concentrations of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) in soil, air, and groundwater, with the goal of determining their utility as groundwater age tracers. The analytical detection limits of HCFC-22 (difluorochloromethane, CHClF2) and HFC-134a (1,2,2,2-tetrafluoroethane, C2H2F4) in 1L groundwater samples are 4.3×10−1 and 2.1×10−1 pmolkg−1, respectively, corresponding to equilibrium gas-phase mixing ratios of approximately 5–6 parts per trillion by volume (pptv). Under optimal conditions, post-1960 (HCFC-22) and post-1995 (HFC-134a) recharge could be identified using these tracers in stable, unmixed groundwater samples. Ambient concentrations of HCFC-22 and HFC-134a were measured in 50 groundwater samples from 27 locations in northern and western parts of Virginia, Tennessee, and North Carolina (USA), and 3 unsaturated-zone profiles were collected in northern Virginia. Mixing ratios of both HCFC-22 and HFC-134a decrease with depth in unsaturated-zone gas profiles with an accompanying increase in CO2 and loss of O2. Apparently, ambient concentrations of HCFC-22 and HFC-134a are readily consumed by methanotrophic bacteria under aerobic conditions in the unsaturated zone. The results of this study indicate that soils are a sink for these two greenhouse gases. These observations contradict the previously reported results from microcosm experiments that found that degradation was limited above-ambient HFC-134a. The groundwater HFC and HCFC concentrations were compared with concentrations of chlorofluorocarbons (CFCs, CFC-11, CFC-12, CFC-113) and sulfur hexafluoride (SF6). Nearly all samples had measured HCFC-22 or HFC-134a that were below concentrations predicted by the CFCs and SF6, with many samples showing a complete loss of HCFC-22 and HFC-134a. This study indicates that HCFC-22 and HFC-134a are not conservative as environmental tracers and leaves in question the usefulness of other HCFCs and HFCs as candidate age tracers.


      PubDate: 2014-08-14T20:40:41Z
       
  • A new approach to Pb evaporation and condensation from zircon enabling
           sub-Ma age determination on Precambrian rocks
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Abin Das , Donald W. Davis
      Sub-million year age precision on low to moderately metamict Precambrian zircon is often reproducible using the isotope dilution thermal ionization technique (ID-TIMS) exploiting U–Pb double spiking for precise mass fractionation correction. However, this requires total dissolution of sample and laborious chemical processing. On the other hand whole grain evaporation techniques of Pb extraction are simple and less time consuming but provide relatively inefficient ionization and are not amenable to double Pb isotope spiking for precise mass fractionation correction, which limits the precision of ages that they can provide (often restricted to ±few million years). A modified evaporation–condensation method is described that overcomes these limitations with the ability to use 202Pb–205Pb double spiking for precise mass fractionation correction in order to achieve sub-million year age precision on low to highly metamict Precambrian samples. Examples are shown of age determinations on previously dated zircon populations from the Marmion tonalite and Nevison Lake rhyolite, western Superior province (U<200ppm), which give 3001.8±0.5Ma and 2998.2±0.3Ma, respectively, and from the black norite and the felsic norite in the Sudbury igneous complex (U>1000ppm), which give 1848.9±0.2Ma and 1851.3±0.5Ma, respectively (all errors are 95% confidence).


      PubDate: 2014-08-10T20:30:57Z
       
  • A complete Os excursion across a terrestrial Cretaceous–Paleogene
           boundary at the West Bijou Site, Colorado, with evidence for recent open
           system behavior
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Jessica Zaiss , Greg Ravizza , Steven Goderis , Justine Sauvage , Philippe Claeys , Kirk Johnson
      The few previously reported values of 187Os/188Os ratios from non-marine K–Pg boundary sections are distinctly higher than the range of 187Os/188Os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust. Here, Re–Os data from the West Bijou continental K–Pg boundary site, located within the Denver Basin, are used to better constrain the Os isotopic composition of fallout from the Chicxulub ejecta plume. For the first time, full vertical profiles of 187Os/188Os ratios and Re and Os concentrations across a continental K–Pg boundary section are reported. Within this section of lignite, the lowest measured 187Os/188Os ratio (0.182) coincides with the K–Pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop. However, sediments with elevated Os concentrations and measured 187Os/188Os below 0.23 extend over a 30cm interval, from ~5cm above the clay-rich boundary interval (~5cm thick) to ≈25cm below. Maximum Ir and Os concentrations occur 3cm and 10cm below the K–Pg boundary, respectively, demonstrating greater diagenetic mobility of Os relative to Ir. Low 187Os/188Os above the K–Pg boundary suggests that accumulation of impact derived Os at this site persisted after the impact event, perhaps due to redistribution of impact debris in the surrounding area. Importantly, calculated initial 187Os/188Os ratios throughout most of the section are impossibly low and require open system behavior, likely in the form of Re addition within the last 10 million years. Within the boundary interval, Re concentrations are among the lowest measured at Bowring Pit and calculated initial 187Os/188Os ratios are very similar to those previously reported from the western interior of the United States K–Pg boundary material (~0.14), suggesting super-chondritic 187Os/188Os is characteristic of the Chicxulub ejecta plume. Beyond implications for our understanding of Os as an impact tracer, this study emphasizes the dramatic effect diagenesis can have on the Re–Os system in organic-rich environments, especially through post-depositional addition of Re.


      PubDate: 2014-08-10T20:30:57Z
       
  • Implications of discordant U–Pb ages on Hf isotope studies of
           detrital zircons
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Martin Guitreau , Janne Blichert-Toft
      Zircon is the ideal tool for unraveling Earth's history because of its refractory nature, being readily datable by the U–Pb isotope system, and owing to the fact that its Hf isotope composition can be precisely determined. However, most analyzed zircons have experienced Pb-loss to various degrees, resulting in present-day measured 207Pb/206Pb ages being younger than that of the time of crystallization, if the loss is ancient. This is of particular importance for ancient zircons from the Archean and the Hadean, notably the Jack Hills zircons. A zircon Lu–Hf and U–Pb isotope evolution model has been developed and shows that Pb-loss at 3700Ma affecting a simple zircon population that crystallized at 4350Ma can reproduce most of the broad ε Hf versus 207Pb/206Pb age trend observed for Jack Hills Hadean zircons. In addition, the model demonstrates that crystals having experienced widely different degrees of Pb-loss (2–80%) appear only slightly discordant (0.1–2.2%), while their apparent 207Pb/206Pb ages are different from that of crystallization by up to 466My. This is insignificant for calculating initial Hf isotope compositions but results in major shifts in initial ε Hf values by up to 11.3 units. This effect is particularly relevant to the global Hadean/Archean detrital zircon record for which several apparent ε Hf versus age trends, similar to that defined by the Jack Hills Hadean zircons, stand out and could be fully or partly artificial. To overcome, or at least significantly reduce, these issues, multiple U–Pb analyses should be undertaken for each detrital zircon. Simultaneous measurement of Hf and U–Pb isotopes by the split-stream technique is the ideal approach because it can provide information about the cause of discordance (Pb-loss or concurrent analysis of two unrelated growth zones) and thereby allow for the most robust and accurate Hf isotope data to be obtained.


      PubDate: 2014-08-10T20:30:57Z
       
  • High precision 87Sr/86Sr measurements by MC-ICP-MS, simultaneously solving
           for Kr interferences and mass-based fractionation
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Jasper G. Konter , Lauren P. Storm
      Sr isotope measurements by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) are complicated by Kr interferences at atomic masses 84 and 86. Particularly the interference on 86Sr is problematic for 87Sr/86Sr measurements, since it is the normalizing stable isotope, and as such also used in the mass-based fractionation correction using 86Sr/88Sr. Although on-peak baseline measurements can subtract an average Kr contribution, signal variations and errors within-run are propagated into the final error, and signal-to-noise ratios are lower than combined Sr+Kr signals. Alternatively, intensities of interference-free 82Kr or 83Kr isotopes can be monitored to correct the interfering Kr masses, although the errors for these signals are amplified and propagated into the 86Kr and 84Kr correction. Instead, our approach depends on the use of the most abundant Kr isotope (84Kr) for correction, which can then be used to estimate interference by 86Kr on 86Sr. This is accomplished by solving how much Kr is needed to explain the difference between I84/I88 and I86/I88 versus 84Sr/88Sr and 86Sr/88Sr (I8×: signal intensity at mass 8×). A solution with the traditional isotope dilution equation would require off-line iteration to include an exponential mass fractionation correction. Instead, we formulate an online exponential correction of the Sr data without iterating, relying on a separate Kr correction that estimates Kr mass fractionation with either the linear law or a series expansion of the exponential law. Both methods advance MC-ICP-MS long-term 87Sr/86Sr precision for NIST SRM 987 to ±16∗10−6 or 22.5ppm 2σ, outperforming an 83Kr-based correction and an on-peak baseline correction applied to the same raw data. Both our methods find identical results for USGS rock standards that agree with published (TIMS) values.


      PubDate: 2014-08-10T20:30:57Z
       
  • Sr and Nd isotopic compositions of apatite reference materials used in
           U–Th–Pb geochronology
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Yue-Heng Yang , Fu-Yuan Wu , Jin-Hui Yang , David M. Chew , Lie-Wen Xie , Zhu-Yin Chu , Yan-Bin Zhang , Chao Huang
      Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol.
      Graphical abstract image

      PubDate: 2014-08-10T20:30:57Z
       
  • Bank storage in karst aquifers: The impact of temporary intrusion of river
           water on carbonate dissolution and trace metal mobility
    • Abstract: Publication date: 14 October 2014
      Source:Chemical Geology, Volume 385
      Author(s): Amy L. Brown , Jonathan B. Martin , Elizabeth J. Screaton , John E. Ezell , Patricia Spellman , Jason Gulley
      Storms can trigger changes in river stage that alter the hydraulic gradients between rivers and adjacent aquifers. In eogenetic carbonate karst aquifer systems, storms enable river water to intrude >1km into the adjacent aquifer systems for days to weeks. This process is similar to bank storage of streams in siliciclastic sediments but can have longer temporal and larger spatial scales. River intrusion triggers changes in mineral saturation states and redox conditions in the aquifer due to the input of low pH, low specific conductivity (SpC), and high dissolved organic carbon (DOC) flood water. To assess the effects of river intrusion into karst aquifers, we measured SpC, temperature, pH, redox state, and concentrations of dissolved major and trace elements through an intrusion event at Madison Blue Spring in northern Florida, USA. River water displaced groundwater in the conduit at least 1km into the aquifer and flowed into the pores of the unconfined aquifer matrix. Distinct Cl− concentrations between river water and groundwater provide estimates for mixing fractions. The location and magnitude of oxidation of organic matter in the subsurface controlled trace metal concentration, redox state and saturation state of the water with respect to calcite (SIcal). Organic matter oxidation in the phreatic conduits was limited by the terminal electron acceptors (TEAs) present in the conduit water. Calcite dissolution and trace metal sorption were limited by the lower surface area to porosity ratios in the conduits than the matrix. Organic matter oxidation was enhanced in the matrix by mixing with matrix waters with available DO and NO3 −, resulting in greater CO2 production and calcite dissolution than in the conduit. After the intruded river water discharged, conditions remained more reducing in the aquifer than baseflow conditions due to the reduction of DO and NO3 − in the matrix water. The organic matter transported into the aquifer during river intrusion drives carbonate dissolution, alters redox state, and impacts trace metal mobility, impacting groundwater and surface water quality.


      PubDate: 2014-08-10T20:30:57Z
       
  • Re-evaluating digestion methods for highly siderophile element and 187Os
           isotope analysis: Evidence from geological reference materials
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Akira Ishikawa , Ryoko Senda , Katsuhiko Suzuki , Christopher W. Dale , Thomas Meisel
      Here we evaluate measurement procedures currently used for the combined determination of highly siderophile element concentrations (HSEs: Re, Ir, Os, Ru, Pt and Pd) and Os isotope ratios in geological samples by isotope dilution mass spectrometry (ID-MS) applying high-temperature sample digestion using inverse aqua regia in closed glass vessels, such as Carius tubes or a high-pressure asher system (HPA). In particular, we address the question of whether an additional hydrofluoric acid dissolution step is required to release HSEs hosted in the silicate portions of rock samples. To do so, we conducted systematic experiments for basaltic (TDB-1 and BIR-1), ultramafic (UB-N and JP-1) and sedimentary (SCo-1, SDO-1 and JCh-1) reference materials with and without HF desilicification. Through the extensive tests on TDB-1—varying apparatus (microwave, Carius tube, high-pressure asher), conditions (temperature, duration, sample size) and protocol (HF desilicification before or after aqua regia attack)—we find the optimum digestion method is to use inverse aqua regia for digesting 1–2g of powder over long durations, such as Carius tubes heated to 240˚C for 72h, followed by an HF desilicification step after CCl4 solvent extraction of Os. For basaltic reference materials, the method provides significantly improved HSE recoveries—particularly Ru—largely due to the use of HF. Strong linear correlations between Os and Ir–Ru–Pt concentrations and isochronous behavior on a 187Re/188Os vs. 187Os/188Os diagram (1240±59Ma, MSWD=1.4) are observed for repeat dissolutions of TDB-1, reflecting sample heterogeneity due to minor minerals enriched in Os, Ir, Ru and Pt—most likely sulphides. In comparison, excellent reproducibilities were obtained for BIR-1: relative standard deviations (RSDs) for 1–2g aliquots were 6.9% Os, 5.1% Ir, 2.0% Ru, 5.1% Pt, 1.5% Pd and 0.7% Re (n=9). Thus, BIR-1 might be a useful candidate reference material for obtaining certified values with small measurement uncertainties. Unlike the basaltic reference materials, extraction of HSEs from ultramafic and sedimentary reference materials is largely independent of the use of HF. The data obtained by Carius tube for UB-N and JP-1 agree well with the literature data obtained using apparently more aggressive digestion techniques. The precision of our data for 0.5–2g aliquots of the sedimentary reference materials also compares favourably with limited data available in the literature.


      PubDate: 2014-07-27T19:19:23Z
       
  • Solid-phase phosphorus speciation in Saharan Bodélé Depression
           dusts and source sediments
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Karen A. Hudson-Edwards , Charlie S. Bristow , Giannantonio Cibin , Gary Mason , Caroline L. Peacock
      Phosphorus (P) is one of the most important limiting nutrients for the growth of oceanic phytoplankton and terrestrial ecosystems, which in turn contributes to CO2 sequestration. The solid-phase speciation of P will influence its solubility and hence its availability to such ecosystems. This study reports on the results of X-ray diffraction, electron microprobe chemical analysis and X-ray mapping, chemical extractions and X-ray absorption near-edge spectroscopy analysis carried out to determine the solid-phase speciation of P in dusts and their source sediments from the Saharan Bodélé Depression, the world's greatest single source of dust. Chemical extraction data suggest that the Bodélé dusts contain 28 to 60% (mean 49%) P sorbed to, or co-precipitated with, Fe (hydr)oxides, <10% organic P, 21–50% (mean 32%) detrital apatite P, and 10–22% (mean 15%) authigenic–biogenic apatite P. This is confirmed by the other analyses, which also suggest that the authigenic–biogenic apatite P is likely fish bone and scale, and that this might form a larger proportion of the apatite pool (33+/−22%) than given by the extraction data. This is the first-ever report of fish material in aeolian dust, and it is significant because P derived from fish bone and scale is relatively soluble and is often used as a soil fertilizer. Therefore, the fish-P will likely be the most readily consumed form of Bodélé P during soil weathering and atmospheric processing, but given time and acid dissolution, the detrital apatite, Fe-P and organic-P will also be made available. The Bodélé dust input of P to global ecosystems will only have a limited life, however, because its major source materials, diatomite in the Bodélé Depression, undergo persistent deflation and have a finite thickness.
      Graphical abstract image

      PubDate: 2014-07-27T19:19:23Z
       
  • The entropy paradox and overstepping in metamorphic reactions
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): G.M. Anderson
      It is commonly asserted that entropy tends towards a maximum at equilibrium in isolated systems. But entropy is only defined in equilibrium states, so in an isolated system the entropy is constant, if it is defined at all. The solution to this paradox is the existence of metastable equilibrium states, so common in mineralogy and petrology. Consideration of such states in a thermodynamic context also involves the concept of constraints and of the affinity, and because the affinity is a measure of the change in Gibbs energy in spontaneous reactions involving metastable phases, the entropy paradox and the overstepping of reactions in metamorphic rocks are closely related subjects.


      PubDate: 2014-07-27T19:19:23Z
       
  • Thermal diffusivity of Fe-rich pyroxene glasses and their melts
    • Abstract: Publication date: 25 September 2014
      Source:Chemical Geology, Volume 384
      Author(s): Anne M. Hofmeister , Alexander Sehlke , Alan G. Whittington
      We provide new measurements of thermal diffusivity (D) of 8 synthetic glasses and melts along the enstatite–ferrosilite compositional binary, 4 remelted crystals which are close to the binary, plus one synthetic ternary composition (Fs15Wo40En45). Thermal diffusivity of these glasses is between 0.50 and 0.62mm2 s−1 at room temperature, decreases to ~0.45 to 0.50mm2 s−1 near 1000K, then drops upon melting at 1100K to ~0.34mm2 s−1. A roughly linear trend in D exists across the En–Fs binary at 298K, with Fe-rich compositions having lower D, whereas at high temperatures all melts have similar D. The trend is not smooth, due to variable amounts of crystallites and residual strain in glasses, both of which affect D-values. Above about 700K, ∂D/∂T is visibly positive for moderate Fe contents (i.e., Fe/(Fe+Mg)=0.1 to 0.3), which we attribute to diffusive radiative transfer being enhanced by electronic–vibronic coupling. All data are well represented by equations of the form FT −G +HT, where F, G and H are fitting parameters. The fitting reveals a weak increase in D at high T for all of our glasses, consistent with weak radiative transfer in the infrared. Although data could only be collected on melts over a narrow temperature range, they also appear to have a positive ∂D/∂T, which is consistent with previous results on less mafic iron-bearing glasses and melts. Our results suggest that Fe-rich melts in the lower mantle could have thermal diffusivity higher than their crystalline counterparts due to the response of D to temperature.


      PubDate: 2014-07-27T19:19:23Z
       
 
 
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