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  Subjects -> EARTH SCIENCES (Total: 631 journals)
    - EARTH SCIENCES (463 journals)
    - GEOLOGY (67 journals)
    - GEOPHYSICS (27 journals)
    - HYDROLOGY (21 journals)
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EARTH SCIENCES (463 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: 13)
Advances In Physics     Hybrid Journal   (Followers: 9)
Aeolian Research     Hybrid Journal   (Followers: 3)
African Journal of Aquatic Science     Hybrid Journal   (Followers: 13)
Algological Studies     Full-text available via subscription   (Followers: 2)
Alpine Botany     Hybrid Journal   (Followers: 5)
AMBIO     Hybrid Journal   (Followers: 13)
Anales del Instituto de la Patagonia     Open Access   (Followers: 2)
Andean geology     Open Access   (Followers: 5)
Annales Henri Poincaré     Hybrid Journal   (Followers: 1)
Annales UMCS, Geographia, Geologia, Mineralogia et Petrographia     Open Access   (Followers: 2)
Annals of Geophysics     Full-text available via subscription   (Followers: 10)
Annals of GIS     Hybrid Journal   (Followers: 18)
Annals of Glaciology     Full-text available via subscription   (Followers: 2)
Annual Review of Marine Science     Full-text available via subscription   (Followers: 11)
Anthropocene     Hybrid Journal  
Anthropocene Review     Hybrid Journal   (Followers: 3)
Applied Clay Science     Hybrid Journal   (Followers: 2)
Applied Geochemistry     Hybrid Journal   (Followers: 7)
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: 10)
Aquatic Conservation Marine and Freshwater Ecosystems     Hybrid Journal   (Followers: 21)
Arctic Science     Open Access   (Followers: 4)
Arctic, Antarctic, and Alpine Research     Full-text available via subscription   (Followers: 9)
Artificial Satellites     Open Access   (Followers: 15)
Asia-Pacific Journal of Atmospheric Sciences     Hybrid Journal   (Followers: 2)
Asian Journal of Earth Sciences     Open Access   (Followers: 20)
Atlantic Geology : Journal of the Atlantic Geoscience Society / Atlantic Geology : revue de la Société Géoscientifique de l'Atlantique     Full-text available via subscription   (Followers: 3)
Atmosphere-Ocean     Full-text available via subscription   (Followers: 7)
Atmospheric and Climate Sciences     Open Access   (Followers: 16)
Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia     Hybrid Journal   (Followers: 12)
Boletim de Ciências Geodésicas     Open Access  
Boreas: An International Journal of Quaternary Research     Hybrid Journal   (Followers: 12)
Bragantia     Open Access   (Followers: 2)
Bulletin of Earthquake Engineering     Hybrid Journal   (Followers: 10)
Bulletin of Geosciences     Open Access   (Followers: 9)
Bulletin of the Lebedev Physics Institute     Hybrid Journal   (Followers: 1)
Bulletin of the Seismological Society of America     Full-text available via subscription   (Followers: 18)
Bulletin of Volcanology     Hybrid Journal   (Followers: 16)
Canadian Journal of Plant Science     Full-text available via subscription   (Followers: 12)
Canadian Mineralogist     Full-text available via subscription   (Followers: 1)
Canadian Water Resources Journal     Hybrid Journal   (Followers: 20)
Carbonates and Evaporites     Hybrid Journal   (Followers: 4)
CATENA     Hybrid Journal   (Followers: 4)
Central European Journal of Geosciences     Hybrid Journal   (Followers: 6)
Central European Journal of Physics     Hybrid Journal   (Followers: 2)
Chemical Geology     Hybrid Journal   (Followers: 11)
Chemie der Erde - Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Geographical Science     Hybrid Journal   (Followers: 4)
Chinese Journal of Geochemistry     Hybrid Journal   (Followers: 3)
Chinese Journal of Oceanology and Limnology     Hybrid Journal   (Followers: 3)
Ciencia del suelo     Open Access  
Climate and Development     Hybrid Journal   (Followers: 11)
Coastal Management     Hybrid Journal   (Followers: 19)
Cogent Geoscience     Open Access  
Comptes Rendus Geoscience     Full-text available via subscription   (Followers: 6)
Computational Geosciences     Hybrid Journal   (Followers: 12)
Computational Mathematics and Mathematical Physics     Hybrid Journal   (Followers: 1)
Computers and Geotechnics     Hybrid Journal   (Followers: 7)
Contemporary Trends in Geoscience     Open Access   (Followers: 2)
Continental Shelf Research     Hybrid Journal   (Followers: 8)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 8)
Contributions to Plasma Physics     Hybrid Journal   (Followers: 2)
Coral Reefs     Hybrid Journal   (Followers: 18)
Cretaceous Research     Hybrid Journal   (Followers: 6)
Cybergeo : European Journal of Geography     Open Access   (Followers: 5)
Depositional Record     Open Access  
Developments in Geotectonics     Full-text available via subscription   (Followers: 3)
Developments in Quaternary Science     Full-text available via subscription   (Followers: 4)
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: 70)
Earth and Space Science     Open Access  
Earth Interactions     Full-text available via subscription   (Followers: 11)
Earth Science Research     Open Access   (Followers: 8)
Earth Surface Dynamics (ESurf)     Open Access   (Followers: 3)
Earth Surface Processes and Landforms     Hybrid Journal   (Followers: 14)
Earth System Dynamics     Open Access   (Followers: 7)
Earth System Dynamics Discussions     Open Access   (Followers: 4)
Earth's Future     Open Access   (Followers: 1)
Earth, Planets and Space     Open Access   (Followers: 3)
Earthquake Engineering and Engineering Vibration     Hybrid Journal   (Followers: 7)
Earthquake Science     Hybrid Journal   (Followers: 8)
Earthquake Spectra     Full-text available via subscription   (Followers: 13)
Ecohydrology     Hybrid Journal   (Followers: 10)
Electromagnetics     Hybrid Journal   (Followers: 1)
Energy Efficiency     Hybrid Journal   (Followers: 12)
Energy Exploration & Exploitation     Full-text available via subscription   (Followers: 4)
Environmental Earth Sciences     Hybrid Journal   (Followers: 12)

        1 2 3 4 5 | Last

Journal Cover   Chemical Geology
  [SJR: 1.927]   [H-I: 123]   [11 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 0009-2541
   Published by Elsevier Homepage  [2812 journals]
  • Monazite Response to Ultrahigh-Pressure Subduction from U-Pb dating by
           Laser Ablation Split Stream
    • Abstract: Publication date: Available online 22 May 2015
      Source:Chemical Geology
      Author(s): Bradley R. Hacker , Andrew R.C. Kylander-Clark , Robert Holder , Torgeir B. Andersen , Emily M. Peterman , Emily O. Walsh , Jonathan K. Munnikhuis
      To assess the response of monazite during subduction of continental crust to mantle depths, U-Pb isotopic ratios and elemental abundances were measured simultaneously by laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) in rocks from the ultrahigh-pressure Western Gneiss Region of the Scandinavian Caledonides. Nearly seventy different samples of quartzofeldspathic basement and overlying metasedimentary rocks were studied. Pre-subduction monazite (chiefly 1.6 Ga and 1.0 Ga) is preserved locally in the structurally lowest, basement rocks because earlier, Precambrian tectonism produced coarse-grained, high-grade rocks that were resistant to further recrystallization in spite of syn-subduction temperatures and pressures of 650–800°C and 2–3.5 GPa. A few of the monazite in the metasedimentary rocks atop the basement preserve syn-subduction U-Pb dates, but the majority continued to recrystallize during post-subduction exhumation and record a general westward decrease in age related to westward-progressing exhumation. The absence of Precambrian monazite in the metasedimentary rock atop the basement suggests that sedimentation postdated the 1.0–0.9 Ga high-grade metamorphism and was late Proterozoic to early Paleozoic.


      PubDate: 2015-05-26T01:42:46Z
       
  • Characterising the stable (δ88/86Sr) and radiogenic (87Sr/86Sr)
           isotopic composition of strontium in rainwater
    • Abstract: Publication date: Available online 22 May 2015
      Source:Chemical Geology
      Author(s): Christopher R. Pearce , Ian J. Parkinson , Jérôme Gaillardet , Benjamin Chetelat , Kevin W. Burton
      Rainwater plays a major role in determining the transfer of elements and pollutants to soils and hydrological catchments, with compositional variations reflecting the extent of mixing between marine (sea-salt), terrestrial (dust and vegetation) and anthropogenic aerosols. While the radiogenic strontium isotope system (87Sr/86Sr) is often used to help constrain the different sources of atmospheric Sr, the degree of variability in the stable strontium isotope system (δ88/86Sr) remains unresolved. In order to determine how atmospheric Sr affects the δ88/86Sr composition of water entering the hydrological cycle this study analysed a suite of rainwater samples collected over the course of a year in central Paris. Rainwater δ88/86Sr compositions were found to range from 0.13 ‰ to 0.32 ‰, with 87Sr/86Sr ratios varying between 0.70796 and 0.71093. No seasonality was observed in δ88/86Sr, whereas 87Sr/86Sr ratios were observed to be generally more radiogenic in the summer and less radiogenic in the winter. Strontium enrichment factors significantly above seawater implied a predominantly terrestrial (non sea-salt) origin of Sr in rainwater, with the δ88/86Sr and 87Sr/86Sr composition of this terrestrial fraction consistent with a mixture of Sr derived from carbonate dust and agricultural fertiliser. Evidence for a ~0.1 ‰ decrease in the δ88/86Sr composition of the River Seine as it passes through Paris suggests that anthropogenic pollutants may contribute to significant regional variations in the composition of Sr in the hydrological cycle. Together these results demonstrate that there is significant potential for both temporal and spatial variations in the δ88/86Sr and 87Sr/86Sr composition of rainwater that need to be taken into consideration when studying δ88/86Sr variations in localised catchments.


      PubDate: 2015-05-26T01:42:46Z
       
  • Noble gases in sulfide deposits of modern deep-sea hydrothermal systems:
           Implications for heat fluxes and hydrothermal fluid processes
    • Abstract: Publication date: Available online 22 May 2015
      Source:Chemical Geology
      Author(s): Zhigang Zeng , Samuel Niedermann , Shuai Chen , Xiaoyuan Wang , Zhaoxue Li
      Studies of noble gases in seafloor sulfide deposits are an important tool for understanding the origin of ore-forming fluids, both enabling the determination of noble gas sources and revealing the degree of fluid-rock interaction and mantle degassing. The noble gas concentrations and isotopic compositions of 27 sulfide, 3 sulfate, and 2 opal mineral aggregate samples have been studied in different hydrothermal fields from the East Pacific Rise, Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, and North Fiji Back-Arc Basin. The helium concentrations and isotopic ratios in the sulfide aggregate samples are variable (4He 0.12 to 22 × 10-8 cm3 STP/g; 3He/4He ~0.6 to 10.4 Ra), and most of the sulfide helium concentrations are higher than those in the opal mineral samples (4He 0.017 and 0.028 × 10-8 cm3 STP/g), suggesting that the low-temperature fluids have lost their mantle helium during cooling. The helium of high (>7 Ra), medium (1–7 Ra), and low (≈1 Ra) 3He/4He ratio samples is derived mainly from the MORB or OIB mantle by magma degassing, from mixing between hydrothermal fluid and seawater during ore-forming processes, and from ambient seawater, respectively. The high 3He/4He ratios (>7 Ra) of sulfides imply that high-temperature sulfides retain the helium isotopic compositions of the primary hydrothermal fluid, whereas low-temperature sulfides, sulfates, and opal minerals do not. The neon, argon, krypton, and xenon concentrations in the sulfide aggregate samples are also variable; in most of the sulfide aggregates, they are significantly lower than in the sulfate and opal mineral samples. It is known that barite and opal minerals are characteristic of low-temperature (<200°C) paragenetic associations, indicating that heavier noble gases (Ne, Ar, Kr and Xe) are enriched under low-temperature conditions. Most of the sulfide, sulfate and opal mineral aggregate samples possess heavy noble gas elemental abundances similar to those of air-saturated seawater and Ne, Ar, and Xe isotopic compositions that span narrow ranges around atmospheric values, an observation which is most easily explained by the dominance of a seawater-derived component. In addition, based on the calculated helium/heat ratios, global helium and heat fluxes to high-temperature hydrothermal vents are approximately 0.05–6 ×104 kg per year and 0.1–12 ×1012 W, meaning that roughly 0.3% of ocean heat is supplied by seafloor high-temperature hydrothermal activity.


      PubDate: 2015-05-26T01:42:46Z
       
  • Molecular Proxies as Indicators of Freshwater Incursion-Driven Salinity
           Stratification
    • Abstract: Publication date: Available online 22 May 2015
      Source:Chemical Geology
      Author(s): Svenja Tulipani , Kliti Grice , Paul F. Greenwood , Lorenz Schwark , Michael E. Böttcher , Roger E. Summons , Clinton B. Foster
      Salinity-related stratification in marine environments is one of the main drivers for the development of persistent anoxia and euxinia. It therefore plays a vital role in the sedimentary preservation of organic matter and has also been associated with Oceanic Anoxic Events during several mass extinctions in Earth’s history. Here we present a novel molecular proxy using relative abundances of methyltrimethyltridecylchromans (MTTCs) as indicators of riverine freshwater incursions into Middle to Late Devonian paleoreefs exposed to conditions of prevailing anoxia, photic zone euxinia and water column stratification. This paper aims at re-instigating a discussion about the origin of these compounds to potentially broaden their utilization as paleoproxies, adding freshwater stratification to the general salinity reconstructions. The co-variation of MTTC abundance profiles and δ13C values with other biomarker parameters in the sediments studied here indicate a link of MTTCs to terrigenous input, pointing towards a previously suggested early diagenetic formation of these compounds from phytol with higher plant-derived alkylphenols.
      Graphical abstract image

      PubDate: 2015-05-26T01:42:46Z
       
  • Differentiating marine vs hydrothermal processes in Devonian carbonate
           mounds using Rare Earth Elements (Kess Kess mounds, Anti-Atlas, Morocco)
    • Abstract: Publication date: Available online 23 May 2015
      Source:Chemical Geology
      Author(s): Fulvio Franchi , Axel Hofmann , Barbara Cavalazzi , Allan Wilson , Roberto Barbieri
      Petrological and geochemical analyses were carried out on Early Devonian Kess Kess mound limestones of the Seheb el Rhassel Group exposed in the Hamar Laghdad Ridge (Tafilalt Platform, Eastern Anti-Atlas, Morocco) in order to evaluate marine vs hydrothermal processes for the origin of the mounds. Hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter-mound facies as cavities, veins and dykes. Shale-normalized rare earth element (REE) patterns for limestones display pronounced light REE depletion compared to middle REE (average PrSN/DySN=0.46) and heavy REE (average PrSN/YbSN=0.55), a super-chondritic Y/Ho ratio (up to 87) and positive La anomaly consistent with precipitation from normal marine seawater. Within our data set the Ce anomaly, as well as U and other trace element concentrations vary with facies providing evidence for variable redox conditions. The presence of positive Ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition. Samples from mound facies and quartz veinlets in the upper part of the group show REE patterns consistent with precipitation from hydrothermal fluids. Early Devonian hypothetical seawater REE patterns were calculated from samples showing normal marine REE patterns (i.e. inter-mound facies) and from samples with weaker marine signature (i.e. fossiliferous mound limestones and plumbing system deposits). Hypothetical patterns are slightly enriched in ƩREE compared to modern open ocean seawater. This study reveals that REE analysis is a powerful tool for understanding polygenetic carbonate systems. It sheds light into the genesis of the Kess Kess mounds by reconstructing fluid pathways and palaeo-redox conditions.


      PubDate: 2015-05-26T01:42:46Z
       
  • Adsorption of monothioarsenate on amorphous aluminum hydroxide under
           anaerobic conditions
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Fan Xiao , Shaofeng Wang , Liying Xu , Ying Wang , Zidan Yuan , Yongfeng Jia
      Thioarsenates have been found to be major arsenic species in some sulfidic waters and their interactions with minerals play an important role in the geochemical cycling of arsenic. This work investigated the sorption behavior of monothioarsenate (mtAsV) on amorphous aluminum hydroxide (am-Al(OH)3) at various pHs and characterized the adsorbed arsenic by using X-ray absorption spectroscopy (XAS) and Fourier transform infrared spectroscopy (FTIR). The uptake of arsenic by am-Al(OH)3 from mtAsV solution (86–16mg·g−1) was found to be much less than that from arsenate (AsV) solution (112–50.5mg·g−1) at all pHs applied (4–10). MtAsV was partly reduced to arsenite with concurrent production of elemental sulfur, and the interaction with am-Al(OH)3 at acidic pH enhanced the reduction, which is attributed to acid-catalysis at the am-Al(OH)3 surface. Transformation of mtAsV to arsenate also occurred during the adsorption process. The linear combination fitting (LCF) of As K-edge X-ray absorption near edge structure (XANES) spectra indicated that the arsenic adsorbed on am-Al(OH)3 was present dominantly as mtAsV, with minor amounts as arsenite and arsenate. The As K-edge extended X-ray absorption fine structure (EXAFS) and FTIR results suggested that bidentate binuclear complexation was involved in the adsorption of mtAsV on am-Al(OH)3. This study is the first documentation of mtAsV sorption on aluminum hydroxides and is necessary to assess geochemical cycling of arsenic in sulfidic natural or mining-impacted environments.


      PubDate: 2015-05-26T01:42:46Z
       
  • Subsurface hydrothermal processes and the bioenergetics of
           chemolithoautotrophy at the shallow-sea vents off Panarea Island (Italy)
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Roy E. Price , Douglas E. LaRowe , Francesco Italiano , Ivan Savov , Thomas Pichler , Jan P. Amend
      The subsurface evolution of shallow-sea hydrothermal fluids is a function of many factors including fluid–mineral equilibria, phase separation, magmatic inputs, and mineral precipitation, all of which influence discharging fluid chemistry and consequently associated seafloor microbial communities. Shallow-sea vent systems, however, are understudied in this regard. In order to investigate subsurface processes in a shallow-sea hydrothermal vent, and determine how these physical and chemical parameters influence the metabolic potential of the microbial communities, three shallow-sea hydrothermal vents associated with Panarea Island (Italy) were characterized. Vent fluids, pore fluids and gases at the three sites were sampled and analyzed for major and minor elements, redox-sensitive compounds, free gas compositions, and strontium isotopes. The corresponding data were used to 1) describe the subsurface geochemical evolution of the fluids and 2) to evaluate the catabolic potential of 61 inorganic redox reactions for in situ microbial communities. Generally, the vent fluids can be hot (up to 135°C), acidic (pH1.9–5.7), and sulfidic (up to 2.5mM H2S). Three distinct types of hydrothermal fluids were identified, each with higher temperatures and lower pH, Mg and SO4, relative to seawater. Type 1 was consistently more saline than Type 2, and both were more saline than seawater. Type 3 fluids were similar to or slightly depleted in most major ions relative to seawater. End-member calculations of conservative elements indicate that Type 1 and Type 2 fluids are derived from two different sources, most likely 1) a deeper, higher salinity reservoir and 2) a shallower, lower salinity reservoir, respectively, in a layered hydrothermal system. The deeper reservoir records some of the highest end-member Cl concentrations to date, and developed as a result of recirculation of brine fluids with long term loss of steam and volatiles due to past phase separation. No strong evidence for ongoing phase separation is observed. Type 3 fluids are suggested to be mostly influenced by degassing of volatiles and subsequently dissolution of CO2, H2S, and other gases into the aqueous phase. Gibbs energies (ΔGr ) of redox reactions that couple potential terminal electron acceptors (O2, NO3 −, MnIV, FeIII, SO4 2−, S0, CO2) with potential electron donors (H2, NH4 +, Fe2+, Mn2+, H2S, CH4) were evaluated at in situ temperatures and compositions for each site and by fluid type. When Gibbs energies of reaction are normalized per kilogram of hydrothermal fluid, sulfur oxidation reactions are the most exergonic, while the oxidation of Fe2+, NH4 +, CH4, and Mn2+ is moderately energy yielding. The energetic calculations indicate that the most robust microbial communities in the Panarea hot springs combine H2S from deep water–rock–gas interactions with O2 that is entrained via seawater mixing to fuel their activities, regardless of site location or fluid type.


      PubDate: 2015-05-26T01:42:46Z
       
  • A new analytical approach to determining Mo and Re speciation in sulfidic
           waters
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Trent P. Vorlicek , Anthony Chappaz , Laura M. Groskreutz , Natalie Young , Timothy W. Lyons
      The distinct reactivities of Mo and Re in oxic versus anoxic waters make these elements ideally suited for use as redox proxies. However, their full exploitation as geochemical tracers requires that their chemical transformations in sulfidic water be well understood. While thermodynamic data have been used to predict Mo and Re speciation within sulfidic waters, these predictions remain unsubstantiated because effective methodologies for separating the thiomolybdate (MoVIOxS4−x 2−) and thioperrhenate (ReVIIOxS4−x −) anions within natural settings have yet to be developed. Anion exchange chromatography (AEC) is often employed to provide separation of environmentally important anions. For example, thiometalates of As and Sb have been quantified in sulfidic geothermal waters using AEC coupled with inductively coupled plasma mass spectrometry (ICP-MS). Unfortunately, AEC methods are incapable of separating the thiomolybdate or thioperrhenate anions due to the extreme retention these thiometalates experience on AEC columns. Because reverse phase ion pair chromatography (RP-IPC) offers greatly diminished retention times, we have developed RP-IPC methods that are capable of separating all stable thiomolybdates and thioperrhenates within 40min. An isocratic method provides effective separation of the thiomolybdates. Thioperrhenates and mixtures of the thiomolybdates and thioperrhenates require gradient methods. Addition of p-cyanophenol (p-CP) to the eluent markedly reduces thiometalate retention and facilitates their separation. Our efforts could lead in the near future to coupling RP-IPC with ICP-MS or multi-collector ICP-MS for characterizing Mo and Re speciation in natural sulfidic waters as well as potential fractionation among Mo and Re isotopes during speciation changes.
      Graphical abstract image

      PubDate: 2015-05-21T05:00:20Z
       
  • High-resolution line-scan analysis of resin-embedded sediments using laser
           ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Rick Hennekam , Tom Jilbert , Paul R.D. Mason , Gert J. de Lange , Gert-Jan Reichart
      Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) line-scanning is a promising technique for producing high-resolution (μm-scale) geochemical records on resin-embedded sediments. However, this approach has not yet been thoroughly tested on sediment samples of known elemental compositions. Here, we address this through the analysis of resin-embedded quartz, calcite, and clay (montmorillonite) sediments spiked with Al, V, Mo, and Ba across a range of concentrations. LA-ICP-MS spot analyses on these samples were compared to data independently obtained by conventional techniques: solution nebulization-ICP-optical emission spectroscopy (SN-ICP-OES), SN-ICP-MS, and X-Ray Fluorescence on fused glass beads. Data were reported as (log-)ratios of the analyte elements normalized to Ca, Si, and Al for calcite, quartz and clay respectively to correct for varying ablation yields. Our LA-ICP-MS data demonstrate close agreement to within 6% of the reference values determined by the conventional techniques, with high correlation coefficients (R2 ≈1.0) across the full range of concentrations, and high precision (<3% relative standard deviations after three repeated analyses). Barium was hosted in both aluminosilicates and carbonates in the clay matrix, giving different yields (elemental fractionation), and leading to variable accuracy (mean deviation of 15%). A selection of the spiked sediments was used to produce artificial laminated sediments to further test the effects of line scanning on LA-ICP-MS accuracy and precision in resin-embedded samples. It appears that LA-ICP-MS line-scan analyses have good accuracy (deviations from the reference values generally <5%) and high precision (relative standard deviations of repeated analyses on the same sample generally <5%) for our target elements. Moreover, the LA-ICP-MS line-scanning closely records the alternating geochemical profiles of our artificial laminations. However, calcium showed a clear tailing signal at the transition from a calcite to a quartz layer, indicating that geochemical signals can be smeared at transitions to sediment layers devoid of analyte element(s). We also analyzed resin-embedded natural sediments for comparison of LA-ICP-MS line-scan data to parallel sub-samples measured by SN-ICP-OES and SN-ICP-MS. In general, the results from LA-ICP-MS line-scans of natural sediments correspond well with reference values and with high reproducibility, corroborating the results of the artificial laminations. The results for Ca, V, Mn, Sr, Mo, Ba, and U (in log-ratios to Al) suggest that these elements can be analyzed semi-quantitatively, showing correlation coefficients (R2) ranging from 0.59 to 0.85 compared to conventional analytical techniques. However, we show that using non-matrix matched calibration standards such as NIST SRM 610 may induce deviations from the LA-ICP-MS values to the reference values. Consequently, we recommend a simple protocol for further data correction in which binned-mean LA-ICP-MS values are calibrated to a parallel series of discrete samples analyzed by these conventional techniques.


      PubDate: 2015-05-21T05:00:20Z
       
  • Microbe–mineral interactions: The impact of surface attachment on
           mineral weathering and element selectivity by microorganisms
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Engy Ahmed , Sara J.M. Holmström
      One of the major gaps within the field of biogeochemistry is the lack of a detailed and deep understanding of the mechanism behind the microbial inducement of mineral dissolution. The association of microorganisms with the mineral surfaces is an important issue for understanding processes like mineral weathering, biomineralization, bioremediation and biofouling. The present study aims to investigate the performance of attached and unattached soil fungal and bacterial species in biotite weathering and in the selectivity of elements from biotite. Sterilized microplate devices were filled with biotite (>2mm) followed by an iron limited liquid growth medium and were inoculated separately with six different microbial species isolated from podzol soil: Erwinia amylovora, Pseudomonas stutzeri, Pseudomonas mendocina, Streptomyces pilosus, Neurospora crassa and Penicillium melinii. The experiment was designed in two set-ups: 1) attached form, in which the microorganisms were inoculated directly to the biotite surface, and 2) unattached form, in which 0.4μm PET track etched devices were used to separate the microbial cells from the biotite surface. Our findings indicate that the surface attached microorganisms led to a greater dissolution of elements from biotite than the unattached microorganisms that was evidenced by 1) higher dissolution of Fe, Al and Si, 2) greater decrease in pH of the liquid growth medium, and 3) relatively higher production of siderophores. Furthermore, there was no significant difference in the capability of element selectivity between the attached and unattached microbial forms. The biotite dissolution was promoted initially by complexation processes and later by acidification processes for most of the attached and unattached microorganisms. Thus, we conclude that despite the mineral dissolution induced by microbial attachment on the mineral surface, the element composition of the biotite and nutritional need of the microorganisms were the main factors affecting the element selectivity.


      PubDate: 2015-05-21T05:00:20Z
       
  • Editorial Board
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403




      PubDate: 2015-05-21T05:00:20Z
       
  • Numerical simulation of porosity and permeability evolution of Mount Simon
           sandstone under geological carbon sequestration conditions
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Liwei Zhang , Yee Soong , Robert Dilmore , Christina Lopano
      A numerical model was developed with the use of reactive transport code CrunchFlow to estimate porosity, permeability and mineral composition changes of Mount Simon sandstone under typical geological carbon sequestration conditions (P =23.8MPa and T =85°C). The model predicted a permeability decrease from 1.60mD to 1.02mD for the Mount Simon sandstone sample in a static batch reactor after 180days of exposure to CO2-saturated brine, which is consistent with measured permeability results. Model-predicted solution chemistry results were also consistent with laboratory-measured solution chemistry data. SiO2 (am) was the primary mineral that causes permeability decrease, followed by kaolinite. Both SiO2 (am) formation and kaolinite formation were attributed to the dissolution of quartz and feldspar. This study shows that the formation of SiO2 (am) and kaolinite in the pore space of host rock is possible under typical CO2 sequestration conditions. SiO2 (am) and kaolinite precipitation at the CO2 plume extent could reduce the permeability of host rock and improve lateral containment of free-phase CO2, contributing to overall security of CO2 storage.


      PubDate: 2015-05-21T05:00:20Z
       
  • Controls on the chemical and isotopic composition of carbonate springs
           during evolution to saturation with respect to calcite
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Pride T. Abongwa , Eliot A. Atekwana
      We investigated the stable carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC) in carbonate springs that evolve chemically to supersaturation with respect to calcite and to isotopic equilibrium with atmospheric CO2(g). The δ13C of DIC (δ13CDIC) will track isotopic fractionation accompanying carbon loss to the atmosphere, precipitation of calcite or carbon exchange with atmospheric CO2(g). We assessed the DIC and δ13CDIC evolution along the flow paths of springs in the field. Since chemical equilibrium is a precondition for isotopic equilibrium, and because it is difficult to follow the evolution of carbonate springs to isotopic equilibrium with atmospheric CO2(g) in field settings, three sets of spring samples were exposed to laboratory atmospheric CO2(g) and allowed to evolve to isotopic equilibrium. One subset of the experimental sample was agitated to simulate mixing in the field. The physical, chemical and carbon isotopic changes in the field and laboratory experiments were complex and varied. Chemical speciation and isotopic mass balance modeling showed that the evolution to calcite supersaturation can be conceptualized in 4 discrete steps, each characterized either by kinetic fractionation, equilibrium fractionation or carbon isotopic exchange with atmospheric CO2(g). These steps sequentially are (1) undersaturation to supersaturation where DIC decreases from CO2(g) loss from solution and small increases in the δ13CDIC (1–2‰) are from kinetic fractionation, (2) saturation to supersaturation where relatively no DIC is lost and small increases in the δ13CDIC (~1‰) are likely due to isotopic exchange of carbon between DIC and atmospheric CO2(g), (3) decreasing supersaturation where DIC concentration decreases and larger increases in the δ13CDIC (~5‰) are from equilibrium isotopic fractionation and (4) increasing saturation where the previous decreasing supersaturation and DIC concentration decreases reverse and increase because of evaporation, and the continued increase in the δ13CDIC (~2‰) is from isotopic exchange of carbon between DIC and atmospheric CO2(g). The unmixed laboratory samples evolved through steps 1, 2 and 3, while the mixed laboratory sample evolved through steps 1, 2, 3 and 4 because agitation of the solution increased the reaction rates and enhanced DIC–atmospheric CO2(g) interaction. The chemical and isotopic evolution of the field samples were limited to steps 1 and 2 because of the relatively short length of flowing springs which limit carbonate evolution to calcite saturation. Our findings suggest that for carbonate springs in contact with atmospheric CO2(g), significant δ13CDIC enrichment that occurs after calcite supersaturation is dominated by equilibrium isotopic effect, despite conditions conducive for calcite precipitation. We hypothesize that the chemical and isotopic behavior observed for the field and laboratory experiments may characterize other carbonate-rich waters (e.g., streams and lakes) evolving in contact with the atmosphere.


      PubDate: 2015-05-21T05:00:20Z
       
  • Tooth enamel sampling strategies for stable isotope analysis: Potential
           problems in cross-method data comparisons
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Hazel Reade , Rhiannon E. Stevens , Graeme Barker , Tamsin C. O'Connell
      The stable isotope composition of a given sample of tooth enamel is a function of the mineralisation process, the isotopic composition of the input signal, and sampling geometry. As the dominant axis of tooth growth is orientated from the occlusal surface to the enamel–root junction, the isotopic composition of an enamel sample that spans the full length of this axis is often taken to represent a whole-tooth or ‘bulk’ average, while serial (or ‘intra-tooth’) samples taken along the same axis are considered to track isotopic variation across the period of tooth formation. A ‘bulk’ sample of enamel should therefore approximate the mean of the ‘serial’ samples, and in teeth that form over a complete annual cycle ‘bulk’ and ‘serial’ sampling methods are often assumed to possess an annual–seasonal relationship. However, this does not take into account the effect of the mineralisation process and sampling geometry on the derived isotope signals. Here we investigate the reproducibility of both the ‘bulk’ and ‘serial’ sampling methods as independent, stand-alone techniques and then explore the assumed relationship between the two. Each method shows good reproducibility in the oxygen (δ18O) and carbon (δ13C) isotope results across paired left and right molars. However, cross method comparisons reveal a systematic offset between bulk and serial results within a single population, with 19 out of 23 δ18O bulk-tooth values being lower, and 22 out of 23 δ13C bulk-tooth values being higher, than the means of their respective intra-tooth sequences. In 4 out of 19 cases for δ18O and 6 out of 22 cases for δ13C the bulk-tooth results fall completely out of the range observed in the corresponding intra-tooth sequence. This finding has implications for cross-method data comparisons and questions the assumed temporal relationship between ‘serial’ and ‘bulk’ isotope data.


      PubDate: 2015-05-21T05:00:20Z
       
  • Comparison of arsenic and molybdenum geochemistry in meromictic lakes of
           the McMurdo Dry Valleys, Antarctica: Implications for oxyanion-forming
           trace element behavior in permanently stratified lakes
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Ningfang Yang , Kathleen A. Welch , T. Jade Mohajerin , Katherine Telfeyan , Darren A. Chevis , Deborah A. Grimm , W. Berry Lyons , Christopher D. White , Karen H. Johannesson
      Water samples were collected for arsenic (As) and molybdenum (Mo) analysis from different depths in Lakes Hoare and Fryxell, both of which are located in the Taylor Valley within the McMurdo Dry Valleys of Antarctica. Sampling depths within each lake were chosen to capture variations in As and Mo concentrations and As speciation in the oxic mixolimnia and anoxic monimolimnia of these meromictic lakes. Arsenic concentrations ranged from 0.67nmolkg−1 to 3.54nmolkg−1 in Lake Hoare and from 1.69nmolkg−1 to 17.5nmolkg−1 in Lake Fryxell. Molybdenum concentrations varied between 5.05nmolkg−1 and 43nmolkg−1 in Lake Hoare, and between 3.52nmolkg−1 and 25.5nmolkg−1 in Lake Fryxell. Concentrations of As and Mo generally increased with depth in the mixolimnion of each lake, consistent with uptake near the ice–water interface by organic particles and/or Fe/Mn oxides/oxyhydroxides, followed by gravitational settling and regeneration/remineralization at depth in the vicinity of the redoxcline. Arsenic concentrations either remained constant (Hoare) or increased with depth (Fryxell) in the anoxic monimolimnia, whereas Mo exhibited dramatic decreases in concentrations across the redoxcline in both lakes. Geochemical modeling predicts that As and Mo occur as thioanions in the anoxic bottom waters of Lakes Hoare and Fryxell, and further that the contrasting behavior of both trace elements reflects the respective reactivity of their thioanions towards Fe-sulfide minerals such as mackinawite (FeS) and/or pyrite (FeS2). More specifically, the geochemical model suggests that Fe-sulfide mineral precipitation in the anoxic monimolimnia of both lakes regulates dissolved sulfide concentrations at levels that are too low for As-sulfide minerals (e.g., orpiment, realgar) to precipitate, whereas mackinawite and/or pyrite react(s) with particle reactive thiomolybdate anions, possibly forming an Fe–Mo–S mineral that precipitates and, hence, leads to Mo removal from solution.


      PubDate: 2015-05-21T05:00:20Z
       
  • Novel 1H-Pyrrole-2,5-dione (maleimide) proxies for the assessment of
           photic zone euxinia
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Sebastian Naeher , Kliti Grice
      1H-Pyrrole-2,5-diones (maleimides) and 1-alkyl-2,3,6-trimethylbenzenes (aryl isoprenoids), degradation products of tetrapyrrole pigments and carotenoids respectively, were analysed and compared with pristane/phytane (Pr/Ph) ratios to reconstruct past redox conditions in three geologic sections. One section comes from the Middle–Late Devonian and was deposited before the Frasnian–Famennian boundary mass extinction. The two other sections span the Late Permian to the Early Triassic as well as the Late Triassic to the Early Jurassic, and recorded the Permian–Triassic (P/T) and Triassic–Jurassic (T/J) extinction events respectively. The 2-methyl-3-iso-butyl-maleimide (Me,i-Bu maleimide) to 2-methyl-3-ethyl-maleimide (Me,Et maleimide) and 2-methyl-3-n-propyl-maleimide (Me,n-Pr maleimide) to Me,Et maleimide ratios (Me,i-Bu/Me,Et and Me,n-Pr/Me,Et ratios) in the studied sections revealed a moderate to strong negative correlation to the aryl isoprenoid ratio (AIR), defined as (C13–C17 1-alkyl-2,3,6-trimethylbenzenes)/(C18–C22 1-alkyl-2,3,6-trimethylbenzenes), indicating that these maleimide ratios can be used as robust, specific indicators of photic zone euxinia (PZE). These results agreed with Pr/Ph ratios, which were used as diagnostic indicators to differentiate between oxic and anoxic conditions. In agreement with previous studies, the novel maleimide proxies suggest that all three mass extinctions were largely characterised by PZE depositional conditions.


      PubDate: 2015-05-21T05:00:20Z
       
  • Using a mathematical model of a weathering clast to explore the effects of
           curvature on weathering
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): M.I. Lebedeva , P.B. Sak , L. Ma , S.L. Brantley
      For the first time, we show that rind thicknesses developed on surfaces of a clast with different values of curvature can be used to estimate the duration of clast weathering. To obtain an analytical expression for the velocity of the curvilinear weathering front on a clast of arbitrary shape, we approximate our previous multi-mineral reactive-diffusion model and explore a simplified 2-D model numerically and analytically. Our analysis documents that with increasing curvature of the weathering front, the mathematical description of weathering advance is equivalent to that derived for advection as the dominant solute transport mechanism, even for the case where transport is occurring by diffusion only. Specifically, for a curvilinear weathering front with constant curvature K <0, diffusivity (D), and porosity (ϕ), the normal component of the weathering advance rate can be calculated using an advection-like term where the advection velocity v can be expressed as v = Dϕ K . Therefore, at points along the rind–core interface with K <0, rind thickness is directly proportional to the absolute value of the curvature of the core–rind interface. The reaction front thickness also increases with K. These inferences are in agreement with field observations. This quantitative analysis allows an assessment of the duration of weathering if certain parameters are known. For example, using the difference in curvature observed at two positions for a clast that weathered in Guadeloupe (0.12mm−1 and 0.018mm−1) and the corresponding rind thickness difference (35.8mm and 20.6mm), we estimated the duration of weathering to be about 118ky, which is consistent with the weathering ages previously determined by U-series isotope disequilibrium.


      PubDate: 2015-05-21T05:00:20Z
       
  • Geochemistry of vein and wallrock carbonates from the Ediacaran system in
           South China: Insights into the origins of depositional and
           post-depositional fluids
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Yan-Yan Zhao , Yong-Fei Zheng
      A combined study of carbon–oxygen isotopes and major-trace elements was carried out for vein and wallrock carbonates from the Lantian Formation of the Ediacaran system in South China. The results provide geochemical constraints on the origins of depositional and post-depositional fluids with respect to recognition of the post-depositional alteration. The Lantian Formation was subdivided into the Upper and Lower carbonate units. For the Upper Unit (UU), petrographic observations indicate that wallrock carbonates only experienced limited post-depositional alteration. The wallrock exhibits δ18O values of −19.2 to −13.1‰ and δ13C values of −9.6 to −8. 9‰ (both relative to VPDB). The vein carbonates are divided into Groups A and B based on their δ18O values and REE+Y patterns. Group A veins show nearly similar δ18O and δ13C ranges to, but different REE+Y patterns from, the wallrock. These data indicate that the origin of fluids for Group A veins may be similar to that of wallrock, which was probably the mixture of continental and marine glacial meltwaters. Group B veins show both different δ18O values and REE+Y patterns from, but similar δ13C values to, the wallrock. This indicates that post-depositional fluids are different from the depositional fluids and thus of external origin. Low δ18O values −23.7 to −18.1‰ for Group B veins indicate that the post-depositional fluids were derived from continental glacial meltwater. There is no correlation between δ18O and δ13C values for these vein and wallrock carbonates, suggesting that the wallrock was not extensively altered by the post-depositional fluids. For the lower unit (LU), veins exhibit different C–O isotope compositions and REE+Y patterns from wallrock carbonate, indicating that post-depositional fluids have different geochemical compositions from depositional fluids and thus are of external origin. The simultaneous changes in δ18O and δ13C values with different laminations in the wallrock carbonates suggest the preservation of the primary geochemical compositions. The δ18O values and REE+Y patterns for the LU veins indicate that the post-depositional fluids were probably the mixtures of continental and marine glacial meltwaters. Therefore, the Lantian carbonates record the incorporation of continental glacial meltwater into the marginal sea of Ediacaran age. Nevertheless, the geochemical screening can provide constraints on the preservation of primary signatures and thus on the origins of depositional and post-depositional fluids in the Neoproterozoic marine carbonates.


      PubDate: 2015-05-21T05:00:20Z
       
  • Isotope Dilution-AMS technique for 36Cl and Cl determination in low
           chlorine content waters
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Camille Bouchez , Julie Pupier , Lucilla Benedetti , Pierre Deschamps , Valéry Guillou , Karim Keddadouche , Georges Aumaître , Maurice Arnold , Didier Bourlès
      The cosmonuclide 36Cl is relevant for hydrological applications due to its conservative chloride form. 36Cl measurements are commonly performed by Accelerator Mass Spectrometry (AMS) while stable chlorine concentration measurements are usually performed by ion chromatography (IC). They can also be achieved by Isotope Dilution-Accelerator Mass Spectrometry (ID-AMS) which offers the advantage of the simultaneous determination of both Cl and 36Cl contents. The range of applicability of this method has been assessed on rock samples but, to our knowledge, no experimental tests were conducted on water samples characterized by low chloride contents. This study aims at investigating the accuracy and precision of the ID-AMS method for such low chlorine water samples for which chloride and 36Cl determinations remain highly challenging. First, internal and external errors were evaluated on the measured ratios and uncertainties on Cl and 36Cl concentrations were assessed at ±5% and ±7% respectively using standard error propagation calculations. Focused on the 0.1 to 10mg·L−1 concentration range, the Cl concentrations of thirty-five gravimetric standards were determined by ID-AMS leading to a 1:1 line (R2 =0.99) and a 1-sigma uncertainty on [Cl−] of ±5%, consistently with the propagated uncertainty. Finally, the optimal sample volume to reach accurate 36Cl and Cl measurements with ID-AMS for water sample concentrations within the 0.1 to 10mg·L−1 range has been theoretically estimated.


      PubDate: 2015-05-21T05:00:20Z
       
  • The densities and dimensions of recoil-track etch pits in mica
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Konstanze Stübner , Raymond Jonckheere , Lothar Ratschbacher
      Measurements of alpha-recoil-track densities in mica as a basis for geological dating depend on an etch model relating the number of tracks per unit volume (N RT) to the counted number of etched tracks per unit area (ρ RT). The model of Gögen and Wagner (2000) implies that ρ RT increases linearly with etch time (t E), so that N RT can be calculated from the slopes or intercepts of step-etch functions ρ RT(t E). This model rests on the assumption that the etch rate of the mica surface (v V) and the horizontal etch pit growth rate (v H) are both constant, in contradiction with experimental results and computer simulations of mineral dissolution. We present results of four different experiments aimed at relating etch pit densities to volumetric track densities. Step-etch data are vulnerable to observation-related artefacts at increasing t E and lack the resolution to confirm or refute the supposed linear increase of ρ RT with t E. The intercepts of regression lines fitted to step-etch data are imprecise and perhaps inaccurate. Intercept estimates based on mirror-image counts and (etch)–anneal–etch experiments indicate that v V increases during the initial etching stages. The (etch)–anneal–etch results show that no pre-etch is in fact required, implying that surface tracks are more resistant to annealing than tracks in the bulk of the mineral. Track-size measurements confirm that v H is also not constant but decreases with increasing etch-pit size. The results show that no recoil-track etch pit in phlogopite etched in 40% HF grows larger than ca. 7μm and that the track-size distribution becomes quasi-invariant at >6min etching. This signifies that there exist accessible etching conditions at which the etch-pit size distribution becomes a fixed, distorted reflection of the size distribution of latent recoil tracks. It is not improbable that track addition and loss also cancel each other out in this equilibrium state, in which case both the etch-pit size distributions and densities ρ RT become independent of etch time.


      PubDate: 2015-05-21T05:00:20Z
       
  • The continuous re-equilibration of carbon isotope compositions of hydrous
           Mg carbonates in the presence of cyanobacteria
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Vasileios Mavromatis , Irina A. Bundeleva , Liudmila S. Shirokova , Christian Millo , Oleg S. Pokrovsky , Pascale Bénézeth , Magali Ader , Eric H. Oelkers
      The hydrous magnesium carbonate minerals dypingite (Mg5(CO3)4(OH)2·5H2O) and nesquehonite (MgCO3·3H2O) were precipitated in the presence and absence of Gloeocapsa sp. and Synechococcus sp. cyanobacteria in batch reactors. The Δ13Cmineral–DIC is similar in both biotic and abiotic systems. Stable carbon isotope analyses of the precipitated minerals and co-existing fluids indicated that the δ13C values of the precipitated carbonates co-vary over time with the δ13CDIC of the fluid phase, even after the precipitation of the carbonate is complete. This observation indicates the continuous isotopic exchange between the carbonates and the fluid, a process that efficiently resets the original δ13C values of the solids. Therefore although cyanobacterial-induced Mg carbonates are 10±5‰ enriched in 13C compared to inorganic carbonates, the δ13C values of natural hydrous Mg carbonates may reflect post precipitation processes and may not be reliable for paleo-environmental reconstructions.


      PubDate: 2015-05-21T05:00:20Z
       
  • Can Mn–S redox cycling drive sedimentary dolomite formation? A
           hypothesis
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Daniel A. Petrash , Stefan V. Lalonde , Gabriela González-Arismendi , Robert A. Gordon , José A. Méndez , Murray K. Gingras , Kurt O. Konhauser
      The formation of dolomite in modern peritidal environments is linked to the degradation of buried microbial mats, with complexation of Ca and Mg by extracellular polymeric substances (EPSs) and alkalinity generation through organic carbon respiration facilitating the nucleation of dolomite precursors. In the past two decades, microbial sulfate reduction, methanogenesis, and methanotrophy have all been considered as potential drivers of the nucleation process, but it remains unclear why dolomite formation could not also occur in suboxic sediments where abundant alkalinity is produced by processes linked to Mn(IV) and/or Fe(III) reduction coupled with the diffusion and reoxidation of reduced sulfur species. Here we report the interstitial occurrence of spheroidal aggregates of nanometer-scale Ca-rich dolomite rhombohedra within suboxic sediments associated with remnant microbial mats that developed in the peritidal zone of the Archipelago Los Roques, Venezuela. Multiple analytical tools, including EPMA, ICP-MS, synchrotron-based XRF and XRD, and spatially resolved XANES microanalyses, show that the dolomite-cemented interval exhibits depleted bulk iron concentrations, but is interstitially enriched in Mn and elemental sulfur (S0). Manganese occurs in several oxidation states, indicating that the dolomite-cemented interval was the locus of complex biological redox transformations characterized by coupled Mn and S cycling. The tight correspondence between sedimentary Mn and MgCO3 concentrations further hints at a direct role for Mn during dolomitization. While additional studies are required to confirm its relevance in natural settings, we propose a model by which coupled Mn–S redox cycling may promote alkalinity generation and thus dolomite formation in manner similar to, or even more efficiently, than bacterial sulfate reduction alone.


      PubDate: 2015-05-21T05:00:20Z
       
  • Bromine speciation in hydrous silicate melts at high pressure
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): B. Cochain , C. Sanloup , C. de Grouchy , C. Crépisson , H. Bureau , C. Leroy , I. Kantor , T. Irifune
      Br speciation in hydrous silicate melts at high pressure has been investigated up to 7.6GPa using X-ray absorption spectroscopy (XAS) at the Br K-edge in a Paris–Edinburgh press. Br in silicate melts is surrounded by an average of 6 Na cations, a number slightly increasing with pressure (5.8 to 6.6), with a Br–Na distance increasing from 3.49 to 3.72Å. Two oxygens, either from a water, an –OH molecule or from the tetrahedral silicate network, with an average Br–O distance of 1.80Å, form the closest coordination shell around Br ions. The persistence of an alkali shell around Br, in a structure similar to crystalline NaBr, throughout the pressure range investigated shows that Br can be retained in the melt structure at relatively high pressure and supports the idea of its deep recycling. Finally, our results confirm that Br could be efficiently degassed with water at low pressures and that Br may also have been efficiently degassed along with water during the early stages of an oxidized magma ocean.


      PubDate: 2015-05-21T05:00:20Z
       
  • Gas blowout from shallow boreholes near Fiumicino International Airport
           (Rome): Gas origin and hazard assessment
    • Abstract: Publication date: Available online 16 May 2015
      Source:Chemical Geology
      Author(s): Maria Luisa Carapezza , Luca Tarchini , Domenico Granieri , Mauro Martelli , Alessandro Gattuso , Nicola Pagliuca , Massimo Ranaldi , Tullio Ricci , Fausto Grassa , Andrea Rizzo , Luca Pizzino , Alessandra Sciarra
      In summer 2013 a toxic and polluting gas blowout (19 tonne day-1 CO2, 95 kg day-1 CH4) occurred from two shallow boreholes drilled at only 50 m from the International Airport of Rome (Italy), in the town of Fiumicino. Another gas blowout occurred in the same period from a borehole located offshore, 2 km away, also generating sea-water acidification; it lasted only a couple of days. Onshore, CO2 was also diffusing from holes within the soil, particularly towards the airport, generating a soil flux up to 1.8 tonne day-1. In 3.5 months ~1500 tonnes of CO2 and 5.4 tonnes of CH4 were emitted in the atmosphere. Temporal monitoring of gas geochemistry indicates that in this area a mixing occurs between shallow and pressurized gas pockets, CO2-dominated, but with different chemical (i.e., He/CH4 ratio) and isotopic (3He/4He, δ13C-δDCH4) characteristics. Numerical simulation of CO2 dispersion in the atmosphere showed that dangerous air CO2 concentrations, up to lethal values, were only found near the vents at a height of 0.2 m. Fiumicino is a high blowout risk area, as CO2 rising through deep reaching faults pressurizes the shallow aquifer contained in gravels confined underneath shales of the Tiber delta deposits. The Fiumicino blowout is a typical example of dangerous phenomenon that may occur in urban context lying nearby active or recent volcanoes and requires quick response on hazard assessment by scientists to be addressed to civil protection and administrators.


      PubDate: 2015-05-21T05:00:20Z
       
  • 40Ar/39Ar ages and residual volatile contents in degassed subaerial and
           subglacial glassy volcanic rocks from Iceland
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): P.L. Clay , H. Busemann , S.C. Sherlock , T.L. Barry , S.P. Kelley , D.W. McGarvie
      Major volatile contents (H2O, CO2, F, Cl, and S) and 40Ar/39Ar ages have been determined in variably degassed rhyolite obsidians from Pleistocene–Holocene aged subaerial and subglacial eruption environments from the Torfajökull volcanic center and the monogenetic volcano at Prestahnúkur (Iceland). Icelandic subglacial rhyolites preserve residual H2O contents of 0.08–0.69wt.%, undetectable CO2, 840–1780ppm F, 430–2000ppm Cl and 6–45ppm S. Most subglacial obsidians have degassed volatile signatures at the time of their eruption under ice. One eruption (Bláhnúkur, Torfajökull) showed H2O contents which exceed those expected for quenching at atmospheric pressures (up to 0.69wt.% H2O) and are consistent with eruption at ~40kbar of pressure or equivalent to under ~450m of ice. Altered and microcrystalline groundmass in some subglacial rhyolites yield variable volatile contents that are likely the result of micro-scale variability and the presence of alteration products. Two subaerial rhyolite obsidians gave low volatile contents consistent with quenching at the surface. New 40Ar/39Ar ages were determined on the subglacially erupted rhyolites and provide broad constraints on the timing and frequency of glacio-volcanism during the Pleistocene. Three subglacial eruptions at Torfajökull yielded ages of 108±22ka (Bláhnúkur), 255±20ka (Hábarmur) and 236±7ka (Háskerðingur) which fit into a range of previously determined ages at Torfajökull of 67–384ka. A new age of 132±19ka was determined for the monogenetic subglacial eruption at Prestahnúkur. Holocene-aged subaerial volcanic glass produced no ages due to large amounts of air-derived 40Ar (84–93%). Preliminary apparent 40Ar/39Ar ages from the limited feldspar phenocrysts available in these crystal-poor samples yield erroneous apparent plateau ages far older than eruption ages suggesting a homogenously distributed excess Ar component. Disturbances to the 40Ar/39Ar geochronometer are correlated with a heterogeneous distribution of volatiles in glass and linked to hydration (e.g., Kaldaklofsfjöll), mild alteration (e.g., Hábarmur) and alteration and/or recrystallization of glass (e.g., Bláhnúkur). Our observations indicate however, that unaltered, subglacially erupted glasses that are partially to wholly degassed are promising materials for the application of the 40Ar/39Ar geochronometer. The precision to which we can date these young glassy samples currently limits the use of 40Ar/39Ar geochronometer as a tool for understanding the exact timing and duration of glaciovolcanism in the recent past.


      PubDate: 2015-05-21T05:00:20Z
       
  • Changes in porosity, permeability, water retention curve and reactive
           surface area during carbonate rock dissolution
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Tobias S. Rötting , Linda Luquot , Jesus Carrera , Diosenia J. Casalinuovo
      Chemical reactions between fluids and the solid phase change porous media by dissolution and precipitation processes that modify the pore space geometry and associated hydraulic and chemical properties (saturated and relative permeability, porosity, capillary pressure curve, pore size distribution, reactive surface area). Knowledge of these properties and how they evolve with time is important for understanding and modelling multi-phase flow or reactive transport. Most published studies for reservoir rocks concentrate on the changes of porosity and saturated liquid permeability, but few data are available on the other parameters, and even less on their evolution during interaction with acidic solutions. We investigate the joint evolution of transport parameters of sedimentary carbonate rocks upon reaction with dilute acid (HCl). We first characterize the initial permeability, porosity and pore size distribution. We then attack the rock samples by permeation with HCl (pH3.5 or 4.1) and characterize them again after partial dissolution. Several dissolution-characterization cycles were performed on each sample in order to study the evolution and interaction of the different parameters. In two coarse calcite cores C1 (dissolved at pH3.5) and C2 (pH4.1) with high initial permeability, significant dissolution occurred only in pores with diameters greater than 0.022mm. Permeability increased very little even though porosities increased by 2.6–5.8%. In fine-grained dolomite cores, dissolution affected a broader range of pore sizes, down to 0.001mm in D1 (pH3.5), and down to 0.0066mm in D2 (pH4.1). A wormhole broke through in D1 after percolation of only 5000 pore volumes of acid. Effective reactive surface areas decreased by a factor of 240–250 in the cores dissolved at pH3.5, but only by a factor of 2–25 in the cores dissolved at pH4.1. A simple 1-D model was unable to simulate the evolution of the reactive surface during wormhole formation. Effective reactive surface area correlated negatively with permeability, and was consistent with the geometric area of the pores that carried most flux, which was calculated from the retention curves.
      Graphical abstract image

      PubDate: 2015-05-21T05:00:20Z
       
  • Kinetics of calcite dissolution in CO2-saturated water at temperatures
           between (323 and 373)K and pressures up to 13.8MPa
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Cheng Peng , John P. Crawshaw , Geoffrey C. Maitland , J.P. Martin Trusler
      We report measurements of the calcite dissolution rate in CO2-saturated water at pressures ranging from (6.0 to 13.8)MPa and temperatures from (323 to 373)K. The rate of calcite dissolution in HCl(aq) at temperatures from (298 to 353)K was also measured at ambient pressure with pH between 2.0 and 3.3. A specially-designed batch reactor system, implementing a rotating disc technique, was used to obtain the dissolution rate at the solid/liquid interface of a single crystal, free of mass transfer effects. We used vertical scanning interferometry to examine the texture of the calcite surface produced by the experiment and the results suggested that at far-from-equilibrium conditions, the measured calcite dissolution rate was independent of the initial defect density due to the development of a dynamic dissolution pattern which became steady-state shortly after the onset of dissolution. The results of this study indicate that the calcite dissolution rate under surface-reaction-controlled conditions increases with the increase of temperature from (323 to 373)K and CO2 partial pressure from (6.0 to 13.8)MPa. Fitting the conventional first order transition state kinetic model to the observed rate suggested that, although sufficient to describe calcite dissolution in CO2-free HCl(aq), this model clearly underestimate the calcite dissolution rate in the (CO2 +H2O) system over the range of conditions studied. A kinetic model incorporating both pH and the activity of CO2(aq) has been developed to represent the dissolution rates found in this study. We report correlations for the corresponding reaction rate coefficients based on the Arrhenius equation and compare the apparent activation energies with values from the literature. The results of this study should facilitate more rigorous modelling of mineral dissolution in deep saline aquifers used for CO2 storage.


      PubDate: 2015-05-21T05:00:20Z
       
  • In-situ iron isotope analyses of pyrites from 3.5 to 3.2Ga sedimentary
           rocks of the Barberton Greenstone Belt, Kaapvaal Craton
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Kazumi Yoshiya , Yusuke Sawaki , Takazo Shibuya , Shinji Yamamoto , Tsuyoshi Komiya , Takafumi Hirata , Shigenori Maruyama
      The Barberton Greenstone Belt (BGB), South Africa, consists of volcano-sedimentary and sedimentary successions deposited between 3.5 and 3.2Ga, which are subdivided into three groups: the Onverwacht, Fig Tree, and Moodies. After deposition, the Barberton Greenstone Belt underwent relatively low-grade tectonothermal events, enabling estimation of surface environmental events and biological evolution in the Middle Archean. Isotopic studies suggest the activity of methanogen, sulfate-reducing bacteria, and photosynthetic bacteria at 3.4Ga. Microbial dissimilatory iron reduction (DIR) is also considered one of the earliest expressions of metabolism on Earth, but the isotopic signal of microbial DIR is still uncertain in the Archean. We performed in-situ iron isotope analyses of individual pyrites in sedimentary rocks from the BGB, using a femtosecond laser ablation multi-collector ICP-MS technique (fs-LA-MC-ICP-MS) to find isotopic evidence for the microbial activity. We obtained a large variation in δ56Fe values, ranging from −1.84 to +3.79‰. Most of the δ56Fe values of pyrites from the Hooggenoeg Complex show positive values, whereas those from the Noisy Complex and subsequent sedimentary sequences show a wide variation from negative to positive δ56Fe values. One of the main differences between these complexes is their depositional depth. The Hooggenoeg Complex was probably deposited in a deep-ocean environment, whereas the Noisy Complex was in shallow water. The negative δ56Fe values of pyrite grains in the Noisy Complex, Onverwacht Group indicate the occurrence of partial reduction caused by microbial DIR in a Middle Archean shallow sea.


      PubDate: 2015-05-21T05:00:20Z
       
  • Geochemical evolution and residence time of porewater in low-permeability
           rocks of the Michigan Basin, Southwest Ontario
    • Abstract: Publication date: 26 May 2015
      Source:Chemical Geology, Volume 404
      Author(s): Tom A. Al , Ian D. Clark , Laura Kennell , Mark Jensen , Ken G. Raven
      Drill cores collected from an 860-m-thick Paleozoic (Cambrian to Devonian) sedimentary sequence of the Michigan Basin in southwest Ontario, Canada, were used to develop vertical profiles of natural tracers in porewater from the sedimentary sequence and build a conceptual model regarding processes and timing of solute migration. In this paper, the conceptual model is quantitatively assessed with numerical simulations of diffusion-dominated solute transport (Cl− and 18O). The conceptual model suggests that the tracer profiles result from mixing between a deep shield brine with isotopic characteristics consistent with water–rock interaction over a long period of geologic time, Ordovician seawater at intermediate depth, and an overlying hypersaline brine formed by evaporation of seawater in a restricted basin during the Silurian. Between the peak of basin consolidation at 300MaBP and the Pleistocene (2.6MaBP), salinity and isotopic concentration gradients are thought to have driven mixing, dominantly by diffusion. Water isotope data indicate that in the Pleistocene, melt water infiltrated to a maximum depth of 330m in the Silurian Salina Formation, likely in response to transient glacial loading and elevated sub-glacial hydraulic pressures. Also in the Pleistocene, over-pressured conditions in deep regions of the basin drove high-salinity brine eastward in the basal Cambrian aquifer, interrupting pre-existing tracer-diffusion profiles. A base case set of boundary and initial conditions provides a good fit between simulated and measured tracer profiles, but requires effective diffusion coefficients (De ) in the Ordovician limestones that are lower than the average values obtained in laboratory measurements by as much as a factor of 10. The base-case values are still within the overall range of measured values. At the formation scale, relatively low in-situ De values are attributed to the intermittent occurrence of gas and liquid hydrocarbons in the Ordovician carbonate units and the effect of lithostatic confining pressure; neither of which are accounted for in the laboratory measurements but both would have the effect of decreasing De for aqueous solutes. The numerical model is most sensitive to the formation-specific De values and geologically reasonable variations in initial and boundary conditions have a relatively small influence on the results. Additional simulations indicate that hydraulic disturbance and ingress of meltwater during the Pleistocene can explain second-order features observed in the measured tracer profiles. This work provides supporting evidence for previous research indicating that the porewater residence time in the Ordovician sequence approaches the age of the rocks.


      PubDate: 2015-05-21T05:00:20Z
       
  • MinKin: A kinetic modeling program for the precipitation, dissolution, and
           phase transformation of minerals in aqueous solution
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Daniel R. Hummer , Peter J. Heaney
      The development of time-resolved, synchrotron scattering techniques has recently enabled the collection of in situ mineral abundance data during mineral–fluid reactions. However, few computational algorithms exist to analyze the kinetics of such reactions. Here we present MinKin (for “Mineral Kinetics”), a global optimization code for Matlab capable of fitting a standard chemical kinetic model to experimental mineral abundance data. MinKin allows users to specify the species and reactions of a geochemical system consisting of a fluid with up to two aqueous species and up to three mineral species, and then uses the global optimization algorithm of Differential Evolution (DE) to calculate the rate constants that minimize the error between the model and the data. Trial calculations reveal that MinKin is able to simultaneously calculate up to six rate constants on a time scale of minutes, with an accuracy of roughly the same magnitude as that of the input data.


      PubDate: 2015-05-21T05:00:20Z
       
  • Effects of ultraviolet radiation on mercury isotope fractionation during
           photo-reduction for inorganic and organic mercury species
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Carla H. Rose , Sanghamitra Ghosh , Joel D. Blum , Bridget A. Bergquist
      Photo-reduction of mercury (Hg) is an important mechanism for removal of both Hg2+ species and monomethylmercury (MMHg) from surface waters. Large mass independent fractionation (MIF) signatures of Hg isotopes preserved in natural samples are thought to reflect MIF produced during aqueous photo-reduction by the magnetic isotope effect (MIE). Recently, Hg MIF signatures in natural samples are being used to quantify photochemical reduction in aquatic systems. However, the fractionation factors used are from laboratory experiments that did not investigate many of the environmental parameters that may affect MIF during photo-reduction. In this study, the effects of different regions of the solar spectrum on the expression of MIF caused by the MIE during Hg2+ and MMHg photo-reduction were investigated to assess how the type of radiation affects the extent and signature of MIF. Photo-reduction in the presence of DOM was carried out through exposure to the full natural solar spectrum unfiltered, with the ultraviolet B (UVB; 290–320nm) portion removed and with both the UVB and the ultraviolet A (UVA; 320–400nm) removed. There is a clear relationship between the expression and magnitude of MIF and the energy of incident radiation for both Hg2+ and MMHg photo-reduction. The experiments indicate that MIF produced during photo-reduction of Hg2+ is significantly influenced by both UVB and UVA radiation. For MMHg photodemethylation, however, UVB radiation is mostly responsible for the MIF with minor contributions from UVA. Overall, there is a lack of correlation between the observed MIF and total photo-reduction in the experiments conducted in this study, which indicates that indirect and other non-MIF producing pathways of photo-reduction are the dominant pathways by which Hg species are being photo-reduced. Extrapolating these experimental results to natural systems is difficult as the experiments were not performed at realistic Hg/DOM ratios and the distribution of ligands that Hg is bound to in the experiments likely differs from natural systems. However, the strong relationship between MIF and energy of incident radiation in this study supports Hg MIF signatures as promising tools for helping to quantify photochemical cycling of mercury, but this study also highlights the need to understand the link between MIF and total photo-reduction before this tool can be fully utilized.


      PubDate: 2015-05-21T05:00:20Z
       
  • Petrology, geochemistry and low-temperature alteration of lavas and
           pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): A. Willcox , I. Buisman , R.S.J. Sparks , R.J. Brown , S. Manya , J.C. Schumacher , H. Tuffen
      Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which preserve extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigation of kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrix assemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primary groundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stage crystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite–vermiculite–montmorrilonite, minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclastic rocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had ~21wt.% SiO2, ~22wt.% CaO, ~23wt.% MgO and Mg# ~70. The IHV are classified as calcite kimberlites. The total volatile concentrations of the primary melt are ~14wt.%, which predominantly consists of CO2 although the H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, low-temperature alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks results from low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X is potentially a new mineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentine minerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose that serpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain the presence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.


      PubDate: 2015-05-21T05:00:20Z
       
  • Hydrothermal activity at the ultraslow- to slow-spreading Red Sea Rift
           traced by chlorine in basalt
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Froukje M. van der Zwan , Colin W. Devey , Nico Augustin , Renat R. Almeev , Rashad A. Bantan , Ali Basaham
      Newly formed oceanic crust is initially cooled by circulating seawater, although where this occurs and over what regions fluids enter the crust is still unclear. Differences in the chlorine (Cl) concentrations between mid-ocean ridge basalt and seawater potentially make Cl a sensitive tracer for this hydrothermal circulation, allowing assimilation of hydrothermal fluids or hydrothermally altered crust by rising magma to be traced by measuring excess Cl in erupted lavas. Such excess Cl has been found in basalts from fast-spreading ridges (Cl concentrations up to 1200 ppm), but not so far on ultraslow- and slow-spreading ridges, where lower Cl values in the basalts (~50–200 ppm) make variations harder to measure. The Red Sea, with its relatively saline bottom water (40–42‰, cf. 35‰ salinity in open ocean water), the presence of axial brine pools (up to 270‰ salinity) and thick evaporite sequences flanking the young rift provides an ideal opportunity to study the incorporation of hydrothermal Cl at an ultraslow- to slow-spreading ridge (max. 1.6 cm/yr). Both absolute Cl concentrations (up to 1300 ppm) and ratios of Cl to elements of similar mantle incompatibility (e.g. K, Nb) are much higher in Red Sea basalts than for average ultraslow- and slow-spreading ridges. An origin of these Cl-excesses by seafloor weathering or syn-eruptive contamination can be excluded, as can mineral/melt fractionation during melting or crystallisation, based on trace element data. Instead, the incorporation of Cl at depth derived from hydrothermal circulation either by direct assimilation of hydrothermal fluids or through mixing of magma with partial melts of the hydrothermally altered crust is indicated. We see no influence of local spreading rate, the intensity of seafloor fracturing or the calculated depth of last crystal fractionation on Cl-excess. Seafloor areas with clear evidence of present or recent hydrothermal activity (brine pool temperatures above ambient, presence of hydrothermal sediments) always show Cl-excess in the local basalts and there is a positive correlation between Cl-excess and intensity of local volcanism (as determined by the percentage of local seafloor showing volcanic bathymetric forms). From this we conclude that Cl-excess in basalts is related to high crustal temperatures and hydrothermal circulation and so can be used to prospect for active or recently extinct hydrothermal systems. Samples recovered within 5 km of a seafloor evaporite outcrop show particularly high Cl-excesses, suggesting addition of Cl from the evaporites to the inflow fluids and that this may be the length scale over which hydrothermal recharge occurs.
      Graphical abstract image

      PubDate: 2015-05-21T05:00:20Z
       
  • Redox architecture of an Ediacaran ocean margin: Integrated
           chemostratigraphic
           (δ13C–δ34S–87Sr/86Sr–Ce/Ce*) correlation of
           the Doushantuo Formation, South China
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Huan Cui , Alan J. Kaufman , Shuhai Xiao , Maoyan Zhu , Chuanming Zhou , Xiao-Ming Liu
      Early diagenetic silicification and phosphatization of the Ediacaran Doushantuo Formation (ca. 635 to 551Ma) in South China offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes, including various algal groups and potentially animals. In order to understand how the ecological and taphonomic distribution of these Ediacaran eukaryotes was controlled by oceanic redox conditions, it is critical to reconstruct the redox architecture of the sedimentary basin. Recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the Doushantuo Formation. One argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments. The other contends that the sulfidic water mass was largely restricted to the intra-shelf basin behind a rimmed margin. These two models make different predictions about the stratigraphic completeness and correlation of the Doushantuo Formation. To test these predictions, we generated high-resolution time-series trends of multiple isotopic and elemental tracers, including δ34S, 87Sr/86Sr and Ce/Ce*, to facilitate an integrated chemostratigraphic correlation between inner shelf (Xiaofenghe), intra shelf (Jiulongwan), and outer shelf (Yangjiaping and Zhongling) sections. Our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section. The euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections. Viewed from our revised chemostratigraphic framework, euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin. Carbonates in the upper Doushantuo Formation at Jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly (i.e., the Shuram Excursion) coincident with a drop in pyrite sulfur isotope values and a significant rise in 87Sr/86Sr from 0.7080 to 0.7090. The integrated stratigraphic data from South China suggest that the onset of the Shuram Excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium, as well as sulfate, to the Ediacaran basin.


      PubDate: 2015-05-21T05:00:20Z
       
  • In situ X-ray diffraction study of decomposition of polycyclic aromatic
           hydrocarbons at pressures of 7–15GPa: Implication to fluids under
           the Earth's and planetary environments
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Artem D. Chanyshev , Konstantin D. Litasov , Anton F. Shatskiy , Eiji Ohtani
      The behavior of polycyclic aromatic hydrocarbons (PAHs) at high pressures and temperatures has been investigated as a part of study of the deep-seated C–O–H fluids in the Earth and planetary interiors. The theoretical calculations of fluid compositions at the Earth's mantle conditions in the simple C–O–H system indicate that dominant fluid species are CH4 and H2O, with subordinate H2, and heavier hydrocarbons. However, calculations of equations of state for a broader range of hydrocarbons predict an appearance of heavy alkanes and PAHs under high pressures (>6–7GPa). Here, we determined stability of the major PAHs (from naphthalene to coronene) using in situ X-ray diffraction in multianvil apparatus at the SPring-8 synchrotron radiation facility (Japan). It was found, that at 7–9GPa, PAHs become unstable at temperatures above 873–1073K. We suggest that stability of PAHs is limited by benzene ring decomposition. The data for coronene only at 15GPa confirms limited temperature stability of PAHs. The PAH decomposition products consist of amorphous hydrogenated carbon at 7–9GPa at 973–1073K. Coronene decomposition products consist of diamond and trans-polyacetylene at 15.5GPa and 973K. Determined PAH decomposition temperatures (873–1073K) are lower than known Earth's geotherms and subduction slab P–T profiles at pressures of 7–9GPa (220–280km depths). According to these data, PAH inclusions in mantle garnet and diamond should be of secondary origin, precipitating from mantle-derived fluids after or prior to kimberlite magma eruption. Limited temperature stability of PAHs restricts the parameters of their formation in meteorite by high-temperature impact events during early Solar System history.
      Graphical abstract image

      PubDate: 2015-05-21T05:00:20Z
       
  • Surface alteration mechanism and topochemistry of iron in tremolite
           asbestos: A step toward understanding the potential hazard of amphibole
           asbestos
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Alessandro Pacella , Marzia Fantauzzi , Francesco Turci , Carlo Cremisini , Maria Rita Montereali , Elisa Nardi , Davide Atzei , Antonella Rossi , Giovanni B. Andreozzi
      Non-occupational, environmental and unintentional exposure to fibrous tremolite, one of the most widespread naturally occurring asbestos, represents a potentially significant geological risk in several parts of the world. The toxicity of amphibole asbestos is commonly related to iron content and oxidation state, but information available on surface iron topochemistry and amphibole alteration mechanism is still rather poor. With the aim to shed a light on this mechanism, two tremolite samples, one from Italy (Castelluccio) and one from USA (Maryland), immersed in a buffer solution (pH7.4) with H2O2 were characterized by a multi-technique approach. X-ray photoelectron spectroscopy (XPS) and high resolution–transmission electron microscopy (HR-TEM) were used to investigate the surface chemistry of the incubated samples and to detect structural modifications of the fibres, while inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the concentration of dissolved elements. An original four-step model for amphibole alteration pathway is proposed. The alteration process starts with an incongruent dissolution of the amphiboles that produces an amorphous, altered surface layer and that is followed by iron oxidation and formation of FeOOH species. Then the congruent dissolution of the altered layer starts and, subsequently, the residual Fe oxi-hydroxides aggregates and insoluble, Fe-rich, amorphous nanoparticles on top of the fibres are formed. The results are compared to those obtained on crocidolite, a highly toxic amphibole asbestos with a 10 to 20 times higher iron content than tremolite. The high chemical reactivity observed in the literature for tremolite appears to be related not only to its iron content and oxidation state, but also to the low nuclearity of iron on the altered surfaces, in contrast to pronounced Fe clusterization at crocidolite surfaces. This is a significant step toward a conceivable explanation of why asbestos tremolite is potentially as toxic as crocidolite.
      Graphical abstract image

      PubDate: 2015-05-21T05:00:20Z
       
  • Extraction of α-cellulose from mummified wood for stable isotopic
           analysis
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Benjamin A. Hook , Jochen Halfar , Jörg Bollmann , Ze'ev Gedalof , M. Azizur Rahman , Julito Reyes , Daniel J. Schulze
      Stable isotope ratios of carbon (δ13C) and oxygen (δ18O) from tree ring cellulose can provide valuable paleoclimatic information at annual and subannual resolution, from time periods long before instrumental climate records. Recently, mummified (non-permineralized) wood was discovered within Canadian Subarctic kimberlites, inviting paleoclimatic investigations of the time in which the trees grew. In the mummified wood, polysaccharides (hemicellulose, α-cellulose) are frequently preferentially degraded, in the low-silica anaerobic burial environment of the kimberlites, resulting in a lignin-rich material. However, some samples from the Ekati Panda kimberlite pipe (ca. 53.3Ma) contain remnant cellulose, demonstrating the extraordinary preservation potential of kimberlites. Preservation of α-cellulose is important because it allows for the construction of stable isotopic proxy records of deep-time paleoclimates at annual to subannual resolution. Established α-cellulose extraction methods [i.e., using a 17.5% sodium hydroxide (NaOH) solution] were unsuitable for this material because all holocelluloses were dissolved. Therefore, we tested variants of two cellulose extraction methods [i.e., Brendel et al. (2000) and Jayme-Wise (Leavitt and Danzer 1993, Loader et al. 1997)] to optimize a procedure for extraction of a consistent yield of mummified wood cellulose for stable isotopic analysis. Stable carbon (δ13C) and oxygen (δ18O) isotopes were measured from cellulose from each extraction method, as well as Extractive-Free Wood and unextracted mummified wood. vitrinite reflectance, used to assess thermal alteration of the unextracted material to estimate post-burial temperatures within the kimberlite, suggested low post-burial peak temperatures (Tpeak =60°C). We detected no mineral contaminants (i.e., iron oxides) in the material using Energy-Dispersive X-ray Spectroscopy (EDX). Despite low cellulose yield (<5%), the Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectra of mummified cellulose samples strongly resembled modern α-cellulose. All cellulose treatments were similar in stable isotope values and ATR-FTIR peaks, but significantly different from unextracted wood and Extractive-Free Wood. As, reported by other sources (Anchukaitis et al. 2008, Brookman and Whittaker 2012), use of the Brendel method may cause cellulose acetylation, introducing an FTIR peak near 1720cm−1, thus complicating hemicellulose detection at a peak near 1725cm−1. For this reason, the Jayme-Wise method is recommended for detection of hemicelluloses in mummified wood. If hemicelluloses are not present in holocellulose, due to groundwater hydrolysis, the NaOH step may be omitted or reduced in concentration and still produce α-cellulose.


      PubDate: 2015-05-21T05:00:20Z
       
  • Contribution of crustal materials to the mantle sources of Xiaogulihe
           ultrapotassic volcanic rocks, Northeast China: New constraints from
           mineral chemistry and oxygen isotopes of olivine
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Yang Sun , Jifeng Ying , Benxun Su , Xinhua Zhou , Ji'an Shao
      Ultrapotassic igneous rocks can generally be divided into two sub-groups based on the tectonic settings in which they formed. The orogenic sub-group occurs in subduction-related tectonic settings, while the anorogenic sub-group is confined to stable continental regimes. The Pleistocene Xiaogulihe ultrapotassic volcanic rocks outcrop in the western part of Heilongjiang province, northeast China, and are of intraplate origin with respect to its tectonic settings. Previous elemental and isotopic investigations have suggested that the mantle source of these volcanic rocks had been modified by continental-derived sediments resulting from an ancient subduction (at least older than 1.5Ga). In this contribution, we performed in-situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry (SIMS). The olivine grains generally have higher δ18O values and CaO contents than those of mantle peridotite xenoliths in the nearby Keluo potassic rocks and show linear correlations between major and trace elements, and Fo, suggesting that they are cognate phenocrysts resulted from fractional crystallization processes. The restricted and non-correlated variations in δ18O with the Fo of these olivine grains imply that the fractional crystallization processes might have negligible influence on their δ18O values. The relatively higher δ18O values of the olivine phenocrysts than the normal mantle imply the addition of an 18O-rich crustal component into their mantle source after ruling out the crustal contamination of the host magmas. We propose that the high-δ18O feature of the olivine phenocrysts was inherited from the subducted crustal component in their mantle source. Given the rapid oxygen isotopic diffusion under high temperature and the long period between mantle metasomatism event and volcanic eruption, it is postulated that the high-δ18O signature could only be preserved in the relatively cold and stable subcontinental lithospheric mantle. Such speculation is consistent with our previous inference that the Xiaogulihe ultrapotassic volcanic rocks were mainly generated from the lower subcontinental lithospheric mantle which had been metasomatized by potassium-rich silicate melts derived from ancient subducted continental-derived sediments.


      PubDate: 2015-05-21T05:00:20Z
       
  • Cadmium isotope fractionation within the soil profile complicates source
           identification in relation to Pb–Zn mining and smelting processes
    • Abstract: Publication date: 5 June 2015
      Source:Chemical Geology, Volume 405
      Author(s): Vladislav Chrastný , Eva Čadková , Aleš Vaněk , Leslaw Teper , Jerzy Cabala , Michael Komárek
      Many cadmium species are among the most toxic of environmental contaminants. Because Cd in many forms is mobile in the environment, identifying its sources is a crucial aspect of remedial intervention. Data from the literature suggest that Cd isotopes could be a useful tool for such identification, but no data about Cd isotopes through entire soil profiles has yet been published. We compared in this paper the Cd isotope composition of three forest and two meadow soil profiles affected by different contamination sources near the Olkusz Pb–Zn smelter in southern Poland together with the main industrial source of possible contamination at that location. In the contaminated forest soil profile, the upper soils reflected the heaviest Cd isotope compositions, while the deeper soil humus layer had the lightest Cd isotope compositions. We found that the overall Cd isotope heterogeneity in industrial wastes was overlapped by the overall Cd isotope fractionation within the forest soil profile near the smelter (ε114/110CdNIST3108 values of 5.14 compared to 11.8, respectively). Both meadow soil profiles had approximately a Cd isotope composition corresponding to the processed ore (Olkusz sphalerite).


      PubDate: 2015-05-21T05:00:20Z
       
  • Geochemistry of lavas from maar-bearing volcanoes in the Oku Volcanic
           Group of the Cameroon Volcanic Line
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): Asobo N.E. Asaah , Tetsuya Yokoyama , Festus T. Aka , Tomohiro Usui , Takeshi Kuritani , Mengnjo J. Wirmvem , Hikaru Iwamori , Eric M. Fozing , Jules Tamen , Gilbert Z. Mofor , Takeshi Ohba , Gregory Tanyileke , J.V. Hell
      Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr–Nd–Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr–Nd–Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source (<6% garnet) and modelled melting results indicate <2% partial melting of the source material. Isotope data plot within the focal zone, extending towards enriched mantle 1 (EM1; e.g. Lakes Oku and Nyos samples). This indicates the involvement of at least three mantle components: depleted mid-ocean ridge basalt mantle, high-μ and EM1 components in the magmatism of the lavas studied. The contributions of these components in different proportions, originating from asthenospheric and subcontinental lithospheric mantle sources, can account for the observed variations in geochemical characteristics. The geochemical characteristics of the studied lavas indicate that the magma source need not necessarily have an abnormal CO2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO2 stability.


      PubDate: 2015-05-21T05:00:20Z
       
  • Clinopyroxene and titanomagnetite cation redistributions at Mt. Etna
           volcano (Sicily, Italy): Footprints of the final solidification history of
           lava fountains and lava flows
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): S. Mollo , P.P. Giacomoni , D. Andronico , P. Scarlato
      For a better understanding of the final solidification history of eruptions at Mt. Etna volcano (Sicily, Italy), we have investigated cation redistributions at the interface between sub-millimetre-sized clinopyroxene and titanomagnetite crystal rims and coexisting melts. The studied products were scoria clasts from lava fountains and rock samples from pahoehoe and aa lava flows. Our data indicate that scoria clasts from lava fountaining were rapidly quenched at the contact with the atmosphere, preserving the original crystal textures and compositions inherited during magma dynamics within the plumbing system. Kinetics and energetics of crystallization were instantaneously frozen-in and post-eruptive effects on mineral chemistry were negligible. The near-equilibrium compositions of clinopyroxene and titanomagnetite indicate that lava fountain episodes were supplied by high-temperature, H2O-rich magmas ascending with velocities of 0.01–0.31m/s. In contrast, magmas feeding lava flow eruptions underwent a more complex solidification history where the final stage of the crystal growth was mostly influenced by volatile loss and heat dissipation at syn- and post-eruptive conditions. Due to kinetic effects associated with magma undercooling, clinopyroxenes and titanomagnetites formed by crystal attachment and agglomeration mechanisms leading to intricate intergrowth textures. The final compositions of these minerals testify to closure temperatures and melt–water concentrations remarkably lower than those estimated for lava fountains. Kinetically-controlled cation redistributions at the crystal–melt interface suggest that the solidification of magma was driven by degassing and cooling processes proceeding from the uppermost part of the volcanic conduit to the surface.


      PubDate: 2015-05-21T05:00:20Z
       
  • Bacteria-mediated reduction of As(V)-doped lepidocrocite in a flooded soil
           sample
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): Aline Dia , Béatrice Lauga , Mélanie Davranche , Anne Fahy , Robert Duran , Bernd Nowack , Patrice Petitjean , Odile Henin , Sébastien Martin , Rémi Marsac , Gérard Gruau
      Understanding the processes involved in the control of arsenic (As) dynamics within soils has become a challenging issue for soil and water quality preservation. Interactions between mineralogical phases, organic ligands and bacterial communities — closely linked to the chemical conditions of the medium — were thus investigated through a geochemical and microbiological experimental study involving the reduction of As(V)-doped lepidocrocite within the soil. Reducing conditions were established as soon as the experiment started, followed by a release of dissolved organic carbon corresponding to a release of acetate. Scanning electron microscopy observations pointed out a large bacterial colonization occurring on the lepidocrocite leading to a 3-dimensionally shaped biodissolution of lepidocrocite. The taxonomic diversity evolved throughout the experiment, and thus it demonstrated the evolution of the metabolic activities of the bacteria. At the beginning, lepidocrocite was mainly colonized by bacteria belonging to the Geobacter genus (Deltaproteobacteria) (26%) and Bacillus and Oxalophagus fermentative related genera (Firmicutes) (72%). After two weeks, Geobacter spp. and Firmicutes represented 54% and 30% of the bacterial community, respectively. Although still dominated by Geobacter spp. (34%) at the end of the experiment, the bacterial diversity had increased. After 3 and 8weeks of incubation, the presence of the arsB and ACR3(1) genes, encoding transporters involved in As detoxification processes, indicated that this community harbored As-resistant or As-transforming genera able to contribute to the transformation of As(V) into As(III).


      PubDate: 2015-05-21T05:00:20Z
       
  • Low-temperature feldspar and illite formation through bioreduction of
           Fe(III)-bearing smectite by an alkaliphilic bacterium
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): Deng Liu , Hailiang Dong , Hongmei Wang , Linduo Zhao
      Biogenic mineral assemblages that form from circumneutral microbial reduction of iron in smectite have been suggested as biosignatures in the geological record. However, mineralogical transformation of smectite mediated by microbes under extreme pH condition is still poorly known. The objective of this study was to understand the reduction capacity of structural Fe(III) in iron-rich smectite (nontronite, NAu-2) by a novel anaerobic alkaliphile (strain CCSD-1) isolated from the deep subsurface, and associated mineralogical changes. The experiments with CCSD-1 were conducted in a growth medium containing the electron shuttle anthraquinone-2,6-disulfonate (AQDS) at pH9.4. The Fe(II) concentration was monitored over the course of the experiment via wet chemistry, and unreduced and reduced nontronites were characterized with X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM). The results indicate that strain CCSD-1 utilizes proteinaceous substrates (yeast extract and tryptone) to reduce structural Fe(III) in smectite with the maximum reduction extent of 26.2%. Mineralogical analysis confirmed that biogenic plagioclase (Na–Ca feldspar) and illite were formed after bioreduction. Our work shows that the interaction between alkaliphile and iron-bearing smectite could account for low-temperature feldspar and illite formation and these minerals may be used as biosignatures in sedimentary rocks.


      PubDate: 2015-05-21T05:00:20Z
       
  • Carbon isotope fractionation during high pressure and high temperature
           crystallization of Fe–C melt
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): V.N. Reutsky , Yu.M. Borzdov , Yu.N. Palyanov
      There is a growing body of experimental evidence that iron carbides can play an important role in the mantle both as a host of carbon and as a redox couple (Fe–C) determining the nature of reduced phases. If carbides are significant in the mantle, it could be of interest to know if any carbon isotope fractionation accompanies carbide crystallization. A series of high-pressure and high-temperature experiments were performed on carbide crystallization from a Fe–C melt. An offset of 2‰ was observed between the δ13C values of Fe3C and Fe–C melt at 6.3GPa and 1400°C. The carbon isotopic compositions of Fe3C and diamond crystallizing from a single carbon source near the peritectic region at 6.3GPa differ by 2.5‰. Fe7C3 was detected as a quench phase during Fe–C melt quenching. Our results have important implications for understanding carbon isotope distributions in iron meteorites and indicate that iron carbide crystallization may be a significant mechanism for carbon isotope heterogeneity in the Earth's mantle.


      PubDate: 2015-05-21T05:00:20Z
       
  • Corrigendum to “Origin of calcareous dust in Argentinean Pleistocene
           periglacial deposits traced by Sr, C and O isotopic compositions, and REE
           distribution” [Chem. Geol. 380 (2014) 119–132]
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Isabelle Techer , Norbert Clauer , Thea Vogt



      PubDate: 2015-05-21T05:00:20Z
       
  • Disequilibrium-induced initial Os isotopic heterogeneity in gram aliquots
           of single basaltic rock powders: Implications for dating and source
           tracing
    • Abstract: Publication date: 16 June 2015
      Source:Chemical Geology, Volume 406
      Author(s): Jie Li , Xuan-Ce Wang , Ji-Feng Xu , Yi-Gang Xu , Gong-Jian Tang , Qiang Wang
      The Re–Os isotopic heterogeneity of mantle-derived rocks has been well documented. However, it is unclear whether and how the Os isotopic heterogeneity of a mantle source can influence Re–Os isotopic dating and source tracing of mantle-derived melts. Here we report that replicate analyses of gram aliquots of single basaltic powders (one of reference material BHVO-2 and three of the Hatu basalts from the western Junggar region, China) show large variations in both Os concentrations and isotopic ratios. More importantly, these replicate data define good apparent Re–Os isochron correlations, yielding ages significantly older than their formation ages, and display remarkable correlations between the 187Os/188Os ratio and the reciprocal of the common Os concentration (i.e., 1/192Os). This indicates that the initial Os isotopic composition in a basaltic magmatic system may be heterogeneous and cannot be homogenized by pulverizing. Theoretical and mathematical deduction demonstrates that the observed apparent Re–Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation, primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and host minerals under mantle conditions. The regressed initial Os isotopic composition may bias the true value of the mantle source. Pulsed mantle melting and magma mixing during ascent are potential processes responsible for such initial Os isotopic heterogeneity in basaltic rock systems. Thus, when using the Re–Os isotopic system to date and to trace the source of basaltic rocks or other rocks with relatively low Os concentrations, it is necessary to consider whether the initial Os isotopic composition was heterogeneous or whether it had reached complete isotopic equilibrium.


      PubDate: 2015-05-21T05:00:20Z
       
  • Effect of manganese oxide minerals and complexes on gold mobilization and
           speciation
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Christine Ta , Joël Brugger , Allan Pring , Rosalie K. Hocking , Claire E. Lenehan , Frank Reith
      (Bio)geochemical processes mediate the dispersion and re-concentration of gold (Au) in Earth surface environments. The fundamental mechanisms underlying the mobilization of Au in surface environments and the chemical speciation of mobile Au remain largely unexplored. For more than half a century authors have argued that highly reactive manganese (Mn) oxides are involved in oxidizing, and hence mobilizing, Au in waters, soils and sediments. However, no systematic study has assessed the mechanisms of Au oxidation under conditions occurring in natural environments. Therefore, this paper explores the links between Mn-oxides and the oxidation/mobilization of Au under acidic to circum-neutral conditions. Using a newly developed high performance liquid chromatography–inductively coupled plasma-mass spectrometry (HPLC–ICP-MS) method, a range of Au(I/III)-complexes occurring in aqueous solution were directly quantified. We show that the oxidation of Au(I) to Au(III) with birnessite occurs in acidic environments, and that the rapid release of Au(III) sorbed (on Mn- or Fe-minerals) via influx of neutral waters transfers Au(III) into solution, making it mobile and transportable. This provides a viable pathway for explaining the occurrence of mobile Au(III)-complexes under neutral to alkaline conditions, as was reported from a Western Australian salt lake environment. In addition, we show that the [(bpy)2MnIII(μ-O)2MnIV(bpy)2]3− complex (bpy=2,2′-bipyridyl), a useful model compound for the interaction of Au with bioorganic Mn in natural systems, mediates the formation of soluble Au(III)-complexes from metallic Au and from Au(I)-complexes under oxic and anoxic conditions. In conclusion, this study shows that reactive Mn-oxide minerals are important drivers of Au mobilization, and further suggests that natural organometallic compounds play an important role for the mobilization of Au in Earth surface environments.


      PubDate: 2015-05-21T05:00:20Z
       
  • Optimizing LA-ICP-MS analytical procedures for elemental depth profiling
           of foraminifera shells
    • Abstract: Publication date: 22 June 2015
      Source:Chemical Geology, Volumes 407–408
      Author(s): Jennifer S. Fehrenbacher , Howard J. Spero , Ann D. Russell , Lael Vetter , Stephen Eggins
      Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is becoming a widespread technique for analyzing elemental ratios in foraminiferal calcite. Here we focus on optimizing LA-ICP-MS for high-resolution depth profiling of elemental ratios through shell walls. This application reveals intrashell variability and provides a unique opportunity to quantify trace element incorporation over short time scales of calcification by an individual foraminifer. High-resolution depth profiling requires careful consideration of both ablation and analytical conditions required to resolve differences in shell chemistry across sub-micron shell thickness. We present laser ablation profiles of NIST SRM 610 standard glass data (in cps) and elemental/Ca ratios (in mmol/mol) from foraminiferal calcite obtained over a range of operating conditions using a Photon Machines 193nm UV excimer laser-ablation system, equipped with a dual-volume ANU HelEx chamber, coupled to an Agilent 7700x quadrupole ICP-MS. Different combinations of energy density, repetition rate, and mass spectrometer cycle time can yield varying elemental profiles. This variability can mimic and/or mask real intrashell trace element heterogeneity in foraminifer shells. At low (<3Hz) laser repetition rates, real intrashell element variation can be obscured depending on the laser energy, whereas using moderate (≥3Hz) laser repetition rates and/or a signal-smoothing device improves the accuracy and precision of intrashell trace element profiles. Shell material is ablated rapidly when using a 5Hz or greater repetition rate and an energy density of 3J/cm2 or greater, resulting in reduced spatial resolution.


      PubDate: 2015-05-21T05:00:20Z
       
  • Scorodite Precipitation in the Presence of Antimony
    • Abstract: Publication date: Available online 27 April 2015
      Source:Chemical Geology
      Author(s): David Kossoff , Mark D. Welch , Karen A. Hudson-Edwards
      The effects of Sb on the precipitation of synthetic scorodite, and the resultant phases formed, were investigated. Nine synthetic precipitates with varying concentrations of Sb, together with As-only and Sb-only end members, were prepared using a scorodite synthesis method, and these were characterized using XRD, SEM, chemical digestion and μXRF mapping. XRD analysis shows that the end members are scorodite (FeAsO4.2H2O) and tripuhyite (FeSbO4), and that the intermediate members are not Sb-substituted scorodite, but instead are physical mixtures of scorodite and tripuhyite, with tripuhyite becoming more prominent with increasing amounts of Sb in the synthesis. Electron microprobe analysis on natural scorodites confirms that they contain negligible concentrations of Sb. With increasing Sb in the synthesis, the morphology of the scorodite changes from rosettes of intergrown crystals to anhedral masses of smaller crystallites. Chemical digestion of the series also became increasingly difficult with increasing Sb content. We conclude that Sb is not taken up in scorodite (perhaps due to its larger ionic radius and different co-ordination with O compared to As), that increasing amounts of Sb in the system affect scorodite morphology, and that tripuhyite is a highly stable and perhaps underestimated Sb-sink.


      PubDate: 2015-05-13T08:29:51Z
       
  • Petrogenesis of mafic collision zone magmatism: The Armenian sector of the
           Turkish–Iranian Plateau
    • Abstract: Publication date: 18 May 2015
      Source:Chemical Geology, Volume 403
      Author(s): Iain Neill , Khachatur Meliksetian , Mark B. Allen , Gevorg Navasardyan , Klaudia Kuiper
      The Turkish–Iranian Plateau grew after the Middle Miocene following the initial Paleogene Arabia–Eurasia collision.
      Authors attribute uplift to break-off of the southern Neo-Tethys slab beneath the Bitlis–Zagros Suture at ~15–10Ma, coupled with continued plate convergence and regional crustal shortening. Since this time there has been an upsurge in mantle-derived collision magmatism over large parts of NW Iran, Eastern Anatolia and the Lesser Caucasus, potentially hundreds of kilometres from the site of southern Neo-Tethys slab break-off, >10Myr after the proposed break-off event. Whole rock elemental and Sr–Nd–Pb–Hf isotope data are presented for <3Ma trachy-basalt to trachy-basaltic andesite lavas erupted in Armenia in the South Caucasus. Samples formed by <5% melting of fertile subduction-modified spinel-facies lithospheric mantle, and few display elemental or isotopic evidence for contamination by the 45-km thick Mesozoic–Paleogene arc crust or South Armenian Block continental crust. Recent magmatic activity in Armenia may not be a direct consequence of southern Neo-Tethys slab break-off 300–450km away, beneath the Bitlis Suture. Late Miocene break-off of a second (northern Neo-Tethys) slab beneath the Pontide Arc may have allowed asthenospheric upwelling over a wider area than was affected by southern Neo-Tethyan break-off. However, whole-scale delamination of mantle lithosphere is ruled out due to the modest degrees of partial melting, a lack of asthenospheric components and limited crustal involvement in magmatism. Small-scale sub-lithospheric convection may be complementary to break-off, causing localised removal of lithospheric mantle and aiding the occurrence of melting for a significant time interval after the break-off event(s). Collision magmas such as those in Armenia represent mantle-derived additions to continental crust, enriched in incompatible elements but with Th/La ratios≤0.2, much lower than those calculated for continental crust (0.25–0.3). Collision magmatism in Turkic-style orogens must be balanced by infra-crustal recycling and delamination to produce bulk continental crust.


      PubDate: 2015-04-01T15:42:18Z
       
 
 
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