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  • 1
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    A high-precision 40Ar/39Ar age for hydrated impact glass from the Dellen impact, Sweden (2013)
    Geological Society (of London)
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    Publication Date: 2013-10-01
    Description: The dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic. However, if the maximum potential of the terrestrial impact crater record is to be realized, accurate and precise ages for crater-forming events are critical. Here we report a high-precision 40 Ar/ 39 Ar age for the Dellen impact structure, Sweden. Previous radio-isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach. A detailed petrographical and mineralogical study provides a solid foundation for interpretation of step-heating 40 Ar/ 39 Ar data, culminating in a statistically robust age of 140.82 ± 0.51 Ma (2; full external precision) for the Dellen impact event, for which data disfavour an inherited argon component. Primary hydration of the impact melt during cooling–quenching and entrapment of molecular water promoted rapid loss of inherited 40 Ar from the impact melt of rhyolitic composition. Duplicate analyses of the water content and D of the glass give similar values for the former (1.9 ± 0.1 μmol mg –1 ) but unexpectedly low values for the latter (–159 ± 8), with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity. This study highlights that the 40 Ar/ 39 Ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events. Supplementary material: Raw 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18633
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 2
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    Hydrothermal fluids in epithermal and porphyry Au deposits in the Central Slovakia Volcanic Field (2014)
    Geological Society (of London)
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    Publication Date: 2014-04-06
    Description: The Neogene Central Slovakia Volcanic Field in the Carpathian arc contains various Au deposits, hosted by central zones of large andesite stratovolcanoes. Fluids involved in mineralization have been studied at three different types of deposit, mostly by fluid inclusion and stable isotope techniques. The Rozália mine in the Štiavnica stratovolcano hosts intermediate sulphidation-style Au–Ag epithermal mineralization in subhorizontal veins related to hydrothermal activity during an early stage of caldera collapse. Associated fluids of low salinity underwent extensive boiling at 280–330 °C on transition from suprahydrostatic towards hydrodynamic conditions at shallow depths ( c. 550 m) from fluids of mixed magmatic and meteoric origin. The Kremnica ore field hosts a large system of low sulphidation-style Au–Ag veins contemporaneous with rhyolite magmatism and situated on resurgent horst faults. Fluids were of low salinity, predominantly of meteoric origin, and showed gradual decrease in temperature along the system ( c. 270–140 °C) related to a decrease in erosion level from c. 500 to c. 50 m. The Biely Vrch Au-porphyry deposit in the Javorie stratovolcano is associated with quartz stockwork in diorite porphyry intrusion. The major type of ore-bearing fluid was high temperature magmatic vapour (720–〈380 °C) accompanied by Fe-rich salt melt. Gold precipitated in a high-temperature but low-pressure subvolcanic environment. Supplementary material: Stable isotope data and fluid inclusion microthermometric data are available at http://www.geolsoc.org.uk/SUP18752
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 3
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    Abundant sulphate in the Neoproterozoic ocean: implications of constant {delta}34S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian (2014)
    Geological Society (of London)
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    Publication Date: 2014-01-05
    Description: The timing and extent of ocean oxygenation is controversial. Proterozoic sulphur isotope datasets often show marked fluctuations over small stratigraphic intervals, suggesting that oceanic sulphate concentrations were much lower than modern values. A large accumulation of Neoproterozoic sulphate (〉8 million tonnes preserved), as stratiform barite rock, is located in the Grampian Highlands near Aberfeldy. Diagenetic/metamorphic alteration has caused pronounced 34 S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in 34 S, mean 36 ± 1.5. We infer that this is representative of contemporaneous seawater sulphate, and that 34 S seawater was constant during deposition of a stratigraphical thickness 〉250 m of mostly fine-grained clastic sediments. Uniformity of 34 S seawater during barite precipitation, even in thick (〉10 m) beds and with the co-occurrence of abundant sulphides incorporating bacteriogenically reduced sulphur, implies no limit to availability of seawater sulphate during hydrothermal exhalative events. Our data, combined with previous 34 S research on Dalradian metasediments, suggest a stability, abundance and constancy of ocean sulphate in the Neoproterozoic. This contrasts with isotopic data using trace sulphate in limestones. It appears that, around the time of the Marinoan glaciation ( c. 635 Ma), the ocean, although stratified at least locally, comprised a substantial reservoir of sulphate-bearing oxygenated seawater.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 4
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    Nerc Radiocarbon Age Measurements Determined By Radiometric Counting 1996–2005 (2010)
    Cambridge University Press
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    Publication Date: 2010-01-01
    Description: We describe a new compilation of radiocarbon age measurements performed by the NERC Radiocarbon Laboratory that is freely available to access over the World Wide Web. The database contains 1000 14C measurements performed using the liquid scintillation counting method between 1996 and 2005, and further results will be added as the information is compiled. Contextual information including sampling location and the nature of sample material is provided, alongside 14C age results and publications codes. Hypertext links provide access to the original 14C age report associated with the samples, providing additional details. The 14C measurements were originally performed for earth and environmental science NERC projects and are therefore likely to be most relevant to the Quaternary research community.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 5
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    Carbon Dioxide Capture Using a Zeolite Molecular Sieve Sampling System for Isotopic Studies (13C and 14C) of Respiration (2005)
    Cambridge University Press
    Details
    Publication Date: 2005-01-01
    Description: A method for collecting an isotopically representative sample of CO2 from an air stream using a zeolite molecular sieve is described. A robust sampling system was designed and developed for use in the field that includes reusable molecular sieve cartridges, a lightweight pump, and a portable infrared gas analyzer (IRGA). The system was tested using international isotopic standards (13C and 14C). Results showed that CO2 could be trapped and recovered for both δ13C and 14C analysis by isotope ratio mass spectrometry (IRMS) and accelerator mass spectrometry (AMS), respectively, without any contamination, fractionation, or memory effect. The system was primarily designed for use in carbon isotope studies of ecosystem respiration, with potential for use in other applications that require CO2 collection from air.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 6
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    Spatial Variability of Bomb 14C in an Upland Peat Bog (2007)
    Cambridge University Press
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    Publication Date: 2007-01-01
    Description: As part of a study investigating the carbon balance of a blanket bog, we made an assessment of the spatial variation of radiocarbon concentrations in the surface layers of a small area of peatland in the north of England. The peat depth at which bomb-14C content was the highest varied considerably between cores sampled from across the site. At several sampling locations, 14C levels 〉100% Modern were confined to the surface 8 cm, whereas bomb 14C was evident at 1 site, located only meters away, to a depth of at least 12–16 cm. Using the layer where 14C levels first exceeded 100% Modern as a chronological reference layer, we estimated the carbon accumulation rate over the last 50 yr for the surface peat at each site (range ∼20 to ∼125 g C m2 yr-1). Our results show that although carbon accumulation over the last 50 yr was similar across the site, variation in the depth to which bomb 14C was evident implied considerable variation in the vertical peat growth rate.
    Print ISSN: 0033-8222
    Electronic ISSN: 1945-5755
    Topics: Archaeology , Energy, Environment Protection, Nuclear Power Engineering , Geosciences
    Published by Cambridge University Press
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  • 7
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    Palaeoproterozoic, rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part I: Sedimentology and major element geochemistry (2004)
    Cambridge University Press
    Details
    Publication Date: 2004-09-01
    Description: The 150-m-thick Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NW Russia, is one of the key formations in the study of a positive δ13Ccarb excursion occurring globally in the Palaeoproterozoic. The KSF formed in an intracratonic rift setting and is sandwiched between two, 2-km-thick subaerially erupted volcanic units. The KSF was previously interpreted as shallow marine, but new data reported here indicate that it is a non-marine unit deposited on a deeply subaerially weathered surface mantling the underlying volcanic rocks. The lowermost part of the KSF represents an alluvial–fluvial plain, followed by a laterally and vertically variable succession of variegated to mottled fine-grained siliciclastic rocks and ‘red beds‘, dolostones containing stromatolite sheets, hydrothermal travertine deposits and abundant desiccation features (e.g.tepees, surfical silicified crusts and dissolution cavities), including probable pseudomorphed evaporites. Measured S and Corg concentrations for the carbonate and siliciclastic rocks are low. Combined, these features indicate that the carbonate rocks of the KSF accumulated in a shallow lacustrine setting. Major types of carbonate facies were formed by: (1) biologically-induced precipitation; (2) evaporitic removal of CO2 in a closed lake environment; and (3) chemical precipitation from thermal springs. Apparently, none of these carbonate facies was in full isotopic equilibrium with atmospheric CO2. This interpretation shows the importance of taking into account the interplay between global and local depositional factors when interpreting the isotopic signature of the KSF dolostones and its implication for the Palaeoproterozoic carbon isotope excursion
    Print ISSN: 1755-6910
    Electronic ISSN: 1755-6929
    Topics: Geosciences
    Published by Cambridge University Press
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  • 8
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    Anorogenic granite magma genesis: new isotopic data for the southern sector of the British Tertiary Igneous Province (1992)
    Cambridge University Press
    Details
    Publication Date: 1992-01-01
    Description: It is now generally accepted that British Tertiary granites contain crustal and mantle components. Genesis principally by differentiation of crustally contaminated basaltic magmas is widely held and silicic melts with some remarkable trace element similarities were generated within different upper crust along the St Kilda/Skye - Carlingford zone.New whole-rock (and mineral) O isotope data for the southern sector of the province (N Arran, Ailsa Craig, Mourne Mountains, Slieve Gullion, etc) reveal that δ18O lies in the range +5·1 to +9·7‰ for most of the analysed granites, meteoric water-rock interaction having been in general less intensive than at Skye and Mull. Nevertheless, highly 18O-depleted country rocks (with δ18O
    Print ISSN: 1755-6910
    Electronic ISSN: 1755-6929
    Topics: Geosciences
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  • 9
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    Palaeoproterozoic, rift-related, 13C-rich, lacustrine carbonates, NW Russia. Part II: Global isotope signal recorded in the lacustrine dolostones (2004)
    Cambridge University Press
    Details
    Publication Date: 2004-09-01
    Description: A comprehensive study of the ∼2200-Ma-old Kuetsjärvi Sedimentary Formation (KSF), NW Russia, was undertaken to contribute to our understanding of palaeoenvironments associated with the global perturbation of the carbon cycle between 2330 and 2060 Ma. Closely spaced drill core samples (n=95) were obtained from a 150-m-thick unit deposited in rift-bound fluvial-deltaic and shallow-water lacustrine settings with a short-term invasion of sea water. Apart from a very few de-dolomitised samples, all other carbonate lithologies are represented by Corg-free, S-poor, quartz-rich dolostones, stromatolites and travertines which have high Sr concentrations (51–1069 ppm) and low Mn/Sr ratios (2·9 ± 2·1). The carbonate succession, excluding travertines, shows high δ13C (+7·5 ± 0·6‰, n=95) with a limited variation (+5·8 to +8·9‰). Fluctuating δ18O values (10·8–20·4‰) were overprinted during diagenesis, regional greenschist-grade and later retrograde metamorphism. Several short-term stratigraphic excursions of δ13C were apparently governed by evaporation and CO2 degassing combined with pulses of12C-rich hydrothermal waters precipitating travertines. However, the 13C-rich nature of the dolostones reflects the global isotopic signal, which was modified in a shallow water lacustrine environment by evaporation, enhanced uptake of 12C by cyanobacteria, and pene-contemporaneous oxidation and loss of organic material. The best proxies to δ13C and 87Sr/86 Sr of coeval sea water recorded in the KSF dolostones are likely to be around +5–6‰ and 0·70406, respectively. The study of the KSF has shown that circumspection is necessary when attempting to model the behaviour and evolution of the global C-cycle in Deep Time. Models which purport to explain global oceanic–atmospheric evolution without first adequately accounting for the possibility that many Precambrian carbonate deposits might be non-marine, or at least influenced by non-marine fluids, should be viewed with caution
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    Topics: Geosciences
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  • 10
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    The isotopic geochemistry of ocean waters through time (1989)
    Cambridge University Press
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    Publication Date: 1989-01-01
    Description: There is a general consensus that the global chemistry of ocean water has not changed markedly during the Phanerozoic. Nevertheless, significant changes have occurred in the geochemical cycles of some elements and patterns of change have been reconstructed, in various forms, through consideration of the isotope ratios 13C/12C, 34S/32S, 87Sr/86Sr and 143Nd/144Nd. There have also been attempts to constrain variations in the isotopic composition of sea water itself through measurements of D/H and 18O/16O, the latter both directly and indirectly. Dissolved constituents in seawater display secular changes in isotopic composition as a consequence of quite different driving mechanisms. δ13C and δ34S variations are broadly correlated and linked by carbon and sulphur exogenic cycle interaction through redox reactions (the “free oxygen cycle”). The 87Sr/86Sr trend is determined by the balance among different Sr inputs to the oceanic pool, which vary in their isotopic composition (limestones, old granitic material and young basaltic material). Neodymium isotope variations are not globally synchronous. Changes in 143Nd/144Nd are influenced by local erosion products from continental landmasses and can therefore be different for coexisting palaeocean basins.
    Print ISSN: 1755-6910
    Electronic ISSN: 1755-6929
    Topics: Geosciences
    Published by Cambridge University Press
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