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  • 1.
    Bellucci, Jeremy
    et al.
    Swedish Museum of Natural History, Department of Geology.
    Nemchin, Alexander
    Swedish Museum of Natural History, Department of Geology.
    Whitehouse, Martin
    Swedish Museum of Natural History, Department of Geology.
    Ross, Kielman
    Swedish Museum of Natural History, Department of Geology.
    Snape, Joshua
    Swedish Museum of Natural History, Department of Geology.
    Pidgeon, Robert
    Geochronology of Hadean zircon grains from the Jack Hills, Western Australia constrained by quantitative scanning ion imaging2018In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 476, p. 469-480Article in journal (Refereed)
    Abstract [en]

    Five Hadean (> 4 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale < 5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of < 10 μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9 Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population.

  • 2.
    Kielman, Ross
    et al.
    Swedish Museum of Natural History, Department of Geology. Stockholm University.
    Nemchin, Alexander
    Curtin University.
    Whitehouse, Martin
    Swedish Museum of Natural History, Department of Geology.
    Pidgeon, Robert
    Curtin University.
    Bellucci, Jeremy
    Swedish Museum of Natural History, Department of Geology.
    U-Pb age distribution recorded in zircons from Archean quartzites in the Mt. Alfred area, Yilgarn Craton, Western Australia2018In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 310, p. 278-290Article in journal (Refereed)
    Abstract [en]

    The U-Th-Pb isotopic data from detrital zircon grains from five samples of Archean quartzite from the Mt. Alfred area of the Illaara greenstone belt in the Yilgarn Craton of Western Australia are presented in this study. The zircon grains are typically fractured and contain both irregular and oscillatory zoned internal structures as revealed by cathodoluminescence imaging. Concordant 207Pb/206Pb ages range between 3109 ± 17 and 3918 ± 16 Ma (2σ), with three main age peaks at ca. 3640, 3690 and 3760 Ma. Older 207Pb/206Pb ages up to 4067 ± 5 Ma are strongly affected by at least one recent disturbance event, however one single-grain discordia yields an upper intercept age of 4107 ± 12 (MSWD = 1.2). A further sixteen zircon grains with multiple analyses define discordia that suggest U-Pb disturbance events in the Neoarchean and the Mesozoic, the latter as a result of invasive low temperature weathering solutions. The notable lack of grains with ages less than ∼3.6 Ga in the Mt. Alfred detrital zircon population differentiates it from other quartzite samples from both the Illaara Formation and the Eoarchean zircon-bearing metasedimentary rocks of the Narryer Terrane. Also, the limited spread of zircon ages between 3640 and 3760 Ma suggests a relatively uniform and possibly local source region. However, no rocks of this age have been found in the Youanmi Terrane. This implies either the distal transport of similarly aged clastic sediments at 3.1 Ga from the Narryer Gneiss Complex (NGC) to the Mt. Alfred area, or the previous existence of NGC-like rocks near the Illaara greenstone belt that are either not currently recognised or have since been destroyed.

  • 3.
    Kielman, Ross
    et al.
    Swedish Museum of Natural History, Department of Geology. Stockholm University.
    Whitehouse, Martin
    Swedish Museum of Natural History, Department of Geology.
    Nemchin, Alexander
    Curtin University.
    Kemp, Anthony
    University of Western Australia.
    A tonalitic analogue to ancient detrital zircon2018In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 499, p. 43-57Article in journal (Refereed)
    Abstract [en]

    A zircon population from an Archean tonalite sample from southern West Greenland hasbeen used as a source analogue in order to test common methods and approaches applied toancient detrital zircon populations. Measurements of U-Th-Pb, oxygen and Lu-Hf isotopes aswell as rare earth element and Ti concentrations were made in these zircon crystals and,where possible, in multiple areas within a single grain. The population is dominated by oscillatory zoned cores aged 3.82 Ga with an isotopically and compositionally distinct rims that formed at 3.59 Ga. We demonstrate that multiple age components may be erroneously inferred from within these oscillatory zoned zircon cores, both from the total population and within individual grains. This has bearing on other zircon-hosted geochemical systems, as temporal correlations may be incorrectly assigned. Oxygen and Lu-Hf isotope compositions are relatively consistent through the population with only a small number of outliers. Ranges in rare earth element and Ti abundances are evident from the total population, from which apparent inverse cooling trends may be inferred. Additionally, we show that even with enhanced filtering of Ti concentrations using light rare earth element abundances,crystallisation temperatures derived from zircon grains of a single, hand sample sized rockcan yield both wide and bimodal results. Since even simple, single “source rock” zirconpopulations may, without careful scrutiny, portray artificially complex results, particular caremust be taken in the interpretation of complex ancient detrital zircon populations.

    The full text will be freely available from 2020-11-05 15:37
  • 4. Sałacińska, Anna
    et al.
    Kusiak, Monika A.
    Whitehouse, Martin J.
    Swedish Museum of Natural History, Department of Geology.
    Dunkley, Daniel J.
    Wilde, Simon A.
    Kielman, Ross
    Swedish Museum of Natural History, Department of Geology.
    Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland2018In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 315, p. 103-119Article in journal (Refereed)
    Abstract [en]

    The Saglek Block of Labrador comprises Eoarchean to Neoarchean lithologies, metamorphosed at high temperature at ca. 2.7 Ga. Here, we investigate the gneisses of Tigigakyuk Inlet, previously identified as the locality exposing the most ancient rocks in the Saglek Block. New geochronological and geochemical results reveal a multistage history. Precise magmatic emplacement ages of 3.75 to 3.71 Ga refine the age of the Uivak Gneiss. Zircon rims and neoblastic grains with low Th/U record metamorphism at ca. 3.6 and 2.8-2.7 Ga. Magmatism between these tectono-metamorphic events is recorded by the presence of meta-mafic dykes in the gneisses, gabbroic enclaves in ca. 2.7 Ga syn-tectonic granitoids, as well as by a ca. 3.56 Ga age for monzonitic gneiss in which metamorphic zircon is present as xenocrysts. Felsic (TTG) magmatism between ca. 3.75 Ga and 3.71 Ga, as well as metamorphism at both ca. 3.6 Ga and 2.8-2.7 Ga, is also recognised in the Itsaq Gneiss Complex of south West Greenland, and is restricted to the Færingehavn Terrane. Our new data enable a more rigorous correlation between these formerly conjugate parts of the North Atlantic Craton.

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