Abstract The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66–56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ~50 m record of lacustrine silt and indicates an age ~10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.
Abundant granitic plutons intruded the eastern Meguma terrane of Nova Scotia in the middle- to late Devonian. Less voluminous diorite–tonalite and gabbro intrusions are associated with the granitic plutons along the northern margin of the terrane adjacent to the Cobequid–Chedabucto fault zone. All plutons contain metasedimentary xenoliths, and the mafic plutons show magma mingling textures with their adjacent granitic plutons. New U–Pb zircon data from autocrystic zircon in 13 samples indicate coeval emplacement of mafic and granitic plutons between ca. 382 and 368 Ma. However, the zircon grains contain numerous inherited domains that range in age from Palaeoproterozoic to Devonian. These inherited ages correspond to detrital zircon U–Pb dates from the Cambrian to Ordovician metasedimentary host rocks. Zircon oxygen isotopic data (δ 18 O) are between +7.4 ± 0.2‰ and +9.3 ± 0.3‰ indicating significant involvement of the crust as the magma source or contaminant. If the high δ 18 O zrn values are a result of contamination, the contaminant was likely the metasedimentary rocks of the Meguma terrane. Hafnium isotopic data from autocrystic zircon have ε Hf ( t) between −6.0 ± 1.5 and +2.1 ± 2.5. The new zircon U–Pb, O, and Hf isotopic data from plutons in the eastern Meguma terrane are indistinguishable from published data from the South Mountain Batholith. The data suggest that Devonian magmatism in the Meguma terrane post-dated the main orogenic event that caused folding and regional metamorphism and involved the same magma source and/or contaminants throughout the terrane.
Elements and compounds circulating in the body are incorporated into hair as it grows. Because of this, hair analyses are increasingly being incorporated in investigations of temporal trends in e.g. hormones and diet in wildlife species such as polar bears (Ursus maritimus). For this study, guard hair (GH; mean length 62 mm) and foreleg guard hair (FGH; mean length 164 mm) were collected from 15 adult male polar bears in western Hudson Bay, Canada. Our aim was to quantify the trace elements arsenic (As), cadmium (Cd), copper (Cu), iron (Fe), lead (Pb), mercury (Hg), selenium (Se), strontium (Sr), and zinc (Zn) every 3 mm in GH and FGH using laser ablation inductively coupled plasma mass spectrometry (laser ablation ICP-MS). Quantitative data was obtained at spatial scales as small as 50 μm – the smallest laser spot diameter used. Mean trace element concentrations increased in the order Pb < As < Cd < Se < Hg < Cu < Sr < Fe < Zn for both GH and FGH (pooled across all individuals). However, mean trace element concentrations were all significantly different between GH and FGH, except for Hg. Hg concentration varied along the length of both GH and FGH; each bear exhibited a unique pattern in Hg variation along the hair. For the remaining eight trace elements, the most common pattern was that of smaller fluctuations near the base of the hair, followed by an increase towards the tip. These fluctuations in trace element concentrations were likley related to hair growth and temporal changes. In this pilot study, we found quantifying trace elements in polar bear hair using laser ablation ICP-MS to be a promising monitoring technique across multiple temporal scales.
Research Article| April 25, 2019 Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga Adrien Vezinet; Adrien Vezinet 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Emilie Thomassot; Emilie Thomassot 2Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, Université de Lorraine, CNRS, 54500 Vandoeuvre-lès-Nancy, France3Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8038 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar D. Graham Pearson; D. Graham Pearson 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Richard A. Stern; Richard A. Stern 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Yan Luo; Yan Luo 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Chiranjeeb Sarkar Chiranjeeb Sarkar 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Geology (2019) 47 (7): 605–608. https://doi.org/10.1130/G46086.1 Article history received: 04 Feb 2019 rev-recd: 25 Mar 2019 accepted: 27 Mar 2019 first online: 25 Apr 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Adrien Vezinet, Emilie Thomassot, D. Graham Pearson, Richard A. Stern, Yan Luo, Chiranjeeb Sarkar; Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga. Geology 2019;; 47 (7): 605–608. doi: https://doi.org/10.1130/G46086.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with δ18O values up to +9‰ at ca. 3.5 Ga, which are the highest values so far measured in any pre–3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon δ18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics ± clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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