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U-Pb age distribution recorded in zircons from Archean quartzites in the Mt. Alfred area, Yilgarn Craton, Western Australia
Swedish Museum of Natural History, Sweden.ORCID iD: 0000-0003-0984-3074
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2018 (English)In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 310, p. 278-290Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2018. Vol. 310, p. 278-290
Keywords [en]
Archean, Yilgarn Craton, Detrital zircon, SIMS, Geochronology
National Category
Geosciences, Multidisciplinary
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-158703DOI: 10.1016/j.precamres.2018.01.016OAI: oai:DiVA.org:su-158703DiVA, id: diva2:1238220
Funder
Knut and Alice Wallenberg Foundation, 2012.0097Swedish Research Council, 2012-04370Available from: 2018-08-13 Created: 2018-08-13 Last updated: 2019-02-19Bibliographically approved
In thesis
1. Assessing the reliability of detrital zircon in Early-Earth provenance studies
Open this publication in new window or tab >>Assessing the reliability of detrital zircon in Early-Earth provenance studies
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Our understanding of the Early Earth and the processes that have shaped its evolution have spawned predominantly from the geochemical and isotopic signatures of a small number of zircon populations around the world. Studies of trace element distributions, Hf and O isotope systematics as well as mineral inclusion chemistry in detrital zircon are combined with U-Pb chronology in order to constrain source rock characteristics. However, previous research has highlighted the potential for primary isotopic and geochemical signatures to be modified after deposition, enhanced by self-induced radiation damage and crystal-plastic deformation. Further complications arise when an unknown number of source rocks contribute to the detrital sediments, or when the source rocks are absent as is the case for Hadean detrital zircon. 

In this thesis, a range of analytical methods are applied to relatively uncharted ancient detrital zircon populations, in addition to a systematic investigation into the behaviour of titanium, rare earth elements, U-Th-Pb, Lu-Hf and O isotopes in a magmatic source-rock analogue to such ancient detrital zircon suites. Three localities are studied: Mt. Alfred, within the Yilgarn Craton of Western Australia (detrital study); the Saglek Block, the western-most extent of the North Atlantic Craton in northern Labrador (detrital study); and a locality south of Isua in southern West Greenland (analogue study). We have utilised a CAMECA ims 1280 Secondary Ion Mass Spectrometer (SIMS) for its high spatial resolution and small volume sampling, except for the Lu-Hf analyses which were carried out using a Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICP-MS).

The main outcomes of this study are threefold. 1) Heavily discordant, ancient detrital zircon populations require extensive data filtering in order to produce reliable data for age comparison. Further, age estimations of pervasive Pb loss even in metamict detrital zircon may be achieved using intragrain discordia intercept ages. Applying this in a regional sense reveals that detrital zircon from Mt. Alfred, Western Australia have a distinct provenance in comparison to other metasedimentary units of the Youanmi Terrane, and bear resemblance to the Mt. Narryer metasediments of the Narryer Terrane. 2) Detrital zircon from metasedimentary rocks exposed to high grade metamorphism in the Saglek Block, Northern Labrador yield predominantly Mesoarchaean age signatures, along with a minor Eoarchaean aged component. Lu-Hf isotope data from these zircon reveal up to five near-chondritic populations. U-Pb-Hf data from two samples of metapelite (L1407 and L1408) suggest that a previous re-assignment of deposition age for this unit to >3.95 Ga is unsubstantiated, undermining later studies based on that interpretation. 3) Geochemical complexities in zircon from an Eoarchaean meta-tonalite, taken as a source analogue to ancient detrital zircon, challenge the assumptions and interpretations drawn from detrital zircon studies. Lu-Hf and O isotope systems display mostly homogenous compositions, despite recognised U-Pb disturbance. Petrogenetic trace element proxies such as REE and Ti yield heterogeneous results, even within individual grains. Discerning magmatic signals from detrital zircon populations can therefore, without careful scrutiny, portray artificially complex results and consequently, lead to false interpretations.

Abstract [sv]

Vår kunskap om jordens tidigaste historia och processerna som har format dess utveckling kommer företrädesvis från geokemiska och isotopkemiska signaturer hos ett fåtal zirkonpopulationer från olika delar av världen. Studier av spårelementsfördelning, Hf- och O-isotopsystematik samt kemidata från mineralinklusioner i detritiska zirkoner kan kombineras med U-Pb kronologi för att bestämma ursprungsbergarternas karaktär. Tidigare forskning har emellertid fäst uppmärksamhet på att primära geokemiska och isotopkemiska signaturer kan modifieras efter depositionen, förstärkt av interna strålningsskador och kristall-plastisk deformation. Ytterligare komplikationer uppstår när ett okänt antal ursprungsbergarter bidrar till detritiska sediment eller när ursprungsbergarterna saknas, vilket är fallet för hadeiska detritiska zirkoner.

I denna avhandling har ett antal analysmetoder tillämpats på relativt outforskade detritiska zirkonpopulationer, tillsammans med systematiska undersökningar av titans, sällsynta jordartselements, U-Th-Pb, Lu-Hf och O-isotopers beteende i en magmatisk ursprungsbergart analog med sådana tidiga detritiska zirkonsviter. Tre lokaler har studerats: Mt. Alfred i Yilgarn-kratonen i västra Australien (detritisk studie); Saglek Block, den västligaste utlöparen av den nordatlantiska kratonen i norra Labrador (detritisk studie); och en lokal söder om Isua i sydvästra Grönland (jämförelsestudie). Vi har använt oss av en CAMECA ims 1280 jonmikrosond (SIMS) för dess höga rumsliga upplösning och goda förmåga att analysera små provvolymer, med undantag av Lu-Hf analyserna, vilka utfördes med laserablation (Laser Ablation Multicollector Inductively Coupled Mass Spectrometer, LA-MC-ICP-MS).

De främsta resultaten från denna studie är följande tre: 1) Starkt diskordanta zirkonpopulationer av hög ålder fordrar omfattande datafiltrering för att generera data som är tillförlitliga för ålderskorrelation. Vidare, åldersuppskattningar av genomgripande blyförlust, även i metamikta, detritiska zirkoner, kan erhållas genom beräkning av diskordia-intercept-åldrar inom enskilda kristaller. När detta tillämpas i ett regionalt perspektiv framkommer det att detritiska zirkoner från Mt. Alfred, Western Australia har en distinkt provenans jämfört med andra metasedimentära enheter i Youanmi Terraine och uppvisar likheter med Mt. Narryer metasediment i Narryer Terraine. 2) Detritiska zirkoner från metasediment som utsatts för höggradig metamorfos i Saglek Block, norra Labrador ger företrädesvis mesoarkeiska ålderssignaturer, tillsammans med en mindre eoarkeisk komponent. Lu-Hf isotopdata från dessa zirkoner visar på upp till fem populationer med nära-kondritisk sammansättning. U-Pb-Hf data från två prover av metapelit (L1407 och L1408) tyder på att en tidigare omtolkning av depositionsåldern för denna enhet till >3.95 Ga är ogrundad, vilket undergräver senare studier baserade på denna tolkning. 3) Geokemisk komplexitet i zirkoner från en eoarkeisk metatonalit, ansedd som en motsvarighet beträffande ursprung till äldre detritiska zirkoner, ifrågasätter antaganden och tolkningar baserade på studier av detritiska zirkoner. Lu-Hf och O-isotopsystem uppvisar mestadels homogena sammansättningar, trots uppvisad U-Pb störning. Petrogenetiska spårelement proxies såsom REE och Ti ger heterogena resultat, även inom enskilda korn. Påvisande av magmatiska signaler från detritiska zirkonpopulationer kan därför, utan noggrann granskning, visa på konstgjort komplexa resultat som följaktligen kan leda till felaktiga tolkningar.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2018. p. 48
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 373
National Category
Geology Geochemistry Geosciences, Multidisciplinary
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-158741 (URN)978-91-7797-386-7 (ISBN)978-91-7797-387-4 (ISBN)
Public defence
2018-09-28, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Accepted.

Available from: 2018-09-05 Created: 2018-08-13 Last updated: 2020-05-11Bibliographically approved

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