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Geochemistry of the Brent Impact Structure, Ontario, Canada

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Geochemistry of the Brent Impact Structure, Ontario, Canada FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde Geochemistry of the Brent impact structure, Ontario, Canada Bart Vleminckx Academiejaar 2010–2011 Scriptie voorgelegd tot het behalen van de graad Van Master in de Geologie Promotor: Prof. Dr. Ph. Claeys Co-promotor: Prof. Dr. M. Elburg Begeleider: Drs. S. Goderis Leescommissie: Prof. Dr. P. Van den haute, Prof. Dr. F. Vanhaecke I. ACKNOWLEDGEMENTS I would like to thank Prof. P. Claeys, Prof. M. Elburg and S. Goderis for their answers, remarks, contributions and making this master dissertation possible; J. Spray for providing the Brent samples; F. Paquay for providing the Os isotope data; V. Renson for providing the Pb isotope data; J. Sauvage for contributing to the platinum group element analysis preparations; J. Belza, E. De Pelsmaeker, D. Debruyne, T. Van der Gucht, and I. Smet for contributing to the major element analysis preparations; M. Van Tomme for everything. Cover art: http://www.mnh.si.edu/earth/text/5_3_2_0.html PAGE I II. NOTE ON STYLE When the contents or structure of a (sub)section is in large measure extracted from one or several works, the references are given introductory to the section. In all other cases in‐text references are used. If not stated else, the reference of a figure’s caption is the same as that of the figure’s source. SI units and prefixes are used following BIPM [2006]. Large numbers are written by the short scale naming system and with the dot as decimal separator. Non‐SI units (with SI prefixes) used are astronomical distance in astronomical units (AU) as distance from the Sun (with Earth set at 1 AU), geological date in annum (a) as years before present (i.e. 1950) and time period in minutes (min), hours (h) or years (y). Chemical element and compound notations are used following IUPAC [2007]. Other abbreviations used are wt.% for weight %, vol.% for volume %, USA for United States of America, USD for USA dollar, SD for standard deviation and SE for standard error. Calendar dates are in the international date format ISO [2004]. If not stated else, sizes of meteorites, craters and impact structures are expressed as their diameters. PAGE II III. DUTCH SUMMARY De eerste vaste stoffen van het zonnestelsel ontstonden 4.567 Ga geleden door condensatie uit de zonnenevel. Door elektromagnetische accretie vormden deze de planetesimalen. Sommige werden zo massief dat ze een kern en mantel met partiële opsmelting vormden. Omdat het zonnestelsel toen nog veel heter was door het samentrekken van de zonnenevel en de jonge zon, konden binnen een bepaalde afstand van de zon, de sneeuwlijn genaamd, geen H‐verbindingen condenseren in ijzen. De planetesimalen die hier ontstonden kregen daarom een samenstelling van voornamelijk metaal en gesteente, en worden asteroïden genoemd. Buiten de sneeuwlijn vormden zich planetesimalen die voornamelijk uit H‐ijzen bestaan met slechts een klein deel metaal en gesteente, kometen genaamd. Onder invloed van de zwaartekracht vormden deze asteroïden en kometen de planeten, dwergplaneten en manen. Er bleven echter ook een aanzienlijk aantal planetesimalen over. Deze zijn niet willekeurig verspreid over het zonnestelsel te vinden, maar verblijven in bepaalde gebieden die begrens worden door zwaartekracht perturbaties van planeten. Zo zijn de meeste kometen te vinden voorbij de planeten in Oortwolk en Kuiper gordel. De asteroïden bevinden zich voornamelijk in de asteroïdengordel tussen 2.1 en 3.3 AU, maar ook andere verblijfsgebieden zijn bekend zoals de Hungarias, Hildas en Cybeles. Niet alle kometen en asteroïden blijven echter in deze gebieden. Diegene die het meest hun verblijfsgebied verlaten zijn de asteroïden uit de asteroïdengordel. Orbitale resonanties met Jupiter zorgen namelijk voor lege gebieden in deze gordel, Kirkwood gaten genoemd. Omloopbanen in deze gebieden hebben perioden gelijk aan een gehele breuk van Jupiters periode en ontvangen dus meer frequent zwaartekracht perturbaties. Door botsingen tussen asteroïden ontstaan fragmenten die deze Kirkwood gaten kunnen bereiken en zo de asteroïdengordel verlaten naar paden doorheen het zonnestelsel. Zo ontvangt de Aarde een deel van deze fragmenten. Wanneer ze de atmosfeer binnen treden zal frictie ontstaan waardoor ze vertragen, opbranden en/of exploderen. Alleen diegene vanaf een bepaalde massa zullen het vaste oppervlak van de Aarde kunnen bereiken. Deze worden meteorieten genoemd. Meteorieten met massa’s vanaf ongeveer 10 Mg zullen zelfs een aanzienlijk deel van hun snelheid PAGE III kunnen behouden en de Aarde inslagen met een dergelijke hoeveelheid kinetische energie, dat deze wordt omgezet in schokgolven. Dit zijn bijna discontinue spanningsgolven met extreme drukken en supersonische snelheden. De resulterende spanningen zijn zo hoog dat de complete meteoriet verdampt en smelt. Ook een hemisferisch volume van doelgesteente, met een straal van 3 tot 4 keer de diameter van de meteoriet, zal smelten. Een nog grotere hoeveelheid van doelgesteente zal geëjecteerd, verplaatst, gebrecciëert en/of gebroken worden. Uiteindelijk zullen de scholgolven een inslagkrater gevormd hebben die 20 tot 30 keer groter is dan de meteoriet. De diepte gaat van 1/3 van de diameter voor kleinere inslagkraters tot 1/6 voor de grotere. Doordat op het einde van de kratervorming instortingen plaatsvinden van de kraterranden, zal de krater breccias bevatten die smelt fragmenten bevatten. Ook smelt lenzen komen voor met een hoog gehalte aan inslagsmelt. Door de oppervlakte processen van de Aarde zullen inslagkraters hun morfologie en lithologie niet behouden. De meesten zijn zelfs onherkenbaar vervormd door tektoniek of totaal geërodeerd. Wat we nu terugvinden van inslagkraters, worden inslagstructuren genoemd. Op de Aarde zijn er nu 178 gekend. Ze hebben ouderdommen van recent als 1947 tot 2.4 Ga en afmetingen van 0.015 tot 300 km. Studies hebben aangetoond dat de inslagsmelt van deze inslagstructuren een meetbare meteoriet contributie bevat, meestal niet groter dan 0.1 wt.%. Deze is gedetecteerd op basis van bepaalde chemische signaturen die afwijken van het doelgesteente. Zo zijn de siderofiele elementen verarmd in de korst omdat ze eerst zijn aangerijkt in de kern en later in de mantel. Vele meteorieten vertonen deze verarming echter niet en slechts een kleine contributie kan een duidelijk verhoging veroorzaken door de lage achtergrond concentraties. Een onderscheid wordt gemaakt tussen de gemiddeld siderofiele elementen Ni, Co and Cr, and de hoog siderofiele platina groep elementen (PGE). Er bestaan ook isotopische signaturen zoals bijvoorbeeld die van Os, die het toelaten om zeer specifiek de hoeveelheid meteoriet contributie te bepalen. Op basis van al deze signaturen kan een inslagstructuur als zodanig geïdentificeerd worden. Meteorieten vertonen ook onderlinge chemische verschillen, en op basis van de siderofiele element verhoudingen kunnen deze bepaald worden. Hierdoor kan bepaald worden welk soort meteoriet de inslagstructuur heeft veroorzaakt. De chondrieten zijn de eerste gevormde planetesimalen. Deze vertonen aquatische en thermische metamorfose, maar hebben geen opsmelting ondergaan. Ze worden onderverdeeld in de koolstofhoudende CI, CM, CR, CB, CV, CK, CO en CH groepen, de gewone H, L en LL groepen, en de enstatiet H en L groepen. Ze vertonen verschillende samenstelling naargelang hun afstand tot de zon. Zo zijn de enstatiet PAGE IV chondrieten het dichts bij de zon gevormd, de gewone iets verder en de koolstofhoudende nog verder, vermoedelijk nabij de sneeuwlijn. Alle meteorieten van een bepaalde groep zijn fragmenten van hetzelfde moederlichaam. Grotere planetesimalen vormden net als de Aarde een kern en ondervonden partieel smelten. Zo zijn er de chondrieten, welke magmatische gesteenten zijn en moeilijk te onderscheiden doordat ze een gelijke geschiedenis hebben als de korst. De ijzermeteorieten zijn fragmenten van de kernen, die zijn vrijgekomen zijn door enorme botsingen. De studie van inslag en meteoriet identificatie heeft zijn belang. Eerst en vooral hebben inslagstructuren een aanzienlijke economische waarde. Maar liefst 15 % van de inslagstructuren wordt winstgevend geëxploiteerd, waaronder een aantal van de grootste mineraalafzettingen ter wereld, maar ook voor olie, gas en/of bouwstenen. Maar inslagstructuren maken nu ook eenmaal, weliswaar een klein, deel uit van de Aarde en hun identificatie is belangrijk om ze te kunnen bestuderen en zo onze kennis van de Aarde te vervolledigen. Echter, op niet of minder actieve plaatsen in het zonnestelsel, maken inslagkraters het grootste deel van het vaste oppervlak uit. Omdat hun afstand hun studie bemoeilijkt kunnen we onze kennis van op de Aarde extrapoleren naar daar. Gezien het economisch potentieel, is hun studie ook van belang voor toekomstige exploratie van de ruimte. Meteorieten worden voor vele doeleinden bestudeerd. Ze bevatten zeldzame informatie over de vorming, componenten, structuur, dynamiek, fluxen, evolutie en huidige staat van het zonnestelsel. Meteorieten die inslagkraters veroorzaken vertegenwoordigen een meer massief bereik dan andere meteorieten. Hun beter hun identificatie, hoe beter hun fluxen, verblijfsgebied, en mogelijkse relaties en moederlichamen kunnen bepaald worden. Verder hebben inslagen ook biologische en maatschappelijke effecten. Het opzet van deze masterproef is om een inslag en meteoriet identificatie toe te passen op Brent inslagstructuur. Hierbij zal nagegaan worden welke het nut is van de verschillende chemische signaturen en hoe ze kunnen ingebracht worden in een multi‐signatuur
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