geochemistry of the brent impact structure, ontario,...

FACULTEIT WETENSCHAPPEN Vakgroep Geologie en Bodemkunde

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

Geochemistry of the Brent impact structure, Ontario, Canada

Bart Vleminckx

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I. ACKNOWLEDGEMENTS

IwouldliketothankProf.P.Claeys,Prof.M.ElburgandS.Goderisfortheiranswers,

remarks, contributions and making this master dissertation possible; J. Spray for

providingtheBrentsamples;F.PaquayforprovidingtheOsisotopedata;V.Renson

forprovidingthePbisotopedata;J.Sauvageforcontributingtotheplatinumgroup

element analysis preparations; J. Belza, E.DePelsmaeker,D.Debruyne, T. Vander

Gucht, and I. Smet for contributing to themajor element analysis preparations;M.VanTommeforeverything.

Coverart:http://www.mnh.si.edu/earth/text/5_3_2_0.html

PAGEII

II. NOTEONSTYLE

Whenthecontentsorstructureofa(sub)sectionisinlargemeasureextractedfrom

one or several works, the references are given introductory to the section. In all

othercasesin‐textreferencesareused.Ifnotstatedelse,thereferenceofafigure’s

caption is the same as that of the figure’s source. SI units and prefixes are used

followingBIPM[2006].Largenumbersarewrittenbytheshortscalenamingsystem

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

Earthsetat1AU),geologicaldate inannum(a)asyearsbeforepresent(i.e.1950)

and time period in minutes (min), hours (h) or years (y). Chemical element and

compoundnotationsareusedfollowingIUPAC[2007].Otherabbreviationsusedare

wt.%forweight%,vol.%forvolume%,USAforUnitedStatesofAmerica,USDfor

USAdollar,SDforstandarddeviationandSEforstandarderror.Calendardatesare

in the international date format ISO [2004]. If not stated else, sizes ofmeteorites,cratersandimpactstructuresareexpressedastheirdiameters.

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III. DUTCHSUMMARY

De eerste vaste stoffen van het zonnestelsel ontstonden 4.567 Ga geleden door

condensatieuitdezonnenevel.Doorelektromagnetischeaccretievormdendezede

planetesimalen.Sommigewerdenzomassiefdatzeeenkernenmantelmetpartiële

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.Deplanetesimalendiehierontstondenkregendaaromeensamenstellingvan

voornamelijk metaal en gesteente, en worden asteroïden genoemd. Buiten de

sneeuwlijn vormden zich planetesimalen die voornamelijk uitH‐ijzen bestaanmet

slechtseenkleindeelmetaalengesteente,kometengenaamd.Onderinvloedvande

zwaartekrachtvormdendezeasteroïdenenkometendeplaneten,dwergplanetenenmanen.

Er bleven echter ook een aanzienlijk aantal planetesimalen over. Deze zijn niet

willekeurig verspreidoverhet zonnestelsel te vinden,maar verblijven inbepaalde

gebiedendiebegrenswordendoorzwaartekrachtperturbatiesvanplaneten.Zozijn

demeestekometentevindenvoorbijdeplaneteninOortwolkenKuipergordel.De

asteroïdenbevindenzichvoornamelijkindeasteroïdengordeltussen2.1en3.3AU,

maar ook andere verblijfsgebieden zijn bekend zoals de Hungarias, Hildas en

Cybeles.Nietallekometenenasteroïdenblijvenechterindezegebieden.Diegenedie

hetmeest hun verblijfsgebied verlaten zijn de asteroïden uit de asteroïdengordel.

OrbitaleresonantiesmetJupiterzorgennamelijkvoorlegegebiedenindezegordel,

Kirkwood gaten genoemd. Omloopbanen in deze gebieden hebben perioden gelijk

aan een gehele breuk van Jupiters periode en ontvangen dus meer frequent

zwaartekrachtperturbaties.Doorbotsingentussenasteroïdenontstaanfragmenten

diedezeKirkwoodgatenkunnenbereikenenzodeasteroïdengordelverlatennaar

padendoorheenhetzonnestelsel.

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.Alleendiegenevanafeenbepaaldemassazullenhetvasteoppervlakvan

de Aarde kunnen bereiken. Deze worden meteorieten genoemd. Meteorieten met

massa’s vanaf ongeveer 10 Mg zullen zelfs een aanzienlijk deel van hun snelheid

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kunnenbehouden endeAarde inslagenmet eendergelijke hoeveelheid kinetische

energie, dat deze wordt omgezet in schokgolven. Dit zijn bijna discontinue

spanningsgolvenmetextremedrukkenensupersonischesnelheden.Deresulterende

spanningen zijn zo hoog dat de complete meteoriet verdampt en smelt. Ook een

hemisferischvolumevandoelgesteente,meteenstraalvan3tot4keerdediameter

van demeteoriet, zal smelten. Een nog grotere hoeveelheid van doelgesteente zal

geëjecteerd, verplaatst, gebrecciëert en/of gebrokenworden.Uiteindelijk zullende

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.Doorde oppervlakteprocessen vandeAarde zullen inslagkratershun

morfologie en lithologie niet behouden. De meesten zijn zelfs onherkenbaarvervormddoortektoniekoftotaalgeërodeerd.

Watwenuterugvindenvaninslagkraters,wordeninslagstructurengenoemd.Opde

Aardezijnernu178gekend.Zehebbenouderdommenvanrecentals1947tot2.4Ga

enafmetingenvan0.015tot300km.Studieshebbenaangetoonddatdeinslagsmelt

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

verarmdindekorstomdatzeeerstzijnaangerijktindekernenlaterindemantel.

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

elementenNi,CoandCr,anddehoogsiderofieleplatinagroepelementen(PGE).Er

bestaan ook isotopische signaturen zoals bijvoorbeeld die vanOs, die het toelaten

omzeer specifiekdehoeveelheidmeteoriet contributie tebepalen.Opbasisvanal

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 kanbepaaldwordenwelksoortmeteorietdeinslagstructuurheeftveroorzaakt.

De chondrietenzijnde eerstegevormdeplanetesimalen.Dezevertonenaquatische

en thermischemetamorfose,maar hebben geen opsmelting ondergaan. Zeworden

onderverdeeldindekoolstofhoudendeCI,CM,CR,CB,CV,CK,COenCHgroepen,de

gewone H, L en LL groepen, en de enstatiet H en L groepen. Ze vertonen

verschillendesamenstellingnaargelanghunafstand totdezon.Zozijndeenstatiet

PAGEV

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

planetesimalenvormdennetalsdeAardeeenkernenondervondenpartieelsmelten.

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 doorenormebotsingen.

De studie van inslag en meteoriet identificatie heeft zijn belang. Eerst en vooral

hebbeninslagstructureneenaanzienlijkeeconomischewaarde.Maarliefst15%van

deinslagstructurenwordtwinstgevendgeëxploiteerd,waarondereenaantalvande

grootstemineraalafzettingenterwereld,maarookvoorolie,gasen/ofbouwstenen.

Maar inslagstructurenmakennuookeenmaal,weliswaareenklein,deeluitvande

Aarde en hun identificatie is belangrijk om ze te kunnen bestuderen en zo onze

kennisvandeAarde tevervolledigen.Echter,opnietofminderactieveplaatsen in

hetzonnestelsel,makeninslagkratershetgrootstedeelvanhetvasteoppervlakuit.

OmdathunafstandhunstudiebemoeilijktkunnenweonzekennisvanopdeAarde

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

meermassiefbereikdananderemeteorieten.Hunbeterhunidentificatie,hoebeter

hun fluxen, verblijfsgebied, en mogelijkse relaties en moederlichamen kunnen

bepaald worden. Verder hebben inslagen ook biologische en maatschappelijkeeffecten.

Het opzet van dezemasterproef is om een inslag enmeteoriet identificatie toe te

passenopBrentinslagstructuur.Hierbijzalnagegaanwordenwelkehetnutisvande

verschillende chemische signaturen en hoe ze kunnen ingebracht worden in een

multi‐signatuur benadering. Twintig stalenwerden verzameld van smelt‐fragment

breccias, een smelt lens en het doelgesteente. Deze werden geanalyseerd voor

hoofdelementen(n=13),Ni,CoenCr(n=20)enPGE(n=13).

Karakterisatie vande aanwezige alteratie toonde chloritisatie, Au‐verarming enK‐

aanrijking aan in de smelt‐fragment breccias. Terwijl dit de correlatie vernietigt

tussendemobieleNi,CoenCr,vertonendeimmobielePGEgeenverandering.Ookde

smelt lens toont alteratie, al zij het inminderemate. Ni, Co en Cr tonen hier een

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goede correlatie. Het gebruik van het Os isotoop systeem leverde meteoriet

contributiewaardenoptot1a2gew.%voorhetdiepstesmelt lensstaal.Tezamen

metdegemiddeldenhoogsiderofieleelementen,weerlegtditdemogelijkheidvan

eeneerdervoorgesteldefractionatie,enimpliceertheteenimpactidentificatie.

Weweerleggenookeenaannamedatergeen(ultra)mafischecomponentaanwezig

is.Hetismogelijkdatsmelt‐fragmentbrecciaseendergelijkecomponentbevatenzo

meedeontbrekendecorrelatieverklaartvoorNi,CoenCr.Desmeltlensbevatmet

zekerheid geen extra component. Een regionale studie van het doelgesteente zou

dezemogelijkheidkunnenbevestigenofuitsluiten,enwanneeraanwezig,heteffectopdesmelt‐fragmentbrecciasbepalen.

Opbasisvaneenmulti‐signatuurbenaderingdoorhetcombinerenvandegemiddeld

en hoog siderofiele elementen kan een precieze meteoriet identificatie worden

bepaaldtotophetgroepniveau:eenIAniet‐magmatischeijzermeteoriet.Terwijlde

Ni/Cr,Co/CrenPd/IrverhoudingenduidenopeenLLofLgewonechondrite,zijnde

Ni/Co, andere PGE en gecombineerde verhoudingen hier niet mee in

overeenstemming. Gebaseerd op een lineair en vergroot PGE patroon, dat

representatiefwordtgeachtvoordemeteoriet,zijnenkeldeniet‐magmatischeIAen

IIIC groepen mogelijk. Als alle siderofiele verhoudingen worden samengebracht,

wijzendezeduidelijkopeenIA.

DeBrentinslagstructuurisideaalvoorhetbestuderenvandeeffectenvanalteratie

opinslagenmeteorietidentificatie,zekervoordesmelt‐fragmentbreccias.Doorzijn

weerstandtegenalteratieenhogemeteorietcontributielevertdesmeltlensdebestestalenvoorinslagenmeteorietidentificatie.

Voor meteoriet identificatie zijn de implicaties duidelijk. De precieze identificatie

wasnietmogelijkgeweestzonderhetgebruikvaneenmulti‐signatuurbenadering.

Terwijl Ni, Co en Cr redelijk gemakkelijk te analyseren zijn, vertonen hun

chondritische verhoudingen overlap met meerdere ijzer meteorieten. Langs de

anderekantmakenzeeenverderediscriminatiemogelijkwanneereenchondrietofijzermeteorietisbepaalddoordePGE.

HetOsisotoopsysteemlijktinstaatomalleeneenimpactteidentificeren,maarhet

omvat een omslachtige analyse. Wanneer deze data met de andere siderofiele

elementen en hoofdelementen word gecombineerd, is het mogelijk om andereprocessenalsalteratieof(ultra)mafischecomponenten,teidentificeren.

Eenalgemeneconclusieisdatvoorimpactenmeteorietidentificatiebeststeedseenmulti‐signatuurbenaderingkangebruiktworden.Hoemeerdata,hoebeter.

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IV. CONTENTS

I.Acknowledgements.................................................................................................................................I

II.NoteonStyle..........................................................................................................................................II

III.DutchSummary..................................................................................................................................III

IV.Contents................................................................................................................................................VII

1.INTRODUCTION__________________________________________________1 1.1.ScientificSetting............................................................................................................................2 1.1.1.TheImpactTheory..............................................................................................................2 1.1.2.ImpactGeochemistry.........................................................................................................3

1.2.StudyObjectives............................................................................................................................4 1.3.Importance......................................................................................................................................4 1.3.1.Economic.................................................................................................................................4 1.3.1.1.NaturalResources......................................................................................................5 1.3.1.2.Tourism..........................................................................................................................6

1.3.2.Scientific..................................................................................................................................6 1.3.2.1.TheSolarSystem........................................................................................................7 1.3.2.2.BiologicalEffects........................................................................................................7

1.3.3.Social.........................................................................................................................................8

2.LITERATURE __________________________________________________10 2.1.ImpactStructures......................................................................................................................10 2.1.1.ImpactMeteorites............................................................................................................11 2.1.1.1.AsteroidsandComets............................................................................................11 2.1.1.2.ResidenceRegions..................................................................................................12 2.1.1.3.VoyagetotheEarth................................................................................................13 2.1.1.4.ThroughtheEarth’sAtmosphere.....................................................................14

2.1.2.ImpactCraters...................................................................................................................15 2.1.2.1.Compression..............................................................................................................16 2.1.2.2.Excavation..................................................................................................................17 2.1.2.3.Modification...............................................................................................................18 2.1.2.4.Post‐impactDevelopment...................................................................................19

2.1.3.ShockMetamorphism.....................................................................................................21 2.1.4.ImpactRocks......................................................................................................................22 2.1.4.1.Basement....................................................................................................................22

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2.1.4.2.Crater‐fill.....................................................................................................................23 2.1.4.3.Proximal‐ejecta........................................................................................................25 2.1.4.4.Distal‐ejecta...............................................................................................................25

2.2.MeteoriteClassification..........................................................................................................26 2.2.1.Chondrites...........................................................................................................................28 2.2.1.1.Carbonaceous............................................................................................................30 2.2.1.2.Ordinary......................................................................................................................31 2.2.1.3.Enstatite......................................................................................................................31

2.2.2.Non‐chondrites..................................................................................................................31 2.2.2.1.Achondrites................................................................................................................32 2.2.2.2.Irons..............................................................................................................................34 2.2.2.3.Stony‐irons.................................................................................................................35

2.2.3.CometaryMeteorites......................................................................................................35 2.3.GeochemicalSignatures..........................................................................................................36 2.3.1.ElementAbundances......................................................................................................37 2.3.1.1.ModeratelySiderophileElements....................................................................37 2.3.1.2.HighSiderophileElements..................................................................................38 2.3.1.3.MajorandTraceElements...................................................................................39

2.3.2.IsotopeRatios....................................................................................................................39 2.3.2.1.OsIsotopeSystem...................................................................................................39 2.3.2.2.CrIsotopeSystem...................................................................................................40 2.3.2.3.OtherIsotopeSystems..........................................................................................40

2.3.3.Multi‐signatureapproach.............................................................................................41 2.4.BrentImpactStructure...........................................................................................................41 2.4.1.ImpactDimensions..........................................................................................................43 2.4.2.TargetRock.........................................................................................................................44 2.4.3.BrecciaLens........................................................................................................................45 2.4.3.1.LithicBreccias...........................................................................................................45 2.4.3.2.Melt‐fragmentBreccias........................................................................................46 2.4.3.3.MeltLens.....................................................................................................................46

2.4.4.Alteration.............................................................................................................................48 2.4.5.GeochemicalIdentification...........................................................................................48

3.METHODS_____________________________________________________50 3.1.Samples..........................................................................................................................................50 3.2.MajorElements...........................................................................................................................51 3.3.Ni,CoandCr................................................................................................................................51 3.4.PlatinumGroupElements......................................................................................................52 3.5.Isotopes..........................................................................................................................................53

4.RESULTS______________________________________________________54 4.1.ElementAbundancesandRatios.........................................................................................54

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4.1.1.MajorElements.................................................................................................................54 4.1.2.Ni,CoandCr.......................................................................................................................56 4.1.3.PlatinumGroupElements.............................................................................................58

4.2.IsotopeRatios..............................................................................................................................61 4.2.1.OsIsotopeSystem............................................................................................................61 4.2.2.PbIsotopeSystem............................................................................................................62

4.3.Multi‐signatureApproach......................................................................................................63

5.DISCUSSION___________________________________________________65 5.1.Alteration......................................................................................................................................65 5.2.ImpactIdentification................................................................................................................66 5.3.MeteoriteIdentification..........................................................................................................68 5.4.Non‐magmaticIronMeteorites...........................................................................................70

6.CONCLUSION__________________________________________________72 6.1.TheBrentImpactStructure..................................................................................................72 6.2.ImplicationsforIdentification.............................................................................................73

V.References................................................................................................................................................X

VI.Appendices.......................................................................................................................................XVII

1. INTRODUCTION

Besides the planets, dwarf planets andmoons, the solar system comprises a vast

amountof smallerbodieswithvaried sizes, compositionsandprovenance regions.

They are defined as small solar system bodies [IAU, 2006]. Throughout time, at

timescalesfarbeyondours,manyoftheseobtaineddynamicalpaths,whichledthem

to the solid surface of other solar system bodies. When not hampered by any

atmosphereorhydrosphere,ormassiveenough, theystriked the target rockswith

suchvelocitythatshockwavesoriginatedfromtheirkineticenergies.Thesecauseda

huge amount of rocks to vaporise, melt, eject, displace, brecciate and/or fracture,

whichresultedintotheformationofacrater,manytimeslargerthanthesmallsolar

systembodies themselvesandbuiltupoutof avarietyofdistinct impact rocks. In

fact,thisprocessofimpactcrateringoccurredoneverysinglesolarsystembody.On

mostofthem,thesolidsurfaceisevenalmostcompletelyformedbyimpactcraters.

Theseshowvarieddiameters,reflectingthevariedsizesoftheimpactedsmallsolar

systembodies.Only few solar systembodieshave theappropriatehydrosphereor

atmosphere, and are active enough, to have surface processeswhich candefy thisshapingforceofimpactcratering.

One such active body is the Earth. Because it has a dense enough atmosphere, a

certainmassrangeofsmallsolarsystembodieswasdeceleratedandlandedonthe

Earth, without impact event. These are called meteorites [IAU, 1961]. The more

massiveoneshowever,namedimpactmeteorites,didreachtheEarth’ssolidsurface

athighvelocity,withimpacteventsasresult.Butbecauseactive,whatwasoncean

impact crater, ismostly processedbeyond recognition as such.However, although

notalwaysvisibletothenakedeye,aminorpartdoessurviveinsomeway.Fewof

them show still some kind of resemblance to the craters they descent from. Off

course,thelargerandyounger,thehigherthepreservation.Thesevariedremnants

of impact craters are called impact structures. At present, 178 are known, which

comeinawiderangeofagesandsizes:fromthe1947SikhoteAlinuptothe2.4Ga

Suavjärviimpactstructure,andfromthe15mHavilanduptothe300kmVredefort

impactstructure(seeAppendixA)[PASSC,2010].Theycanbestudiedinmanyways

andtomanyends.Geochemistryisonesuchwayanditformsthetopicofthismasterdissertation.

1.1. SCIENTIFICSETTING

Althoughimpactcrateringisnowconsideredtobeanimportantprocessinthesolar

system–perhapseventhemostimportantatpresent–andmanyimpactstructures

are known, this knowledge is rather recent. Before, impact cratering, craters and

structures,wereoftenignoredandevenridiculed.Geochemicalstudiesevenneeded

the acquiring of faraway impact rocks from the Moon first, before analyses wereconductedonimpactstructures.

1.1.1. THEIMPACTTHEORY

ThefirstappearanceofimpactstructuresinliteraturewasbyRobertHookein1665.

He suggested an impact origin for theMoon’s surface features,which for the first

time were described as depressions by Galileo Galilei in 1609. However, his

revolutionaryimpacthypothesiswaseasilydiscardeddueto(1)thelackofpossible

projectiles, the interplanetary universe was considered empty back then; and (2)

multipleexplanationsbymoreconventional,volcanism‐related,processes[Koeberl,2001].

Atthebeginningofthe19thcentury,notlongafterthefirstsuggestionsbyE.Chladni

in1794,itbecameknownthatsmallsolarsystembodiesexistandthatsomeofthem

becomemeteorites[Howard,1802andVauquelin,1803].Withthisnewknowledge,

theimpacthypothesiswasdevelopedfurtherinthe19thandfirstquarterofthe20th

century[vonPaulaGruithuisen,1828;Proctor,1873;Gilbert,1893;Öpik,1916;Ives,

1919;Wegener, 1921; and Gifford, 1924]. But still, it was supressed by themore

conventional volcanic theory [Koeberl, 2001 and references therein]. It was only

from the 1920s from studies of theBarringer impact structure byD.M. Barringer

[PASSC, 2010 and references therein] and theoretical studies, that the hypothesis

gainedmoreacceptance[Grieve,1991;Koeberl,2001;French&Koeberl,2010;and

referencesinall].CombiningthesedifferentstudiesbyBaldwin[1949],ledtoafirst

complete meteorite impact hypothesis that would challenge the volcanic theory

increasingly.

In1965,aspecialmeetingwasorganisedbytheNewYorkAcademyofSciencesfora

debatebetweenthevolcanicandimpactcamps.Itillustratesthegeneralacceptance

of an impact theory,whichoccurred in the1960sand1970s [Bucher,1963;Dietz,

1963; French, 1968, 1990;Nicolaysen& Reimold, 1990; Sharpton&Grieve, 1990;

French & Koeberl, 2010; and references in all]. This by (1) the exploration of the

solarsystembyroboticandmannedspacecraft,whichrevealedimpactcrateringas

an important process in the solar system [Taylor, 1982, 1992; and French, 1998];

and (2) the ability to unambiguously identify impact structures and thus to

distinguish them from more conventional geological structures. This impact

identificationisbasedongeochemistry,andthepresenceofpermanentpetrological

and mineral changes, unique to impact cratering, called shock metamorphism

[French & Koeberl, 2010 and references therein]. It was only in 1977 that E. M.

Shoemakerdeclaredimpactcrateringasafundamentalprocessofthesolarsystem,

andbecamethe famouspioneerofplanetarygeologyand impactcrateringstudies,

heisnowknownas[Reimold&Koeberl,2008].Thesuggestionthatanimpactevent

causedthemassextinctionattheCretaceous‐Palaeogeneboundary,byAlvarezetal.

[1980], made awareness of the importance of impact events complete, and even

resulted into the attention ofmany others than earth and planetary scientists. At

present, the study of impact cratering, craters and structures has developed anextendedliteraturethatisstillincreasingathighrate.

1.1.2. IMPACTGEOCHEMISTRY

Geochemical studies on impact structures only started when lunar impact rocks

were brought to the Earth by the USA Apollo missions between 1969 and 1972

[Ganapathy et al., 1970a, 1970b, 1972, 1973, 1974; Morgan et al., 1972, 1974a,

1974b; Anders et al., 1973; and Higuchi & Morgan, 1975]. These studies all

concludedthatduringtheformationofalunarimpactcrater,asmallamountofthe

smallsolarsystembodyisincorporatedintotheformedimpactrocks.Morganetal.

[1975]confirmedthis forthemelt‐bearingimpactrocksfromimpactstructureson

theEarth.Here,theamountofthemeteoritecontributionisusuallylessthan1wt.%

[Koeberl,1998],althoughsometimesmuchhighervaluesarefound(upto8wt.%for

the Clearwater East impact structure [McDonald, 2002]). The presence of the

meteorite contribution was deduced from multiple geochemical signatures in the

impactrocksthatweredifferentfromthetargetrock.Thisdifferenceoccursbecause

meteorites show a distinct difference in chemical composition from the crust, a

resultoftheirdifferentdifferentiationhistory.Thesegeochemicalsignaturescanbe

usedfor(1)meteoriteidentification:thechemicalcharacterisationofthemeteorite,

because meteorites show also mutual differences in composition, including their

chemical, and are classified this way into multiple levels; and (2) for impact

identification:whenameteoritecontributionismeasured,thisidentifiesorconfirms

offcourse,theimpactoriginoftheimpactstructure.

The first meteorite identification studies could only identify few impacts and few

meteorites,mostlyonly toa first level [Koeberl,1998].Butover the last tenyears,

some improvementshavebeenmade,especially regarding toanalytical techniques

andisotopesystems[Koeberl,2007andGoderisetal.,2009].Atpresent,meteorite

identification studies are conducted for 51 impact structures, of which only ten

delivered a precise classification [PASSC, 2010]. Recently, a new approach of

combiningthemultiplegeochemicalsignaturesisused,thesocalledmulti‐signature

approach [Koeberl et al., 2007 and Tagle et al., 2007]. This to classify moremeteoritestoamorepreciselevel.

1.2. STUDYOBJECTIVES

In this master dissertation we conduct a meteorite identification, as precisely as

possible,onobtainedmelt‐bearingimpactrocksfromtheBrentimpactstructure.In

addition,thisisusedasacasestudytoreviewthedifferentgeochemicalsignatures

for meteorite identification, and discuss their applicability and place in a multi‐signatureapproachandforimpactidentification.

1.3. IMPORTANCE

Thestudyofimpactcratering,cratersandstructures,includingimpactandmeteorite

identification,has its importance,andnotonly fromascientificpointofview.Also

social and economic aspects are involved. Especially the economic importance of

impact structures is increasing significantly in thecapitalised societywe live in. In

turn,economicprospectingandexplorationhasitbenefitsforthescientificpart,for

examplebyprovidingtheinvolvedscientistwithsamplesfromdrillholesorevenby

discoveringimpactstructures.

1.3.1. ECONOMIC

The economic importance of impact structures, makes impact identification even

more significantly. However, this economic part is still addressed only shortly or

sometimes even overlooked, when reviewing impact structures. Probably because

someimpactstructureswerealreadyexploitedsincebeforetheywereidentifiedas

such.Butthevaluesaresubstantial.Grieve[2005]estimatesthatimpactstructures

haveavalueofmorethanUSD18billionperyear inNorthAmericaalone.Mostofthiseconomicpotentialliesinnaturalresources.

1.3.1.1. NATURALRESOURCES

Approximately25%oftheimpactstructurescontainnaturalresources,two‐thirdof

theseareexploited.Thismeansthatmorethan15%ofallimpactstructuresareof

economic profit, which implicates a very high probability in mineral and

hydrocarbon exploration standards [Grieve & Masaitis, 1994 and Hawke, 2004].

Thesenumbersarealsovery important towards future space exploration,because

impact craters are the most frequent geological features in the solar system.

Accordingtotheirtimeofformation,impact‐structure‐relatednaturalresourcesarepro‐,syn‐orepigenetictotheimpactevent[Masaitis,1989].

Progenetic natural resources were already present before the impact event, but

because of it, are displaced, exposed and/or have an increased preservation

potential.NotablearetheVredefortimpactstructure,whichistheworld’slargestAu

deposit and accounts for more than one‐third of the world’s historic production

[Phillips& Law, 2000; andGrieve, 2005], and the Carswell impact structure, fromwhichalready590millionkgofUismined,worthUSD1.5billion[Grieve,2005].

Thesyngeneticnaturalresourcesaredirectlyformedbytheimpactevent.Outofthe

melt that is formed during it, magmatic mineral deposits can form. However,

enrichment is only sufficient for economicmining in very large impact structures.

That’swhyonlyonesuchimpactstructureisknown:Sudbury.Ontheotherhand,it

is the largest mineral deposit known. Its production and reserves account for

approximately1.655trillionkgofNi,Cu,Co,Au,Ag,Se,Te,Ru,Rh,Pd,Os,IrandPt

[Ames & Farrow, 2007]. Also mineral phase transitions occur during an impact

event. This way, when carbon‐rich rocks are present, diamonds can form out of

graphite or by crystallization of coal. Such impact diamonds are present in many

impact structures and even in large quantities. However, their concentrations are

low,whichmakes exploitation currently not profitable (e.g. the Zapadnaya impact

structurecontainsapproximately17,5billionkg,worthUSD90million,butonlyat1

ppm[Hawke,2004]).Amorepossibleplace for impactdiamondminingareplacer

deposits.OnesuchplaceisprospectedontheEbeliakhriver,approximately200km

eastofthePopigaiimpactstructure[Shelkovetal.,1998andHawke2004].

Epigenetic natural resources originate after an impact event, but because of

conditions that were created during the impact event. Impact structures remain

heatedatdeeperlevelsforthousandsofyearsorevenmillionsifverylarge[Ameset

al.,2000andOsinskietal.,2001].Thisheatcanproducehydrothermalsystemsthat

formsubstantialmineraldeposits,suchasthecarbonateandsulphidedepositsofthe

Haughton impact structure [Osinskietal.,2001].Alsoaminorpartof theSudbury

depositsareofhydrothermalorigin[Grieve&Masaitis,1994].Evenmoreimportant

are hydrocarbons. Due to the extended fractionation and brecciation, impact

structures form excellent reservoirs. Also traps and seals can be created by the

structuraldisplacement.Forexample,intheGulfofMexico,themajorpartofthe4.8

billionm³oiland425billionm³gasoftheCantarellfieldisrelatedtotheChicxulub

impactstructure[Grieve,2005].Inaddition,whenthecraterispreserved,theimpact

structure itself can form a basin, in which hydrocarbon‐potential layers can be

deposited, trapped and sealed. Many impact structure also act as fresh water

reservoirs this way. Some even deliver hydroelectricity. For example, the hydro

power station at theManicouagan impact structure produces 4500GWhper year.

This is enough to supplypower to a small city andworth approximatelyUSD400millionperyear[Hawke,2004].

Besides these, impact structures are also exploited for their rocks. They canbe as

well pro‐, syn as epigenetic. These include building stones, evaporites, diatomitesandoil‐shales[Westbroek&Stewart,1996andFrench,1998].

1.3.1.2. TOURISM

Inadditiontonaturalresources, impactstructurescanalsobeofeconomicinterest

bytherelativenewformoftourism.Eachyear,theyarevisitedbytensofthousands

of people, all imposedby their scientific newness, dimensions, devastatingpowers

and sometimes beautiful nature. Local economies try to benefit from this and

develop tourist centres with information and hiking trails, to attract even more

people(e.g.Ries,Tswaing,SudburyandBarringerimpactstructures)[Hawke,2004].

1.3.2. SCIENTIFIC

ImpactstructurescontributetotheEarth’scrust,andalthoughtheyonlyconstitutea

minorpartof it,knowledgeof themisnecessary tocomplete theunderstandingof

the whole Earth someday, which is what many scientists so desperately seek. In

addition, the formation of large impact structures includes a significant rebound

uplift,whichbringsdeepseatedrockstothesurface.Thismakesimpactstructures

the ideal place to study these otherwise hard to find rocks. But impact structureshavealsosomescientificimportanceoutsidetheEarthandeventowardslife.

1.3.2.1. THESOLARSYSTEM

As already stated above, impact cratering is a fundamental process in the solar

system, itplaysan important role in the formationanddevelopmentofmostsolar

systembodies.Thesurfaceofourownmoonforexample,consistsforover80%out

of impact craters, the largest 2500 km in size [Bennet et al., 2008 and Reimold&

Koeberl,2008].ButexceptfortherelativelycloseMoon,theseexoticworldsarestill

hard to reachoreven sample.Therefore, theknowledgewehave fromour impact

structures, isextrapolatedtothem.Somedetailswillbedifferent,becausedifferent

gravities,atmospheresand targetrocksarepresent,but thegeneralprincipleswill

besimilar.Infact,knowledgeofimpactcrateringisprobablyessentialtothesuccess

offuturespaceexploration,knowingitseconomicimportance.

Meteoritesarestudiedformanyreasons.Forexample,theyprovideinformationon

how thesolarsystem is formed, fromwhat it isbuild, itsdynamicsand fluxes,but

alsoonitsevolutionandpresentstate.Whilemeteoritessampleacertainmassrange

ofsmallsolarsystembodies,impactmeteoritesareproficientbecausetheysamplea

moremassiverange.Themorepreciselyameteoriteisidentified,thebetteritsfluxrate,provenanceregionandpossibleparentbodycanbedetermined.

1.3.2.2. BIOLOGICALEFFECTS

Impact cratering has also some biological effects. In fact, impacts of small solar

systembodiesareprobablyevenresponsiblefortheoriginoflifeinthefirstplace.A

small solar system body that impacts upon another solar system body can spall

fragmentsofthisbody,ofwhichthemostacceleratedinturn,canbecomesmallsolar

systembodies.Somescientistssuggestthatlifeoriginatedelsewhereandthatitwas

transportedtotheEarthbysuchfragments,probablyfromMars,orevenfromoutof

the solar system.Others say lifeoriginatedonsmall solar systembodies.Evidence

existsof complexorganicmolecules that resideon small solar systembodies from

theouterpartsofthesolarsystem,includingaminoacids,whichareessentialforlife

asweknowit.LifecouldhaveoriginatedhereandthenbroughttotheEarth.Amore

supported hypothesis is that these small solar system bodies supplied the raw

materials,oratleastapartofthem,thatwereneededforlife.But,theoriginoflife

happened on the Earth. These solar system bodies probably brought also a large

portionofthewaterpresentontheEarth[Horneck&Rettberg,2007andBennetet

al.,2008].

Ontheotherhand,impacteventsarealsodestroyersoflife.Thiswasfirstsuggested

with evidence by Alvarez et al. [1980],who found ameteorite contribution in the

global clay layer that forms the Cretaceous‐Palaeogene boundary. This implicated

the clay to be impact ejecta and thus an impact origin for the ~ 65.5 Ma mass

extinction.Themeteorite contributionwas confirmed in later studiesatnumerous

locationsaroundtheworld[Schulteetal.,2010andreferencestherein].Morethan

ten years later, the impact structurewas identified: the 170 km Chicxulub impact

structure inMexico [Hildebrand et al., 1991]. The local and regional effects of the

impact event include the air blast and heat from the impact event, tsunamis, and

earthquakes. Global effects include forest fires ignited by spalled fragments, the

injection of huge amounts of dust into the upper atmosphere, which may have

inhibited photosynthesis for as much as twomonths, and the production of huge

quantitiesofN2Ofromatmosphericheating,whichistoxic,astronggreenhousegas,

andharmfulfortheozonelayer.However,themostimportanteffectwasduetothe

compositionof the targetrocks:approximately3kmofcarbonatesandevaporites.

Thisway, the impactevent resulted into thereleaseofvastamountsofCO2,alsoa

greenhouse gas, and sulphur species, which are toxic and block sunlight. Al this

resulted into immediate devastation and short‐term global cooling. Estimates

suggestasmuchasa15°Cdecrease inaverageglobal temperatures.Thiswasthen

followed by a long‐term warming from the release of the greenhouse gasses.Conditionsthatweretoohardtoendureformanylifeforms[Osinski,2008].

1.3.3. SOCIAL

Understanding the process of impact cratering, should not only be of interest to

earth and planetary scientists, but to whole society. This because it is still active.

Imagine what would happen to our society if an impact event should take place.

Smaller impactmeteoriteswould only cause local effects. But the global effects by

larger ones, as described in the previous section, could be catastrophic. Although

such large impact events happen at timescales far beyond ours, there is an

approximate1in10.000chancethatasmallsolarsystembodyof2kmwillimpact

the Earth in the next 100 y,which is enough for devastating effects at regional toglobalscale[French,1998].

The scientific findings on a possible impact event, its hazards and consequences,

receives more and more public attention these days. This is helped by the more

spectacularmilestoneslikethe“dinosaur”extinctionbyAlvarezetal.[1980]orthe

1908explosionnearthePodkamennayaTunguskariverinKrasnoyarskKrai,Siberia.

The latter devastated some 2000 km² of forests and the resulting air blast was

measured around the world. It is now known that it was the break‐up of an

approximately40mmeteorite.Ifthishadhappenedafewhourslater,thewholecity

ofSaintPetersburgwouldbedestroyed [Cohen,2008andSteel, 2008].Evenmore

appealingwasthe1994recordedcrashof the fragmentedsmall solarsystembody

Shoemaker‐Levy 9 onto Jupiter. Its resultingmarks, some initially as large as the

Earth, were visible for several months [Hockey, 1994]. The footage was spread

wholeover theworld.However, toooften thispublicattentionon impactevents is

onlyexploitedforsensationalpurposes.

Those known small solar system bodies that have the potential for impacting the

Earth,haveaspecialstatusandarereferredtoasnear‐Earthobjects(NEO’s).They

receive a value of zero to ten on an impact hazard and public warning scale: the

Torino Scale.Values eight to ten represent certain impactswith respectively local,

regional and global effects. The highest value ever reached was four: an impact

probabilityof2.4%withregionaleffects.Currently,onlytwoNEO’shaveavalueof

one, meaning an impact probability above zero, but still extremely unlikely

[Morrisonetal.,2004].

The identification of more impact structures could adjust our estimations of the

impactrateandextentof impactevents.Thiscould improveourknowledgeonthe

impactchanceswearesubjectedto.Meteoriteidentificationcanalsoindicatewhich

solarsystembodiestolookfor.

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2.1.1. IMPACTMETEORITES

Small solar system bodies are defined as those bodies that orbit the Sun,without

sufficientmasstoobtainanearlyroundshape[IAU,2006].Theyrangeinsizefrom

microscopicdustparticles[Scott&Krot,2005]overtensofkilometrestoseveral100

kmwithextremesup toapproximately550km[Tedesco&Desert,2002].Manyof

them, especially the smaller part, are fragments of others, larger ones, originatedfromspallationbyimpactsorfragmentationbycollisions.

2.1.1.1. ASTEROIDSANDCOMETS

Chambers[2007]andBennetetal.[2008]

Adivisionintoasteroidsandcometscanbemadeamongsmallsolarsystembodies.

AlthoughtheInternationalAstronomicalUnionusesobservationalcriteriatodefine

theirdifferences,thelimitationswiththis[Brownlee,2007]preferustouseamore

theoretically distinctionbased on their location in the solar systemandderivativedifferenceincomposition.

Followingthenebulartheory,bothasteroidsandcometsareleftoversfromthefirst

rocks that formed in thesolarsystem.Thesearecalledplanetesimalsandaccreted

by electromagnetic forces from the solids that condensed out of the cooling solar

nebula. Thedifference in location and composition between asteroids and comets,

arises from their different place of origin, approximately 4.565 Ga [McKeegan &

Davis,2005].Duetoheatingbythegravitationalcontractionofthesolarnebulaand

theyoungSun,thesolarsystemwasmuchhotterbackthenthantoday.Thiscaused

hydrogencompoundsonlytocondenseintoicesbeyondacertaindistancefromthe

Sun,calledthesnowline,whichwassituatedbetweenthepresentorbitsofMarsand

Jupiter. That’s why, theoretically, asteroids are said to be planetesimals formed

inside the snowline. They aremade out of rock andmetalmostly,with almost no

ices.Cometsincontrast,areplanetesimalsformedoutsidethesnowlineandconsist

mainlyoficesofmultiplehydrogencompounds,mixedwithminormetal,rock,and

complex organic compounds. In addition, asteroids are known to show mutualdifferencesincomposition,alsorelatedtotheirdistanceofformation.

When planetesimals reached sizes exceeding 1 km, gravitational forces took over,

whichproducedevenlargerplanetesimals.Thisledtotheformationoftheplanets,

dwarf planets andmoons. During and afterwards, the leftover planetesimals, thus

asteroids and comets, experienced gravitational tugs from the planets. With each

encounter, these gravitational perturbations increasingly changed their orbits to

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Otherminorasteroidresidenceregions,allboundbyKirkwoodgaps,arethatofthe

Hungariasat1.8to1.9AU,theCybelesat3.3to3.5AU,theHildasat3.9to4.0AU

andtheTrojans[McSween,1999].TheseTrojanasteroidsarefoundontheorbitof

Jupiterandconsistoftwosets,one60°beforeJupiterandone60°behind.Recently,

between 2001 and 2008, also seven Neptune trojans have been discovered [MPC,

2010] and many more are expected to exist. However, they could all resemble

cometsmorethanasteroidsorbeamixofboth[Sheppard&Trujillo,2006].

Most of the outer solar system comets reside in a vast spherical region at an

immense distance of about 50000 AU, called the Oort cloud,which is assumed to

consist of the comets that were swung out of the planetary region of the solar

system. Perhaps it even contains some asteroids thisway [Weissman, 1999]. It is

assumedtocontainmorethan1012cometslargerthan1kmwithatotalmassofone

tofiftyEarthmasses[Stern&Weissman,2001].Asmallernumberofcometscanbe

foundinaneclipticregionbeyondtheorbitofNeptune,calledtheKuiperbelt,which

is estimated to contain about105 comets larger than50km [Brownlee, 2007], the

equivalentofapproximatelyone‐tenthEarthmass[Delsanti&Jewitt,2007].Alsoin

the Oort cloud and Kuiper belt, the number of smaller bodies is estimated to be

immense.

2.1.1.3. VOYAGETOTHEEARTH

To become meteorites, small solar system bodies have to leave their residence

regionsandobtainanimpactcoursewiththeEarth.Thisrequirestheirorbitstobe

altered into more eccentric ones that cross the Earth’s orbit. As seen above, this

happened frequently in the early days of the solar system by gravitational

perturbationsfromplanets,buttoday,thisprocesshasmostlyspentitsforce.Itstill

onlyaccountsforthecometsfromtheKuiperbeltandaminorpartoftheasteroids

[McSween,1999andWeissman,1999].

The comets from the Oort cloud are also being pushed away by gravitational

perturbations.However,thesearenotprovidedbyplanets,astheOortcloudcomets

areatsuchgreatdistance.Insteadtheyarebeingpushedawaybypassingstarsand

molecularcloudsduringthesolarsystem’sorbitintheMilkyWaygalaxy[Weissman,1999andMorbidelli,2008].

Themajorpartoftheasteroidsthatleavethereresidenceregiondosoasfragments.

Typical relative velocities for asteroids are in the order of 5 km/s, which upon

impactsandcollisions,canresultintofragmentswithvelocitiesuptoapproximately

100m/s [McSween,1999]. Suchvelocitieswill spread the fragmentsout along the

orbit of their parent bodies, but aremost of the timenot enough for ejection into

moreeccentricorbitsand leavingtheresidenceregionsthisway[Wetherill,1976].

However, some fragments can reach nearby Kirkwood gaps, by which they are

swungaway,possiblyonanimpactcoursetotheEarth.InparticulartheKirkwood

gapofthe3:1orbitalresonancewithJupiterandthev6precessionresonancewith

Saturn in the inner asteroid belt are assumed to be responsible for a significant

contributiontotheasteroidfluxreachingtheEarth[Morbidellietal.,2002].

In fact, such resonances are probably responsible for almost all meteorites. This

becausethemajorpartofthesmallsolarsystembodiesthatreachedtheEarthare

asteroids,which is reflected in theNEO population. The contribution of comets is

assumedtoberatherlow,buttheestimationsvary.Stokesetal.[2003]estimatethis

contribution tobeonly1%,whileBottke&Morbidelli [2006]estimate thatof the

Kuiperbeltcometstobelessthan10%andthatoftheOortcloudcometstobe5%

at the most. However, Levison et al. [2001] estimate the latter as 1 %, while

Weissmanetal.[2002]suggest10to30%.

2.1.1.4. THROUGHTHEEARTH’SATMOSPHERE

WhensmallsolarsystembodiesreachtheEarth,theyhavestillonepassagetomake

beforebecomingmeteorites:theEarth’satmosphere.Althoughitisnotinagreement

withtheInternationalAstronomicalUnion’sdefinition,whichisbasedonsize[IAU,

1961],wehereuse themoreworkabledefinitionbywhichanysmall solar systembodyiscalledameteoroidassoonasitenterstheEarth’satmosphere.

The maximum velocity of small solar system bodies is approximately 42 km/s. A

highervelocitymakesaclosedorbitaroundthesunimpossibleandthusresultsinto

leaving the solar system.TheEarth’sorbital velocity is approximately30km/s, so

theentryvelocityofameteoroidismaximum72km/s.However,thisvalueassumes

aretrogrademotion,whichisonlyobservedforfewcomets[McSween,1999].These

haverelativevelocitiesupto60km/s[French,1998].Theminimumvelocityis11.2

km/s, the Earth’s escape velocity. Typical entry velocities are 15 to 25 km/s forasteroids[Chybaetal.,1994]and30km/sforComets[Reimold&Koeberl,2008].

Ataltitudesbelow100km,theairdensitybecomeshighenoughtocreatesignificant

friction,whichcausesthemeteoroidtodecelerateandthesurroundingairtoionize.

Thelatterproduceslightandiscalledameteor.Thefrictionalsomeltstheexteriorof

themeteoroidandthisformsathinlayerofglass,calledafusioncrust.Lesscoherent

meteoroidswill explode and break up this way into smallermeteoroids, which isoftenaccompaniedwithanairblast[McSween,1999].

Less massive meteoroids burn up by this friction, leaving only some microscopic

dustparticlestosettle.However,although lessmassive, theyaccountforthemajor

partofsmallsolarsystembodiesthatreachtheEarth.ItisestimatedthattheEarth

receivesbetween80and215Mgeachdaythisway[Kortenkamp&Dermott,1998

andMcSween,1999].Moremassivemeteoroidswillonlybedeceleratedtofreefall

velocitiesofafew100m/sandcanreachtheEarth’ssurfaceintact[McSween,1999].

Upon impact, these meteorites will form mechanical penetration craters, onlyslightlylargerthanthemselves[French,1998].

Onlymeteoroids exceeding approximately 10Mg [McSween, 1999], depending on

velocity, composition and shape, are able to retain a part of their entry velocity.

Thosethatimpactwithvelocitiesexceedingapproximately3km/s,dependingonthe

target rock andmeteorite lithology, will not only produce a penetration crater at

impact,butduetotheextremevelocities,alsocauseshockwavestoform.Thesewill

excavate a much larger impact crater, around 20 to 30 times the meteorite’s

diameter,dependingontheimpactmagnitude(i.e.themeteorite’smassandvelocity,

and the impactangle)and target rock [Bjork,1963;Gault etal., 1968;andFrench,1998].

2.1.2. IMPACTCRATERS

Melosh[1989]andFrench[1998]

Inthissection,theformationof impactcratersandstructures isdiscussed.Wewill

limit us to continental impacts, because only a few marine impact structures are

known[PASSC,2010].AmarineexampleandthedifferenceswithcontinentalimpactcratersisgivenbyLindstrometal.[1996,2005].

Impact craters form at random intervals, which depend on the solar system

dynamics that deliver impact meteorites. Because impact cratering is active over

large time‐scales, an average cratering rate can be established. Assuming an

approximateconstantimpactrateforthelast3Gy[Grieve&Shoemaker,1994and

Bottke&Morbidelli, 2006], Bland [2005] and Bland & Artemieva [2006] estimate

thata1kmimpactcraterisformedevery16000y,a10kmcraterevery200kyanda

250 km crater every 143 My. However, there are many uncertainties concerning

theserates,seeGrieve[1984],Morrisonetal.[1994],French[1998],Hughes[2000]andAsheretal.[2005]forothercalculations.

Theformationprocessofimpactcratersisstillnotfullyunderstood.Thisbecauseit

isverycomplexandcan’tbereproducedintotal.Alsonoimpacthasbeenrecorded

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PAGE16

impact

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PAGE17

datthe

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eteorite

Whentheinitialandrarefactionshockwavesreachtheenergylevelbelowwhichno

morerockcanbeexcavatedbyejectionordisplacement,theexcavationstageends.

Atthispoint,thetransientcraterreachesitsmaximumextent.Calculationsindicate

that the excavation takes 6 s for a 1 km transient crater and 90 s for a 200 kmtransientcrater.

2.1.2.3. MODIFICATION

All initial and rarefaction shock waves will decay to seismic waves beyond the

transient crater rim and will play no further part in the formation of the impact

crater. Gravity and rock mechanics take over now. Modification starts with the

depositionofthemeltandejecta.Butalsothetransientcraterchanges,dependingon

its size and the target rock. This leads to a division into four transitionalmorphologicaltypesofimpactcraters:simple,complex,ringandmulti‐ring.

Thesmallesttransientcratersdevelopsimpleimpactcraters.Themodificationonly

includesaminorcollapseof thewalls,making the impactcraterup to20% larger

than the transient. This results into a roughly parabolic shape with a depth to

diameterratioofapproximately1/3.Theslumpedrocks,togetherwiththemeltand

ejecta,will fill thecrateruptoapproximately50%.Forsimple impactcraters, this

crater‐fill iscalledthebreccia lens. It isobservedtocompriseonlyaminorpartof

ejecta.

Themodification of larger craters ismore complex. In crystalline rocks they form

above approximately 4 km and in sedimentary rocks above approximately 2 km

[Dence, 1972]. It includes the central rebound uplift of basement rocks by

approximately one‐tenth of the final crater’s diameter and amajor collapse of the

craterwalls,resultinginterracesboundedbynormalfaults,abroadflatfloorandan

impactcraterupto50to70%largerthanthetransient[Therriaultetal.,1997].This

results intoadepthtodiameterratioofapproximately1/5to1/6[Koeberl,2008].

The slumped rocks, melt and ejecta form an annular deposit around the central

rebounduplift. Theejecta contribution is larger thanwith the simple type. Impact

craters that are above approximately 25 to 40 kmwill have a depression in their

central peak, forming a central ring, because their uplift overshot gravitationalstability[Hawke,2004].

The largest craters form multiple rings. However, there is still a debate on its

definition[Hawke,2004andreferencestherein].Herewefollowthesystematicsof

Melosh [1989], which state that, based on lunar impact structures, a multi‐ring

impactcratershouldoccuraboveapproximately100kmontheEarth.However,no

multi‐r

impactburied

Figure5.crater(mejectedr

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2.1.2.4

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PAGE19

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PAGE20

t crater

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2.1.3. SHOCKMETAMORPHISM

French[1998]andFrench&Koeberl[2010]

Thestressesproducedbytheshockwavesarefarhigherthanthestrengthofrocks.

This results in permanent petrological and mineral changes in the rocks, all are

calledshockmetamorphism[French,1968].BecausenootherprocessesontheEarth

produce such stresses, some of these changes are unique to impact cratering and

usedfortheidentificationofimpactstructures.Asalreadystatedabove,theyplayedanimportantroleinthegeneralacceptanceoftheimpacttheory.

Shockpressurescanriseuptoseveral100GPaattheimpactpoint,dependingonthe

impact magnitude. Because the shock waves lose their energy rapidly, the shock

pressures will decrease exponentially from here. In the displacement zone they

reachamaximumofapproximately30GPa.Neartheeventualcraterrim,theshock

waveswillhavelostmorethan99%oftheirenergy[GaultandHeitowit,1963]and

passintoseismicwaveswithpressuresbelow2GPaandvelocitiesbetween5and8

km/s, depending on the target rock. These pressures are only sufficient for

fracturingandbrecciationandnotforpermanentdeformation.

Between 2 and 10 GPa shatter cones form. These aremegascopicmultiple sets of

striatedfracturesthatformpartialtocompletecones.Howtheyexactlyform,isnot

well understood. They can be up to severalmetres in size and are observed in all

kinds of rock, but the finer the rock is grained, the better the shatter cones are

formed.Itisassumedthattheconespointinthedirectionofwhichtheshockwaves

came, independent from any existing bedding. Because the onlymegascopic ones,shatterconesarethebestshockmetamorphiceffectsforimpactidentification.

Alsohighpressurepolymorphs form.Next to the formationofdiamonds, alsohigh

pressurepolymorphsofquartzoccur. It transformstostishoviteatpressuresof12

to 15 GPa and to coesite at 30 GPa [Stöffler & Langenhorst, 1994]. This in

contradiction with the lithospheric conditions where coesite forms at lower

pressures than stishovite. Especially stishovite is useful for impact structure

identification,because ithasneverbeenfoundelsewhere.Recentstudiesalsohave

identifiedotherhighpressurepolymorphslikereidite,apolymorphofzircon,anda

TiO2polymorph,possiblyderivedfromrutileoranatase[Glassetal.,2002;Jackson

etal.,2006;andWittmannetal.,2006].

Shockwaves also producemicroscopic planar structures inmanyminerals. These

are used as standardmicroscopic impact identification criteria, especially those in

quartz,whicharealsobeststudied.Thisbecausequartzisabundantinawiderange

ofrocks,andisresistanttoalterationandmetamorphism.Mostimportantareplanar

deformation structures, which are multiple sets of closed, extremely narrow

(typicallylessthan3μm),parallel(typically2to10μmspaced)planarregionsofan

amorphous phase. They form at shock pressures between 8 and 35 GPa and

comprisemultipletypes,seeFrench[1998]andFrench&Koeberl [2010] formore

on these. Also parallel sets of planar fractures develop. This at shockpressures of

only5to8GPa.Thefracturesaretypically5to10μmwideandspaced15to20μm.

Rarely, similar cleavageoccurs also fromnon‐impactprocesses, therefore theyare

onlyanindicationforapossibleimpact.

At higher shock pressures, minerals are partial or complete transferred into an

amorphous phase,without changing the fabric. These are called diaplectic glasses.

Theyaremostcommoninquartz(35to50GPa)andfeldspar(30to45GPa)[Grieve,

2005].Shockpressuresbetween40and60GPaproducetemperaturesabove1000

°C andwill cause selectivemineralmelting. This differswith equilibriummelting,

whichhappensgraduallyandstartsatthemineralboundaries.Withshockmelting,

the melting occurs instantaneously and also inside the mineral. Shock pressures

above approximately 60 GPa are sufficient for complete rock melting and thoseabove100GPaforvaporising.

2.1.4. IMPACTROCKS

French[1998]andreferencestherein

The formation of an impact crater produces a wide variety of rocks out of the

meteoriteandtargetrock.Allarecalledimpactrocks.Asalreadystatedabove,they

experiencedprocessesoffracturing,brecciating,excavating,meltingandvaporising.

Based on their final location, they are divided into basement, crater‐fill, proximal‐

ejecta and distal‐ejecta rocks. For all breccias counts that being monomict orpolymictdependsonthevariationoftargetrocks.

2.1.4.1. BASEMENT

The displacement zone consists of target rock, forced down‐and outwards. This

happensinlargeblocks,tenstohundredsofmetresinsize, insidewhichthetarget

rockstratigraphyandfabriccanbepreserved.Theserocksarestronglythinnedand

will form the floor of the impact crater. They are called the parautochthonous

basement. Shock pressures can exceed 30 GPa in the centre of the impact crater.

Here,thehighshockmetamorphicmicroscopicplanarstructurescanbefound.The

majorpartoftheparautochthonousbasementarelessshocked,oftenonlyshowingshattercones,nexttofracturingandbrecciating.

Atdeeperlevels,theparautochthonousbasementwillpassintotargetrockthatwas

not displaced, but fractured and brecciated in situ. These are called the

autochthonousbasement.Theyshownoshockmetamorphiceffects,whichmakesit

hardtodistinguishthemfromnon‐impactbreccias.Thisautochthonousbasementin

turn,willpassintorocks,whicharenotaffectedbytheimpactevent.Thishappensat

depthsofafew100mforsmallestimpactcratersandafewkmforthelargestones.

Suchdepths implicate thateven for impact structuresoferosion level seven, some

impactrockscanbepreserved.

Duringthemodificationstageoflargetransientcratersintocomplexandmulti‐ring

impactcraters, therapidreboundmovementsof thebasement intoacentraluplift,

canagain,causefracturingandbrecciating,whichgivesthemacomplexhistory.Alsopseudotachylitecanformthisway.

Inside the basement, also dikes can be found. These are formed during the

excavation stage by rocks from the top of the displacement zone or during the

modification stage by crater‐fill rocks, that intruded into fractures of the

parautochthonousbasement.Examplesareknownofuptoakilometrelong(e.g.the

StatenIslandsimpactstructures[Dressler&Sharpton,1997]).

2.1.4.2. CRATER‐FILL

Asalreadystatedabove,thecraterisfilledduringthemodificationstage.Insimple

craters this forms a breccia lens and in complex craters a more annular deposit,

around the central rebounduplift.This crater‐fill consistsoutof four components:

(1) slumped rocks from the lowly shocked displacement zone; (2) highly shocked

ejecta; (3) slumped ejecta; and (4) impact melt. The crater‐filling process is both

rapid and chaotic, but mixing of the different components is not complete. The

crater‐fillthereforecontainsvaried,butdistinctive,impactrocks.

Lithicbreccias form themajorpartof the crater‐fill in all impact craters.Theyare

freeofmeltandconsistofrockandmineralfragmentsinasimilarclastic,butfiner‐

grained,matrix.Thevariedshapedclastsarepoorlysortedandvaryfrommillimetre

tometrescale.Mostof thematerial in the lithicbreccias isderived fromthe lowly

shockedregionsaroundthewallsandrimof the transientcrater,distinctiveshockmetamorphiceffectsarethereforeonlyrarelyobserved.

Figure7.Hawke[2

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PAGE24

1987]and

ve [1987]

t that is

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upper parts of the crater‐fill, and sometimes also in the lower centre of simpleimpactcraters.

Where themelt ispresent asmatrix, thebreccias are calledmelt‐matrix.Themelt

component makes out 25 to 75 vol.% here. It ranges from glass to completely

crystalline igneous rock. The target rock fragments and minerals are frequently

highly shocked with partial melting. Melt‐matrix breccias often grade into impact

meltswithlittleornotargetrockclasts.Thesehavetheappearanceofconventional

igneousrocks.Fortheimpactoriginofimpactstructures,thiswasforalongtimean

evidence for the volcanic camp, until the discovery of an irrefutable meteorite

contribution. Such, almost pure, impactmelts surrounded bymelt‐matrix breccias

arecalledimpactmeltlenses.Theyarerareinsimpleimpactcratersbutcommonatthesurfaceofthecrater‐fillsinlargerimpactcraters.

2.1.4.3. PROXIMAL‐EJECTA

Of all ejected rocks, approximately 50 vol.% is deposited inside a distance of two

timestheimpactcrater’sradiusfromtheimpactcentre.Neartherim,thesewillform

acontinuousproximalejectablanketthatmaybetenstohundredsofmetresthick,

depending on the impact magnitude. At greater distances, this blanket becomes

thinneranddiscontinuous.Approximately90vol.%ofallejectedrocks,isdeposited

less than a distance of 5 times the impact crater’s radius from the impact centre.Ejectarocksconsistoflithicandmelt‐fragmentbrecciasthatarehighlyshocked.

2.1.4.4. DISTAL‐EJECTA

Approximately10vol.%oftheejectedrocksreachesdistancesgreaterthan5times

theradiusoftheimpactcrater.Thesearecalleddistalejectaandareusuallylessthan

a few centimetre thick. The finest part can be transported by the atmosphere to

regionalorevenglobaldistances(e.g.theChicxulubimpactevent).Distal‐ejectaisin

average higher shocked than proximal‐ejecta. It comprises a peculiar and much‐

studiedvariety: tektites.Thesearesmall (millimetre tocentimetrescale)bodiesof

pureglass thatareknowntobeejectedonly froma fewimpact structuresandare

spreadoverareas,calledstrewnfields, thatmaybethousandsofkilometres insize.

Probably,theyareformedonlywiththehighestimpactmagnitudes.

Whileallimpactangles(excepttheverylow,probablybelow15°)producethesame

circular morphology that impact craters show, the impact angle has much more

effect on the distribution of distal‐ejecta, which will show an asymmetry

proportionaltoalowerimpactangle[Hawke,2004].

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momento

Earth,isalyonthera

and groups

mutual d

so in min

eswith a g

doxygenis

omposedm

mostentir

amoment

tameteor

the last th

of40Kto4

mperature

sagain.Al

ed.Fromth

nonthe fr

onstopsw

bsorbscos

becalcula

henthisisa

of separatio

lsobasedoadioactive

(modified fro

differences

neralogy a

genetic rel

sotopecom

mostlyofs

elyofmeta

inthemet

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hermaleve40Ar.Thisb

edropsbe

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hen,severa

ragment’s

whenthefr

smicradiat

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addedtot

on isknow

ontheradiisotopes.

om McSween

in comp

and petro

levance. Ch

mpositions

silicatean

allicFeNi;

PAGE27

teorite’s

ntbody

entofa

because

lowthe

omenta

alstable

surface,

ragment

tion.By

btained

thetime

wn.The

ioactive

n [1999];

position,

logy. A

hemical

s.There

ndoxide

and(3)

stony‐i

intoch

non‐ch

groups

meteor

relevan

meteor

said to

commo

the m

Mittlefemorec

Figure1insteadoandfindi

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chondr

chondrdomina

2.2.1

Bydefi

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pyroxe

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hondritesa

hondritesa

is used. B

rites with

nceisthat

rites are c

o be ungro

onmeteori

meteorite c

ehldt[200ompleteon

1.Relativemoffindsbecauingchance.

be seen i

rites. Altho

ritedominaantchondr

1. CHON

inition,cho

gularshape

ene and so

nearlyeq

andachond

andaredif

By conven

comparab

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alled grou

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itegroups

classificati

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meteorite typuseabundan

in Figure

ough the r

anceisproriticcompo

NDRITES

ondriteme

edinclusio

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fferentiate

ntion, a gro

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pendixB co

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&Krot[200

peabundancecesofthelat

11, the ma

relative ab

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S

eteoritesa

onsare0.0

lic FeNi. B

rtionsofs

hondrites,

ed fromch

oup is def

stry, mine

ofacomm

teorites no

ontains th

ainderoft

McSween

05],Krote

esof falls (mtterarebiase

ajor part o

bundances

caseformsmallsolar

remadeo

01to10mm

Besides ch

ilicatesan

ironsand

hondrites.A

fined as ha

eralogy an

monparen

ot related

e chemica

thissection

[1999],

tal.[2007

modified fromedduetoad

of the met

s must hav

mostofgeorsystembo

utofchon

minsizea

ondrules,

dmetal.S

stony‐iron

A further c

aving at le

d petrolog

tbody.Tw

to any oth

l composit

ncontains

Haack &

7],andrefe

mMcSween [1ifferenceinp

teorite fall

ve change

logicaltimodies.

drulesma

ndconsist

chondrites

tonesare

nsarealso

classificati

east five un

gy. Their

wotofour

hermeteo

tions of th

sabriefre

& McCoy

erencesin

1999]). Fallspreservation

ls consists

d over tim

meandrefl

ainly.These

tmostlyof

s contain

PAGE28

divided

o called

ion into

npaired

genetic

related

rite are

hemost

eviewof

[2005],

allfora

areusedpotential

s out of

me, this

ectsthe

eround

olivine,

diverse

proportions of three other components. These are refractory inclusions, metallic

FeNi and matrix material. Refractory inclusions are tens of micrometre to

centimetres in size and can be divided into Ca‐Al‐rich inclusions (CAIs) and

amoeboidolivineaggregates(AOAs).ThemetallicFeNi ispresentasgrainsuptoa

millimetre in size and is also found inside the chondrules. Both chondrules,

refractory inclusions andmetallic FeNi are formed by high temperature processes

involvingcondensationoutof thesolarnebula,accretion,–partial–shock‐melting

andrapidcoolinginminutestohours.Theyarethefirstandthusoldestmaterialsof

the solar system, formed between 4.567 and 4.564 Ga [McKeegan&Davis, 2005].

Thematrixmaterialisfine‐grainedfrom5upto10µmandvolatile‐rich.Itconsists

ofsilicates,oxides,sulphides,metallicFeNi,organicmaterialandtherarepre‐solar

systemgrains[Zinner,2005].Asseenabove,theplanetesimalsformedoutofthese

fourcomponentsbyelectromagneticaccretion.Thisstarted4.565Ga [McKeegan&

Davis,2005]andlastingsomefewMy.Allothersolidsinthesolarsystemoriginated

out of them. Chondritemeteorites represent these first rocks to form in the solarsystem.

Chondrites are given a petrologic type from one to six, indicating their degree of

metamorphism.Thisoccurredapproximatelyinthe60Myfollowingtheirformation.

Chondrites of type three are the least altered. Towards type six heating

metamorphismoccurredbytheshort‐livedisotopes26Alandtoalesserextent60Fe,

but without experiencing melting. Chondrites towards type one suffered aqueous

metamorphismwith the formation of hydratedminerals. Many chondrites consist

out of breccias, formed by collisions and impacts. Herein, almost no mixes of

chondrite groups are found, but mixed petrologic types are often. This – and the

occurrenceof shockmetamorphiceffects– suggests theexistenceofparentbodies

witharangeinmetamorphicgrades,thatwerebrecciated,fracturedandexcavated,

by collisions and impacts. Cosmic‐ray exposure and gas‐retention ages show that

thesehappenatrandomandstill today.Asseenabove, theyprovideaway for the

fragments to escape from theirparentbodiesand residence regions,possiblyonavoyagetotheEarth.

Chondrite meteorites include fifteen groups and fifteen ungrouped chondrites.

However, two groups, the R (Rumuruti‐like) and K (Kakangari‐like), are actual

grouplets. In addition to groups, chondrite groups are also placed in clans, to

illustratetheirrelatedtimeandplaceoforigininthesolarsystem.BoththeRandK

chondrites form a clan, the other thirteen groups belong to the carbonaceous (C),

ordinary (O) or enstatite (E) clan. Following the temperature and density

dependenceoftheoxidationstate,whichincreasesinthesequenceofE,K,O,RandC

chondr

andRc

probab

Somechondr

Figure1ThehoriMeanOc

2.2.1.1

Theirn

theCI

otherg

(Karoo

contain

mostp

chemic

referen

petrolo

other c

charact

CI;(2)

rites,Eand

chondrites

bly near th

characterritesarelis

2.BulkoxygezontalaxisineanWater.T

1. CARBON

nameisan

(Ivuna‐like

groupsare

nda‐like),

nthehighe

rimitivech

calcompos

nce compo

ogic type o

carbonace

terizedby

oxygeniso

dKchondr

sat interm

he snowlin

istics ofstedbelow

enisotopecondicatestheTheterrestria

NACEOUS

nhistorical

e)andCM

theCR(R

CO (Orna

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osition. Ho

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ous chond

(1)refract

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ritesareas

mediatedis

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the comw.

ompositionoδ18Oandthelsampleline

lmismatch

(Mighei‐li

Renazzo‐lik

ans‐like)

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owever, d

are the o

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entabunda

withδ17Oe

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ected in th

bonaceous

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elements.T

sentationo

iswhy th

tensive aq

drite group

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ancesthat

equalorlo

medclosest

hondrites t

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s, ordinar

(modifiedfroothrelativetoonationline.

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bon(i.e.1.5

ike),CV(V

5‐like). Th

Theyarea

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eyareofte

ueous me

p that lack

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owerthanm

ttotheSu

thefarthes

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omMcSweenotheVienna

onlytwo

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he CI cho

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enusedas

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nd three a

xceedthos

minus2‰

PAGE30

n,theO

staway,

ositions.

nstatite

n[1999]).Standard

groups,

%).The

ike),CK

ondrites

obethe

relative

sarock

sm (i.e.

ules. All

and are

seofthe

‰(with

someexceptions);and(3)refractoryinclusionabundancesequalorhigherthan0.1vol.%.

2.2.1.2. ORDINARY

Theordinarychondriteclancontainsapproximately80%ofallthemeteoritefalls.It

is divided into three groups: the H, L and LL. These letters refer to the bulk Fe

contents, with the H chondrites having high total Fe contents, the L chondrites

havinglowtotalFecontentsandtheLLchondriteshavinglowmetallicFerelativeto

totalFeaswellaslowtotalFecontents.AlsothroughtheH,LandLLsequence,the

oxidation state increases and the siderophile element (i.e. those showing affinities

with Fe) abundances decrease. All three groups are characterized by (1) Mg‐

normalizedrefractorylithophileabundancesof±0.85timesthoseofCIchondrites;

(2)oxygenisotopiccompositionplottingabovetheterrestrialfractionationline;(3)

highabundanceofchondruleswithnon‐porphyriticandFeO‐richchondrulesbeing

commonandAl‐richchondrulesbeingrare;(4)ararityofCAIsandAOAs;and(5)a

largerangeindegreeofmetamorphism,petrologictypesthreetosix,withoftenalsoevidenceforminoraqueousmetamorphism.

2.2.1.3. ENSTATITE

This clan comprises two groupswith different contents ofmetallic Fe: EH andEL.

Additionally, there isoneungroupedEchondrite:LEW87223.Allarecharacterized

by (1) a unique mineralogy indicating formation under extremely reducing

conditions; (2) bulk oxygen isotopic compositions that plot along the terrestrial

fractionation line, close to thatof theEarthand theMoon; (3) abundant enstatite‐

richchondrules;(4)ararityofCAIs;(5)verylowabundanceoffine‐grainedmatrix

andchondrulerims;and(6)alargerangeindegreeofmetamorphism,i.e.petrologic

typesthreetosix.

2.2.2. NON‐CHONDRITES

Planetesimalsthatbecamelargerthan10kmstartedtodifferentiate.Intheinteriora

core formed and at the exterior igneous rocks originated by partial melting and

fractional crystallization. This caused isotopic homogenization and chemical

compositions to deviate to various degrees from chondritic values. Non‐chondrite

meteorites contain virtually none of the components found in chondrites. Partial

melting occurred in variable degrees, which also led to a distinction between

primitive and differentiated non‐chondrites. As already stated above, the non‐

chondrchondr

2.2.2.1

Thepri

showig

metam

primiti

winona

Figure13[1999]).Standard

Theaca

olivine,

Cl‐apat

tothat

acapulc

grained

acapulc

impact

and ch

grained

those o

rites are drites),irons

1. ACHON

imitiveach

gneousor

morphosed

ive achond

aite,brachi

3.BulkoxygeThehorizondMeanOcean

apulcoites

, Cr‐diops

tite,chrom

ofordinar

coites are

d(540–70

coitesand

t event on

hemical co

d equigran

of E and H

divided intsandstony

DRITES

hondritess

metamorp

chondrites

drites can

initeandu

enisotopecomtalaxisindicnWater.

andlodra

ide, Na‐pl

miteandgra

rychondrit

fine‐grain

0mm).Co

mostof t

a common

mposition

nular rocks

H chondrit

to the stoy‐irons.

stillhavea

phictextur

sorresidu

be assign

ureilitegro

mpositionofcatestheδ18O

nitesaree

agioclase,

aphite.Alt

tes,themi

ned (150–

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he lodrani

nparent b

, but recry

s. Theirm

tes. FeNi‐F

ony achon

approximat

res.Theya

uesofvery

ned to fiv

oup.

f mostprimitiOandthever

equigranul

FeNi‐meta

thoughthis

ineralcom

230 mm),

exposurea

ites,possib

body.Wino

ystallized

mineral com

FeS veins

drites (inc

telychond

arethough

ylowdegr

e groups:

iveachondritrticalaxisth

larrocksc

al, schreib

sminerala

mpositions

whereas

agesarebe

bly indicat

onaitesha

textures.

mpositions

are comm

cluding th

driticbulkc

httobeimp

eesofpar

the acapu

tegroups(moeδ17O,both

omposedo

bersite, tro

assemblage

andabund

the lodra

etween5.5

ting sampl

ave a chon

They are

s are inter

mon and co

he primitiv

compositio

pactrocks

rtialmeltin

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oilite, whit

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dances,dif

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5and7Ma

ling froma

ndriticmin

fine‐ to m

rmediate b

onstitute t

PAGE32

ve non‐

ons,but

s,highly

ng.Most

dranite,

McSweenheVienna

yroxene,

tlockite,

rsimilar

ffer.The

coarse‐

aforall

a single

neralogy

medium‐

between

the first

partial

grained

olivine

phosph

olivine

fromo

Cr2O3 c

reduce

and (5

chondr

The di

howard

Mars (

Nakhlit

fragme

Figure 1McSweenViennaS

The an

genera

olivine,

mostly

nearly

melts of

d (0.1–2.7

and min

hatesandF

and pyro

therachon

contents in

d rims on

5) a large

ritemixing

fferentiate

dites, eucr

(the lherzo

tes,anddu

entbreccia

14. Bulk oxygn[1999]).ThtandardMea

ngrites are

llybasaltic

,andanor

outofne

FeO‐free

a chondri

7 mm) eq

nor augite

FeNi‐metal

xenewith

ndritesby

n olivine;

olivine (a

range in

gline.

ed achond

ritesandd

olitic and

uniticChas

s)areclas

gen isotopehehorizontalanOceanWat

e medium

ccomposit

rthiticplag

arlyFeO‐f

diopside

itic precur

quigranular

e, plagioc

l.Ureilites

minor da

(1)highC

(3) high (

and someti

oxygen is

drites com

diogenites.

basaltic S

signite)an

sifiedhere

compositionaxisindicateter.

m‐ to coars

tionandco

gioclase.Au

freeenstat

and forst

rsor. The

r dunitic

clase, orth

areC‐bear

ark intersti

CaOconte

(up to 5w

imespyro

sotopic co

prise also

Inadditio

hergottite

ndtheMoo

e.

of most diffestheδ18Oan

se‐grained

onsistmai

ubritesare

tite,witha

terite. Sim

brachinite

wehrlites,

hopyroxen

ringultram

itialmater

nts inoliv

wt.%) C co

oxene)wer

omposition

o five grou

onalso the

s, the clin

on(basalts

ferentiated andthevertica

d (up to 2

inlyofCa‐A

ehighlyre

aminoram

milarities i

es are me

consistin

e, chromi

maficrocks

rial. They

vineandpi

ntents; (4)

re in conta

ns which

ups: the a

e rareSNC

nopyroxeni

,basalticb

achondrite gralaxistheδ17

2‐3 mm)

Al‐Ti‐richp

educedbre

mountofa

n mineral

edium‐ to

ng domina

ite, Fe‐sul

scompose

are disting

igeonite; (

) the pres

actwith gr

plot along

angrites, a

Cmeteorite

itic and w

brecciasan

roups (modi7O,bothrelat

igneous r

pyroxene,

ecciasthat

albiticplag

logy and

PAGE33

coarse‐

antly of

lphides,

doutof

guished

(2)high

sence of

raphite;

g the C

ubrites,

es from

wehrlitic

ndmelt‐

fied fromtivetothe

ocks of

Ca‐rich

consist

gioclase,

oxygen

isotopi

howard

these m

studies

asteroi

Vesta’s

orthopy

from v

breccia

amoun

eucritexenolit

2.2.2.2

Figure15[1999]).Standard

Irons a

exposu

collisio

mustb

Ironm

c compos

dites, eucr

meteorites

s,thiscoul

d belt. Th

s crust [

yroxene c

varied,mor

ated surfac

ts of nobl

e and diogths.

2. IRONS

5.BulkoxygeThehorizondMeanOcean

are fragme

uresuchde

onand/orm

beacorefr

meteoritesa

sitions sug

rites and d

s originate

dbethe5

hey formed

[Drake, 2

cumulates

re shallow

ce regolith

le gases im

genite frag

enisotopecotalaxisindicnWater.

ents of th

eeplyseate

multiplela

romwhich

areclassifi

ggest that

diogenites

ed from th

530km4V

d by an im

001]. Dio

with mino

w, pyroxen

h,which is

mplanted b

gments, wi

mpositionofcatestheδ18O

he cores fo

edcores,th

argeimpac

h,atleastp

iedbased

t aubrites

are also

he same p

Vesta,loca

mpact and

ogenites

or plagioc

e‐plagiocla

s formedb

by cosmic

ith some

fsomestony‐Oandthever

ormed in

heplanete

cts.Ifanyp

partly,the

on theiro

are rela

classified

parent bod

tedneart

d represen

sample t

clase and

ase basalts

by former

radiation

minor imp

‐ironandironrticalaxisth

the differ

esimalsmu

parentbod

overlying

oxygen isot

ted to E

as the HE

dy. Based

he3:1Kirk

nt differen

he deepe

olivine. Th

s. Howard

impacts. T

and cons

pact melt

ngroups(moeδ17O,both

rentiated p

usthaveex

ysurvived

gsilicatero

topesand

chondrite

ED group b

on spectr

rkwoodgap

nt depths

est parts,

he eucrite

dites are fr

They conta

sist mostly

and C ch

odifiedfromrelativetoth

planetesim

xperienced

dandstille

ocksarest

siderophi

PAGE34

es. The

because

roscopy

pinthe

from 4

being

es come

rom the

ain high

y out of

hondrite

McSweenheVienna

mals. To

dsevere

exists,it

tripped.

le trace

elementcompositions.85%ofthemcanbecategorisedintothirteengroups:IAB,IC,

IIAB, IIC, IID, IIE, IIF, IIIAB, IIICD, IIIE, IIIF, IVA, and IVB.Numbers I to IV indicate

decreasing contents of Ga and Ge, the two most volatile siderophile elements.

Chemical evidence indicates that while some groups were formed by fractionalcrystallization,calledthemagmaticironmeteorites,othersarenot.

Probably,someirongroupsaregeneticallyrelatedtosomeachondritegroups,some

werepossiblyderivedfromasingleparentbody:(1)silicateinclusionsinIABirons

(and probably also those in IIICD irons) are linked through oxygen isotopic and

mineralcompositions to thewinonaites; (2) thebulkoxygen isotopiccompositions

of silicates in IIE irons are similar to those of H chondrites; and (3) IIF irons are

probably formed in the same region of the solar system as some stony‐ironpallasites,somemighthaveacommonparentbody,othersdefinitelynot.

2.2.2.3. STONY‐IRONS

Thepallasitesandmesosideritesmakeoutthestony‐irons.Botharefurtherdivided

into groups. Pallasites are composed of roughly equal amounts of silicate

(dominantly Mg‐rich olivine), metal and troilite. They are regarded to be derived

from the core‐mantle boundary, which presumes a close relationship to the iron

meteorites.Mesosideritesarebrecciaswith the samecomponentsandproportions

as the pallasites, but the silicate component consists mainly out of pyroxene and

plagioclase, that have a crustal signature. Probably, they formed bymixing duringcollisions.

2.2.3. COMETARYMETEORITES

[Gounelleetal.,2008andreferencestherein]

Atpresent,allmeteoritesaredeterminedtohaveanasteroidorigin,exceptforthose

fewfromtheMoonandMars.Itisassumedthatcometsdonotoronlyforaminor,

not known, part, contribute. This because they are already a small fraction of the

smallsolarsystembodiesthatreachtheEarthandinaddition,havearathersmall

coherencedue to their composition, bywhichmost don’t survive passage through

theEarth’s atmosphere.Themostmassive form impact craters and aredestroyed,exceptforsomepossiblespalledfragmentswithlittlepreservationpotential.

RecentlyGounelleetal.[2008]arguedthatthebestcandidatesforcometsaretobe

found among the CI chondrites. They postulate that there is a continuum in

composition between asteroids and comets rather than a sharp distinction. This

because there is no reason for the snowline to have always occupied the same

location,nortodefineanabrupttransitionbetweenice‐poorandice‐richbodies.CI

chondritesmightsamplethiscontinuum.

2.3. GEOCHEMICALSIGNATURES

[Tagle&Hecht,2006andKoeberl,2007]

The Earth is formed out of planetesimals with chondritic bulk compositions. But

already early in its history, it started to differentiate into a metallic core and

primitive silicate mantle by the gravitational separation of Fe. Because trace

elements show affinities towards major elements, those which do so towards Fe,

were also partitioned into the core [Lorand et al., 2008]. As already stated above,

such elements are called siderophile. Relative to the Earth’s chondritic bulk

composition, they are enriched in the core and depleted in the primitive mantle.

Basedon theirabundances in themantleand theoreticalvaluespredictedbycore‐

mantleequilibriumpartitioningmodels, the later impact‐additionofapproximately

0.8wt.%ofchondriticrocktotheprimitivemantleissuggested[Halliday,2004].By

partialmelting,alsotheprimitivemantledifferentiated,bywhichamantleandcrust

was formed.Again, traceelements showdifferentbehaviour in thisprocess.Those

thatprefer to remain in themantlearecalledcompatible, those thatpreferentially

enterthemeltarecalledincompatibleandwillrisewiththemelttoformthecrust.

Those trace elements that are siderophile and compatible, will now be highlydepletedinthecrust.

Meteoritesareplanetesimalrockswithanearlierand/ordifferentevolutionthanthe

Earth. Therefore, theydonot show such depletion of compatible siderophile trace

elements and/or have different isotope ratios of these (see Appendix B). Because

depleted, even a smallmeteorite contributionwill createmeasurable geochemical

signaturesoftheseelementsthisway,whichoffcourse,canbeusedforimpactand

meteorite identification.Thesegeochemical signatures aremostlymeasured in the

melt‐bearing impact rocks from the crater‐fill, because these contain the highest

meteoritecontribution[Palmeetal.,1978,1981;McDonaldetal.,2001;McDonald,

2002;Tagle andClaeys, 2005]. But also themeteorite contribution in the ejecta is

measurable (e.g. Alvarez et al. [1980]). However, for most impact structures, theejectarocksareeroded.

2.3.1. ELEMENTABUNDANCES

Adivisioncanbemadebetweenthecompatiblemoderatelyandhighlysiderophile

elements. While the metal/silicate partition coefficients of the latter are above

approximately10000, thoseof themoderatelyareoneor twoordersofmagnitude

lower[Lodders,2003].Theyarediscussedseparately,becausetheyareanalysedby

different methods and form each an independent way for impact and meteoriteidentification.

2.3.1.1. MODERATELYSIDEROPHILEELEMENTS

Ni, Co and Cr are moderately siderophile elements. Cr in addition, also has a

lithophile character, this implicates an affinity towards O and because Si is also

lithophile,towardssilicaterocks.Averagecrustalvaluesareapproximately51ppm

for Ni, 25 ppm for Co and 119 ppm for Cr [McDonough& Sun, 1995]. The upper

continentalcrusthasvaluesofapproximately47ppmNi,17.3ppmCoand92ppm

Cr[Rudnick&Gao,2003].Whenasignificantenrichmentoftheseelements,relative

tothetargetrocks,ismeasured,thisindicatesanimpactorigin.

When the meteorite contribution exceeds 0.1 wt.%, also the element ratios will

significantly be affected. If accounted for the indigenous composition of the target

rock, it will be possible to distinguish between a chondrite and iron meteorite

[Koeberl,1998].ThisbecausechondritesshowCrabundancesof2575to3810ppm

and Ni/Cr ratios that vary between roughly 2 to 7 [Tagle and Berlin, 2008]. Cr

contentsinironmeteoritesareafewordersofmagnitudelowerandtheNicontents

arehigher[Buchwald,1975].ThismeansironsdisplaymuchhigherNi/Crratios,up

to three orders of magnitude. The same is applicable for the Co/Cr ratios. The

absence of a Ni and Co enrichment, accompanied by elevated Cr values, and low

Ni/Cr and Co/Cr ratios, could be interpreted as the result of the impact of an

achondritemeteorite[Palme,1980].

A more precise meteorite identification is possible by comparing the Ni/Cr andCo/Crratioswithmeteoritedatabases(seeAppendixB).

Becauseonlymoderately siderophile, somecrustal target rockscanhaverelatively

high indigenousconcentrationsofNi,CoandCr,whichoffcourse isnotpreferable

forimpactandmeteoriteidentification.Aninterfering(ultra)maficcomponentmay

also not be present or should be extracted. Thismight be difficult whenmultiple

typesoftargetrockandhighdepthsareinvolvedandespeciallywhenfractionationprocessesareinvolved.

2.3.1.2. HIGHSIDEROPHILEELEMENTS

The platinum group elements (PGE) Ru, Rh, Pd, Os, Ir and Pt are high siderophile

elementsandsharesimilarphysicalandchemicalproperties.Becauseofcomparable

properties, Re and Au are often associated with them. Most meteorites have PGE

contentsapproximately10000ordersofmagnitudehigherthanthecrust(0.022ppb

Irfortheuppercontinentalcrust)and100higherthanthemantle(3.5ppbIrforthe

uppermantle)[Peucker‐Ehrenbrink&Jahn,2001].Whenasignificantenrichmentof

PGErelativetothetargetrockispresent,thisimplicatesanimpactorigin.

Iristhebestknownelementwhenitcomestometeoriteidentification.Thisbecause

of the first meteorite contribution studies, which relied on analysis by neutron

activation methods. Ir was relative to the other PGE the most easy and sensitive

elementtoanalyse.Withoutdissolutionofthesamples,itwaspossibletomeasureIr

values as low as only 0.1 ppb this way. By inductively coupled plasma mass

spectrometry (ICPMS) with isotope dilution or nickel sulphide fire assay pre‐

concentration, all PGE can now be measured together. Because the PGE show an

inhomogeneous distribution (they form micrometre scale nuggets), large enoughsamplesareneeded(>1g).

WhenPGEconcentrationsareplottedonalogarithmic‐scaleCI‐normalizeddiagram

(mostcommonlyinorderofdecreasingcondensationtemperature),arelativelyflat

pattern points towards a chondritic meteorite, while sloping indicates an iron.

However,alsoamantlecomponentor thepresenceofmultiplevaried target rocks

(e.g. the Bosumtwi impact structure [Goderis et al., 2007]),which are not easy to

determine,canaffectthePGEpattern.

More recently, a different approach based on the determination of PGE ratios has

madepossiblethepreciseidentificationofthemeteorite,withoutaccountingforthe

target component [McDonald et al., 2001; McDonald, 2002; Tagle & Claeys, 2005;

TagleandHecht,2006;Goderisetal.,2009;andTagleetal.,2009].Linearregression

analysis can be carried out by plotting the PGE ratios on a X‐Y diagram. This

producesamixinglinebetweenthetargetrockandmeteorite.Itsslopewillonlybe

slightlyaffectedbythetargetrock,evenforlowmeteoritecontributions,becauseof

the extremedifferences in PGE abundances between crustal target rocks and thus

willbecharacteristicforthemeteorite.AlsothenuggeteffectsofthePGEareruled

outthisway.Becausecondensationprocessesinthesolarnebulamainlycontrolled

the fractionation of PGE in chondrites, elements with large differences in

condensation temperatures present the strongest variations in element ratios.

Elementratiosoflowcondensationtemperatureelements(RhorPd)combinedwith

thosewith higher condensation temperatures (Os, Ir, Ru), therefore offer the bestdiscriminationbetweendifferentmeteorites.

2.3.1.3. MAJORANDTRACEELEMENTS

Meteoritecontributionsduetoimpacteventsaretoosmalltosignificantlyaffectthe

majorelementcompositionsoftheimpactrocks.A5wt.%contributionofa5wt.%

SiO2 ironmeteoritewould only lower a 65wt.% SiO2 target rock to a value of 62

wt.% SiO2. Such depletion would probably still not fall significantly outside the

uncertainty range. However, together with trace elements compositions, mixing

calculationscanbemadewhenall targetrocksarechemicallycharacterised.Major

andtraceelementscompositionsarealsoneededtodetectalterationandunknown,

possibly (ultra)mafic, target components which can interfere with the siderophile

signatures.Afterall, impacteventsexcavateandmixa largevolumeof targetrock,alsotodepthswhichareoftennotsampledfortargetrocks.

2.3.2. ISOTOPERATIOS

Small isotope variations inducedby the addition of almost indiscerniblemeteorite

contributionsintoimpactrocksarenowmeasuredwithhighprecisionthankstothe

development of isotope geology, ion exchange chromatographic separation, and

thermalionizationandmulti‐collector(MC)ICP‐MS.TheOsandCrisotopesystemsarethemostcommonusedones.

2.3.2.1. OSISOTOPESYSTEM

Oshassevennaturaloccurringisotopes,whichareallstable.Theseare184Os,186Os,187Os, 188Os, 189Os, 190Os, and 192Os. They have relative abundances of respectively

0.024%, 1.600%, 1.510%, 13.286%, 16.252%, 26.369%, and 40.958% [Faure,

1986].187Osisradiogenic,itisproducedbythebeta‐decayof187Re,whichhasahalf‐

lifeof4.16x1010y[Walkeretal.,2002].Asalreadystated,bothOsandRearePGE,

thus highly siderophile and depleted in the primitive mantle. But while Os is

compatibleduringpartialmelting,Reisincompatible.Thisresultsintheenrichment

ofOs inthemantleandRe inthecrust.Therefore,crustalrockshaveahighRe/Os

ratio,andconsequentlyalsohigh187Oscontentsrelativetoothernon‐radiogenicOs

isotopes(e.g.188Os).Theoldertherocks,thehighertheseratios.Thecommonlyused187Os/188Os average for the upper continental crust is 1.4 [Peucker‐Ehrenbrink &

Jahn, 2001]. Meteorites have high amounts of both Re and Os, and except for the

differentiated achondrites, Os even exceeds Re. This resulted into low 187Os/188Os

ratiosthatonlychangedslowlythroughouttime.Therationowrangesfrom0.11to

0.18 in meteorites with an average of approximately 0.13 for the chondritemeteorites[Koeberl&Shirey,1993andTagle&Hecht,2005].

While the Os meteorite contribution is already significant relative to the low Os

contentsofcrustaltargetrocks,itinducesanevenmoresubstantialdecreaseinthe187Os/188Osratioof the impactrock,relativetothetargetrock.This isnotusedfor

meteorite identification, but for impact identification. Os isotopes can precisely

detect the amount of the meteorite contribution (except for some differentiated

achondrites).Offcourse,olderimpactrocksusuallyrequireaRecorrectiontoobtain

the exact initial 187Os/188Os ratio andalso a significantmantle componentmustbe

excluded from the target rocks. However, because most chondritic and iron

meteorites stillhave twoordersofmagnitudehigherPGEcontents than theupper

mantle,ittakesa100timesgreatermantlethanmeteoritecontribution,forthesameratios.Suchsignificantmantlecontributioniseasilydetected.

2.3.2.2. CRISOTOPESYSTEM

Cr has 4 natural occurring isotopes. These are 50Cr, 52Cr, 53Cr and 54Cr. The latter

threearestable.Theyhaverelativeabundancesofrespectively4.345%,83.789%,

9.501%and2.365%[Faure,1986].53Crisradiogenic,itisproducedbythedecayof53Mn,whichhasahalf‐lifeof3.7x106y.Becauseofthisrelativeshorthalf‐life,53Mn

hasalreadyextinct fora longtime.However, itexistedduringthe formationof the

first solids and planetesimals. Due to heterogeneities or mass fractionation, these

developed differences in their 53Cr/52Cr (52Cr is not radiogenic) which remained

unchangedafterall53Mndecayed.BecausetheEarthformedafterwards,itcontains

novariationsof the53Cr/52Crratio.Anydeviationofa53Cr/52CrratiototheEarth’svalue,implicatesthereforeameteoritecontribution.

Because Cr was not completely scavenged from the primitive mantle due to its

partiallithophilecharacter,meteoriteidentificationbythe53Cr/52Crratio,requiresa

relativelyhighmeteoritecontributionintheorderofafewwt.%,dependingonthe

Crcontentsofthetargetrock(approximately1.2wt.%ofmeteoritecontributionis

needed for 185 ppm Cr in the target rock). If such contribution is present, it candiscriminatebetweentheironandchondritegroups.

2.3.2.3. OTHERISOTOPESYSTEMS

AlsotheisotopesystematicsofW,Pb,NdandSrhavebeenproposedforimpactand

meteoriteidentification.However,theirusesarestillunclearbecauseofthevariable

degreesofsuccessthatweremade.Morestudiesareneededonthesetodetermine

the information their signatures could reveal and how this informationwill differfromothersignatures.

2.3.3. MULTI‐SIGNATUREAPPROACH

CombiningNi,CoandCrwithIrtoNi/Ir,Co/IrandCr/Irratios,canalsodiscriminate

between ironandchondritegroups.Butadditionally, a strongcorrelationbetween

themoderatelyandstronglysiderophileelementssupportsacommonoriginandcan

excludethepresenceofaninterferingtargetcontributionaswellassignificantpost‐

impact fractionation and remobilisation [Palme, 1980]. When no target rock

composition is available, the Ni, Co and Cr contents can be retrieved this way by

linearregressionanalysis.ThisbecauseoftheextremelowcrustalabundanceofIr,

whileNi,CoandCrhavemuchhigherabundances.ForIr,anuppercontinentalcrust

orevenzeroconcentrationisassumed.

Also the isotope systems can be used together with the siderophile element

abundances and ratios. Estimations by these on the amount of meteorite

contributioncanbecomparedwithvaluesobtainedbytheprecise188Os/187Osratio.

Thiscanvalidatetheconclusionsdrawnfromthesiderophileelementsabundances

andratios.Recently,togetherwiththePGEratios,alsodatabasesonthe188Os/187Os

ratios of meteorites are constructed [Fischer‐Gödde et al., 2010]. The Cr isotope

systemcanbeused, togetherwith theabsenceofmoderately siderophileelements

and PGE, to discriminate an achondritemeteorite better, or when the siderophileelementsprovidenodecisiveanswer.

2.4. BRENTIMPACTSTRUCTURE

TheBrentimpactstructureisnamedafterthenearbyvillageofBrentandlocatedin

Ontario, Canada, near the northernboundary of theAlgonquin Provincial Park, 75

kmeastof lakeNipissing (46°05’N,78°29’W). It isvisibleasa circulardepression,

approximately3kmindiameterandatitsdeepestpoint60mbelowthesurrounding

terrain [Dence, 1972]. At its edges, it is partly filled by two curved lakes, named

GilmourandTecumseh.AlreadyfromitsdiscoverybyJ.A.Robertsin1951andfirst

appearances in literature, itwasconsidered tobean impactstructure [Bealsetal.,

1956;Beals,1958;andMillmanetal.,1960].Butbeforethegeneralacceptanceofthe

meteor

as the

centre

1971].

Figure 1structure

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PAGE42

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PAGE43

learethe

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dGrieve

f3.8km

,1976].

Observational data by Grieve & Cintala [1981] confirm this diameter and depth,

supplementary indicatingarimheightof200mandvolumeof7.4km3.Fromthis,

Garvin&Grieve[1982]estimateatransientcraterwithadiameterof3.3km,3.1km

attheoriginalsurface[Grieve&Cintala,1982],andrimheightof250m,resultingin

avolumeof5.7km³.Withabulkingfactorofapproximately10%forthetransition

from crystalline rock to breccia [Innes & Beals, 1961], this implies a volume

difference between the transient and final crater of 1.9 km³,which represents the

amountofrocksthatslumpedduringthemodificationstageandformedthebreccia

lens.Thisisinagreementwiththeobserved2.1km³ofbreccialens[Grieve&Cintala,

1982],whichpossiblyalsoincludesaminor,unrecognizable,partoffall‐backejecta.

Denceetal. [1976]estimate thatat least0.5km³of theexcavated target rockwas

exposed to shock pressures above 60 GPa, thus resulting into vaporisation and

melting.Of this, approximately0.022km³, or4.4%, is incorporated in thebreccia

lens [Grieve & Cintala, 1981]. Robertson and Grieve [1977] estimate that shock

pressureswereintheorderof23GPaat thedeepestpointof theimpactcrater.At

the rim of the impact crater these are estimated to have been 0.3 GPa [Grieve &Cintala,1982].

2.4.2. TARGETROCK

TheBrentmeteoriteimpactedonanigneous‐metamorphicbasementcomplexofthe

GrenvillestructuralprovinceoftheCanadianShield.Thetargetrockismesoperthite

and microcline gneiss of granodioritic composition with a minor contribution of

maficgneiss[Grieve,1978].RareCambrianalnoitedikesarealsopresent[Hartunget

al, 1971 and Grieve & Dence, 1978]. This target rock can physically be treated as

homogeneous and isotropic [Grieve&Cintala, 1982 andDence, 2004]. Its regionalsettingisdescribedbyCurrie[1971]andDence&Guy‐Bray[1972].

Table1.Elementconcentrationsofthegneiss(mesoperthite)andalnoitetargetrocksattheBrentimpactstructure. The PGE, Re andAu are derived from target rock clasts [Palme et al., 1981]. The others arecomingfromparautochthonousbasementandsurfacesamples[Grieve,1978].FeOistotalFe.

wt.% SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2OGneiss 63.30 0.91 15.01 06.52 0.20 01.18 01.78 4.61 4.92Alnoite 37.80 3.37 11.25 10.70 0.24 10.50 11.35 1.80 4.02ppm Sc V Cr Co Ni Cu Zn Rb SrGneiss 04.8 020 012 08.0 010 16 0120 136 136Alnoite 21.7 300 350 88.1 327 70 1200 040 664ppb Zr Ba Ge Se Pd Au Re Os IrGneiss 900000 938000 0774 027.7 <5.3 <0.26 0.021 <0.040 <0.06Alnoite 300000 500000 3586 178.7 <4.6 <0.30 0.585 <0.034 <0.01

Thech

analno

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Howev

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2.4.3

Thecra

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Figure1[1978]asedimentfragmentdiagonalbasemen

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1. LITHIC

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BRECCIAS

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PAGE45

senceof

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2.4.3.2. MELT‐FRAGMENTBRECCIAS

Atthetopofthebreccialens,aroundtheaxis,melt‐fragmentbrecciasarepresentas

lenses between the lithic breccias. The target rock clasts here, experienced shock

pressuresofapproximately25to45GPa.Theupperpartofthelenseshaveimpact

meltcontentsofaverage13vol.%,butsomezonesreachupto30vol.%.Thishigh

melt‐fragment part is intersected between 264 and 427 m in 1‐59 and its radial

extent is approximately 1.1 km. The lower part of the lenses, intersected between

427and606min1‐59,occursonlyoccasionallyin1‐67,suggestingaradialextentof

about200m.Theyhaveanaverageimpactmeltcontentofonly6to7vol.%[Grieve,

1978andGrieve&Cintala,1981].Suchlowmelt‐fragmentvaluesarealsofoundfor

the edges of the upper part and for one melt‐fragment breccia layer that isintersectedbetween741and763min1‐59and1‐67.

The impactmelt ispresentas alterated inclusion‐rich clastsof glass.Mostof them

aresmaller than5cm,butsomerangeup to25cm [Grieve&Cintala,1982].They

havesinuousorcontortedshapesandcanoccuraschilledrindsaroundtargetrock

clasts.Whenmixedwiththetargetrockclasts, theywereat least inaplasticstate,

butbecause they lackdistinctiveaerodynamic shapes, it is thereforeassumed that

they have never been ejected [Grieve et al., 1977]. Togetherwith the observation

that the melt‐fragment breccias are not continuous throughout the crater, the

breccialensisinterpretedasbeingforthemajorpartderivedfromtheslumpingof

displaced rocks during the modification stage [Grieve, 1978], as is expected for

simpleimpactcraters.

Thetargetrockclaststhatsufferedthehighestshockpressures,arepartiallymelted

andvesiculated,andshowheavyalteration.Alsosixzonesofmelt‐fragmentbreccia

withavesiculatedmatrixoccur in1‐59.These rangeup to5m in thickness.Their

centralportionsconsistoflessthan25%recrystallizedfeldsparinclusionsinafine‐

grainedmatrixoffeltedfeldsparlathsupto0.45mminlength.Thevesiclesareupto

1.25cmindiameterandcanbefilledwithzeolitesorlesscommonlybarite[Grieve,

1978]. The vesiculated target rock clasts are distinguishable from the vesiculated

matrixbyabrick‐redcolourandtheabsenceofinclusions[Grieve&Dence,1978].

2.4.3.3. MELTLENS

[Grieve,1978]

Near thebaseof thebreccia lens,a34mmelt lens is intersected in1‐59,between

823and857m.Itsexactradialextentisnotknown.However,in1‐67,200mtothe

west,itisnotfound.Instead,azoneofpyroxenehornfelsisintersectedbetween824

and840m.Becausethepyroxenehornfelspresentin1‐59extendsonly10maway

fromthemeltzone,itisassumedthatthemeltlenscomescloseto1‐67,implyinga

radialextentofonlyslightlylessthan200m.Assuminga190mradiusandlenticular

shape, Grieve& Cintala [1982] estimate the volume of themelt lens to be 0.0019

km³, of which 50 to 55 vol.%, or approximately 0.001 km³, is impact melt. This

impliesthatonly4.5vol.%ofalltheimpactmeltinthebreccialensinsituatedinthe

meltzone[Grieve&Cintala,1981],whileitwaspreviouslyestimatedtobe1vol.%byDence[1971]and2vol.%byGrieve[1978].

Theimpactmeltismicrocrystallineneartheupperandloweredgeofthemeltzone,

but thegrain size increases rapidly to thecentre.At4m from theupperedge, the

average modal composition is reached, which is 70 vol.% feldspar, 13 vol.%

mesostasis sheet silicate, 7 vol.% pyroxene, 3 vol.% amphibole, 3 vol.% quartz, 3

vol.%opaquesand1vol.%apatite.Thefeldsparsaretabularandabove0.35mmin

size. They have well‐developed cores and rims. The core consists of an altered

intergrowthofK‐andNa‐feldspar.Between843and850m,thefeldsparcoresalso

containzonesofunalteredfeldspar.TheserangeincompositionfromK‐oligoclaseto

anorthoclase.Thefeldsparrimsarealwaysunalteredsanidine,oftenwithmarginal

perthite and needles of apatite. The compositions of the feldspars from the target

rockgneissarewithinthecompositionalrangeoftheintergrownK‐andNa‐feldspar

cores.ItisbelievedbyGrieve[1978]thatthelatterarexenocrystalfeldsparsofthe

targetrockgneiss.TheK‐oligoclaseisconsideredtoderivefromthemeltphase.The

pyroxeneisferro‐augiteandinterstitialtothefeldsparandoftenassociatedwithand

altered to interstitial sheet silicate. The amphiboles occur as grains with a

arfvedsonite coreand riebeckiteovergrowth.Thequartz is anhedral an interstitial

andwhereincontactwiththemesostasis,ithasareactionrim.Themesostasissheet

silicate is microcrystalline and of unknown composition. It may represent altered

interstitialglass.

Theother45to50vol.%ofthemeltzoneconsistsoutoftargetrockinclusions.Most

ofthemaremicroscopicrecrystallizedfeldspar.Theymakeup20to95vol.%ofthe

upper5mandlower8mofthemeltzone.Theserockscanbedefinedasmelt‐matrix

breccias.Within the central21mof themelt zone, the target rock inclusionsvary

from 16 to less than 1 vol.%, whichmakes this zone almost pure impact melt. A

number ofmacroscopic, up to 1m in size, recrystallized target rock xenoliths arealsopresent.

2.4.4. ALTERATION

The Brent structure is reported to have experienced a significant amount of

alteration,especiallyresultingindepletedSiO2andenrichedK2Ocontents[Grieve&

Dence, 1978]. The high K2O contents up to 14wt.% are encountered in themelt‐

fragmentbreccias andmelt lens.Theyarenotproportional to theamountofmelt,

buttotheshockpressuresthetargetrockclastsandinclusionsexperienced,witha

threshold of approximately 20 GPa. The lower shocked lithic breccias with

approximately 5wt.% (~ target rock) do not show elevated K2O contents. This is

reported by Grieve [1978], who states that this K enrichment arose from alkali

exchange between feldspars and saline aqueous solutions during post‐impact

cooling. Because Lozej & Beales [1975] concluded that the Brent impact event

occurred on the edge of a transgressive shallow tropical sea and that the impact

craterwas inundated soon after formation, it is likely that such saline fluidswere

presentandprovidedasourceofK.Thethresholdisprobablyrelatedtotheonsetof

diaplectic glass formation. Palme et al. [1981] also explain depleted Re and Aucontentsthiswaybyhydrothermalmobilisation.

2.4.5. GEOCHEMICALIDENTIFICATION

A few geochemical impact andmeteorite identification studies have already been

conductedfortheBrentimpactstructure.Grieve[1978]reportedcorrelatingNiand

Crvaluesuptorespectively575ppmand120ppm,indicatinganimpactorigin.The

targetrocksinclusionsinthemeltlenscontained35ppmNiataverage.Thisexcess

of 25 ppm relative to the target rock reported in Table 1 was explained as

contamination ofmelt in the analysed samples, or as the actual Ni content of the

targetrockinclusionsbyincorporationofcondensedmeteoriticvapourormelt.An

average Ni content of 253 ± 133 ppm for the impact melt in the melt lens was

obtained this way and a Ni/Cr ratio of 2.76 ± 0.36. Assuming that the meteorite

contributioncontrolsthisratio,achondritemeteoriteandmorespecificanordinaryLwassuggested.

Table2.SomesiderophileelementconcentrationsoftwosamplesfromthemeltlensusedbyPalmeetal.[1981](moresampleswereused).Thesamplenamecorrespondstothedrillholeanddepthinfoot.

Os(ppb) Ir(ppb) Pd(ppb) Au(ppb) Ni(ppm) Co(ppm) Cr(ppm)59‐2773.3 1.26 2.68 0.055 0.15 151 10.5 7359‐2800.2 9.22 9.57 18.1 0.93 320 21.1 103

Basedona flatCI‐normalisedpatternof thesiderophileelements listed inTable2,

Palmeetal.[1981]alsosuggestachondritemeteorite.ALorLLordinarychondrite

isderived,basedonaNi/Crratioof2.70±0.26 for the impactmelt.A target rock

valueof20ppmNiand23ppmCrisusedtoobtainthisratio.Meltlensanalysesfor

PGEbyEvansetal.[1993]arelistedinTable3.ThesePGEvaluesaresaidtobenotin

agreementwithaLorLLordinarychondrite.ThePt/Irratioof1.95suggestaCIor

ironmeteorite.However, theLLdata to comparewithare limitedand thePGECI‐

normalisedpatternisrelativelyflat.TogetherwiththedataofPalmeetal.[1981],an

increaseofPGEabundanceswithincreasingdepthisreported,possiblyanindication

forafractionationofthemeteoritecontributionduringpost‐impactcoolingorbythehydrothermalalteration.ThiscouldexplainthePGEdeviation.

Table3.SomesiderophileelementconcentrationsofthetwosamplesfromthemeltlensusedbyEvansetal.[1993].Thesamplenamecorrespondstothedrillholeanddepthinfoot.

Pt(ppb) Pd(ppb) Ru(ppb) Ir(ppb) Au(ppb) Rh(ppb)59‐2778.9 11 12 5.9 1.8 6.7 2.059‐2781.0 15 8.8 7.8 3.0 11 2.2

A mixing model by Grieve & Dence [1978], based on major and trace elements,

indicates that themelt lens corresponds to amixof97.8% target rock and2.2%

meteorite,whilevaluesofrespectively98.4%and1.6%wereobtainedbyGrieve&

Cintala[1981].

3. METHODS

To determine the geochemical signatures from the meteorite contribution in the

impact rocks of the Brent impact structure, twenty samples from the Dominion

Observatorydrillingareobtained,includingtargetrock,melt‐fragmentbrecciasand

meltlensrocks.Foralltwentysamples,Ni,CoandCraredetermined.Forthirteenof

themalso themajorelementsandPGEareanalysed.For fewselectedsamples, the

whole rock isotope ratios are measured of the common used Os and recentlyproposedPbisotopesystems.

3.1. SAMPLES

Table4.ThetwentyusedsamplesfromtheBrentimpactstructures.Givenarethedepth(m),mass(g)andthekindofimpactrock.Samplenamesarederivedfromtheirdrillholeanddepthinfoot,asinliterature.*ThesesampleshaveadarkgreencolourandarelabelledaschloritizedbasedonthinsectionobservationsbytheEarthPhysicsBranchoftheDepartmentofEnergy,MinesandResources,Ottawa,Canada.

Sample Depth Mass ImpactRock59‐951 289.86 34.47 Melt‐fragmentbreccia59‐951.5 290.02 43.90 Melt‐fragmentbreccia59‐1050 320.04 51.37 Melt‐fragmentbreccia59‐1379 420.32 45.81 Melt‐fragmentbreccia*59‐1929.5 588.11 28.32 Melt‐fragmentbreccia59‐2762 841.86 33.55 Meltlens59‐2771 844.60 49.24 Meltlens59‐2781 847.65 42.46 Meltlens59‐2794 851.61 40.49 Meltlens59‐2800 853.44 48.61 Meltlens59‐2880 877.82 35.00 Targetrock67‐888 270.66 64.95 Melt‐fragmentbreccia*67‐926 282.24 51.58 Melt‐fragmentbreccia67‐926.5 282.40 30,76 Melt‐fragmentbreccia67‐933.5 284.53 26.33 Melt‐fragmentbreccia67‐1169 356.31 32.76 Melt‐fragmentbreccia*67‐1446 440.75 49.00 Melt‐fragmentbreccia67‐2630 801.62 27.12 Targetrock67‐2702 823.57 40.79 Melt‐fragmentbreccia67‐3813 1162.20 41.98 Targetrock

Thetwentysampleshavemassesbetween26.33and67.95g.Elevenarefromdrill

hole 1‐59, five of themaremelt‐fragment breccias, another five are from themelt

lens.Onesampleisatargetrockclastfromthehighlyshockedlithicbrecciasbeneath

themeltlens.Theotherninesamplesarefromdrillhole1‐69.Sixofthesearemelt‐

fragmentbreccias,oneisatargetrockclastsfromalithicbreccia,andoneisatarget

rock from the parautochthonous basement. Sample names are derived from their

drill hole and depth in foot. This to be in agreement with sample names used inliterature.

3.2. MAJORELEMENTS

Thirteen samples are analysed for Si, Ti, Al, Fe, Mn, Mg, Ca, K, Na and P. This by

inductively coupled plasma atomic emission spectroscopy at Ghent University. To

determine the accuracy, theBCR‐2 (United StatesGeological Survey,Denver, USA)

and SARM‐44 (SouthAfricanBureau of Standards, Pretoria, SouthAfrica) certified

standardsareaddedtotheanalysis.

Eachpowdered sample isdried at110 °C. Loss on ignition (LOI) isdeterminedby

heatingat850°Cfor2h.Afterwards,toapproximately200mg,a1gfluxof1:1Li‐

meta‐/‐tetraborate(LiBO2/Li2B407) isaddedtothesamples.Theresultingmixtures

arehomogenisedand transferred tohighpuritygraphite crucibles.These are than

heatedat800to1000°C.Whenameltisformed,thisisdissolvedby2vol.%dilutedHNO3andstirredtoacceleratethereaction.

3.3. NI,COANDCR

To obtain Ni, Co and Cr concentrations, all twenty samples are quantitatively

analysed for the trace elements 52Cr, 59Co, 60Ni, 68Zn, 71Ga, 84Sr, 85Rb, 86Sr, 89Y, 91Zr,93Nb, 115In, 118Sn, 121Sb, 137Ba, 140Ce, 146Nd, 172Yb, 205Tl, 206Pb, 232Th and 238U. This is

done by ICP‐MSwith a XSERIES 2 fromThermo Scientific. In and Tl are added as

internal standards. To determine the accuracy, the PM‐S (Service d'Analyse des

RochesetdesMinéraux,Nancy,France)andDNC‐1(UnitedStatesGeologicalSurvey,

Denver,USA)certifiedstandardsareaddedtotheanalysis.Eachsampleismeasured

twiceontwodifferentdaystoenlargetheanalyticalconfidence.SamplepreparationcomprisestheaciddigestionprocedureasdescribedbyTagleetal.[2007].

Approximately 100 mg of each powdered sample is transferred into a 15 ml

perfluoroalkoxy(PFA)beaker,dischargedwithananti‐electrostaticgun.Todissolve,

first2ml14MHNO3isaddedslowlyandthen4ml22MHF.Thecoveredbeakeris

put on a 125 °C hot plate for 24 to 48 hours to accelerate the reaction. After 45

minutesofcooling,thebeakerisopenedandputona75to90°Chotplateforone

night to evaporate the solution. After adding 1ml of 6 µg/ml In and 1ml of 3.25

µg/mlTlasinternalstandards,thesolutionswerebroughttonear‐drynessandthis

solutionisevaporatedona75to90°Chotplate.Theresidueistakenupina1ml7M

HClO4and1mlsaturatedH3BO3watersolution.Thisagain,isevaporatedona75to

90 °C hot plate. Themetal rich residue is dissolved in aqua regia (1:3 HNO3:HCl).

Afterdryingdown,2dropsof5%H2O2 followedby2ml14NHNO3areaddedto

ensure oxidation of all Fe to Fe3+. The solutions are dried down again 30minutes

later.Theprecipitate isdissolved in10ml14MHNO3andheatedona120°Chot

plate for30min toaccelerate the reaction.Finally, the solutionsarequantitatively

transferred into a 250 ml polymethylpentene (PMP) volumetric flask and dilutedwithultrapure18.2MΩ/cmmilli‐qwater.

3.4. PLATINUMGROUPELEMENTS

ThirteensamplesareanalysedforthePGE99Ru,101Ru,102Ru,103Rh,105Pd,106Pd,108Pd,191Ir,193Ir,194Pt,195Ptand196Pt.Osvolatilisesduringtheappliedsamplepreparation.

Inaddition, also 197Au ismeasured.This isdoneby ICP‐MSatGhentUniversity. In

and Tl are added as internal standards. To determine the accuracy, the PGE‐poor

KTB‐FDO and PGE‐richWPR‐1A standards are added to the analysis, both are in‐

housestandards.PGEconcentrationsarenearorbelowthedetectionlimitsinmost

crustalrocks.Therefore,apre‐concentrationmethodisappliedtobecomeaprecise

quantitativeanalysis.Weusethenickel‐sulphide fireassayprocedureasdescribedbyPlessen&Erzinger[1998]andTagle&Claeys[2005].

Approximately20g(10g is theminimumtoaccountwithcertainty for thenugget

effect [Tagle & Claeys, 2005]) of each powdered sample is transferred into a

polypropylenecontaineranda fireassay flux isadded:12gofdehydratedsodium

carbonate(Na2CO3),24gofdehydratedsodiumtetraborate(Na2B4O7),2gofcalcium

fluoride(CaF2),2gofsulphur(S),and2gofnickel(Ni).Thismixtureishomogenized

withanagatepestleandtransferredintoafireclaycrucible.Thecruciblesisplaced

inafurnaceandheatedto1150°Cforanhour.Herebythemixturemeltsanddueto

theflux,aheavyNiSphaseformsinwhichthePGEarepartitioned.Thiswillsinkto

thebottomandfuseintoabutton.Oncecooled,thecrucibleisbrokenwithahammer

and the NiS button is removed by hand. The button is dissolved in a 300 ml

Erlenmeyerflaskin200mlof38vol.%HCl.TheErlenmeyerflaskiscoveredtoavoid

splashingandplacedina90°Cwaterbathinsideafumehood.Afterapproximately

24hthedissolutioniscompleteandthecooledsolutionisfilteredthroughamicro‐

filter disc of glass (brand: Schott; porosity: 4) in a Büchner funnel into a Büchner

flaskthatisconnectedtoawateraspirator.TheBüchnerfunneliswashedwithHCl

at the end (40 vol.%, twice distilled). The PGE containing residue (only the NiS

dissolves in thewarmHCl) isdissolvedwithamixtureof twopartsHCl (32vol.%,

twice‐distilled)andonepartH2O2 (30vol.%,analyticalgrade) in threesubsequent

steps.Finally, thesolution ispassedthrougha filterpaper, transferredtoa250ml

beakerandevaporatedonahotplatetoavolumeof1a2ml.Afteraddingafinal5

mlofHCland2mlofH2O2,thesolutionisevaporatedto1a2ml,cooledanddiluted

with2%HCltoavolumeof10ml.

3.5. ISOTOPES

Inadditiontothemajorelements,PGEandNi,CoandCr,alsowholerockratiosfor

theisotopesystemsofOsandPbwereobtainedforselectedsamples.Theseanalyses

were conducted with MC‐ICP‐MS by respectively F. Paquay at the University ofHawaiiandV.RensonattheVrijeUniversiteitBrussel.

4. RESULTS

4.1. ELEMENTABUNDANCESANDRATIOS

4.1.1. MAJORELEMENTS

Table5 lists the results from themajor elements analysis. The concentrations are

calculatedbyconstructingacalibrationcurvewithquantitativeanalysesofamethod

blank, and the BHVO‐2, AGV‐2, QLO‐1, DTS‐2, GSP‐2 (United States Geological

Survey,Denver,USA),JSy‐1(GeologicalSurveyofJapan,Ibaraki,Japan),Mica‐Fe,BX‐

N,IF‐G(CentredeRecherchesPetrographiquesetGeochimiques,Nancy,France)andNIM‐L(Mintek,Randburg,SouthAfrica)certifiedstandards.

Table5.Resultsofthemajorelementanalysis.Standarderrorsare0.20wt.%forSiO2,0.01wt.%forTiO2,0.11wt.%forAl2O3,0.03wt.%forFe2O3,0.01wt.%forMnO,0.01wt.%forMgO,0.01wt.%forCaO,0.04wt.% for K2O, 0.02 wt.% for Na2O, and 0.01 wt.% for P2O5. Limits of detection are 0.003 wt.%. Fe2O3includesallFe.

wt.% SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO K2O Na2O P2O5 LOI59‐951 54.62 0.75 17.96 10.21 0.07 4.84 0.33 10.28 0.24 0.13 3.5059‐1050 52.18 1.74 16.57 13.08 0.15 5.50 0.60 8.97 0.66 0.26 3.1059‐1379 33.77 1.28 17.13 34.31 0.26 11.64 0.95 0.08 0.87 0.36 7.9659‐1929.5 53.52 0.79 23.05 8.52 0.19 2.50 0.51 8.70 1.17 0.18 2.1959‐2762 53.58 0.71 24.04 8.35 0.18 1.68 1.90 6.31 3.00 0.18 1.5659‐2771 56.91 0.75 19.44 9.65 0.28 1.43 2.60 4.89 4.26 0.19 1.0459‐2781 57.45 0.80 19.19 9.22 0.24 1.48 2.52 5.62 4.11 0.20 1.0759‐2800 51.66 0.89 24.00 10.03 0.20 2.49 2.37 5.72 3.01 0.23 1.4767‐888 33.89 0.68 18.02 31.87 0.15 14.25 0.65 0.06 0.76 0.17 8.4467‐926 54.83 0.59 18.78 9.87 0.05 4.35 0.22 10.46 0.47 0.11 2.7567‐1169 33.08 1.59 17.68 33.17 0.29 11.46 1.33 0.08 0.89 0.52 7.7867‐1446 62.03 0.66 14.51 9.87 0.12 2.83 0.94 5.56 2.23 0.14 2.6967‐2702 55.68 1.02 15.47 11.65 0.19 4.56 0.60 9.59 0.35 0.27 3.12

Theobtainedvalues for theBCR‐2(54.27±0.46wt.%SiO2,2.26±0.02wt.%TiO2,

13.45±0.01wt.%Al2O3,13.80±0.10wt.%Fe2O3,0.20±0.01wt.%MnO,3.58±0.02

wt.%MgO,7.13±0.05wt.%CaO,1.77±0.01wt.%K2O,3.15±0.01wt.%Na2O,and

0.36±0.01wt.%P2O5)andSARM‐44(34.8±0.74wt.%SiO2,1.86±0.03wt.%TiO2,

59.81±0.16wt.%Al2O3,2.15±0.19wt.%Fe2O3,0.03±0.01wt.%MnO,0.13±0.05

wt.%M

0.10±

BCR‐2

wt.%F

±0.05

(34.84

MnO,0

P2O5) v67‐116

Figure 1normalisbyGrievGao,200

SiO2 co

Al2O3 fr

0.29wt

to10.4

MgO,0.12±

0.01wt.%

(54.1±0.8

Fe2O3,0.2±

wt.%K2O,

wt.% SiO

0.10wt.%M

values. SAR69.BCR‐2w

19. Target‐nosationarethae[1978].Th3].Fe2O3incl

oncentratio

from14.51

t.%,MgOf

46wt.%,Na

±0.09wt.%

P2O5)cert

8wt.%SiO

±0.01wt.%

,3.16±0.

2, 1.83 wt

MgO,0.14

RM‐44wawasmeasu

ormalised resatofGrieve[evaluesforludesallFe.

ons vary f

1 to24.04

from1.43t

a2Ofrom0

%CaO,1.7

tifiedstand

O2,2.26±0

%MnO,3.5

11wt.%N

t.% TiO2, 5

wt.%CaO,

smeasureuredwitht

sults of the[1978]listedtheupperco

from 33.08

wt.%,Fe2to14.25w

0.24to3.01

79±0.02w

dardsarei

0.05wt.%T

59±0.05w

Na2O,and0

58.80 wt.%

,0.18wt.%

ed togethetheothers

major elemeinTable1.Pontinentalcru

8 to 62.03

O3 from9

wt.%,CaOf

1wt.%,an

wt.%K2O,

ingoodag

TiO2,13.5

wt.%MgO,

0.35±0.0

% Al2O3, 2

%K2O,0.05

rwith samamples.

ent analysis.P2O5isnotreust(UCC)are

wt.%, TiO

.87 to34.3

from0.22t

ndP2O5fro

0.03±0.0

reementw

±0.2wt.%

7.12±0.1

2wt.%P2O

.06 wt.%

5wt.%Na2mples51‐1

The targetportedbecaueaddedasa

O2 from 0.

31wt.%,M

to2.60wt.

m0.11to

6wt.%Na

withthepu

%Al2O3,13

11wt.%Ca

O5)andSA

Fe2O3, 0.0

2O,and0.1

1379, 67‐8

rock valuesuseitisnotmreference[R

.66 to 1.74

MnO from

.%,K2Ofro

0.52wt.%

PAGE55

a2O,and

ublished

3.8±0.2

aO,1.79

ARM‐44

03 wt.%

10wt.%

888and

used formeasuredRudnick&

4 wt.%,

0.05 to

om0.06

%.Figure

19showsthetarget‐normalisedresultsofthemajorelementanalysis.However,no

target rock is measured because no target rock samples were available at the

momentoftheanalysis.Therefore,thetargetrockusedfornormalisationisthatof

Grieve [1978], listed in Table 1. Because lithic breccias, deep parautochthonous

basement(≥911m)andsurfacesamplesareused for theanalysis(n=10), this isassumedtobearepresentativetargetrock.

Threemelt‐fragmentbrecciasamples(59‐1379,67‐888and67‐1169;markedredin

Figure 19) show a total different composition than the other samples. They have

averagecontentsof33.58wt.%SiO2,1.18wt.%TiO2,17.61wt.%Al2O3,33.12wt.%

Fe2O3,0.23wt.%MnO,12.45wt.%MgO,0.98wt.%CaO,0.07wt.%K2O,0.84wt.%

Na2O, and 0.35 wt.% P2O5. Their LOI amounts 8.06 wt% at average and is

significantly higher than the other samples. Relative to the target rock of Grieve

[1978]thesevaluesimplicateanenrichmentinFeandMg,anddepletioninSi,Ca,K

andNa.Asalreadymentioned inTable4, these threedarkgreencolouredsamples

werelabelledaschloritized,whichisconsistentwiththevaluesweobtained.

Only the samples from the melt lens (59‐2762, 59‐2771, 59‐2781 and 59‐2800;

marked blue in Figure 19) show a flat pattern that resembles the target rock of

Grieve [1978], but still a depletion in Si, Ti and Na exists, while the others are

enriched.Fivemelt‐fragmentbrecciasamples(59‐951,59‐1050,59‐1929.5,67‐926

and67‐2702;markedgreeninFigure19)areenrichedinMgandK,anddepletedin

Si,CaandNa.Melt‐fragmentbrecciasample67‐1446(markedpurple inFigure19)

shows concentrations intermediate to the melt lens and melt‐fragment breccia

samples.OnlyitsSicontentisdistinctwith62.03wt.%SiO2,beingthehighestofallsamplesandclosesttothetargetrockofGrieve[1978].

4.1.2. NI,COANDCR

Table 6 lists the results from the Ni, Co and Cr analysis. The concentrations are

calculated by a one‐point calibration with a quantitative analysis of the BE‐N

certified standard (Service d'Analyse des Roches et desMinéraux, Nancy, France).

ObtainedvaluesforthePM‐S(126±2ppmNi,51±1ppmCoand314±3ppmCr)

andDNC‐1(269±3ppmNi,59±1ppmCoand286±3ppmCr)certifiedstandards

areingoodagreementwiththepublishedPM‐S(115±3ppmNi,49±2ppmCoand

314±6ppmCr)andDNC‐1(247±12ppmNi,57±3ppmCoand270±9ppmCr)

values.ThequantifiedNiconcentrationsvaryfrom6.4to311ppm,Cofrom1.4to35

ppm,andCr from30to194ppm.Figure20 showstheNiandCrvalues inaNi/Cr

diagram.Thethreetargetrocksamples(59‐2880,67‐2630,67‐3813)allhavelowNi,

Co and Cr concentrations, most of them even below the limit of quantification.

Therefore,alsothetargetrockofGrieve[1978](10±4ppmNi,8±2ppmCoand12±2ppmCr)isshowninFigure20.

Table6.ResultsoftheNi,CoandCranalysis.Standarderrorsare3ppmforNi,1ppmforCoand3ppmforCr.Limitsofquantificationare3ppmforNi,1ppmforCoand5ppmforCr.Also theNi/CrandCo/Crratiosarereported.

Ni(ppm) Co(ppm) Cr(ppm) Ni/Cr Co/Cr Ni/Co59‐951 129 4.6 79 1.63 0.06 28.0459‐951.5 109 5.7 37 2.95 0.15 19.1259‐1050 81 35 113 0.72 0.31 2.3159‐1379 9.8 3.6 <LOQ 2.7259‐1929.5 28 2.1 30 0.93 0.07 13.3359‐2762 187 12 64 2.92 0.18 15.5859‐2771 201 13 68 2.96 0.19 15.4659‐2781 186 13 68 2.74 0.19 14.3159‐2794 235 15 85 2.76 0.17 15.6759‐2800 311 20 127 2.45 0.16 15.5559‐2880 <LOQ 3.4 <LOQ 67‐888 135 1.4 <LOQ 96.4367‐926 97 7.9 156 0.62 0.05 12.2867‐926.5 103 8.5 194 0.53 0.04 12.1267‐933.5 97 12 198 0.49 0.06 8.0867‐1169 10 2.9 <LOQ 3.4567‐1446 <LOQ 2.3 <LOQ 67‐2630 6.4 4.1 <LOQ 1.5667‐2702 94 7.4 64 1.47 0.12 12.7067‐3813 <LOQ <LOQ <LOQ

This enrichment in Ni, Co and Cr relative to the target rock of Grieve [1978]

implicatesanimpactidentificationandalsoprecludesadifferentiatedachondriteas

impact meteorite. The samples of the melt‐fragment breccias do not show a

correlation,butthemeltlenssamplesdo,suggestingacommonsource.Thisisalso

noticed for the Co/Cr and Ni/Co ratios (for the latter also some melt‐fragment

brecciasamplesshowcorrelationwiththemeltlenssamples).Takenthetargetrock

intoaccount,thisresultsinto2.93±0.16Ni/Cr,0.10±0.01Co/Crand29.85±3.00

Ni/Co ratios for the impact meteorite. The Ni/Cr and Co/Cr ratios rule out the

possibilityofanironmeteoritebecausetheseallhaveratiosonetothreeordersof

magnitude higher. TheNi/Cr ratio points to a LL (2.64 ± 0.21) or L (3.22 ± 0.19)

ordinary chondrite, while the Co/Cr ratio approaches the LL (0.13 ± 0.02). No

chondrite meteorites are known with a lower Co/Cr. The Ni/Co ratio is not

consistent with a L or LL ordinary chondrite. It does not fall into the narrow

chondrite range (19.34 ± 3.15 to 22.86 ± 2.83) and only corresponds to the iron

meteorites[Schmieder&Buchner,2010].Ofthemeltlenssamples,59‐2800hasthe

highest

contrib

equal t

ratioa

Figure20Grieve[1

4.1.3

Table 7

constru

KTB‐FD

values.

their P

quantif

target

momen

t Ni, Co an

bution.Wit

to thatof t

valueof25

0.Ni/Crdiagr1978],listedi

3. PLAT

7 lists the

uctingaca

DO and W

Thechlor

PGE conce

fication. Fi

rock is m

ntofthea

nd Cr valu

thonly thi

theLL.Als

5.08isobt

ramofthereinTable1.

INUMG

results of

alibrationc

WPR‐1A in‐

ritizedsam

entrations

igure 21 s

measured b

nalysis.Al

ues and th

is sample,

so theCo/

ained,whi

esultsoftheN

GROUPE

f the PGE

curvewith

‐house sta

mples59‐13

were no

hows the

because n

sothetarg

herefore s

aNi/Crra

/Crratioof

ichstilldoe

Ni,CoandCr

ELEMEN

analysis. T

standard

andards ar

379,67‐88

ot detected

CI‐norma

no target

getrockv

should con

atioof2.6

f0.10 is in

esnotsign

analysis.The

TS

The conce

solutions.

re in good

88and67‐

d or did

lised resu

rock samp

aluesbyP

ntain the h

2 is found

nagreeme

nificantlyfi

etargetrockv

ntrations

Theobtain

d agreeme

1169aren

not exce

lts of the

ples were

Palmeetal

highest me

d,which is

ent.For the

itthechon

valuesuseda

are calcul

nedvalues

ent with p

notlistedb

ed the lim

PGE analy

available

l.[1981], l

PAGE58

eteorite

almost

eNi/Co

ndrites.

arethatof

ated by

sforthe

previous

because

mits of

ysis. No

at the

istedin

Table 1, are not used as a reference in Figure 21, because they are onlymaxima.

Probablytheyalsoarenotrepresentativeforthetargetrock,becauseobtainedfrom

amelt‐fragmentbrecciatargetrockclast.Theuppercontinentalcrustisshownasa

referenceinstead.

Table7.ResultsofthePGEanalysis.Standarderrorsare0.03ppbforIr,0.09ppbforRu,0.08ppbforPt,0.02ppbforRh,0.03ppbforPdand0.03ppbforAu.Limitsofdetectionare0.02ppbforIr,0.03ppbforRu,0.02ppbforPt,0.01ppbforRh,0.05ppbforPdand0.05ppbforAu.Limitsofquantificationare0.06ppbforIr,0.11ppbforRu,0.06ppbforPt,0.03ppbforRh,0.17ppbforPdand0.16ppbforAu.

ppb Ir Ru Pt Rh Pd Au59‐951 5.02 11.46 13.01 2.15 6.58 1.5359‐1050 0.22 1.64 1.66 0.31 1.52 0.7259‐1929.5 1.27 3.48 3.70 0.65 2.21 0.2359‐2762 2.31 5.53 6.42 1.14 3.27 0.8859‐2771 2.78 6.42 7.83 1.31 3.94 1.1359‐2781 2.66 6.25 7.12 1.21 3.57 0.6259‐2800 6.21 13.37 15.16 2.57 8.27 1.4767‐926 6.41 17.06 19.63 3.48 8.96 0.7567‐1446 0.07 0.31 0.13 0.02 0.21 0.1867‐2702 2.64 4.92 6.30 0.96 3.81 0.86

Irconcentrationsvaryfrom0.07to6.41ppb,Rufrom0.31to17.06ppb,Ptfrom0.13

to19.63,Rh from0.02to3.48ppb,Pd from0.21to8.96ppb,andAu from0.18to

1.47ppb.Allminimumvaluesare found in sample67‐1446andallmaxima in67‐

926. Only Au has a maximum in sample 59‐2800, which for the other PGE is the

secondhighestsample.Exceptforsample59‐951,allsampleshaveAuvaluesbelow

thatoftheuppercontinentalcrust.Theseseemtobedepletedinthemelt‐fragmentbrecciasamples,butnotinthemeltlenssamples.

ThemeasuredPGE concentrations are significantlyhigher than any crustal values.

Thisimplicatesanimpactidentificationandrulesoutadifferentiatedachondriteas

meteorite.MostsampleshaveaflatPGEpattern,whichsuggestsachondriteimpact

meteoriteandnotaniron.Onlysamples59‐1050and67‐1446haveanoddpattern.

Whilethelatterhasnocorrelation,theformerseemstobeintermediatetotheupper

continental crust and might represent a lesser meteorite contribution. When the

upper continental crust from [Rudnick & Gao, 2003] is used as target rock andsubtractedfromsample59‐1050,itformsamoreflatpattern.

A linear regression analysis is carried out on the obtained PGE concentrations.

SlopesandinterceptsaredeterminedforthePd/Ir,Rh/Ir,Pt/Ir,Ru/Ir,Pd/Rh,Pt/Rh,

Ru/Rh, Pt/Pd, Ru/Pd and Pt/Ru ratios. The results are listed in Table 8 and

visualisedinFigure22.Nexttothechloritizedsamples,alsosample67‐1446isnot

listed,becauseitdoesnotshowacorrelationtotheothersamples.Theseothernine

sampletheyha

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PAGE60

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PAGE61

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PAGE62

771and

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PAGE63

ncanbe

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Table11.ResultsoflinearregressionanalysisonNi,CoandCrwithIr.ThesedataareobtainedwiththeAnalysisToolPakRegressionfromMicrosoftOffice2010Excel.Foursamplesareused.

Y X Slope(ppm) SE Intercept(ppm) SE R2

Ni Ir 32880 2 106.49 8.25 0.991Co Ir 2025 1 7.43 0.2 0.999Cr Ir 16551 1 23.98 2.32 0.997

TheresultsfromthePbisotopesystemscanbeplottedagainstIr(andtheotherPGE)

toseeifacorrelationappearsthatwouldsuggestacommonsourceofPb.However,

nocorrelationisfound.

5. DISCUSSION

Thismasterdissertationhas theobjective to conduct ameteorite identification, as

precisely as possible, for the Brent impact structure. If possible, implications are

drawn from the results, regarding to the applicability of the different chemical

signatures on impact en meteorite identification. Special attention is paid to the

multi‐signature approaches. Twenty melt lens andmelt‐fragment breccia samples

areobtainedfromthebreccialensandanalysedformajorelements(n=13),andthemoderately(n=20)andhigh(n=13)siderophileelements.

5.1. ALTERATION

Onarrival, threedarkgreencolouredmelt‐fragmentbrecciasamples(59‐1379,67‐

888and67‐1169)werelabelledaschloritized,basedonthinsectionobservationsby

theEarthPhysicsBranchoftheDepartmentofEnergy,MinesandResources,Ottawa,

Canada. Because no target rock is analysed, these samples are comparedwith the

target rock values obtained by Grieve [1978], which are assumed to be

representative because obtained from lithic breccias, deep parautochthonous

basement(≥911m)andsurfacesamples(n=10).Asingletargetrockcomponentis

proposed and identified as a mesoperthite gneiss of granodioritic composition,

whichisconfirmedbyPalmeetal.[1981].ThesamplesshowelevatedvaluesofFe,

Mg and the LOI, and depleted Si, Ca, K and Na contents (Figure 19), which is

consistentwithchloritization.WhiletheBrent impactstructureisalreadyreported

to be highly altered [Grieve, 1978; Grieve & Dence, 1978; and Dence, 2004],

chloritizationisnotspecificallymentionedbefore.

Alsotheothermelt‐fragmentbrecciassamples(n=5)showadeviatedpatternfrom

the target rock in their major elements composition (Figure 19). Most of this

signatureissimilartothechloritizedsamples,albeittoalowerlevel.However,they

stronglydifferinK2Ocontents.Whilethatofthechloritizedsamplesisdepletedwith

0.07 ± 0.01 wt.%, the other melt‐fragment breccias show 9.06 ± 0.31 wt.% K2O,

which ishigher than the target rockvalue (4.92±0.08wt.%).These elevatedK2O

contents are already explainedbyGrieve [1978],who foundvaluesup to14wt.%

whichareproportionaltotheexperiencedlevelsofshockpressure(withathreshold

of20GPa).Thisisprobablycausedbyalkaliexchangebetweenfeldsparsandasaline

aqueous solution that was present during post‐impact cooling [Lozej & Beales,1975].

Themajorelementsanalysisof themelt lenssamples (n=4) resembles the target

rockthemost.TheystillshowadepletionofSi,TiandNa,andanenrichmentofthe

other major elements, suggesting some alteration, but no spikes like the melt‐

fragmentbrecciasamples(Figure19).Therefore,themeltlensisassumedtobeless

altered than the melt‐fragment breccias, which can be expected for its coherent

lithology. Grieve [1978] reports that the alteration in themelt lens occurs on the

highestshockedtargetrockclasts.AnincreaseinPGEwithdepthisnoticedbyEvans

etal.[1993]inthemeltlenssamples,basedonowndataanddatafromPalmeetal.

[1981]. It isexplainedbyafractionationof themeteoritecontributionduringpost‐

impact cooling and/or the hydrothermal alteration.While our PGE values for the

melt lens samples (n = 4) are in agreementwith this observation (Figure21), we

found another explanation, as will be seen below. Following Palme et al. [1981],

hydrothermalalterationisalsoresponsiblefordepletedAuconcentrations,whichis

consistent with our data and seems likely due to the mobility of Au. The lack of

correlation for the Pb isotope is probably also because of alteration, taken the

mobilityofPbintoaccount.

Onemelt‐fragment sample (67‐1446) shows deviating values between that of the

melt lens and melt‐fragment breccia samples (Figure 19), probably reflecting anintermediateordifferentalteration.

WecanconcludethattheBrentimpactstructureissignificantlyaltered.Probablyan

earlyfeldsparalterationoccurred,madepossiblebythepresenceofasalineaqueous

solutionduringpost‐impactcooling.Thiswasfollowedbyotheralterationprocesses,includingchloritization.Themeltlensistheleastaltered,whichcanbeimportant.

5.2. IMPACTIDENTIFICATION

Arelativeeasyanalysisforimpactidentificationisthatofthemoderatelysiderophile

elementsNi,CoandCr.TheanalysedsamplesfromtheBrentimpactstructureshow

a significant enrichment of these elements relative to the target rock of Grieve

[1978].When theNi/Cr, Co/Cr andNi/Co ratios are plotted (Figure20), themelt‐

fragment breccias (n = 8) show no correlation (probably due to alteration as

suggestedbythemajorelements),butthemeltlenssamplesdo(n=5),implicatinga

common meteorite source. However, ultramafic alnoite and (ultra)mafic gneiss

componentsarereportedbyGrieve[1978]andPalmeetal.[1981],whichcouldbe

responsibleforelevatedNi,CoandCrcontents.Whilesuchcontributionisdiscarded

by Grieve [1978] and Palme et al. [1981], based on mixing calculations and V

contents, we do not find the evidence conclusive because based only on a few

samplesandalsonoerrorsareprovidedconcerning theVconcentrations.Perhaps

new target rockdata shouldbe compiled to explore thispossibilityof (ultra)mafic

components. After all, based on the surrounding rocks, it is difficult to exclude a

minorcomponentfromthelargevolumeoftargetrockthatcontributestoanimpact

structure. Local enrichments can alwaysoccur.Because alnoite clasts are found in

themelt‐fragment breccias by Grieve [1978] and Palme et al. [1981], it is always

possiblethatsmall(ultra)maficclastsarepresentinoneormoreofoursamplesand

absence inallothers,partlyexplaining thescatterof themelt‐fragmentbreccias.A

moreuniformcontribution(aspartofthemelt)couldbepartlyresponsibleforthedepletedSicontentsthatallanalysedsamplesshow.

The more difficult to analyse PGE also show high concentrations, except for the

chloritizedsamples(forwhichtheyarenotdetectedorquantified)andsample67‐

1446,which has very low values, probably due to the alteration (sample 67‐1446

alreadyshoweddeviatingvaluesforthemajorelements).Notargetrockismeasured

tocomparewithandalsothetargetrockvaluesofPalmeetal.[1981]arenotused

because they are only maxima and obtained from a melt‐fragment breccia target

rock clast, thus probably even not representative for the target rock due to

alteration.However, all samples (n = 9) show correlating PGE concentrations that

are significantly higher than any known crustal values (except for the alteredAu)

(Figure 21), implicating a common meteoritic source. This means that the PGE

concentrations of the included melt‐fragment breccias (n = 5) did not change by

alteration(which isconsistentwiththeirrelative immobility).Theydonotexclude

the presence of (ultra)mafic components because they even exceed theconcentrationsthataretypicalformostuppermantlecomponents.

LikethePGE,alsotheNi,CoandCrconcentrationsincreasewithdepthinthemelt

lens (Table 6 and Figure 20). An explanation by alteration or an increasing

(ultra)mafic component seems unlikely. Higher values would suggest a higher

meteoritecontribution,but fromwhat isknownof impactmechanisms,noprocess

canaccountforthisdepthrelation.Thisdoesnotmakesitabetteroptionthanthe

suggestion of Evans et al. [1993] of fractionation during post‐impact cooling.However,Osisotopescanbringsolution.

While it is also relatively difficult to analyse, the Os isotope system is highly

straightforward when it comes to impact identification. It is known to be very

sensitive inreflectingtheexactmeteoritecontribution.This isdonebyplottingthe187Os/188OsratiosagainsttheOsconcentrationsandcomparingthemwiththemixing

linefromTagle&Hecht[2006]betweentheuppercontinentalcrustandmeteorites

(Figure23).Valuesareobtainedfrom0.01wt.%forsample59‐1050(whichalready

showed a PGE signature between the othermelt‐fragment breccias and the upper

continentalcrust),0.5wt.%forsamples59‐951.5and59‐2771,andbetween1and2

wt.%forsample59‐2800.Thisrejectsthefractionationoption.Whileitcanexplain

thehigherOsconcentrations,itcannotaccountforthedifferenceinthe187Os/188Osratios.

Asaconclusion,ameteorite identification for theBrent impactstructure iscertain

because of a conclusive multi‐signature meteorite contribution detection by the

moderately and highly siderophile elements, togetherwith the Os isotope system.

While the latter is probably sufficient on its own, the moderately and highly

siderophile elements, together with the major elements, can be necessary to

characterize the target rock and used samples when this information is not yetavailable.

5.3. METEORITEIDENTIFICATION

Based on the impact identification, a differentiated achondrite can be excluded as

impact meteorite, because this would not result into such distinctive meteorite

contributions. No data exists on primitive achondrites in the used databases,

probably because these are very rare. The highly siderophile elements show a

relative flat CI‐normalisedpattern (Figure21),which is confirmedwhen including

the Os values from the Os isotope system analysis and moderately siderophile

elements(Figure24).BasedontheNi/Cr(2.93±0.16)andCo/Cr(0.10±0.01)ratios

ofthemeltlenssamples(n=5)aLLordinarychondrite(2.64±0.21Ni/Crand0.13

± 0.02 Co/Cr) is suggested. This is in agreement with the previous meteorite

identificationofGrieve[1978]asaL(2.76±0.36Ni/Cr)andPalmeetal.[1981]asa

L or LL (2.56 ± 0.26 Ni/Cr). Both fall closer to the LL if compared with recentdatabases(seeAppendixB).

Figure 2normalisRudnickfromTag

Howev

range(

ratios,

canbe

LLbyc

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for the

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(29.85 ±

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PAGE69

used forCC) fromoritesare.

hondrite

dCo/Cr

NiorCo

nttothe

for the

entative

± 0.14)

donot.

e,which

ftheLL,

pattern

usionof

[1993],

stionof

fractionation or alteration would explain the flat CI‐normalised pattern, but wealreadyexcludedthesepossibilities.

Table12.ComparisonwiththeobtainedsiderophileratiosfortheBrentimpactstructure(X/Y),andtheIAandIIICnon‐magmaticironmeteorites.TheNi/CrandNi/IrvaluesarefromTagle[2004].

Y X X/Y IA IIICPd Ir 1.22 1.52 3.02Rh Ir 0.46 0.60 1.17Pt Ir 2.65 2.58 3.25Ru Ir 2.31 2.05 3.03Pd Rh 2.56 2.57 2.59Pt Rh 5.72 4.36 2.79Ru Rh 5.00 3.47 2.60Pt Pd 2.17 1.70 1.08Ru Pd 1.90 1.35 1.00Pt Ru 1.14 1.26 1.07Ni Cr 2.93 3.07 2.73Ni Ir 32880 31000 119000

When the CI‐normalised PGE patterns are plotted on a linear scale instead of a

logarithmic,andmagnifiedbyafactorof500,aspikedpatternappears(Figure25).

BecauseoftheextremelowtargetrockPGEcontents,theresultingpatternisalmost

notaffectedbythetargetrock.ThisisshowninFigure25forsample59‐2800.This

meansthepatternishighlycharacteristic fortheimpactmeteorite.Ascanbeseen,

theflatpatternoftheLLdoesnotreflectatallthatofthemeltlenssamples.Onlythe

twonon‐magmaticironmeteoritegroupsIAandIIICcanfitthePGEpatternsofthe

melt samples, while the closely related IB do not. When these two groups are

compared to the obtained meteorite discriminating data, almost all ratios do

significantlybetterfittheIA(Table12).WethereforeconcludethattheBrentimpactstructureisproducedbytheimpactofaIAnon‐magmaticironmeteorite.

5.4. NON‐MAGMATICIRONMETEORITES

[Goderis,2009andreferencestherein]

Thenon‐magmaticiron(NMI)meteoritesaredividedintotheIA,IB,IIIC,IIID,andIIE

groups.Theyare, as the stony‐irons, composedof an ironanda silicatephase, the

latteroccurringessentiallyaslargeorsmallinclusionsdispersedinthemetalphase.

Silicate inclusions occur in NMI, which can vary significantly between different

meteorites, as well as between different sections of the same meteorite. They

underwent a complex and poorly understood formation history (including

metamorphism,partialmelting, incompletedifferentiationandcrystal segregation)

that is reflected in the high variability of the amount of silicate inclusions. After a

catastrophic impactbreak‐upin theasteroidbelt,gravitationalreassemblingof the

debris probably produced a heterogeneous mixture of 5 different components:

metal, a sulphide‐richphase, a chondritic‐silicatephase,partialmelt, and residues.

AllNMIshowmixedfractionsofthesecomponents.

Atpresent,fourotherimpactstructuresareidentifiedasbeingproducedbyaNMI:Lockne,Rochechouart,SääksjärviandGardnos.

6. CONCLUSION

Because all analysis in this master dissertation, as well from this study as from

literature, were conducted on melt‐bearing impact rocks from a breccia lens, all

following conclusions apply only to such impact rocks and consequently have no

implicationsforimpactandmeteoriteidentificationwithejectarocks.

6.1. THEBRENTIMPACTSTRUCTURE

CharacterizationofthealterationpresentattheBrentimpactstructure,revealedat

leastthepresenceofachloritization,Au‐depletionandK‐enrichmentprocessinthe

melt‐fragment breccias. While this totally scattered the mobile Ni, Co and Cr, the

immobilePGEarenotaffected.Alsothemeltlensshowsalteration,albeittoalesser

degree.Ni,CoandCrstillhaveastrongcorrelationhere.TheuseoftheOsisotope

systemdeliveredameteoritecontributionupto1a2wt.%forthedeepestmeltlens

sample. Together with the moderately and siderophile elements, it rejects the

possibilityofafractionationduringpost‐impactcooling,assuggestedbyEvansetal.

[1993],andimplicatesanimpactidentification.

We also reject the assumption of Grieve [1978] and Palme et al. [1981] that no

(ultra)mafic components are present. However, if present, they can only be

responsible for (apart of) theNi, Co andCr scatter in themelt‐fragmentbreccias.

Themeltlensisforcertainnotaffected.Amoreregionaltargetrockstudyshouldbe

conductedtoexaminethepossibilityof(ultra)maficcomponentsandtheireffectonthemelt‐fragmentbreccias.

Based on a multi‐signature approach by combining the moderately and highly

siderophile elements, a precise meteorite classification into the IA non‐magmatic

ironsispossible.WhiletheNi/Cr,Co/CrandPd/IrratiospointtoaLLorLordinary

chondrite,theNi/Co,otherPGEandcombinedsiderophileratiosdonot.Basedona

linearandmagnifiedPGEpatternthatisassumedtoberepresentativefortheimpact

meteorite, the IA or IIIC non‐magmatic irons are the only possibility. When allsiderophileratiosaretakenintoaccount,theIAisbyfarthebestfittinggroup.

6.2. IMPLICATIONSFORIDENTIFICATION

The Brent impact structure is the ideal case study for the effects of alteration on

impact and meteorite identification, especially for melt‐fragment breccias which

showconsistentPGEvalues,butscatterforthemoderatesiderophileelements.Due

to its resistance against alteration and high meteorite contribution, the melt lensprovidesthebestsamplesforimpactandmeteoriteidentification.

For meteorite identification, the implications of the Brent impact structure are

straight forwarded, no correct meteorite identification would have been possible

withouttheusedmulti‐signatureapproach.WhileNi,CoandCrarerelativelyeasyto

analyse,thechondriticvaluesoftheirratiosaresharedbymultipleirongroups.On

theotherhand, theymake itpossible todiscriminate furtherwhena chondriticor

ironmeteoriteisdeterminedbythePGE.

The Os isotope system seems sufficient on its own for impact identification.

However, its analysis is relative difficult compared to the moderately siderophile

elements. When the moderately and highly siderophile, and major elements are

added, this is the best way to describe all interfering processes like alteration,

(ultra)maficcomponentsandfractionation.

BecausethePbisotopesystemisscatteredduetoalteration,otherimpactstructures

than Brent should be examined for Pb isotopes to develop its use for impact and

meteoriteidentification.Inaddition,alsothecommonCr,andproposedW,NdandSr

isotope systems could be analysed and compared with the now well establishedimpactandmeteoriteidentificationdataoftheBrentimpactstructure.

A general conclusion for impact andmeteorite identification is: themoredata, thebettertheresults.Amulti‐signatureapproachisalwaysrecommended.

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V. REFERENCES

Alvarez,L.W.,Alvarez,W.,Asaro,F.&Michel,H.V.[1980]. Extraterrestrial Cause for theCretaceous‐Tertiary Extinction. Science 208,1095‐1108.

Ames,D.E.&Farrow,C.E.G. [2007].Metallogenyof the Sudbury mining camp, Ontario. In:Goodfellow, W. D. (ed.), Mineral Deposits ofCanada: A Synthesis of Major Deposit‐Types,DistrictMetallogeny,theEvolutionofGeologicalProvinces,andExplorationMethods.GeologicalAssociation of Canada, Mineral DepositsDivision,SpecialPublication5,329‐350.

Ames,D.E.,Gibson,H.L.&Watkinson,D.H.[2000].Controls on major impact‐inducedhydrothermal system, Sudbury structure,Canada. Lunar and Planetary Science XXXI,1873.

Anders,E.,Ganapathy,R.,KriihenbiihlU.&MorganJ.W. [1973].Meteoriticmaterial on theMoon.TheMoon8,3‐24.

Asher,D. J.,Bailey,M.,Emel’yanenko,V.&Napier,B. [2005]. Earth in the cosmic gallery.Observatory125,319‐322.

Asphaug, E. [2009]. Growth and Evolution ofAsteroids. Annual Review of Earth andPlanetarySciences37,413‐448.

Baldwin, R. B. [1949]. The Face of the Moon.UniversityofChicagoPress,239pp.

Beals, C. S. [1958]. Fossil meteorite craters.ScientificAmerican199,32‐39.

Beals,C.S.,Ferguson,G.M.&Landau,A.[1956].Asearch for analogies between lunar andterrestrial topography on photographs of theCanadianshield:PartIandPartII.JournaloftheRoyalAstronomical SocietyofCanada50,203‐211and250‐261.

Bennet, J., Donahue, M., Schneider, N. & Voit, M.[2008]. The Cosmic Perspective, 5th edition.PearsonAddison‐Wesley,864pp.

BIPM [2006]. The International System of Units(SI), 8th edition. International Bureau ofWeightsandMeasures,180pp.

Bjork,R.L.[1963].ReviewofPhysicalProcessesinHypervelocity Impact and Penetration. RANDCorporation,51pp.

Bland, P. A. [2005]. The impact rate on Earth.PhilosophicalTransactionsoftheRoyalSocietyA363,2793‐2810.

Bland,P.A.&Artemieva,N.A. [2006].TherateofsmallimpactsonEarth.Meteoritics&Planetary

Science41,607‐631.Bottke,W.F.&Morbidelli,A. [2006].Theasteroid

and comet impact flux in the terrestrial planetregion: a brief history of the last 4.6 Gy.Planetary Chronology Workshop 2006, LunarandPlanetaryInstitute,2pp.

Brownlee, D. E. [2007]. Comets. In: Davis, A. M.(ed.), Treatise on Geochemistry, vol. 1:Meteorites,CometsandPlanets,Onlineedition.Elsevier,29pp.

Bucher, W. [1963]. Cryptoexplosion structurescausedfromwithoutorfromwithintheEarth?(“Astroblemes” or “Geoblemes”?). AmericanJournalofScience261,597‐649.

Buchwald, V. F. [1975] Handbook of IronMeteorites.UniversityofCaliforniaPress,1418pp.

Chambers,J.E.[2007].PlanetFormation.In:Davis,A.M. (ed.), Treatise on Geochemistry, vol. 1:Meteorites,CometsandPlanets,Onlineedition.Elsevier,17pp.

Chyba, C. F., Owen, T.C.& Ip,W.H. [1994]. Impactdelivery of volatiles and organic molecules toEarth. In: Gehrels, T. (ed.), Hazards Due toComets and Asteroids, 9‐58. University ofArizonaPress,1300pp.

Cohen, D. [2008]. The day the sky exploded. NewScientist28,38‐41.

Currie,K.L. [1971].A studyofpotash fenitizationaround the Brent Crater, Ontario, a Paleozoicalkaline complex. Canadian Journal of EarthSciences8,481‐497.

Currie, K. L. & Shafiqullah,M. [1967]. Carbonatiteandalkaline igneous rocks in theBrentCrater,Ontario.Nature215,725‐726.

Delsanti, A. & Jewitt, D. [2007]. The Solar SystemBeyondThePlanets.InstituteforAstronomy,27pp.

Dence, M. R. [1971]. Impact melts. Journal ofGeophysicalResearch76,5552‐5565.

Dence, M. R. [1968]. Shock zoning at Canadiancraters: Petrography and structuralimplications. In: French, B. M. & Short, N. M.(eds.), Shock Metamorphism of NaturalMaterials, 339‐362. Mono Book Corporation,644pp.

Dence,M.R.[1972]Thenatureandsignificanceofterrestrialimpactstructures.Proceedingsofthe24th InternationalGeologicalCongress15,77–89.

PAGEXI

Dence,M.R.[1973].Dimensionalanalysisofimpactstructures.Meteoritics8,343‐344.

Dence, M. R. [2004]. Structural evidence fromshock metamorphism in simple and compleximpactcraters:Linkingobservations to theory.Meteoritics&PlanetaryScience39,267‐286.

Dence, M. R. & Guy‐Bray, J.V. [1972]. Someastroblemes, craters and cryptovo1canicstructures inOntario andQuebec. Proceedingsof the InternationalGeologicalCongress24,61pp.

Dence, M. R., Grieve, R. A. F. & Robertson, P. B.[1976].Terrestrial impact structures:Principalcharacteristics and energy considerations. In:Roddy,D. J., Pepin,R.O.&Merrill, R. B. (eds.),Impact and Explosion Cratering, 247‐276.PergamonPress,1315pp.

Dietz, R. S. [1963]. Cryptoexplosion structures: Adiscussion. American Journal of Science 261,650‐664.

Drake, M. J. [2001]. The eucrite/Vesta story.Meteoritics&PlanetaryScience36,501‐513.

Dressler, B. O. & Sharpton, V. L. [1997]. Brecciaformation at a complex impact crater: SlateIslands, Lake Superior, Ontario, Canada.Tectonophysics275,285‐311.

Evans, N. J., Gregoire, D .C., Grieve, R. A. F.,Goodfellow, W. D. & Veizer, J. [1993]. Use OfPlatinum‐Group Elements For ImpactorIdentification ‐ Terrestrial Impact Craters AndCretaceous‐Tertiary Boundary. Geochimica etCosmochimicaActa57,3737‐3748.

Faure,G.[1986]PrinciplesofIsotopeGeology,2ndedition.Wiley,608pp.

Fischer‐Gödde, M., Becker, H. & Wombacher, F.[2010]. Rhodium, gold and other highlysiderophile element abundances in chondriticmeteorites. Geochimica et Cosmochimica Acta74,356‐379.

Fiske,P.S.,Hargraves,R.B.,Onstott,T.C.,Koeberl,C. & Hougen, S. B. [1994]. Pseudotachylites oftheBeaverhead impactstructure:Geochemical,geochronological, petrographic, and fieldinvestigations.In:Dressler,B.O,Grieve,R.A.F.,& Sharpton, V. L. (eds.), Large MeteoriteImpacts and Planetary Evolution, 163‐176.GeologicalSocietyofAmerica,348pp.

French, B. M. [1968]. Shock metamorphism as ageological process. In: French, B.M. & Short,N.M. (eds.), Shock Metamorphism of NaturalMaterials, 1‐17. Mono Book Corporation, 644pp.

French, B. M. [1990]. 25 years of the impact‐volcanic controversy: Is there anything newunder the sun or inside the Earth? Eos,Transactions of the American GeophysicalUnion71,411‐414.

French, B. M. [1998]. Traces of Catastrophe: AHandbook of Shock‐Metamorphic Effects in

Terrestrial Meteorite Impact Structures. LunarandPlanetaryInstitute,120pp.

French,B.M.&Koeberl,C. [2010].Theconvincingidentification of terrestrial meteorite impactstructures:Whatworks,whatdoesn’tandwhy.EarthScienceReviews98,123‐170.

Ganapathy, R., Keays R. R., Laul J. C. & Anders, E.[1970a]. Trace elements in Apollo 11 lunarrocks: implications for meteorite influx andorigin of moon. Geochimica et CosmochimicaActaSupplement1,1117‐1142.

Ganapathy, R., Keays R. R. & Anders E. [1970b].Apollo 12 lunar samples: Trace elementanalysis of a core and the uniformity of theregolith.Science170,533‐535.

Ganapathy,R.,LaulJ.C.,Morgan, J.W.&AndersE.[1972].Moon:Possiblenatureof thebodythatproduced the Imbrian basin, from thecompositionofApollo14samples.Science175,55‐59.

Ganapathy, R., Morgan, J. W., Krahenbiihl U. &Anders, E. [1973]. Ancient meteoriticcomponentsinlunarhighlandrocks:Cluesfromtrace elements in Apollo 15 and 16 samples.ProceedingsoftheLunarScienceConference4,1239‐1261.

Ganapathy,R.,Morgan,J.W.,Higuchi,H.,Anders,E.&Anderson,A.T.[1974].Meteoriticandvolatileelements in Apollo 16 rocks and in separatedphases from 14306. Proceedings of the LunarScienceConference5,1659‐1683.

Garvin,J.B.&Grieve,R.A.F.[1982].Ananalyticalmodel for terrestrial simple craters:Brent andMeteor. Lunar and Planetary Science 13, 251‐252.

Gault,D.E.&Heitowit,E.D.[1963].ThePartitionofEnergy for Hypervelocity Impact CratersFormed in Rock. Proceedings of theHypervelocityImpactSymposium6,419‐456.

Gault,D.E.,Quaide,W.L.&Oberbeck,V.R.[1968].ImpactCrateringMechanicsandStructures. In:French, B. M. & Short, N. M. (eds.), ShockMetamorphism of Natural Materials, 87‐99.MonoBookCorporation,644pp.

Gifford, A. C. [1924]. TheMountains of theMoon.NewZealandJournalofScienceandTechnology7,129‐142.

Gilbert, G.K. [1893]. TheMoon’s Face:A Studyofthe Origin of its Features. Bulletin of thePhilosophical Society of Washington 12, 241‐292.

Glass,B.P.,Lu,S.&Leavens,P.B. [2002].Reidite:an impact‐produced high‐pressure polymorphofzirconfoundinmarinesediments.AmericanMineralogist87,562‐565.

Goderis,S.[2006].Geochemieendistributievandeplatinagroepelementen in de impactstructurenvanBosumtwi(Ghana,Pleistoceen)enGardnos(Noorwegen, grens Proterozoïcum‐

PAGEXII

Paleozoïcum).UniversiteitGent,103pp.Goderis, S., Tagle, R., Schmitt, R. T., Erzinger, J. &

Claeys, P. [2007] Platinum group elementsprovide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwicrater,Ghana.MeteoriticsandPlanetaryScience42,731‐741.

Goderis, S., Kalleson, E., Tagle, R., Dypvik, H.,Schmitt, R., Erzinger, J. & Claeys, P. [2009]. Anon‐magmatic iron projectile for the Gardnosimpactevent.ChemicalGeology258,145‐156.

Gounelle,M.,Morbidelli,A.,Bland,P.A.,Spurný,P.,Young, E. D. & Sephton, M. [2008]. MeteoritesfromtheOuterSolarSystem?In:Barucci,M.A.,Boehnhardt,H., Cruikshank,D.P.&Morbidelli,A. (eds.), The Solar System beyond Neptune,525‐541.ArizonaUniversityPress,592pp.

Grahn, Y. & Ormö, J. [1995]. Microfossil dating ofthe Brent meteorite crater, southeast Ontario,Canada. Revue deMicropaléontologie 38, 131‐137.

Grieve, R. A. F. [1978]. The melt rocks at Brentcrater, Ontario, Canada. Proceedings of theLunar and Planetary Science Conference 9,2579‐2608.

Grieve,R.A.F.[1984].TheImpactCrateringRateinRecent Time. Journal of Geophysical Research89,403‐408.

Grieve, R. A. F. [1987]. Terrestrial impactstructures. Annual Reviews of Earth andPlanetaryScience15,245‐270.

Grieve, R. A. F. [1991]. Terrestrial impact: Therecordintherocks.Meteoritics26,175‐194.

Grieve,R.A. F. [2005].Economicnatural resourcedeposits at terrestrial impact structures. In:McDonald,I.,Boyce,A.J.,Butler,I.B.,Herrington,R.J. & Polya, D.A. (eds.) Mineral deposits andearth evolution, Geological Society of LondonSpecialPublication248,1‐29.GeologicalSocietyofLondon,263pp.

Grieve,R.A.F.&Cintala,M.J.[1981].BrentCrater,Ontario: Observation and theory. Lunar andPlanetaryScience12,362‐364.

Grieve,R.A.F.&Cintala,M.J.[1982].Amethodforestimating the initial impact conditions ofterrestrial cratering events, exemplified by itsapplication to Brent crater, Ontario.ProceedingsoftheLunarandPlanetaryScienceConference.12B,1607‐1621.

Grieve, R. A. F. & Dence, M.R. [1978]. PrinciplecharacteristicsoftheimpactitesatBrentcrater,Ontario,Canada.LunarandPlanetaryScience9,416‐418.

Grieve, R. A. F. & Masaitis, V.L. [1994]. Theeconomicpotentialofterrestrialimpactcraters.InternationalGeologicalReview43,105‐151.

Grieve, R. A. F. & Pilkington, M. [1996]. Thesignature of terrestrial impacts. Journal ofAustralian Geology and Geophysics 16, 399‐

420.Grieve, R. A. F. & Shoemaker, E.M. [1994]. The

recordofpast impactsonEarth. In:Gehrels,T.(ed.), Hazards Due to Comets and Asteroids,417‐462.UniversityofArizonaPress,1300pp.

Grieve, R. A. F., Dence, M. R. & Robertson, P. B.[1977]. Cratering processes: As interpretedfromtheoccurrenceofimpactmelts.In:Roddy,D. J., Pepin,R.O.&Merrill, R. B. (eds.), Impactand Explosion Cratering, 791‐814. Pergamon,1315pp.

Grunert,N.[2000].NamibiaFascinationofGeology.KlausHessPublishers,17pp.

Haack,H.&McCoy,T.J.[2005].IronandStony‐ironMeteorites. In: Davis, A. M. (ed.), Treatise onGeochemistry, vol. 1: Meteorites, Comets andPlanets,325‐346.Elsevier,737pp.

Halliday, A. N. [2004]. Mixing, volatile loss andcompositional change during impact‐drivenaccretionoftheEarth.Nature427,505‐509.

Hartung, J. B., Dence,M. R. & Adams, J. A. [1971].Potassium‐argon dating of shockmetamorphosed rocks from the Brent impactcrater, Ontario, Canada. Journal of GeophysicalResearch76,5437‐5448.

Hawke,P.J.[2004].Thegeophysicalsignaturesandexploration potential of Australia's meteoriteimpact structures. University of WesternAustralia,314pp.

Higuchi, H. & Morgan, J. W. [1975]. Ancientmeteoritic component in Apollo 17 boulders.ProceedingsoftheLunarScienceConference6,1625‐1651.

Hildebrand, A. R., Penfield, G. T., Kring, D. A.,Pilkington, M., Camargo, A., Jacobsen, S. B. &Boynton, W. V. [1991] Chicxulub Crater: APossible Cretaceous/TertiaryBoundary ImpactCrater on the Yucatan Peninsula, Mexico.Geology19,867‐871.

Hockey, T.A. [1994]. The Shoemaker‐Levy9 Spotson Jupiter:TheirPlace inHistory.Earth,MoonandPlanets66,1‐9.

Horneck,G.&Rettberg,P.[2007].CompleteCourseinAstrobiology.Wiley,434pp.

Howard, E. C. [1802]. Experiments andobservations on certain stony substances,whichatvarioustimesaresaidtohavefallenonthe earth; alsoonvariouskindsofnative iron.PhilosophicalTransactions92,168‐212.

Hughes, D. W. [2000]. A new approach to thecalculation of the cratering rate of the Earthover the last 125±20 Myr. Monthly Notices oftheRoyalAstronomicalSociety317,429‐437.

IAU [1961]. GeneralAssemblyXI, Commission22.InternationalAstronomicalUnion,38pp.

IAU [2006]. Definition of a Planet in the SolarSystem, Resolution B5. InternationalAstronomicalUnion,2pp.

Innes, M. J. S. & Beals, C. S. [1961]. Profile of the

PAGEXIII

fossil crater at Brent, Ontario. Journal of theRoyalAstronomicalSocietyofCanada55,258.

IUPAC [2007]. Quantities, Units and Symbols inPhysicalChemistry‐theIUPACGreenBook,3thedition.InternationalUnionofPureandAppliedChemistry,248pp.

ISO[2004].Dataelementsandinterchangeformats– Information interchange – Representation ofdates and times, 3th edition. InternationalOrganizationforStandardization,40pp.

Ives, H. E. [1919]. Some Large‐Scale ExperimentsImitatingtheCratersoftheMoon.AstrophysicalJournal50,245‐250.

Jackson,J.C.,HortonJr., J.W.,Chou,I.M.&Belkin,H. E. [2006]. A shock‐induced polymorph ofanatase and rutile from the Chesapeake Bayimpact structure, Virginia, U.S.A. AmericanMineralogist91,604‐608.

Koeberl, C. [1998]. Identification of meteoriticcomponent in impactites. In: Grady, M. M.,Hutchinson,R.,McCall,G. J.H.&Rothery,R.A.(eds.), Meteorites: Flux with Time and ImpactEffects,133‐153.TheGeologicalSocietySpecialPublications140,278pp.

Koeberl, C. [2001]. Craters on the moon fromGalileo to Wegener: A short history of theimpact hypothesis, and implications for thestudyofterrestrial impactcraters.Earth,MoonandPlanets85‐86,209‐224.

Koeberl, C., [2007]. The geochemistry andcosmochemistry of impacts. In: Davis, A. M.(ed.), Treatise on Geochemistry, vol. 1:Meteorites,CometsandPlanets,Onlineedition.Elsevier,52pp.

Koeberl, C. & Shirey S. B. [1993]. Detection ofmeteoritic component in Ivory Coast tektiteswith Rhenium‐Osmium isotopes, Science, 261,595‐598.

Koeberl, C., Shukolyukov, A. & Lugmair, G.W.[2007].Chromiumisotopicstudiesofterrestrialimpact craters: Identification of meteoriticcomponents at Bosumtwi, Clearwater East,Lappajärvi, and Rochechouart: Earth andPlanetaryScienceLetters256,534‐546.

Kortenkamp,S.J.&Dermott, S.F. [1998].Accretionof Interplanetary Dust Particles by the Earth.Icarus135,469‐495.

Krot, A. N., Keil, K. & Scott, E. R. D. [2007].Classification of Meteorites. In: Davis, A. M.(ed.), Treatise on Geochemistry, vol. 1:Meteorites,CometsandPlanets,Onlineedition.Elsevier,52pp.

Levison,H.F.,Dones,L.&Duncan,M.J.[2001].TheoriginofHalley‐typecomets:ProbingtheinnerOort Cloud. The Astronomical Journal 121,2253‐2267.

Lindström, M., Sturkell, E. F. F., Törnberg, R. &Ormö, J. [1996]. The marine impact crater atLockne, central Sweden. Journal of the

GeologicalSocietyofSweden118,193‐206.Lindström, M., Shuvalov, V. & Ivanov, B. [2005].

Lockne crater as a result of marine‐targetobliqueimpact.PlanetaryandSpaceScience53,803‐815.

Lodders, K. [2003]. Solar system abundances andcondensation temperatures of the elements:TheAstrophysicalJournal591,1220‐1247.

Lorand, J.,Luguet,A.&Alard,O. [2008].Platinum‐groupelements:anewsetofkeytracersfortheEarth’sinterior.Elements4,247‐252.

Lozej, G. P. & Beales, F. W. [1975]. TheunmetamorphosedsedimentaryfilloftheBrentmeteorite crater, south‐eastern Ontario.CanadianJournalofEarthSciences12,606‐628.

Masaitis, V. L. [1989]. The economic geology ofimpact craters. International Geology Review31,922‐933.

McDonald, I. [2002] Clearwater East impactstructure: A re‐interpretation of the projectiletype using new platinum‐group element data.Meteoritics&PlanetaryScience37,459‐464.

McDonald, I., Andreoli, M. A. G., Hart, R.J. &Tredoux, M. [2001]. Platinum‐group elementsin the Morokweng impact structure, SouthAfrica: Evidence for the impact of a largeordinary chondrite projectile at the Jurassic‐Cretaceous boundary. Geochimica etCosmochimicaActa65,299‐309.

McDonough, W. F. & Sun, S. [1995]. Thecomposition of the Earth. Chemical Geology120,223‐254.

McKeegan,K.D.&Davis,A.M. [2005].EarlySolarSystem Chronology. In: Davis, A.M. (ed.),Treatise on Geochemistry, vol. 1: Meteorites,CometsandPlanets,431‐460.Elsevier,737pp.

McSween, H. Y. Jr. [1999]. Meteorites and TheirParent Planets, 2nd edition. CambridgeUniversityPress,310pp.

Melosh H. J. [1989]. Impact Cratering: A GeologicProcess.OxfordUniversityPress,245pp.

Melosh,H. J.& Ivanov,B.A. [1999]. Impact cratercollapse.AnnualReviewofEarthandPlanetarySciences27,385‐415.

Millman,P.M.,Liberty,B.A.,Clark, J.F.,Willmore,P. L. & Innes,M .J. S [1960]. The Brent crater.PublicationsoftheDominionObservatory24,1‐43.

Mittlefehldt,D.W.[2005].Achondrites.In:Davis,A.M. (ed.), Treatise on Geochemistry, vol. 1:Meteorites, Comets and Planets, 291‐324.Elsevier,737pp.

Mittlefehldt, D. W., McCoy, T., Goodrich, C. A. &Kracher, A. [1998]. Nonchondritic meteoritesfromtheasteroidalbodies.In:Papike,J.J.(ed.),Planetary materials, 4.1‐4.195. MineralogicalSocietyofAmerica,1059pp.

Morbidelli, A. [2008]. Origin and dynamicalevolution of comets and their reservoirs.

PAGEXIV

ObservatoiredelaCôted’Azur,86pp.Morbidelli,A.,Bottke,W.F.,Froeschlé,C.&Michel,

P. [2002]. Origin and Evolution of Near‐EarthObjects.In:Bottke,W.F.,Paolicchi,P.,Binzel,R.P. & Cellino, A. (eds.), Asteroids III, 409‐422.UniversityofArizonaPress,785pp.

Morgan,J.W.,Laul,J.C.,Kriihenbiihl,U.,GanapathyR. & Anders, E. [1972]. Major impacts on theMoon:Characterization from trace elements inApollo 12 and 14 samples. Proceedings of theLunarScienceConference3,1375‐1395.

Morgan, J.W.,Ganapathy,R.,Higuchi,H.&AndersE. [1974a].Proceedingsof theSoviet‐AmericanConference on Cosmochemistry of the Moonand Planets. The Lunar Science Institute, 764pp.

Morgan, J. W., Ganapathy, R., Higuchi, H.,Krahenbiihl, U. & Anders, E. [1974b]. Lunarbasins:Tentativecharacterizationofprojectiles,frommeteoriticelementsinApollo17boulders.ProceedingsoftheLunarScienceConference5,1703‐1737.

Morgan,J.W.,Higuchi,H.,Ganapathy,R.&Anders,E.[1975].Meteoriticmaterialinfourterrestrialmeteorite craters. Proceedings of the LunarScienceConference6,1609‐1623.

Morrison, D., Chapman, C. R. & Slovic, P. [1994].Theimpacthazard.In:Gehrels,T.(ed.),HazardsDuetoCometsandAsteroids,59‐91.UniversityofArizonaPress,1300pp.

Morrison,D.,Chapman,C.R.,Steel,D.&Binzel,R.P.[2004].ImpactsandthePublic:CommunicatingtheNatureoftheImpactHazard.In:Belton,M.J.S.,Morgan,T.H.,Samarasinha,N.H.&Yeomans,D.K.(eds.),MitigationofHazardousCometsandAsteroids, 353‐390. Cambridge UniversityPress,436pp.

MPC[2010].ListofNeptuneTrojans.MinorPlanetCenter.WWW(2010‐11‐16):

http://www.minorplanetcenter.org/iau/lists/NeptuneTrojans/

MS [2010]. Meteoritical Bulletin Database,MeteoriticalSociety.WWW(2010‐10‐13):

http://tin.er.usgs.gov/meteorNicolaysen, L. O. & Reimold, W. U. [1990].

Cryptoexplosions and catastrophes in thegeological record, with a special focus on theVredefortstructure.Tectonophysics171,1‐422.

O’Keefe, J. D. & Ahrens, T. J. [1993]. PlanetaryCratering Mechanics. Journal of GeophysicalResearch98,17.011‐17.028.

Öpik, E. J. [1916]. Remarque sur le théoriemétéoriquedes cirques lunaires. Bulletin de laSociétéRussedesAmisde l’Etudede l’Univers3.21,125‐134.

Osinski,G.R. [2008].Meteorite impact structures:thegoodandthebad.GeologyToday24,13‐19.

Osinski,G.R., Spray, J. G.&Lee,P. [2001]. Impactinduced hydrothermal activity within the

Haughton impact structure, artic Canada:Generation of a transient, warm, wet oasis.MeteoriticsandPlanetaryScience36,731‐745.

Palme,H. [1980].Themeteoriticcontaminationofterrestrial and lunar impact melts and theproblemofindigenoussiderophilesinthelunarhighland. Proceedings of the Lunar andPlanetaryScienceConference11,p481‐506.

Palme,H., Janssens,M. J.,Takahashi,H.,Anders,E.&Hertogen,J.[1978].Meteoriticmaterialatfivelarge impact craters. Geochimica etCosmochimicaActa42,313‐323.

Palme, H., Grieve, R. A. F. & Wolf, R. [1981].Identification of the projectile at the Brentcrater and further considerations of projectiletypes at terrestrial craters. Geochimica etCosmochimicaActa45,2417‐2424.

PASSC [2010]. Earth Impact Database. PlanetaryandSpaceScienceCentre.

http://www.passc.net/EarthImpactDatabase/Peucker‐Ehrenbrink, B. & Jahn, B. M. [2001].

Rhenium‐osmium isotope systematics andplatinum group element concentrations: Loessand the upper continental crust. GeochemistryGeophysicsGeosystems4,8911.

Phillips,G.N.&Law,J.D.M.[2000].WitwatersrandGoldfields: Geology, Genesis and Exploration.Societyof EconomicGeologyReviews13, 439‐500.

Plessen,H.G.,Erzinger,J.[1998].Determinationofthe platinum‐group elements and gold intwenty rock reference material by inductivelycoupled plasma‐mass spectrometry (ICPMS)after pre‐concentration by nickel fire assay.GeostandardsNewsletter22,187‐194.

Proctor, R. A. [1873]. The Moon: Her Motions,Aspect, Scenery, andPhysicalCondition.AlfredBrothers,314pp.

Reimold,W.U.&Koeberl,C. [2008].Catastrophes,Extinctions and Evolution: 50 Years of ImpactCrateringStudies.GoldenJubileeMemoiroftheGeologicalSocietyofIndia66,69‐110.

Robertson P. B. & Grieve R. A. F. [1977]. Shockattenuationat terrestrial impact structures. In:Roddy,D.L.,Pepin,R.O.&Merrill,R.B. (eds.),Impact and Explosion Cratering, 687‐706.Pergamon,1315pp.

Rudnick,R.L.&Gao,S.[2003].Compositionofthecontinental crust. In: Holland, H. D. (ed.),Treatise onGeochemistry, vol. 3: TheCrust, 1‐64.Elsevier‐Pergamon,702pp.

Ryan,D.[2003].WWW(2010‐10‐13): http://www.physics.mcgill.ca/~dominic/updat

e03/Flying/Schmieder,M.&Buchner,E. [2010]. Possible iron

meteoritic contamination in impact meltparticles from the Steinheim basin (Baden‐Württemberg, Germany). Lunar and PlanetaryScienceConference41,2103.

PAGEXV

Schneider, G. [2004]. The Roadside Geology ofNamibia.GebruderBorntraeger,294pp.

Schulte,P.,Alegret,L.,Arenillas,I.,Arz,J.A.,Barton,P.J.,Bown,P.R.,Bralower,T.J.,Christeson,G.L.,Claeys,P.,Cockell,C.S.,Collins,G.S.,Deutsch,A.,Goldin, T. J., Goto, K., Grajales‐Nishimura, J.M.,Grieve, R. A. F., Gulick, S. P. S., Johnson, K. R.,Kiessling,W.,Koeberl,C.,Kring,D.A.,MacLeod,K. G., Matsui, T., Melosh, J., Montanari, A.,Morgan,J.V.,Neal,C.R.,Nichols,D.J.,Norris,R.D., Pierazzo, E. , Ravizza, G., Rebolledo‐Vieyra,M.,Reimold,W.U.,Robin,E. , Salge,T.,Speijer,R.P.,Sweet,A.R.,Urrutia‐Fucugauchi, J.,Vajda,V.,Whalen,M.T.&Willumsen,P.S.[2010].TheChicxulubAsteroidImpactandMassExtinctionat theCretaceous‐PaleogeneBoundary.Science327,1214‐1218.

Scott,E.R.D.&Krot,A.N. [2005].Chondritesandtheircomponents.In:Davis,A.M.(ed.),TreatiseonGeochemistry,vol.1:Meteorites,CometsandPlanets,143‐200.Elsevier,737pp.

Sharpton,V.L.&GrieveR.A. F. [1990].Meteoriteimpact, cryptoexplosion, and shockmetamorphism: A perspective on the evidenceattheK/Tboundary.In:Sharpton,V.L.&Ward,P. D. (eds.), Global Catastrophes in EarthHistory: An Interdisciplinary Conference onImpacts, Volcanism, and Mass Mortality, 301‐318.Geological.SocietyofAmerica,631pp.

Shelkov, D. A., Verchowsky, A. B., Milledge, H. J.,Kaminsky,F.V.&Pillinger,C.T.[1998].C,N,ArandHestudyofimpactdiamondsfromEbeliakhalluvialdepositsandPopigaicrater.MeteoriticsandPlanetaryScience33,985‐992.

Sheppard, S. S. & Trujillo, C. A. [2006]. A ThickCloud of Neptune Trojans and Their Colors.Science313,511‐514.

Steel, D. [2008]. Tunguska at 100. Nature 453,1157‐1159.

Steiger,R.H.&Jäger,E.[1977].SubcommissiononGeochronology:Conventionontheuseofdecayconstants in geo‐ and cosmochronology. EarthandPlanetaryScienceLetters36,359‐362.

Stern, S. A. & Weissman, P. R. [2001]. Rapidcollisional evolution of comets during theformation of the Oort cloud. Nature 409, 589‐591.

Stöffler, D. & Langenhorst, F. [1994]. Shockmetamorphism of quartz in nature andexperiment: I. Basic observation and theory.Meteoritics29,155‐181.

Stokes, G.H., Yeomans,D. K., Bottke,W. F., Jewitt,D., Chesley, S. R., Kelso, T. S., Evans, J. B.,McMillan,R.S.,Gold,R.E.,Spahr,T.B.,Harris,A.W.&Worden,S.P. [2003].StudytoDeterminetheFeasibilityofExtendingtheSearchforNear‐Earth Objects to Smaller Limiting Diameters.NASAOfficeofSpaceScience,154pp.

Tagle, R. [2004]. Platingruppenelemente in

Meteoriten und Gesteinen irdischerImpaktkrater: Identifizierung derEinschlagskörper. Humboldt‐Universität zuBerlin,173pp.

Tagle, R. & Berlin, L. [2008]. A database ofchondrite analyses including platinum groupelements,Ni,Co,Au,andCr:implicationsfortheidentification of chondritic projectiles.MeteoriticsandPlanetaryScience43,541‐559.

Tagle,R.&Claeys,P.[2005].Anordinarychondriteimpactor for the Popigai crater, Siberia.Geochimica et Cosmochimica Acta 69, 2877‐2889.

Tagle,R.&Hecht,L.[2005].Evaluationofchemicalmethods for projectile identification interrestrial and lunar impactites. Lunar andPlanetaryScience36,1927.

Tagle, R., & Hecht, L. [2006] Geochemicalidentification of projectiles in impact rocks:Meteoritics and Planetary Science 41, 1721‐1735.

Tagle, R., Claeys P., Öhman, T., Schmitt, R. T. &Erzinger, J. [2007].TracesofanH‐chondrite inthe impact melt rocks from Lappajärvi impactstructure, Finland. Meteoritics & PlanetaryScience42,1841‐1854.

Tagle, R., Schmitt, R. T., and Erzinger, J. [2009]Identificationoftheprojectilecomponentintheimpact structures Rochechouart France andSääksjärvi, Finland. Implications for theimpactor population for the Earth. GeochimicaetCosmochimicaActa73,4891‐4906.

Taylor, S. R. [1982]. Planetary Science: A LunarPerspective.LunarandPlanetaryInstitute,481pp.

Taylor,S.R.[1992].SolarSystemEvolution:ANewPerspective. Cambridge University Press, 307pp.

Tedesco, E. F.&Desert, F. X. [2002]. The InfraredSpace Observatory Deep Asteroid Search. TheAstronomicalJournal123,2070‐2082.

Therriault A.M., Grieve R. A. F. & Reimold,W. U.[1997].OriginalsizeoftheVredefortStructure:Implications for thegeological evolutionof theWitwatersrand Basin. Meteoritics & PlanetaryScience32,71‐77.

Vauquelin, L. N. [1803]. Memoire sur les pierresdite tombees du ciel. Annales des Chimie 45,225‐245.

von Engelhardt, W. [1990]. Distribution,petrography and shock metamorphism of theejectaoftheRiescraterinGermany–Areview.Tectonophysics171,259‐273.

von Paula Gruithuisen, F. [1828]. Analekten fürErd‐ und Himmelskunde, Erstes Heft. Joh.Palm’schenBuchhandlung,660pp.

Walker,R.J.,Horan,M.F.,Morgan,J.W.,Becker,H.,Grossman, J. N. & Rubin, A. E. [2002].Comparative 187Re‐187Os systematics of

PAGEXVI

chondrites: Implications regarding early solarsystemprocesses.GeochimicaetCosmochimicaActa66,4187‐4201.

Wegener, A. [1921]. Die Entstehung derMondkrater.FriedrichVieweg&Sohn,48pp.

Weissman, P. R. [1999]. Diversity of comets:formation zones and dynamical paths. SpaceScienceReviews90,301‐311.

Weissman, P. R., Bottke, W. F. & Levison, H. F.[2002]. Evolutionof Comets intoAsteroids. In:Bottke, W. F., Paolicchi, P., Binzel, R. P. andCellino, A. (eds.), Asteroids III, 669‐686.UniversityofArizonaPress,785pp.

Westbroek, H. H. & Stewart, R. R. [1996]. Theformation,morphology,andeconomicpotential

ofmeteorite impact craters.CREWESResearchReport8,34.1‐34.26.

Wetherill, G.W. [1976].Where do themeteoritescome from? A re‐evaluation of the Earth‐crossingApolloobjectsassourcesofchondriticmeteorites. Geochimica et Cosmochimica Acta40,1297‐1317.

Wittmann, A., Kenkmann, T., Schmitt, R. T. &Stöffler, D. [2006]. Shock‐metamorphosedzircon in terrestrial impact craters.MeteoriticsandPlanetaryScience41,433‐454.

Zinner,E.K.[2005].PresolarGrains.In:Davis,A.M.(ed.), Treatise on Geochemistry, vol. 1:Meteorites,CometsandPlanets,17‐39.Elsevier,737pp.

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VI. APPENDICES

AppendixA.Listofall178knownimpactstructuresonEarth[PASSC,2010].Given

arethenameoftheimpactstructure,itslocation,diameterinkmandageinMa.

Name Location Diameter(km) Age(Ma)Acraman SouthAustralia,Australia 90 ~590AmeliaCreek NorthernTerritory,Australia 20 600–1640Ames Oklahoma,USA. 16 470±30Amguid Algeria 0.45 <0.1Aorounga Chad 12,6 <345Aouelloul Mauritania 0.39 3.0±0.3Araguainha Brazil 40 244.40±3.25Avak Alaska,USA 12 3–95BP Libya 2 <120Barringer Arizona,USA 1.18 0.049±0.003Beaverhead Montana,USA 60 ~600Beyenchime‐Salaatin Russia 8 40±20Bigach Kazakhstan 8 5±3Boltysh Ukraine 24 65.17±0.64Bosumtwi Ghana 10.5 1.07Boxhole NorthernTerritory,Australia 0.17 0.0054±0.0015Brent Ontario,Canada 3.8 396±20Calvin Michigan,USA 8.5 450±10CampoDelCielo Argentina 0.05 <0.004Carswell Saskatchewan,Canada 39 115±10Charlevoix Quebec,Canada 54 342±15ChesapeakeBay Virginia,USA 90 35.5±0.3Chicxulub Yucatán,Mexico 170 64.98±0.05Chiyli Kazakhstan 5.5 46±7Chukcha Russia 6 <70ClearwaterEast Quebec,Canada 26 290±20ClearwaterWest Quebec,Canada 36 290±20CloudCreek Wyoming,USA 7 190±30ConnollyBasin WesternAustralia,Australia 9 <60Couture Quebec,Canada 8 430±25Crawford SouthAustralia,Australia 8.5 >35CrookedCreek Missouri,USA 7 320±80Dalgaranga WesternAustralia,Australia 0.02 ~0.27Decaturville Missouri,USA 6 <300DeepBay Saskatchewan,Canada 13 99±4Dellen Sweden 19 89.0±2.7DesPlaines Illinois,USA 8 <280

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Dhala India 11 1700–2100Dobele Latvia 4.5 290±35EagleButte Alberta,Canada 10 <65Elbow Saskatchewan,Canada 8 395±25El'gygytgyn Russia 18 3.5±0.5Flaxman SouthAustralia,Australia 10 >35FlynnCreek Tennessee,USA 3.8 360±20Foelsche NorthernTerritory,Australia 6 >545Gardnos Norway 5 500±10Glasford Illinois,USA 4 <430Glikson WesternAustralia,Australia 19 <508GloverBluff Wisconsin,USA 8 <500GoatPaddock WesternAustralia,Australia 5.1 <50GossesBluff NorthernTerritory,Australia 22 142.5±0.8Gow Saskatchewan,Canada 4 <250Goyder NorthernTerritory,Australia 3 <1400Granby Sweden 3 ~470Gusev Russia 3 49.0±0.2Gweni‐Fada Chad 14 <345Haughton Nunavut,Canada 23 39Haviland Kansas,USA 0.01 <0.001Henbury NorthernTerritory,Australia 0.15 0.0042±0.0019Holleford Ontario,Canada 2.35 550±100IleRouleau Quebec,Canada 4 <300Ilumetsä Estonia 0.08 ~0.0066Ilyinets Ukraine 8.5 378±5Iso‐Naakkima Finland 3 >1000Jänisjärvi Russia 14 700±5JebelWaqfasSuwwan Jordan 5.5 56–37Kaalijärv Estonia 0.11 0.004±0.001Kalkkop SouthAfrica 0.64 0.25±0.05Kaluga Russia 15 380±5Kamensk Russia 25 49.0±0.2Kamil Egypt 0.045 ?Kara Russia 65 70.3±2.2Kara‐Kul Tajikistan 52 <5Kärdla Estonia 7 ~455Karikkoselkä Finland 1.5 ~230Karla Russia 10 5±1KellyWest NorthernTerritory,Australia 10 >550Kentland Indiana,USA 13 <97Keurusselkä Finland 30 <1800Kgagodi Botswana 3.5 <180Kursk Russia 6 250±80LaMoinerie Quebec,Canada 8 400±50Lappajärvi Finland 23 73.3±5.3LawnHill Queensland,Australia 18 >515Liverpool NorthernTerritory,Australia 1.6 150±70Lockne Sweden 7.5 455

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Logancha Russia 20 40±20Logoisk Belarus 15 42.3±1.1Lonar India 1.83 0.052±0.006Lumparn Finland 9 ~1000Macha Russia 0.3 <0.007Manicouagan Quebec,Canada 100 214±1Manson Iowa,USA 35 73.8±0.3MapleCreek Saskatchewan,Canada 6 <75Marquez Texas,USA 12.7 58±2Middlesboro Kentucky,USA 6 <300Mien Sweden 9 121.0±2.3MishinaGora Russia 2.5 300±50Mistastin NewfoundlandandLabrador,Canada 28 36.4±4Mizarai Lithuania 5 500±20Mjølnir Norway 40 142.0±2.6Montagnais NovaScotia,Canada 45 50.50±0.76Monturaqui Chile 0.46 <1Morasko Poland 0.1 <0.01Morokweng SouthAfrica 70 145.0±0.8MountToondina SouthAustralia,Australia 4 <110Neugrund Estonia 8 ~470NewQuebec Quebec,Canada 3.44 1.4±0.1Newporte NorthDakota,USA 3.2 <500Nicholson NorthwestTerritories,Canada 12.5 <400Oasis Libya 18 <120Obolon' Ukraine 20 169±7Odessa Texas,USA 0.16 <0.05Ouarkziz Algeria 3.5 <70Paasselkä Finland 10 <1800Piccaninny WesternAustralia,Australia 7 <360Pilot NorthwestTerritories,Canada 6 445±2Popigai Russia 100 35.7±0.2Presqu'ile Quebec,Canada 24 <500Puchezh‐Katunki Russia 80 167±3Ragozinka Russia 9 46±3RedWing NorthDakota,USA 9 200±25RiachãoRing Brazil 4.5 <200Ries Germany 24 15.1±0.1RioCuarto Argentina 4.5 <0.1Rochechouart France 23 214±8RockElm Wisconsin,USA 6 <505RoterKamm Namibia 2.5 3.7±0.3Rotmistrovka Ukraine 2.7 120±10Sääksjärvi Finland 6 ~560Saarijärvi Finland 1.5 >600SaintMartin Manitoba,Canada 40 220±32SantaFe NewMexico,USA 9.5 <1200SerpentMound Ohio,USA 8 <320SerradaCangalha Brazil 12 <300

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Shoemaker WesternAustralia,Australia 30 1630±5Shunak Kazakhstan 2.8 45±10SierraMadera Texas,USA 13 <100SikhoteAlin Russia 0.02 0.000063Siljan Sweden 52 376.8±1.7SlateIslands Ontario,Canada 30 ~450Sobolev Russia 0.05 <0.001Söderfjärden Finland 6.6 ~600Spider WesternAustralia,Australia 13 >570SteenRiver Alberta,Canada 25 91±7Steinheim Germany 3.8 15±1Strangways NorthernTerritory,Australia 25 646±42Suavjärvi Russia 16 ~2400Sudbury Ontario,Canada 250 1850±3SuvasvesiN Finland 4 <1000Tabun‐Khara‐Obo Mongolia 1.3 150±20Talemzane Algeria 1.75 <3Tenoumer Mauritania 1.9 0.0214±0.0097Ternovka Ukraine 11 280±10TinBider Algeria 6 <70Tookoonooka Queensland,Australia 55 128±5Tswaing SouthAfrica 1.13 0.220±0.052Tvären Sweden 2 ~455UpheavalDome Utah,USA 10 <170VargeãoDome Brazil 12 <70Veevers WesternAustralia,Australia 0.08 <1Vepriai Lithuania 8 >160±10Viewfield Saskatchewan,Canada 2.5 190±20VistaAlegre Brazil 9.5 <65Vredefort SouthAfrica 300 2023±4Wabar SaudiArabia 0.11 0.00014Wanapitei Ontario,Canada 7.5 37.2±1.2WellsCreek Tennessee,USA 12 200±100WestHawk Manitoba,Canada 2.44 351±20Wetumpka Alabama,USA 6.5 81.0±1.5Whitecourt Alberta,Canada 0.04 <0.0011WolfeCreek WesternAustralia,Australia 0.87 <0.3Woodleigh WesternAustralia,Australia 40 364±8Xiuyan China 1.8 >0.05Yarrabubba WesternAustralia,Australia 30 ~2000Zapadnaya Ukraine 3.2 165±5ZelenyGai Ukraine 3.5 80±20Zhamanshin Kazakhstan 14 0.9±0.1

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AppendixB.Siderophileelementabundancesandratiosformeteorites[Tagle,2004;

Tagleetal,2009;andFischer‐Göddeetal.,2010].

ppm Ni Co Cr Ni/Cr SD Co/Cr SD Ni/Co SDCI 10863 521 2796 3.87 0.25 0.19 0.01 20.87 1.62CM 12396 583 3059 4.01 0.30 0.19 0.02 21.27 1.78CO 13564 688 3503 3.96 0.09 0.20 0.02 19.73 1.85CV 13629 641 3557 3.76 0.12 0.18 0.01 21.26 1.45CK 12518 647 3627 3.45 0.40 0.18 0.02 19.34 3.15CR 13794 666 3810 3.72 0.39 0.17 0.02 20.71 2.55CH 25716 1125 3393 7.65 0.69 0.33 0.06 22.86 2.83K 17172 786 2575 9.72 5.01 0.31 0.26 21.85 5.06R 14366 700 3563 3.99 0.19 0.20 0.01 20.53 1.03H 16846 763 3470 4.38 0.42 0.22 0.03 22.07 2.99L 12936 571 3650 3.22 0.19 0.16 0.02 22.67 3.33LL 9585 452 3521 2.64 0.21 0.13 0.02 21.19 3.12EH 18006 856 3076 5.79 0.36 0.28 0.03 21.04 1.95EL 14676 727 3148 4.77 1.03 0.23 0.04 20.18 2.86ppb Os Ir Ru Pt Rh Pd AuCI 502 478 725 970 136 570 140CO 758 726 1047 1265 0 772 184CM 671 617 877 1147 156 624 166CV 830 760 1078 1458 169 717 146CK 814 770 1119 1274 177 665 130CR 655 621 918 1115 745 124CH 1155 1066 1566 1596 249K 673 628 960 209R 658 603 908 121H 835 749 1010 1446 193 789 213L 565 525 765 1074 127 628 153LL 391 339 496 686 82 449 119EH 673 565 937 1207 173 858 331EL 639 569 830 1103 143 723 240IIIC 1550 4702 5036 1808 4684 IA 2380 4887 6135 1406 3611 IB 1210 1869 3828 640 3525 IIE‐A 6618 9750 11377 2490 3415 IIE‐M 1313 3633 6370 1580 2963 IIID 91.00 155 801 677 23362

Os/Ir SD Ru/Ir SD Pt/Ir SD Rh/Ir SDCI 1.06 0.04 1.52 0.07 2.03 0.15 0.29 0.014CM 1.12 0.09 1.50 0.10 1.84 0.09 0.26 0.013CO 1.08 0.05 1.49 0.08 1.66 0.23CV 1.06 0.03 1.52 0.08 1.90 0.12 0.28 0.014CK 1.07 0.05 1.45 0.14 2.02 0.07 0.28 0.014CR 1.05 0.04 1.49 0.07 1.94 0.16CH 1.08 0.04 1.47 0.05 1.50 0.27

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Os/Ir SD Ru/Ir SD Pt/Ir SD Rh/Ir SDK 1.07 0.02 1.53 0.02R 1.06 0.05 1.48 0.07H 1.07 0.04 1.51 0.07 2.08 0.06 0.31 0.004L 1.07 0.04 1.49 0.09 2.05 0.12 0.33 0.007LL 1.08 0.04 1.55 0.07 2.10 0.12 0.34 0.006EH 1.13 0.04 1.62 0.16 2.06 0.04 0.33 0.017EL 1.11 0.08 1.53 0.14 2.01 0.04 0.33 0.020

Pd/Ir SD Au/Ir SD Pt/Pd SD Ru/Rh SDCI 1.19 0.10 0.30 0.04 1.69 0.23 4.95 0.25CM 1.07 0.09 0.27 0.02 1.70 0.13 5.08 0.25CO 0.94 0.11 0.26 0.01 1.80 0.06CV 0.93 0.07 0.19 0.02 2.10 0.15 5.01 0.25CK 0.97 0.05 0.16 0.05 1.82 0.09CR 1.18 0.17 0.20 0.05 1.69 0.24CH 0.90 0.05 0.23 0.07 2.00 0.10K 0.34 0.09R 0.23 0.09H 1.10 0.10 0.28 0.03 1.86 0.07 4.73 0.12L 1.22 0.14 0.30 0.03 1.70 0.19 4.43 0.17LL 1.49 0.21 0.35 0.03 1.40 0.17 4.40 0.16EH 1.62 0.09 0.57 0.05 1.27 0.05 4.94 0.25EL 1.31 0.07 0.43 0.04 1.53 0.09 4.98 0.02

Pt/Ru SD Ru/Pd SD Pt/Rh SD Pd/Rh SDCI 1.37 0.06 1.15 0.01 7.36 0.17 4.18 0.21CM 1.23 0.14 1.45 0.22 7.18 0.36 4.08 0.20CO 1.24 0.07 1.46 0.13CV 1.38 0.17 1.65 0.08 6.93 0.35 3.38 0.17CK 1.30 0.06 1.41 0.07 7.51 0.38 3.52 0.18CR 1.30 0.03 1.30 0.20CH 0.94 0.17H 1.38 0.04 1.33 0.06 6.77 0.14 3.63 0.09L 1.38 0.11 1.16 0.10 6.54 0.12 4.08 0.16LL 1.35 0.11 1.04 0.16 6.46 0.21 4.70 0.24EH 1.33 0.03 0.96 0.03 6.39 0.32 5.04 0.25EL 1.32 0.09 1.16 0.10 6.25 0.40 4.11 0.28

Ir/Pd SD Ni/Ir SD Co/Ir SD Cr/Ir SDCI 0.84 0.07 23.02 1.15 1.10 0.07 5.92 0.20CM 0.93 0.07 20.48 1.29 0.96 0.05 5.06 0.34CO 1.06 0.13 18.70 1.64 0.95 0.03 4.83 0.38CV 1.07 0.08 17.97 1.13 0.85 0.02 4.69 0.20CK 1.03 0.05 16.21 1.97 0.84 0.09 4.70 0.36CR 0.85 0.12 22.18 2.02 1.07 0.09 6.13 0.63CH 1.11 0.06 24.13 1.23 1.06 0.12 3.18 0.42K 27.34 4.86 1.25 0.19 4.10 3.50R 23.58 1.03 1.15 0.03 5.85 0.23

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Ir/Pd SD Ni/Ir SD Co/Ir SD Cr/Ir SDH 0.91 0.08 22.48 2.20 1.02 0.10 4.63 0.45L 0.82 0.09 25.84 2.83 1.14 0.11 7.29 0.42LL 0.67 0.09 28.50 2.41 1.35 0.16 10.47 0.84EH 0.62 0.03 31.41 2.42 1.49 0.08 5.37 0.40EL 0.76 0.04 25.92 3.02 1.28 0.10 5.56 0.98

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