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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
PAGEI
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.
PAGEIII
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
PAGEIV
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
PAGEVI
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.
PAGEVII
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
PAGEVIII
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
PAGEIX
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
PAGE1
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.
PAGE2
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;
PAGE3
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.
PAGE4
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
PAGE5
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
PAGE6
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.
PAGE7
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
PAGE8
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
PAGE9
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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PAGE10
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PAGE11
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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PAGE12
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PAGE13
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
PAGE14
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].
PAGE15
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
during
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rized part
etrockforntsofit.
odifiedfromF
ent on the
bining theo
us, the for
othreesuctal.,1968]
netratethe
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wavesare
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he rocks fr
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French[1998
PAGE16
impact
oretical,
rmation
ccessive].
etarget
ofshock
een1014
93] (for
enearly
ies.One
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ck, also
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rom the
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eteorite
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Thecom
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1999].
this is
takeles
2.1.2.2
While
rarefac
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rock.B
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Figure 4Hawke [arrowsinimpactev
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retical transieporised, melt.Thetopofthemoretheh
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modified fromejected anderepresentstzewillexceed
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Melosh&
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ansient cra
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PAGE17
datthe
brief.It
by the
Ivanov,
eof45°,
ewould
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risesthe
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ater, an
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eteorite
PAGE18
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
The m
Howev
smallerlargest
2.1.2.4
Related
volcani
ring types
t structure(Chicxulub
.Schematiccmodifiedfromrocks.Thearr
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4. POST‐IM
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and 15miatersofaf
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EVELOPMEN
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PAGE19
he three
fort) or
ngimpactmpedand
opment.
minfor
that the
ntation,
rmation,
areall
into th
affected
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Asedim
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layerlarge
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.Schematicc&Pilkington[1
mentaryba
thecrater’
mplete im
ure [von E
ures is bu
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.Beaverhe
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awke,2004
smakesis
pactcrater
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eteoritefra
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crosssection1996]andHa
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4].
assumable
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ke, 2004].
ved;(2)eje
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eve & Ma
eby,theyc
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canloseth
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usingsizea
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Impact cra
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abeltheam
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crater,byw
ccurred in
by sedim
pproximate
94]. Impa
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al.,1994]).
ccrustisg
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otallydestr
tes a unde
andagedis
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aters are m
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ments (e.g.
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errepresen
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act crater
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PAGE20
t crater
y to be
crater.
affected,
impact
Grieve&
are (1)
ved;(3)
ved;(5)
‐fill not
ifiedafter
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PAGE21
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
PAGE22
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.
PAGE23
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
Figure8andHaw
A smal
formed
vol.%.
Where
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aters,whe
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PAGE24
1987]and
ve [1987]
t that is
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PAGE25
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].
2.2
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PAGE26
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valontheon,butonl
types, clans a
tes show
differ al
difference
dancesand
onesarec
consistalm
reflectinga
hemomen
Itreflects
thedecay
henthetem40Arstart
nbedefine
cradiation
rproductio
osphereab
rationcan
reage.Wh
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
rite’sparen
hermaleve40Ar.Thisb
edropsbe
lsothemo
hen,severa
ragment’s
whenthefr
smicradiat
ated.Theo
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
As can
chondr
chondrdomina
2.2.1
Bydefi
toirreg
pyroxe
ronshave
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
ofbeingf
alled grou
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itegroups
classificati
05],Scott&ne.
meteorite typuseabundan
in Figure
ough the r
anceisproriticcompo
NDRITES
ondriteme
edinclusio
omemetal
ualpropor
drites.Ach
fferentiate
ntion, a gro
ble chemis
fragments
uplets.Met
pendixB co
.Therema
ion. See
&Krot[200
peabundancecesofthelat
11, the ma
relative ab
obablytheositionofs
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
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minsizea
ondrules,
dmetal.S
stony‐iron
A further c
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tbody.Tw
to any oth
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Haack &
7],andrefe
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teorite fall
ve change
logicaltimodies.
drulesma
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chondrites
tonesare
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classificati
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gy. Their
wotofour
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tions of th
sabriefre
& McCoy
erencesin
1999]). Fallspreservation
ls consists
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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
PAGE29
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
estconcen
hondritegr
sition form
osition. Ho
one), they
ous chond
(1)refract
otopiccom
ritesareas
mediatedis
ne. This is
the comw.
ompositionoδ18Oandthelsampleline
lmismatch
(Mighei‐li
Renazzo‐lik
ans‐like)
ntrationsof
roupandb
mostelem
owever, d
are the o
drites hav
toryeleme
mpositionw
ssumedto
stancesan
also refle
mmon carb
ofmostchondeverticalaxisistheEarth’
h,although
ike),areri
ke),CB(Be
and CH (
fvolatilee
bestrepres
ents.This
due to int
only chond
ve petrolog
entabunda
withδ17Oe
haveform
dtheCch
ected in th
bonaceous
dritegroupsstheδ17O,bosmassfractio
hallcarbo
ichincarb
encubbin‐l
(ALH85085
elements.T
sentationo
iswhy th
tensive aq
drite group
gic types
ancesthat
equalorlo
medclosest
hondrites t
heir chemi
s, ordinar
(modifiedfroothrelativetoonationline.
nbearing,
bon(i.e.1.5
ike),CV(V
5‐like). Th
Theyarea
ofthesolar
eyareofte
ueous me
p that lack
of two an
equalorex
owerthanm
ttotheSu
thefarthes
ical compo
ry and e
omMcSweenotheVienna
onlytwo
5to6wt.%
Vigarano‐li
he CI cho
assumedto
rsystem’s
enusedas
etamorphis
ks chondru
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
PAGE31
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–
osmic‐raye
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
ulcoite, lo
odifiedfromrelativetoth
oforthopy
oilite, whit
eisrather
dances,dif
anites are
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
PAGE35
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
PAGE36
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.
PAGE37
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.
PAGE38
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
PAGE39
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
PAGE40
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
PAGE41
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
In195
Domini
Depart
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drilling
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approx
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reached
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ion Obser
tmentofE
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PAGE42
tructure
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xtensive
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Figure1circulard
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PAGE43
learethe
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teiger&
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ssumed
ns were
eteorite
etration
dGrieve
f3.8km
,1976].
PAGE44
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
gneiss
Howev
alnoitesiderop
2.4.3
Thecra
lens,as
distinctfragme
Figure1[1978]asedimentfragmentdiagonalbasemen
2.4.3.1
Most o
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shockeexperie
emicalcom
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3. BREC
ater‐fillro
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8.SchematicandDence [2t fill is marktpartofthe striped lensnt,beneathth
1. LITHIC
of the brec
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the targehmentoft
CCIALEN
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BRECCIAS
ccia lens is
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et rock isthemelt‐be
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Brent imp
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s compose
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ouldbeattr
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ck. The cl
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inTable1
iderophile
ributedto
[1981] sta
to have apactrocks.
turescomp
Althoughv
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ructurewithedbyhorizonic breccia zoripedlenses,ack. Themarabove23GP
lithic brec
asts are c
pecially be
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ate that the
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verycompl
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ccias. They
coarse and
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atthepres
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y contain s
d are only
melt lens
breccias d1977].
PAGE45
senceof
e tothe
ibution.
e of the
ce on a
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il,three
as,melt‐
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did not
PAGE46
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
PAGE47
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.
PAGE48
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
PAGE49
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].
PAGE50
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
PAGE51
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].
PAGE52
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
PAGE53
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.
PAGE54
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
PAGE56
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,
PAGE57
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
PAGE59
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
Figure21
&Berlin[Goderis,
Table 8.ToolPak
YPd
Rh
Pt
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Pd
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Ru
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es do showaveacomm
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Results of lRegressionfr
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w a correlamonsource
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PAGE60
tes that
atofTagle
IIIB irons
e Analysis
86
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61
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The Ir/
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otherPGE
angeofthe
/Rh,Ru/Pdarecciasamplelarcorrelatio
TOPER
OTOPES
resultsof
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alcrustand
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andPt/Rudies(thechlorions.
RATIO
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2006].Sam
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PAGE61
ensation
± 0.14).
thatdo
s samplesTheother
50,pure
ttheOs
een the
mple59‐
ibution.
Theoth59‐105
Table9.R
187Os/188
2SDOs(ppb)2SD
Figure 2concentr(UCC)anand primMorokwetogether
4.2.2
Nocon
Table10
206Pb/204
SE207Pb/204
SE208Pb/204
SE207Pb/206
SE208Pb/206
SE
her sampl50at0.5w
Resultsofthe
8Os
23. Determinration [TaglendCImeteorimitive upperengimpactswiththeUCC
2. PBIS
clusionsca
.Resultsofth
59‐954Pb 20.23
0.0014Pb 15.65
0.0004Pb 41.97
0.0026Pb 0.773
0.0006Pb 2.074
0.000
es fallneat.%andsa
eOsisotopea
59‐951.50.12630.00142.2440.26
ation of the & Hecht, 20itesisshown.mantle aretructures.OnC,LCC,PUMa
OTOPES
anbedraw
hePbisotope
51.5 5900 190 0.071 158 0.000 402 0.093 0.801 0.066 2.004 0.0
ar theprimample59‐2
analysis.
59‐0.50.00.00.8
meteorite c006]. On the.Alsothemidplotted as antherightthandCImeteor
SYSTEM
wnfromth
eanalysis.
‐1050.43060008.56260008.7406002280090000010967800005
mitiveuppe2800betwe
‐10505251006705285
contributione left the mixd‐oceanridgereference, to
hevaluesforrites.
M
ePbisotop
59‐277117.39200.000615.43590.000637.81850.00190.887470.000012.174470.00005
ermantleeen1and2
59‐2770.12670.00312.9760.69
by plotting txing line betwebasalts(MOogetherwiththeBrentim
pesystem
59‐280018.29040.001115.48790.000938.87530.00250.846730.000012.125450.00005
with samp2wt.%.
7171
the 187Os/188
ween the upORB),lowercoh samples frompactstructur
alone.
67‐88818.1450.000915.4760.000838.4690.00200.85280.00002.12010.0000
ples59‐27
59‐28000.12550.00147.4040.44
8 ratio againpper continenontinentalcrom the Chicxresamplesa
8 67‐53 19.99 0.0062 15.68 0.0099 41.30 0.0086 0.7801 0.0014 2.0605 0.00
PAGE62
771and
st the Osntal crustrust(LCC)xulub andareshown
9269979008607400731500208041000165950004
4.3
Whena
constru
relative
thecrudeplete
Figure24arethato[2003],t
Alinea
and int
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fragmechondr
. MUL
allsiderop
ucted to vi
eflatchon
ustandtheedsignatur
4.CI‐normaliofTagle&BethosefortheI
arregressio
terceptsar
in Table 1
entbrecciaritemeteor
LTI‐SI
philedataa
isualise th
ndriticpatt
eothersamre.
isedconcentrerlin[2008].TIBandIIIDir
onanalysis
redetermi
1. Only th
asshownorites,asca
GNAT
arecombin
here enrich
tern,excep
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rationsofallThevaluesforonsfromTag
siscarried
ined for th
hemelt len
ocorrelationbeseeni
TUREA
ned,aCI‐n
hment (Fig
ptforsamp
dsample6
siderophileortheuppercgle[2004].
doutbyco
heNi/Ir,C
ns samples
on.All thrinAppendi
APPRO
normalised
gure24). A
ple59‐105
67‐1446w
elements.Thcontinentalcr
ombiningN
o/IrandC
s have bee
eeratiosfixB.
OACH
dsiderophi
All sample
50whichi
hichshow
heCIvaluesurust(UCC)ar
Ni,Coand
Cr/Irratio
en used, b
falloutside
ilepattern
es seem to
isinterme
wsadeviat
usedfornormrefromRudn
CrwithIr
s.Theres
because th
etherang
PAGE63
ncanbe
have a
diateto
tingand
malisationnick&Gao
.Slopes
ultsare
hemelt‐
eof the
PAGE64
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.
PAGE65
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
PAGE66
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
PAGE67
[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
PAGE68
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
meltsl
for the
matche
Togeth
isalike
alsofal
ofthes
aLor
whopr
25. CI‐normasation are th&Gao[2003gle[2004]an
ver, the Ni
(19.34±3.
ifCriscon
notrepres
coincidenc
enssampl
e impact m
esexactlyt
herwithth
elypossibi
llinthera
siderophile
LLbased
roposean
alised PGEhose of Tagle3].ThevaluedTagleetal.
/Co ratio
15to22.8
nsideredn
sentative,
ce.Howeve
es,meanin
meteorite.
theLifthe
eNi/Cora
ilitybecaus
angeofthe
eelements
onPGEco
ironorCI
concentratioe & Berlin [s fortheIIIC[2009].The
(29.85 ±
6±2.83).
notreprese
whichwou
er, theNi/
ngtheyall
The best
emeteorit
atio,theya
setheNi/C
eironmete
sdoesnot
oncentratio
chondrite
ons of the m2008], thoseC,IAandIBimeltlenssam
3.00) doe
Thiscane
entativefo
uldimply
/Ir,Co/Ira
shareaco
discrimin
teischond
aremorein
Cr,Co/Cra
eorites.Ho
agreewith
ons, is alre
basedon
melt lens sae for the uppiron,andLLmplesarema
s not fall
excludethe
ortheimpa
theirratio
andCr/Ira
ommonsou
nating PGE
dritic,butt
ndicativef
andPd/Irr
owever,the
hanironm
eady repor
a1.95Pt/
amples. Theper continenordinarychognifiedbyaf
into the n
euseofthe
actmeteor
withCrw
all showco
urceanda
E ratio Pd/
theotherP
foraniron
ratios,next
eflatCI‐no
meteorite.
rtedbyEv
Irratio.Th
CI valuesntal crust (Uondritemeteofactorof500.
narrow ch
eNi/Crand
rite.AlsoN
wouldpoin
orrelation
arereprese
/Ir (1.22
PGEratios
nmeteorite
ttothatof
ormalised
Thepreclu
vanset al.
heirsugge
PAGE69
used forCC) fromoritesare.
hondrite
dCo/Cr
NiorCo
nttothe
for the
entative
± 0.14)
donot.
e,which
ftheLL,
pattern
usionof
[1993],
stionof
PAGE70
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
PAGE71
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.
PAGE72
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.
PAGE73
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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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
PAGEXXI
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
PAGEXXII
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
PAGEXXIII
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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