Geochemistry and Ar- Ar geochronology of impact-melt ...bcohen/publications/Cohen_MAPS2005.pdf ·...

Meteoritics amp Planetary Science 40 Nr 5 755ndash777 (2005)Abstract available online at httpmeteoriticsorg

755 copy The Meteoritical Society 2005 Printed in USA

Geochemistry and 40Ar-39Ar geochronology of impact-melt clasts in feldspathiclunar meteorites Implications for lunar bombardment history

Barbara Anne COHEN1 Timothy D SWINDLE2 and David A KRING2

1Institute of Meteoritics Department of Earth and Planetary Sciences University of New Mexico Albuquerque New Mexico 87131 USA

2Department of Planetary Sciences University of Arizona Tucson Arizona 85722 USACorresponding author E-mail bcohenunmedu

(Received 14 May 2004 revision accepted 05 April 2005)

AbstractndashWe studied 42 impact-melt clasts from lunar feldspathic regolith breccias MacAlpine Hills(MAC) 88105 Queen Alexandra Range (QUE) 93069 Dar al Gani (DaG) 262 and DaG 400 fortexture chemical composition andor chronology Although the textures are similar to the impact-melt clasts identified in mafic Apollo and Luna samples the meteorite clasts are chemically distinctfrom them having lower Fe Ti K and P thus representing previously unsampled impacts The 40Ar-39Ar ages on 31 of the impact melts the first ages on impact-melt samples from outside the region ofthe Apollo and Luna sampling sites range from sim4 to sim25 Ga We interpret these samples to havebeen created in at least six and possibly nine or more different impact events One inferred impactevent may be consistent with the Apollo impact-melt rock age cluster at 39 Ga but the meteoriteimpact-melt clasts with this age are different in chemistry from the Apollo samples suggesting thatthe mechanism responsible for the 39 Ga peak in lunar impact-melt clast ages is a lunar-widephenomenon No meteorite impact melts have ages more than 1σ older than 40 Ga This observationis consistent with but does not require a lunar cataclysm

INTRODUCTION

The bombardment history of the Earth-Moon system hasbeen debated since the first recognition that the circularfeatures on the Moon may be formed by impact processes(Baldwin 1949) In the early 1970s 40Ar-39Ar and U-Pbisotopic analyses of Apollo 15 16 and 17 highland rocks(Turner et al 1973 Tera et al 1974) revealed surprisingwidespread isotopic disturbances at 39 Ga Argon and leadlosses (and correlated disturbances in the Rb-Sr system) wereattributed to metamorphism of the lunar crust by an enormousnumber of collisions with asteroids andor comets in a briefpulse of time (lt200 Myr) in what was called the lunarcataclysm This event occurred after the saturation of thehighlands and the creation of some 25 large old lunar basins(Wilhelms 1987) However this single pulse was postulatedto have created gt15 large basin structures and resurfacedsim80 of the Moon

Lunar impact history has been recently revived as a topicof debate (Ryder 1990 Hartmann et al 2000) largely fueledby the application of laser 40Ar-39Ar dating to impact-meltsamples (Dalrymple and Ryder 1993 1996 Cohen et al2000 Culler et al 2000 Fernandes et al 2000) Impact-melt

rocks created in the impact itself are the most reliable rocksto record the impact date impact-melt rocks form in thebottoms of craters where pressure-release melting of targetmaterial occurs (Melosh 1989) Craters of all sizes createsome melt from glass soil coatings to impact glass spherulesglassy breccias and crystalline melt rocks Large lunar craterscreate enough melt to be lined by a melt sheet where a slowcooling rate facilitates recrystallization and degassingproducing crystalline clast-free melt rocks The minimumsize at which lunar craters produce clast-free crystallineimpact-melt rocks appears to be 5ndash7 km based on coolingrate considerations (see Deutsch and Stˆffler 1987) ejectedmelt samples cool more quickly and require a larger parentcrater to belong to a large enough melt pool or ejecta blanketto cool slowly enough to crystallize Thus crystalline impact-melt clasts can be assumed to be products of impact eventslarger than sim5 km on the Moon

Because impact-melt rocks are fine-grained remeltedmixtures of the target materials they are rarely suitable fordating methods that rely on mineral separation such as U-Pbor Rb-Sr but are well suited for the 40Ar-39Ar method The40Ar-39Ar system is only fully reset by a combination oftemperature and time Glassy splashes veins and agglutinates

756 B A Cohen et al

are so rapidly quenched that 40Ar often does not have time toescape leading to irregular degassing profiles anduninterpretable ages (McConville et al 1988 Cohen et al2001) Passage of the shock front and subsequent cooling isalso rapid preventing total degassing in shocked samples(Deutsch and Schpermilrer 1994 Bogard et al 1995) At the otherextreme rocks exposed to a small elevation in temperature fora long time often show characteristic signs of 40Ar diffusiveloss without complete resetting such as granulitic impactites(Cushing et al 1999) and rocks exposed to daily thermalcycling on the lunar surface (Turner 1971 Turner et al 1971)Crystalline impact-melt rocks which have been completelymelted and slowly cooled are the most useful in revealingabsolute impact datesmdashcritical information in determining thelarge-scale bombardment history of the Moon

Crystalline clast-free impact-melt rocks occur in thereturned Apollo and Luna sample collections as rocks clastsin breccias and soil fragments They can be distinguishedfrom lunar igneous rocks by their unusual chemicalcompositions (combinations of target materials unable to bederived from igneous fractionation processes) and uniquetextures (haystack poikilitic) The largest example 14310(3439 g) has nearly identical 40Ar-39Ar and internal Rb-Srisochron ages of 388 Ga (York et al 1972 Mark et al 1974McKay et al 1979) Melt rocks 684156 (544 g total) also havea well-constrained Rb-Sr internal isochron of 384 plusmn 001 Ga(Papanastassiou and Wasserburg 1971) and a concurrent 40Ar-39Ar age (Stettler et al 1973) demonstrating the ability of the40Ar-39Ar technique to accurately date impact-melt rocksSmaller rocks and breccia clasts are not large enough to permitmineral separation and the majority of impact ages have beenobtained using only 40Ar-39Ar systematics The availability oflaser heating in 40Ar-39Ar work inspired new very precise agedeterminations of Apollo 15 and 17 impact-melt samples(Dalrymple and Ryder 1993 1996) These samples wereinferred to have been formed in the Serenitatis basin at 389 Gaand give an upper limit on the age of the Imbrium basin of 385Ga Formation of the Crisium basin at 389 Ga has beeninferred by dating melt rocks in the Luna 20 collection(Podosek et al 1973 Swindle et al 1991) Together with theinferred radiometric age of Nectaris at sim392 Ga and craterdensity age of Orientale at le375 Ga (Wilhelms 1987 Stˆfflerand Ryder 2001) five large basins were formed on the Moonwithin 200 Ma though subtle variations in age and traceelement chemistry among the dated samples may argue formore or fewer impact events creating all the samples(Dalrymple and Ryder 1993 Korotev 1994 Haskin et al1998)

The preponderance of 39 Ga impact-melt rock ages andthe lack of older samples of unequivocal impact origin arehighly suggestive of an extraordinary bombardment event atsim39 Ga However the Apollo and Luna sample sites were allin or near an area called the Procellarum KREEP terrain(PKT) identified by the surface expression of elevated

thorium levels and by inference other incompatible elementssuch as K P and rare earths (KREEP) (Jolliff et al 2000) Theimpact-melt rocks that have been dated from these collectionsare predominantly mafic KREEP-rich samples which affordenough radiogenic elements to be feasibly dated but whichmay be dominated by the large amount of melt created in thenearside basins Nectaris Crisium Serenitatis and ImbriumHaskin et al (1998) and Korotev (2000) have argued that themafic KREEP-bearing impact-melt rocks in the Apollo 14ndash17samples are a special product of impacts into the PKT and notproduced by impact melting of the lower crust under typicalfeldspathic highlands These samples may not includeevidence of earlier events and thus the age distributionobtained from these samples may not reflect the global impactrecord on the Moon Lunar meteorites which were onlyrecognized after the Apollo missions provide a newopportunity to test the hypothesis These are samples of theMoon randomly ejected from the lunar surface (Warren andKallemeyn 1991) without enough velocity to escape the Earth-Moon system They experienced only mild shock duringlaunch (Warren 1994) and typically landed on Earth afterle1 Ma in space (Warren 1994 Gladman et al 1995)Meteorites that are chemically distinct from the Apollo maficimpact-melt samples may not have been affected by nearsideequatorial basin impacts Instead impact-melt clasts withinthese meteorites may have been formed in large impact eventsin other regions of the Moon as suggested by Taylor (1991)for MacAlpine Hills (MAC) 88105

There are two objectives in this study The first is toidentify impact-melt clasts in lunar highland meteorites thatare chemically different from the mafic KREEP-rich Apolloand Luna impact-melt rocks Candidate samples have texturessimilar to well-known rocks of impact origin establishingtheir origins in slowly cooled impact-melt sheets but arefeldspathic and lacking in basaltic or KREEPy contributionsindicating their formation either before the formation of thePKT or far from the Apollo sampling sites The secondobjective is to determine the ages of the impact-melt clastsusing 40Ar-39Ar techniques The impact-melt samples occur assmall (generally lt500 microm) clasts in the meteorite breccias sospecial handling techniques for extracting and analyzing thesamples had to be developed for this task These techniquesallow us to date a large number of clasts within a singlemeteorite looking for multiple impact events in a single rockand contributing 31 new impact-melt sample ages astatistically significant number to test the lunar cataclysmhypothesis If the Apollo and Luna samples are biased becauseof their proximity to one or more large nearside basins wewould expect to see a more random distribution of lunarimpact-melt clast ages from the feldspathic breccia meteoritesWe gave a brief report on these sample ages in Cohen et al(2000) In this paper we give a full treatment of thegeochemical and geochronological data that led to theconclusions in that paper For more details see Cohen (2000)

Impact melt clasts from lunar feldspathic regolith breccias 757

METEORITES

Four meteorites were obtained for this study Antarcticfinds MacAlpine Hills (MAC) 88105 and Queen AlexandraRange (QUE) 93069 and Libyan desert finds Dar al Gani(DaG) 262 and DaG 400 All are lunar feldspathic regolithbreccias containing abundant impact melt (15ndash50 byvolume) Figure 1 shows the studied clasts inphotomicrographs and backscattered electron image mosaicsTable 1 gives details of our thin sections

MAC 88105 (Lindstrom 1989) is a fine-grained regolithbreccia with abundant angular feldspathic clasts and vesicularmelt veins In thin section MAC 88105 is a microbreccia ofsmall mineral grains and rock clasts in a brown glassy matriximpact-melt and glass clasts make up sim50 of the rock byvolume with a range of compositions including Al-rich andbasaltic types (Jolliff et al 1991 Koeberl et al 1991Lindstrom et al 1991 Neal et al 1991 Taylor 1991)Regolith components are rare and solar-wind abundance islow (Eugster et al 1991) indicating an origin in an immatureregolith

QUE 93069 (Lindstrom 1994 Bischoff 1996) has a lightgray matrix with abundant millimeter-sized white-to-grayclasts In thin section QUE 93069 is a microbreccia from amature anorthositic regolith (Kring et al 1995) containingplagioclase grains granular clasts impact-melt clasts andglass spherules in a brown glassy matrix Pale browndevitrified glass clasts were identified by Koeberl et al(1996) Korotev et al (1996) and Grier et al (1995)

DaG 262 (Bischoff et al 1998 Jolliff et al 1999 Flossand Crozaz 2001 Cahill et al 2004) is a well-consolidatedbreccia consisting of a fine-grained matrix with abundantclasts of granular anorthosite and crystalline impact-meltclasts melt veins and metal grains The breccia is moderatelyweathered and contains terrestrial weathering products suchas calcite-filled veins

Dar al Gani 400 (Zipfel et al 1998 Bukovanska et al1999 Semenova et al 2000 Floss and Crozaz 2001 Cahillet al 2004) also contains terrestrial weathering products Thebulk meteorite is a well-consolidated dark-gray breccia withmineral fragments granular rock fragments and impact-meltclasts

The cosmic ray exposure (CRE) histories of MAC 88105(Nishiizumi et al 1991 Warren 1994) and QUE 93069(Thalmann et al 1996) are sufficiently different to rule outsource-crater pairing with each other or with feldspathicbreccia meteorites Y-821928219386032 ALHA 81005 andY-791197 though QUE 93069 is paired with mixed highlandmare breccia QUE 94269 (Nishiizumi et al 1996 Polnau andEugster 1998) QUE 93069 is also similar to DaG 262 inchemistry and CRE age (Eugster et al 2000) but glassspherules are abundant in QUE 93609 and lacking in DaG262 (Bischoff et al 1998) possibly indicating their origins indifferent places on the Moon Though the CRE history of

DaG 400 is not well constrained solar wind abundancedifferences between it and DaG 262 (Scherer et al 1998) ruleout pairing of these two meteorites Thus at least threedistinct areas on the Moon are sampled by these fourmeteorites each containing a variety of clasts from a few kmarea (Warren 1994) However clasts from a single eventcould have found their way to more than one of the brecciasbeing studied

GEOCHEMISTRY

Technique

We analyzed one 100-microm thick section of each meteoritethis thickness ensures enough material can be extracted for40Ar-39Ar work (tens of micrograms from each clast) buttransmitted light still penetrates the section and allows us tosee that the extracted samples were uniform throughout thesection depth Crystalline clast-poor impact-melt sampleswere identified on the basis of their textures Petrographicallythey are very fine-grained to cryptocrystalline and milky (notisotropic glasses) and range in color from bright white toshades of tan and dark brown We used backscattered-electron(BSE) imaging to texturally classify the melt clasts using theimpact-melt rock nomenclature of Stˆffler et al (1985) asmuch as possible (Fig 2) Table 2 shows the characteristics ofclasts studied by each technique Most of the clasts in thisstudy are microporphyritic a few are poikilitic andapproximately five clasts have a striated or ldquohaystackrdquotexture believed to be an impact-derived texture (Lofgren1977) While poikiloblastic textures can arise during thermalmetamorphism the Apollo collection contains poikiliticclasts that are unequivocally impact-melt rocks (Dalrympleand Ryder 1996) Therefore poikilitic clasts were alsoincluded in the study despite the possibility that they mightnot be impact-melt samples Their inclusion did not changeany of our conclusions

A Cameca SX-50 microprobe at the University ofArizona was calibrated with mineral standards and operatedwith a beam current of 20 nA voltage of 15kV andintegration time of 20 seconds per element (Na was the firstelement analyzed) Two olivine and two plagioclase feldsparstandards were analyzed after every calibration andintermittently during data collection A focused beam wasused for point analyses of feldspathic phenocrysts (generally5ndash30 microm in size) the beam was defocused to 10 microm for bulkanalyses Both beam sizes gave identical analyses of standardminerals In addition rhyolitic glass and tektite glassstandards were analyzed to show that no significantvolatilization (lt10) of K or Na occurs with a 10 microm beamunder the microprobe conditions used The analyticaluncertainty for each oxide based on counting statistics isreported in Table 3

A set of points (5ndash50) set up in a grid pattern over a single

758 B A Cohen et al

Fig 1 Grayscale transmitted-light photomicrographs (left) and BSE mosaics (right) of the thin sections in this study The photomicrographsare all at the same scale indicated by the 1 cm scale bar in (c) The microcore locations are shown in the photomicrographs each site has abright ring where material was etched away to define a sample the size of the darker middle circle as shown in the example in (a) The BSEmosaics are enlarged to show the impact-melt clasts identified in each section (outlined areas)

Table 1 Details of the thin sections studiedMeteorite Area of thin section (cm2) Number of melt clasts identified Impact melt by volume

MAC 88105 075 9 43QUE 93069 sim1 11 14DaG 262 025 16 45DaG 400 15 gt40 44


clast was analyzed with the 10 microm beam and all good analyses(95 lt total lt105) were averaged together to arrive at thebulk composition Table 4 shows the average composition andstandard deviation within each clast The standard deviation isgenerally not an accurate measurement of analyticaluncertainty because each impact-melt clast is heterogeneouson varying scales but it does give an estimate of the sampleheterogeneity The defocused beam technique yields absoluteelemental abundances that are different from true abundancesbecause of the interaction of excited elements from multiplephases (Warren 1997) We applied only standard ZAFcorrections to our defocused beam analyses thus there maybe a systematic uncertainty of a few percent However we areinterested in comparing clasts with each other and identifyingclusters so systematic errors of a few wt are notmeaningful

Results

A CIPW norm was calculated from the bulk compositionof each clast and the normative feldspar content andcompositions are shown in Table 2 All identified clasts are

feldspathic containing gt80 normative feldspar (simAn96)and all are olivine- and pyroxene-normative Figure 3 showsmajor-element relationships among the meteorite clastsLunar feldspathic breccias can be described to first order interms of three compositional parameters (Korotev et al2003) the Al2O3 concentration (anticorrelated with FeO +Mg) which reflects the ratio of plagioclase to iron-bearingminerals the concentrations of incompatible elements (forwhich K2O is a proxy in Fig 3a) and the MgOFeO ratio(Fig 3b) which increases with increasing olivinepyroxene inthe mafic mineral fraction

Figure 3 shows that by and large the impact-melt clastswithin each meteorite are compositionally similar to the bulkcomposition of the meteorite As noted by Delano (1991) therange of compositions of impact-melt clasts within any singlemeteorite is smaller than the range of Apollo 16 regolithbreccia compositions (Fig 3) implying that thecompositional diversity within the few-kilometer area ofmeteorite assembly is much less than the Apollo 16 site Thecompositional diversity of the Apollo 16 breccias arises fromtheir proximity to the PKT and to nearside maria where maficand KREEP-rich material was distributed by the nearside

Fig 2 BSE images of impact-melt clast textures (a) and (b) are microporphyritic (400 K 400 I) (c) is striated (262 A) and (d) is poikilitic(262 D) Dark gray is plagioclase light gray is pyroxene and white is olivine The scale bar in all panels is 20 microm

760 B A Cohen et al

Table 2 impact-melt clast characteristics

Name of clastClast size(microm) Texture

Norm wt feldspar Microprobe 40Ar-39Ar

MAC A 400 times 2000 Poikilitic 79 times timesMAC B 3000 times 4000 Microporphyritic 91 times times MAC C 700 times 600 Microporphyritic 96 times times MAC D 500 times 1000 Microporphyritic 92 times times MAC E 500 times 1000 Microporphyritic 95 times times MAC F 2000 times 2000 Microporphyritic 89 times times MAC F2 1000 times 1000 Microporphyritic ndash ndash times MAC G 1000 times 1000 Microporphyritic 86 times times MAC H 2000 times 3000 Microporphyritic 93 times times MAC I ndash ndash ndash ndash times QUE A 150 times 100 Microporphyritic 83 times ndashQUE B 800 times 500 Microporphyritic to glassy 87 times ndashQUE D 1000 times 750 Microporphyritic 86 times times QUE D2 ndash ndash ndash ndash times QUE E 600 times 600 Poikilitic ndash ndash times QUE F 500 times 500 Microporphyritic 77 times times QUE F2 200 times 300 Poikilitic 86 times ndashQUE G 400 times 800 Poikilitic 72 times times QUE I 500 times 500 Microporphyritic 90 times times QUE K ndash ndash ndash ndash times 262 A 500 times 400 Striated 61 times times 262 D 350 times 200 Poikilitic ndash ndash times 262 E 200 times 200 Poikilitic 69 times ndash262 F 1000 times 700 Microporphyritic 82 times times 262 G ndash Microporphyritic ndash ndash times 262 H 150 times 250 Microporphyritic 87 times times 262 I 200 times 100 Striated 82 times times 262 J 500 times 500 Microporphyritic 89 times ndash262 M ndash Microporphyritic 64 times ndash262 N 1000 times 500 Microporphyritic ndash times ndash262 O 400 times 300 Poikilitic ndash times times 262 P 1500 times 500 Glassy 72 times times 262 Q2 5000 times 5000 Microporphyritic 80 times times 262 R 1000 times 1000 Microporphyritic 87 times times 400 A1 ndash Microporphyritic 80 times times 400 A2 ndash Microporphyritic 78 times ndash400 B ndash Microporphyritic 76 times ndash400 C1 ndash Microporphyritic 92 times times 400 C3 ndash Microporphyritic ndash ndash times 400 D ndash Microporphyritic 73 times times 400 E ndash Microporphyritic 82 times ndash400 G ndash ndash 79 times ndash400 H ndash Microporphyritic 84 times ndash400 I ndash Microporphyritic 82 times ndash400 J ndash Microporphyritic 87 times ndash400 K ndash Microporphyritic 88 times ndash400 L ndash Microporphyritic 81 times ndash400 L1 ndash Microporphyritic 98 times ndash400 L9 ndash Crystalline plagioclase ndash ndash times 400 L15 ndash Poikilitic ndash ndash times400 P ndash Microporphyritic 82 times ndash400 Q ndash Microporphyritic 84 times times 400 R ndash Microporphyritic ndash times ndash400 T ndash Microporphyritic 82 times times400 T2 ndash Microporphyritic 85 times times


basins and worked into the regolith In contrast the clasts inthis study have extremely low K2O and P2O5 contents (bothlt005 wt) ruling out their origin in the PKT (Fig 3a) Maficimpact-melt clasts have been reported in both MAC 88105and QUE 93069 (Jolliff et al 1991 Koeberl et al 1991Lindstrom et al 1991 Neal et al 1991 Taylor 1991 Koeberlet al 1996 Korotev et al 1996) few mafic clasts have beenreported in DaG 262 or DaG 400 (Bischoff et al 1998Fernandes et al 2000 Semenova et al 2000) No clasts in thisstudy show evidence of containing any significant proportionof basaltic material having low FeO and MgO (Fig 3b) andTiO2 (lt05 wt) On the other hand the diversity of Mgrsquo(molar MgMg + Fe) among the clasts (Fig 3c) suggests thatmultiple impacts may be responsible for creating the impact-melt samples because the impact melting process might beexpected to very effectively homogenize Mgrsquo in the meltproducts

The major element chemistry of the impact-melt clasts inthis study implies either that the source terrain for eachbreccia is well within the feldspathic highlands ie far fromthe incompatible-rich PKT or mafic South PolendashAitken basinterrane or that the melt rocks formed before the widespreaddistribution of material from these terranes Additionally thedifferences among both meteorites and clasts within eachmeteorite implies that these clasts were not all created in asingle impact event but rather formed in several differentevents However within the uncertainties of our analyticaltechniques we could not discern groups of chemicallyidentical clasts thus we were unable to use major elementchemistry and texture to distinguish families of feldspathicimpact-melt samples created in common impact events

GEOCHRONOLOGY

Technique

Because the impact-melt clasts can be quite small(sim100 microm) and fine-grained mineral separates cannot be usedfor dating On the other hand contribution of Ar fromadjacent clasts or matrix is a concern for in situ laser heatingIn well-consolidated glass-bound breccias such as the

meteorites in this study crushing is not an effective way toextract impact-melt clasts because the breccia may not breakcleanly along clast boundaries Therefore we employed amicrocorer to extract individual melt clasts from the thicksection for dating (Cohen 2000) We extracted core samplesfrom the 100-microm thick sections with diameters ge100 microm inorder to obtain enough material When impact-melt clastscontained fragments of possibly relict materials the selectedmicrocore avoided these fragments Each microcore wasdrilled and removed from the section using distilled waterWithin a few hours of being cored each sample was baked ina 200 degC oven for approximately one hour to dry and degasany epoxy that might have remained though the epoxy stillproduced problems as discussed later Table 2 comparessome characteristics of the extracted impact-melt clasts

Microcored samples were placed into wells in pure(99999) aluminum metal discs and irradiated for 500 hr inlocation L-67 of the Phoenix-Ford Memorial Reactor at theUniversity of Michigan The irradiation took place over sixweeks in five separate doses to produce a J-factor of 733 times10minus2 The standards Ga1550 biotite (979 plusmn 09 Ma)(McDougall and Roksandic 1974) and MMhb-1 hornblende(5231 plusmn 26 Ma) (Renne et al 1998) as well as pure CaF andK2SO4 salts were simultaneously irradiated

The sample discs were placed into a laser port covered byan optically transparent silica window A Liconix 5000-seriescontinuous Ar ion (514 nm) laser step-heated each sample for30 sec at each step Heating steps were achieved by varyingthe laser current between 10 and 17 A rastering the beamover the sample surface and for the lowest-temperature stepby defocusing the 10 A laser beam over the sample Thoughthe actual temperature achieved at each step was notmeasured it is not necessary to the experiment The minimumtemperature achieved (with the 10 A defocused beam) issim800ndash900 degC known because the samples began to glowsamples generally fused between 13 A and 17 A Thus allsteps in these experiments are high-temperature steps relativeto conventional 40Ar-39Ar experiments The small size andlow potassium content of the samples limited the number ofheating steps that could be performed per sample The samplegas was incrementally expanded into a volume of sim850 cm3

400 U ndash Microporphyritic 84 times ndash400 V ndash Microporphyritic 81 times times 400 V2 ndash Microporphyritic 79 times 400 W ndash Microporphyritic ndash ndash times 400 AA ndash Microporphyritic ndash times times 400 BB ndash Microporphyritic 80 times times 400 DD ndash Striated-porphyritic ndash ndash times400 FF ndash ndash ndash ndash times400 HH ndash Microporphyritic 79 times ndash400 JJ ndash Microporphyritic 81 times ndash

Table 2 Continued impact-melt clast characteristics



762 B A Cohen et al Ta

ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

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412

007

481

167

036

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010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

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246

004

271

18

006

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006

Ran

ge

sdd

02ndash

44

001

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90

2ndash15

80

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19

001

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90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

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60

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007

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4

a The

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f sep

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used

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95

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Tabl

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Con

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K2O

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5To

tal

764 B A Cohen et al

including an SAES SORB-AC getter The getter was offduring the DaG 262 runs leading to a low signal-to-noiseratio and larger uncertainties

Sample analysis took place in several batches over twomonths System blanks were measured each day averagedover each analysis batch and subtracted from the data Thevariation in blank levels over the entire data collection periodwas le50 The average of all blank runs at masses 36 to 40 isin order 137 times 10minus13 149 times 10minus13 310 times 10minus14 189 times 10minus13and 363 times 10minus11 ccSTP The contribution from HCl at masses36 and 38 was sim003 thus this negligible correction wasnot considered further

All isotopic corrections in the samples and standardsdepending on the time since irradiation were based on the lastday of irradiation this method has no effect on the amount of39Ar but systematically underestimates the amount of 37Ar bysim50 (irradiation took place over approximately one half-lifeof 37Ar) for all samples and standards However because all

salts monitors and samples were irradiated together andcorrected to the same date corrections based on measuredratios will still yield all isotopes in the correct ratiosTherefore the relative KCa ratios from one step to another orone sample to another are accurate Sample K2O contentscalculated using 39Ar content are similar to but sometimeshigher than the K2O contents found using the microprobeWe are not sure what causes the discrepancies however in nocases are the calculated values more than a factor ofapproximately 3 higher than the measured values and in nocase is the apparent K2O content gt01 wt

Cosmic ray contributions of 38Ar and 36Ar weresubtracted by deconvolving the 38Ar36Ar signatures fromcosmic-ray-spallation (36Ar38Ar = 066 Hohenberg et al1978) and terrestrial atmosphere (36Ar38Ar = 019) Thiscorrection completely subtracted all 36Ar from five samples(400D 400L9 400T 400BB and 400FF) Note that in thesecases 36Ar = 0 but still has an associated uncertainty this

Table 4 40Ar-39Ar dating results

SampleWeight (microg)

of 39Ar in plateau

Uncorrected plateau agea plusmn1σ (40Ar36Ar)i Isochron ageb plusmn1σ Best-estimate agec plusmn1σ

MAC A 42 64 2531 plusmn 1502 2474 plusmn 1549MAC B1 55 12 3012 plusmn 173 2944 plusmn 189MAC C 25 82 3347 plusmn 209 3298 plusmn 217MAC D 22 93 3597 plusmn 269 3248 plusmn 251MAC F 121 29 3807 plusmn 60 18 plusmn 19 3730 plusmn 103MAC F2 100 47 3942 plusmn 92 3905 plusmn 96MAC G 35 81 3176 plusmn 263 3415 plusmn 272MAC H 44 77 3910 plusmn 149 52 plusmn 42 3360 plusmn 575MAC I 39 72 4038 plusmn 102 39 plusmn 19 3763 plusmn 108QUE E 19 441 3850 plusmn 225 3341 plusmn 663QUE F 16 35 3015 plusmn 423 2993 plusmn 430QUE G 4 521 2516 plusmn 2593 1838 plusmn 2488QUE I 24 63 2727 plusmn 1019 2685 plusmn 1035QUE K 27 57 3814 plusmn 437 3727 plusmn 459262 H 3 61 4123 plusmn 475 4120 plusmn 474262 Q2 25 51 3550 plusmn 169 3520 plusmn 174262 R 27 54 2429 plusmn 171 2429 plusmn 170400 A1 13 37 2577 plusmn 352 2556 plusmn 354400 C1 37 25 2622 plusmn 613 2619 plusmn 611400 C3 12 61 3407 plusmn 85 41 plusmn 51 3312 plusmn 152400 D 24 52 3235 plusmn 169 3235 plusmn 129400 L9 3 100d 2453 plusmn 532 2455 plusmn 519400 L15 34 61 2924 plusmn 191 2914 plusmn 190400 Q 61 64 3467 plusmn 69 3395 plusmn 84400 T 31 64 3335 plusmn 213 3335 plusmn 213400 T2 21 57 2744 plusmn 369 2697 plusmn 381400 W 80 70 3610 plusmn 35 17 plusmn 07 3390 plusmn 95400 AA 48 51 3074 plusmn 104 09 plusmn 51 3070 plusmn 197400 BB 50 82 2967 plusmn 84 2967 plusmn 84400 DD 48 48 3111 plusmn 141 16 plusmn 75 3100 plusmn 178400 FF 30 56 2788 plusmn 271 2788 plusmn 234

aAges uncorrected for trapped Ar representing the upper limit to sample ages bAfter the isochron-defined (40Ar36Ar)i was subtractedcAfter the best-estimate ratio (40Ar36Ar = 3 plusmn 3) was subtracteddPlateau comprises a single heating step


uncertainty was propagated when any further correctionsinvolving isotopes ratioed to 36Ar were attempted (such as thetrapped-argon corrections discussed below)

Figure 4 shows argon release data for each sample Aplateau in a sample (steps shown in bold) consisted ofconsecutive steps with apparent ages within 2σ of each otherexcept when a step contained no measurable 40Ar above thebackground and less than 2 of 39Ar released (eg the thirddegassing step in MACC) We calculated sample ages byadding together the gas released in each plateau step andusing this total amount of 40Ar and 39Ar to calculate a singleintegrated age and associated uncertainty rather thancalculating some other weighted average of the individualheating step ages Including the heating steps where theapparent age was zero lowers the age by less than 1 in anycase Most samples showed a high apparent age in the firstheating step We attribute this to degassing of atmosphericargon from bubbles in epoxy clinging to the microcore anddiscount the first heating step in all cases including thosesamples in which the first heating step was within 2σ of thefollowing steps (sample 400 L9 had three total degassingsteps the first two released 40Ar and 36Ar but no 39Ar abovethe background and so they do not appear in Fig 4v Wediscarded these steps in the age calculation) It should benoted that we took the most conservative approach withregard to individual data points within each sample someheating steps as well as some entire samples were excludedfrom the data set when the data were not robust for anyreason However the ages calculated using these questionabledata did not differ substantially from the ages reported andwould not affect our conclusions

In samples with remaining excess 36Ar after the cosmicray correction a three-isotope plot was constructed todetermine the amount of trapped Ar A modified York fitaccounting for uncertainty in both isotope ratios wascalculated based on the same temperature steps used in theplateau Not every individual temperature step released 36Arabove background thus the same number of degassing pointsis not always available in the isochron We were able to fitmeaningful isochrons to seven samples (MACF MACHMACI 400C3 400W 400AA and 400DD) and deduce anage based on the slope of the data (Table 4) In these cases theplateau age (after subtracting the initial 40Ar36Ar) is exactlythe same within uncertainty as the age calculated from theisochron

Due to the relatively large uncertainties in the remainingsamples the (40Ar36Ar)i was largely unconstrained Toaddress this we developed three possible scenarios aterrestrial atmosphere correction (40Ar36Ar = 2955) a recentsolar wind exposure correction (40Ar36Ar le 10 Eugster et al2001) and a conservative best estimate correction based onthe average of the isochron intercepts which propagates anuncertainty through the subtraction (40Ar36Ar = 3 plusmn 3) Thebest estimate corresponds to surface exposure lt3 Ga ago

Fig 3 Major element chemistry of the impact-melt clasts in thisstudy shown as a function of Al2O3 content a) K2O (wt) b) MgO+ FeO (wt) and c) Mgrsquo (molar Mg[Mg + Fe]) Each pointrepresents the average composition of a single melt clast Shown forcomparison are fields encompassing whole-rock analyses offeldspathic lunar meteorites (Al2O3 = 25ndash31 wt [Korotev et al2003]) Apollo 16 feldspathic regolith breccias (Al2O3 = 20ndash34 wt[McKay et al 1986]) and Apollo mafic impact-melt breccias(Al2O3=12ndash22 wt [Korotev 2000])

766 B A Cohen et al

Fig 4 Argon release patterns and KCa ratios for impact-melt samples in MAC 88105 (andashi) QUE 93069 (jndashn) DaG 262 (ondashq) and DaG 400(rndashee)


Fig 4 Continued Argon release patterns and KCa ratios for impact-melt samples in QUE 93069 (jndashn) DaG 262 (ondashq) and DaG 400 (rndashee)

768 B A Cohen et al

(Eugster et al 2001) a conservatively wide range Allowing ahigher ratio would mean an earlier exposure but would pushthe corrected ages younger sometimes requiring thephysically unrealistic case of exposure before formation Theterrestrial atmosphere correction was usually a grossovercorrection subtracting all 40Ar and causing apparent agesto become zero The recent solar wind correction did notchange the apparent age in any sample by more than 001The best estimate correction can be considered a reasonableguess of the amount of trapped lunar Ar present in sampleswith no terrestrial atmosphere Ar The magnitude of thiscorrection depends on the amount of 36Ar remaining in thesample but generally caused less than a 10 reduction in

sample age Ages calculated using this correction are thusreasonable estimates of the sample ages (Table 4) that takeinto account propagation of uncertainty in the ages

Five samples had no 36Ar contribution to subtract sevenhad data with which we were able to construct an isochronand calculate the 40Ar36Ar ratio to be subtracted We couldnot deconvolve the relative contributions of the varioustrapped-argon components in the remaining 30 samples Theages reported in Table 4 reflect the different techniques wewere able to use to arrive at the sample ages However one ofthe goals of the study is to be able to compare clast ageswithin and among the meteorites For this purpose we mustuse a sample age data set that is internally self-consistent

Fig 4 Continued Argon release patterns and KCa ratios for impact-melt samples in DaG 400 (rndashee)


Therefore we approach this problem twice first using alldata without correcting for (40Ar36Ar)i and second using theages corrected using the derived isochron 40Ar36Ar ratios forthe seven samples and the ages calculated using the best-estimate correction (40Ar36Ar = 3 plusmn 3) for all other samplesAges derived from the uncorrected data must be consideredupper limits to the true sample ages with smaller uncertaintiesthan may be warranted Any corrections would lower theapparent sample ages an effect that only strengthens our mainconclusions that no impact-melt rocks older than 40 have yetbeen found and that many impact-melt clasts from lunarmeteorites have ages much younger than the canonicalcataclysm

Results

The data set reported in Cohen et al (2000) consisted ofonly the uncorrected sample ages in this paper we reportboth data sets but for consistency refer to the best estimate(40Ar36Ar corrected) sample ages in our discussion unlessotherwise indicated We also explore the effect on our resultsof using two data subsets those samples with large plateausand those samples with the smallest uncertainties

Table 4 lists ages derived from the data before and afterthe (40Ar36Ar)i corrections (determined by the isochron orbest estimate correction) were applied Each sample age canbe represented by a Gaussian distribution having a widthproportional to the 1σ uncertainty and a unit area under thecurve As the uncertainty in age decreases the individualsamplersquos curve becomes narrower and higher at the peak Theindividual Gaussian curves for the samples add together toproduce an ideogram (Fig 5) the advantage of which over atraditional age histogram is that the ideogram accounts for theuncertainty associated with each sample A normaldistribution fitted to each peak in the ideogram identifies theage (and associated uncertainty) of a single impact event thatcould have created the group of samples (Table 5) In thisapproach we disregarded peaks made up of only one sampleor peaks with very low probabilities (wide distributions)though these could represent still other impact events Thesame samples make up the age clusters in both theuncorrected and corrected data sets with the exception ofQUE 93069 the samples of which make two distinct groupsin the uncorrected data but one in the corrected data

Two clear peaks arise in the age distribution of impact-melt clasts within MAC 88105 a group of four clasts at379 plusmn 014 Ga within 1σ of the presumed age of theOrientale basin (Stˆffler and Ryder 2001) and three clasts at323 Ga This supports Taylorrsquos (1991) suggestion thatmultiple impact-melt populations exist within MAC 88105though chemically the two groups could not be distinguishedin this study The one poikilitic clast is included in theyounger group and is not statistically different in age from theother impact-melt clasts though it is slightly different

chemically The QUE 93069 samples fall into a single normaldistribution with a large uncertainty 238 plusmn 069 Gaindicating that any textural or compositional differences seenin the major element analyses have no correlation with the ageof the samples

Because of a technical problem with the SAES getteronly three clasts from DaG 262 had interpretable ages andtwo of the three do not overlap until the 3σ level The oldestclast is 41 Ga but with a large uncertainty of 05 Ga Theother two clasts are younger at 35 Ga and 24 Ga There wasno obvious difference in chemistry or texture among thesethree clasts The thirteen impact-melt clasts in DaG 400 rangemore widely in age than do the ten in MAC 88105 Manydifferent ages are recorded although some of these agesoverlap at the 1σ level The youngest group (259 plusmn 045 Ga)contains four samples including one crystalline-plagioclaseclast the oldest and youngest ages of which overlap at the 1σlevel but form a broad distribution Better defined groupsoccur at 304 plusmn 021 Ga (six samples) and 336 plusmn 014 Ga(three samples)

The fact that MAC 88105 QUE 93069 and DaG 400 arenot paired with each other and probably come from differentplaces on the Moon implies that each of the impact eventsrepresented in these meteorites is a different event on thelunar surface Thus in these three meteorites we findevidence for six or seven different impact events (Table 5)Including the two well-dated clasts from DaG 262 asrepresenting two more impact events raises this number toeight or nine though as mentioned in the Meteorites sectionDaG 262 and QUE 93069 may be from a similar source areaon the Moon but not paired In addition there are severalsamples in MAC 88105 and DaG 400 that do not fall into anyof the groups which may represent distinct events Thereforewe conclude that these four meteorites contain evidence for atleast six to nine different impact events on the Moon

Figure 6a shows an ideogram of all the impact-meltsamples dated in this study along with a histogram of theinferred impact events (from the corrected ages) that weresampled in each meteorite The peak of the meteorite impact-melt clast ideogram is much younger than the 39 Ga peakobserved in the Apollo samples We considered the possibilityof a systematic error in the data collection or reductionhowever the simultaneous analysis of two different fluxmonitors argues against this possibility In addition duringthe analysis of the lunar samples a clast from the H chondriteOurique was dated using the same techniques and reductionyielding an age of 445 Ga (Kring et al 2000) Some meltsamples contained mineral fragments that may have beeneither phenocrysts or incompletely degassed relic mineralsbut fragments of older rock entrained in the melt would yieldolder apparent sample ages not younger

We also considered shock exposure and recoil effectswhen interpreting the 40Ar-39Ar data Shock and solar heatingon the lunar surface can partially degas or rearrange 40Ar

770 B A Cohen et al

within a rock causing it to lose radiogenic 40Ar from its leastretentive sites (Deutsch and Schpermilrer 1994) and producing adisturbed step-heat profile where low temperature sites showa young apparent age The most retentive sites may remainundisturbed yielding a plateau in the high temperature stepsThis profile is commonly seen in lunar rocks (Turner 1971Turner et al 1971 Turner 1972 McDougall and Harrison1999) However it was rarely observed in the samples studiedhere for which resolvable low temperature data are lackingand often overwhelmed by atmospheric Ar outgassing fromepoxy The high temperature steps in these experiments yieldplateaus therefore shock degassing was not considered

further Samples that have experienced recoil showcharacteristically high ages in their low temperature releasesteps where fine-grained less retentive sites have lost their39Ar (Turner and Cadogan 1974 McDougall and Harrison1999) and low ages in the high temperature steps reflecting39Ar that was implanted into more retentive sites Epoxydegassing dominated the first gas release step in almost everysample in this study overwhelming any possible signature ofrecoil but a complementary downturn in the highesttemperature steps was not observed In a single phase singlegrain-size sample recoil can cause a net volume loss of 39Arfrom the outside surfaces to a mean depth of 0082 microm

Fig 5 Age ideograms for impact-melt clasts in each meteorite using corrected sample ages Each ideogram (solid curve) is a sum of all theGaussian curves representing clast ages The dashed curves are normal distributions fit to the ideogram peaks yielding an average age for eachgroup of clasts The y-axis scale (relative probability) is the same in each panel

Table 5 Inferred impact event ages based on normal distributions fit to ideogramsMeteorite Clasts Event (Ga)a Event (Ga)b

MAC 88105 C D G 3226 plusmn 244 3364 plusmn 294MAC 88105 F F2 H I 3794 plusmn 140 3915 plusmn 144QUE 93069 E F G I K 3275 plusmn 692 2969 plusmn 548 3841 plusmn 288DaG 400 A1 C1 L9 T2 2590 plusmn 447 2614 plusmn 448DaG 400 D L15 AA BB DD FF 3035 plusmn 214 3037 plusmn 174DaG 400 C3 Q T 3358 plusmn 143 3433 plusmn 98

aUsing isochron- or proxy-corrected agesbUsing uncorrected ages two age clusters are apparent in QUE 93069 uncorrected data


(Turner and Cadogan 1974 Huneke and Smith 1976) theaffected volume in a typical microcore is less than 1 of thesample and not considered significant

The impact-melt clasts in this study have similar texturesto clasts dated earlier (Turner et al 1971 Podosek et al 1973Cadogan and Turner 1977 Swindle et al 1991 Dalrympleand Ryder 1993 1996) which had ages around 39 Ga but aremuch finer-grained The major differences between this andprevious studies are the sample composition and mass Ourtechnique allows us to conduct step-heating experiments onsmaller more feldspathic samples than in previous studies(factors of 10ndash100 less total K2O) greatly expanding thenumber of datable samples within a single meteorite butmaking detailed interpretation of the argon spectra

impossible The plateaus in our samples average sim50 of the39Ar released comparable to plateaus in other studies but theheating steps in this study are much coarser and fewer stepsmake up the plateaus Fernandes et al (2000) also foundrelatively young (29ndash35 Ga) ages for DaG 262 componentsincluding one definite impact-melt clast

We also considered two subsets of our results (Figs 6band 6c) Excluding samples whose plateau encompassesfewer than two steps andor less than 50 of all the 39Arreleased (Fig 6b) leaves 22 samples remaining with an agedistribution from 24 to 40 Ga Including only those sampleswith age uncertainties less than 200 Myr (Fig 6c) produces adistribution of 15 samples with ages between 24 and 39 GaUsing these subsets of data decreases the certainty of

Fig 6 Ideogram summing ages from samples in all meteorites solid curve is corrected data dashed curve shows uncorrected data forcomparison Two subsets of the data are also shown b) only samples with plateaus consisting of more than one step andor more than 50 of39Ar released c) only samples with uncertainties le200 Myr The number of discrete impact events (Table 5 binned in 150-Myr intervals) isshown in the histogram beneath the curve Though the data subsets in (b) and (c) preclude identification of some inferred events the spreadin ages is similar in all representations

772 B A Cohen et al

assigning impact events within each meteorite using the dataset in Fig 6b allows identification of four events and the dataset in Fig 6c shows three events However the overalldistribution is similar in all three treatments of the data setfrom the peak of Apollo sample ages at 39 Ga to distinctlyyounger ages We are able to clearly identify two impactevents at 39 Ga represented by multiple samples using theuncorrected data This is unlikely to be simply coincidentalwith the Apollo sample age spike though we cannot rule outthis possibility We next consider interpretations of the sampleages and their implications for lunar bombardment history

DISCUSSION

The lunar meteorite impact-melt clasts in this study areprobably derived from the feldspathic highlands of the Moonand thus represent an aspect of lunar impact history that is notavailable using the Apollo and Luna samples Thesefeldspathic impact-melt clasts and bulk meteorite have FeOcontents le45 wt the remotely sensed lunar highlands mean(Lucey et al 1995) No distinct populations of impact-meltclasts within a single meteorite could be distinguished usingmajor element chemistry or texture but diversity in the clastsrsquoMgrsquo suggest that each meteorite samples more than oneimpact event This suggestion is borne out in the clast ageswhich range from 24 to 41 Ga However there is no apparentcorrelation of age with any macroscopic characteristicincluding texture grain size or normative mineralogySiderophile elements (Korotev 1994 Norman et al 2001)may be better able to distinguish groups of impact-meltsamples from the same event

The oldest impact event recorded in these lunarmeteorites (MAC 88105 379 plusmn 014 Ga) occurred at roughlythe same time as the events represented by the peak in theApollo sample age distribution These impact events areconsistent with the basin age for Imbrium at 392ndash394 Ga(Stˆffler and Ryder 2001) The meteoritic evidence thusappears to support Ryderrsquos (1990) argument that a low flux ofimpactors existed prior to sim39 Ga since no impact-meltrocks can be found anywhere on the Moon from that era In analternative explanation for the lack of old impact-melt rocksHartmannrsquos (1975) stone wall hypothesis and Grinspoonrsquos(1989) model postulate that the lunar crust is completely resetandor destroyed by multiple generations of crater saturationprior to 39 Ga However this argument cannot be completelyvalid because rocks are resistant to shock resetting (Deutschand Schpermilrer 1994) and impact-melt rocks are notmechanically weaker than other rocks so are not likely to bepreferentially pulverized

Hartmann (2003) modified his ldquostone wallrdquo hypothesispostulating that material in the upper layers of themegaregolith does become pulverized but material storeddeeper in the crust is occasionally excavated by large impactsproviding old anorthositic and plutonic rocks to be sampled at

the surface In this scenario impact-melt rocks are in factpreferentially pulverized because they always reside at thelunar surface at the time of their creation whereas plutons andprimary crust can be stored at depth and excavated at a latertime In this case melt rocks from large old impact basinseither exist now as very small clasts in the regolith (smallerthan sim200 microm) or as larger rocks buried under deep regolithand impact-melt rocks at the surface reflect only the mostrecent impacts For this scenario to work the volume ofsurviving impact melt must be much less than the volume ofigneous plutons and primary anorthositic crust at the samelevel in the crust so that for a given number of old igneousrocks at the surface a lower number of impact-melt fragmentswould be expected The relative volume of plutons and themechanics of large scale regolith development are largelyunconstrained and more work needs to be done on thisscenario (Chapman et al 2002 Chapman et al 2004)

Another possible reason that the Apollo-site impact-meltrocks only date a few basins could be that the sheer volume ofimpact melt created in basin forming events overwhelms thevolume created in smaller craters To address this possibilitywe calculated the impact-melt volume produced from basinsand smaller craters over the surface of the Moon (Fig 7) Thenumber of craters with diameter 1ndash300 km was calculatedusing the present day flux curve scaled by a factor of 50 toapproximate the inferred flux at 40 Ga (Neukum et al 2001)and scaled to the entire lunar surface Craters gt300 km are the42 known or suspected lunar basins listed by Wilhelms(1987) all of which formed earlier than sim38 Ga Thiscalculation thus compares the instantaneous number ofle300 km craters created at 40 Ga to the basins as if they wereall instantaneously created at the same time which is meant tobe illustrative rather than a realistic scenario Each transientcrater diameter was calculated from the final crater diameter(Croft 1985 Melosh 1989 Kring 1995) using scaling lawsfor simple craters up to a crater diameter of 18 km and forcomplex craters for larger craters (including multiringedbasins) Using alternative scaling laws (eg Kring 1995) hasa negligible effect on this calculation The volume of impactmelt was calculated based on the transient crater diameter(Cintala and Grieve 1998) the calculation of which is similarfor a wide range of projectile types and impact velocities Thevolume calculated in this way is comparable to that modeledby Pierazzo et al (1997) for typical asteroid impact velocities(20 kmsec) thought to be most important in the lunar case(Kring and Cohen 2001 Swindle and Kring 2001) Thevolume of melt created by each crater was multiplied by thenumber of craters of that size to arrive at the total amount ofimpact melt created in each size bin

This calculation illustrates that basin-generated impactmelt should be dominant in and near the basins themselves asthey may eject a sizeable fraction of their impact melt out intoejecta blankets (Warren 1996) The extent and thickness ofthe Imbrium ejecta blanket has been offered as an explanation


for the dominant 39 Ga age in the Apollo impact-melt rocksassuming most or all the mafic impact-melt rocks in theApollo collection were created in the Imbrium event anddistributed to collection sites in its ejecta blanket (Haskin1998) However it is improbable that Imbrium or otherimpacts into the PKT created the feldspathic KREEP-poorclasts in the feldspathic meteorites particularly the 39 Gaclasts in MAC 88105 In fact more than half the identifiablelunar basins formed in the feldspathic highlands and soshould have created a large amount of feldspathic KREEP-poor impact melt in and near these basins The feldspathichighlands are not currently covered by the Th-rich ejectablanket meaning the feldspathic breccia meteorites with thelowest Th contents such as those used in this study stand thebest chance of incorporating feldspathic basin melt into them

Because the youngest basin Orientale is sim38 Ga(Wilhelms 1987 Stˆffler and Ryder 2001) impact-meltsamples with younger ages necessarily come from smallercraters Smaller craters are widespread over the lunar surfaceand generate an appreciable volume of impact melt only anorder of magnitude less than all the basins combined (Fig 7)Some 20ndash30 craters gt100 km and more than 60 smaller freshcraters can be geologically identified as post-Imbrian in age(Wilhelms 1987 Grier et al 2001) These smaller cratersrsquo agesare reflected in the lunar spherules from Apollo 14 soils (Culleret al 2000) A cluster of well-defined spherule ages at 39 Gais attributable to the proximity of the Apollo 14 soil sample tothe Imbrium created Fra Mauro formation but the spheruleages reflect smaller impacts occurring throughout time

The lack of very recent impact-melted material in thesemeteorites compared with the ubiquity of recent melt

spherules in the Apollo 14 soil implies that the meteoritebreccias were closed to input of new material at around thetime of the youngest clast within them (sim25 Ga) The lunarejection time based on cosmic ray exposure (016 Ma forQUE 93069 027 Ma for MAC 88105 015 Ma for DaG 262)(Warren 1994 Thalmann et al 1996 Bischoff et al 1998) ismuch later than the apparent breccia closure time Thisimplies that either the breccia was lithified at 25 Ga andlaunched by a separate impact event or that the brecciacomponents were buried deeply and effectively closed to latermaterial at around 25 Ga possibly allowing for a singleimpact event to cause both lithification and launch Becausethe breccia site remained open to new materials until wellafter the near-side basin forming impacts and yet contain noKREEPy clasts derived from the PKT these meteorites musthave formed in locations well removed from the nearsidebasins which ejected Th-rich material hundreds of kilometersover the lunar surface

This work intended to test the lunar cataclysm hypothesisby measuring the ages of a large number of impact-meltsamples Out of 31 different samples representing at leastseven to nine different impact events no impact-melt clastwith an age more than 1σ older than 39 Ga was found Thecontinued absence of impact-melt samples older than 39 Gasupports though does not prove the existence of a lowimpactor flux prior to 39 Ga The original ldquolunar cataclysmrdquoas proposed by Tera et al (1974) envisioned an increase inthe lunar cratering flux over the time span of sim200 Myr Ryder(1990) has argued that based on the stratigraphy and apparentages of the lunar basins the cataclysmic event lasted for aslittle as 10ndash20 Myr However the peak in the lunar meteorite

Fig 7 The relative impact-melt volume contributions from lunar basins and smaller craters Filled squares show the volume of impact meltgenerated by all craters in each size bin (D) open circles show the cumulative amount of impact melt created by all craters of diameter D andless The number of craters up to 300 km diameter is calculated from the crater curve (see text) roughness in the trends among the larger basinsreflects the small numbers of currently visible basins at these sizes In particular there are no visible basins with diameters between 1200 km(Imbrium) and 2500 km (South PolendashAitken) While basins generate the most impact melt smaller craters create appreciable volumes ofimpact melt

774 B A Cohen et al

impact-melt clast age distribution at 35 Ga is 400 Myryounger than the basinsrsquo age of 39 Ga Neither the smoothdecline or terminal cataclysm hypothesis as we currentlyunderstand them predict the peak of ages of impact-meltsamples from lunar meteorites and spherules at 35 Ga Ifthese impact-melt clasts were created in the same cataclysmiccratering event as the lunar basins a number of smallerimpactors must have been available until 35 Ga possibly asa tailing off of the major cataclysmic impactor flux thoughcrater density observations appear to preclude this possibility(Stˆffler and Ryder 2001) It is interesting to note howeverthat evidence may exist for several impact events on the Earthat this time (Byerly et al 2002)

It may be that assembly of the meteorites in the upperlayers of the lunar regolith skews the clast population towardsmall young local impact events whereas hand samplecollection may favor large chunks of melt rocks derived fromthe largest nearby impact More analyses of impact-meltclasts in breccias or soil samples identified using the samecriteria as in the meteorites along with modeling of impact-melt distribution in the lunar regolith may be able to clarifythis discrepancy Another test would be to directly sample theimpact-melt sheet of a large lunar basin The South PolendashAitken basin with a diameter of 2500 km probably createdmore impact melt than all other lunar craters combined(Fig 7) Though no South PolendashAitken basin impact-meltrock has yet been identified in the Apollo Luna or meteoritecollections a large amount of melt probably still resides onthe basin floor (Pieters et al 2001) and could be directlysampled by a robotic mission (Duke 2003)

Impact ages in ordinary chondrites and HED meteorites(Bogard 1995) and the martian meteorite ALH 84001 (Ashet al 1996) tungsten isotopic anomalies in the Isuametasediments on the Earth (Schoenberg et al 2002) and theinferred age of Mercuryrsquos Caloris basin (from crater countingbased on the lunar flux Neukum et al 2001) suggest that the39 Ga event affected the entire inner solar system (Kring andCohen 2001) More age data on asteroidal impact-melt rocksand Mars are essential to clarifying this implication If truethe population of impactors could not have been in geocentricorbit An event of this magnitude cannot be explained by ourcurrent view of solar system formation and evolution butrequires an extraordinary and perhaps singular event (seereviews by Hartmann et al 2000 Dones 2002) Afteraccretion of the terrestrial planets impact by the leftoverplanetesimals would have occurred within 100 Myr of lunarformation (Morbidelli et al 2001) too early to causewidespread bombardment at 39 Ga Breakup of a main beltasteroid near a resonance can produce showers lasting from 5to 80 Myr (Zappala et al 1998) but to produce several basinforming impacts on the Moon collisional disruption of anasteroid larger than Ceres is required and is dynamicallyunlikely at 39 Ga (Wetherill 1975) Other models such asscattering of planetesimals by the late formation of Uranus

and Neptune (Levison et al 2001) effects of a short-livedfifth terrestrial planet (Chambers and Lissauer 2002) andinteraction of our solar system with galactic objects (Napierand Clube 1979) may be able to produce a cataclysmicbombardment of consistent magnitude and timing but requirefurther work and tests related to their viability

The lunar cataclysm hypothesis if true has far-reachingconsequences Because the Earth has a larger gravitationalcross-section than the Moon the number of impacts occurringon Earth would have been at least an order of magnitudelarger than on the Moon The impact cataclysm is also nearlycoincident with the earliest isotopic evidence of life 385 Ga(Mojzsis and Harrison 2000) suggesting that the bombardingasteroids may have affected the biologic evolution of EarthThe effect may have been detrimental by destroying existinglife or organic fragments (Maher and Stevenson 1988 Sleepand Zahnle 1998) or beneficial by delivering precursormolecules (Pierazzo and Chyba 1999) and providing suitableenvironments for evolution (Kring 2000) Either way acatastrophic bombardment of the Earth-Moon system musthave affected the origin and evolution of life

AcknowledgmentsndashThis work was partially supported by aNASA Space Grant Fellowship (BAC) NASA grantsNAG5-4767 (T Swindle) and NAG5-4944 (D Kring) andNASArsquos astrobiology program via a subcontract fromArizona State University to the University of Arizona(D Kring) We appreciate sample loans from A Bischoff atthe Institute of Planetology University of Mcedilnster Germanyand J Zipfel at Max-Planck-Institut fcedilr Chemie GermanyThis manuscript benefited from reviews by Don BogardRandy Korotev Ernst Zinner Marc Caffee and ananonymous reviewer and from discussions with SuzanneBaldwin Bill Hartmann and Graham Ryder We used theNASA Astrophysical Data System Abstract Service

Editorial HandlingmdashDr Marc Caffee

REFERENCES

Ash R D Knott S F and Turner G 1996 A 4-Gyr shock age for amartian meteorite and implications for the cratering history ofMars Nature 38057ndash59

Baldwin R B 1949 The face of the Moon Chicago University ofChicago Press pp 239

Bischoff A 1996 Lunar meteorite Queen Alexandra Range 93069A lunar highland regolith breccia with very low abundances ofmafic components Meteoritics amp Planetary Science 31849ndash855

Bischoff A Weber D Clayton R N Faestermann T Franchi I AHerpers U Knie K Korschinek G Kubik P W Mayeda T KMerchel S Michel R Neumann S Palme H Pillinger C TSchultz L Sexton A S Spettel B Verchovsky A B WeberH W Weckwerth G and Wolf D 1998 Petrology chemistryand isotopic compositions of the lunar highland regolith brecciaDar al Gani 262 Meteoritics amp Planetary Science 331243ndash1257


Bogard D D 1995 Impact ages of meteorites A synthesisMeteoritics 30244ndash268

Bogard D D Garrison D H Norman M Scott E R D and Keil K1995 39Ar-40Ar age and petrology of Chico Large-scale impactmelting on the L chondrite parent body Geochimica etCosmochimica Acta 591383ndash1399

Bukovanska M Dobosi G Brandstpermiltter F and Kurat G 1999 Daral Gani 400 Petrology and geochemistry of some majorlithologies (abstract) Meteoritics amp Planetary Science 34A21

Byerly G R Lowe D R Wooden J L and Xie X 2002 AnArchaean impact layer from the Pilbara and Kaapvaal cratonsScience 2971325ndash1327

Cadogan P H and Turner G 1977 40Ar-39Ar dating of Luna 16 andLuna 20 samples Philosophical Transactions of the RoyalSociety of London A 284167ndash177

Cahill J T Floss C Anand M Taylor L A Nazarov M A andCohen B A 2004 Petrogenesis of lunar highlands meteoritesDhofar 025 Dhofar 081 Dar al Gani 262 and Dar al Gani 400Meteoritics amp Planetary Science 39503ndash529

Chambers J E and Lissauer J J 2002 A new dynamical model forthe lunar late heavy bombardment (abstract 1093) 33rd Lunarand Planetary Science Conference CD-ROM

Chapman C R Cohen B A and Grinspoon D H 2002 What arethe real constraints on commencement of the Late heavybombardment (abstract 1627) 33rd Lunar and PlanetaryScience Conference CD-ROM

Chapman C R Cohen B A and Grinspoon D H ForthcomingWhat are the real constraints on the existence and magnitude ofthe late heavy bombardment Icarus

Cintala M J and Grieve R A F 1998 Scaling impact melting andcrater dimensions Implications for the lunar cratering recordMeteoritics amp Planetary Science 33889ndash912

Cohen B A 2000 Geochemistry and 40Ar-39Ar geochronology oflunar meteorite impact-melt clasts PhD thesis The Universityof Arizona Tucson Arizona USA

Cohen B A Snyder G A Hall C M Taylor L A and NazarovM A 2001 Argon-40ndashargon-39 chronology and petrogenesisalong the eastern limb of the Moon from Luna 16 20 and 24samples Meteoritics amp Planetary Science 361345ndash1366

Cohen B A Swindle T D and Kring D A 2000 Support for thelunar cataclysm hypothesis from lunar meteorite impact-meltages Science 2901754ndash1756

Croft S K 1985 The scaling of complex craters Proceedings 15thLunar and Planetary Science Conference pp 828ndash842

Culler T S Becker T A Muller R A and Renne P R 2000 Lunarimpact history from 40Ar39Ar dating of glass spherules Science2871785ndash1788

Cushing J A Taylor G J Norman M D and Keil K 1999 Thegranulitic impactite suite Impact melts and metamorphicbreccias of the early lunar crust Meteoritics amp Planetary Science34185ndash195

Dalrymple G B and Ryder G 1993 40Ar39Ar age spectra of Apollo15 impact-melt rocks by laser step-heating and their bearing onthe history of lunar basin formation Journal of GeophysicalResearch 9813085ndash13095

Dalrymple G B and Ryder G 1996 Argon-40argon-39 age spectraof Apollo 17 highlands breccia samples by laser step heating andthe age of the Serenitatis basin Journal of Geophysical Research10126069ndash26084

Delano J W 1991 Geochemical comparison of impact glasses fromlunar meteorites ALH A81005 and MAC 88105 and Apollo 16regolith 64001 Geochimica et Cosmochimica Acta 553019ndash3029

Deutsch A and Schpermilrer U 1994 Dating terrestrial impact eventsMeteoritics 29301ndash322

Deutsch A and Stˆffler D 1987 Rb-Sr analyses of Apollo 16 melt

rocks and a new age for the Imbrium basin Lunar basinchronology and the early heavy bombardment of the moonGeochimica et Cosmochimica Acta 511951ndash1964

Dones L 2002 Dynamics of possible late heavy bombardmentimpactor populations (abstract 1662) 33rd Lunar and PlanetaryScience Conference CD-ROM

Duke M B 2003 Sample return from the lunar South PolendashAitkenBasin Advances in Space Research 312347ndash2352

Eugster O Beer J Burger M Finkel R C Hofmann H JKrahenbcedilhl U Michel T Synal H A and Wolfli W 1991History of the paired lunar meteorites MAC 88104 and MAC88105 derived from noble gas isotopes radionuclides and somechemical abundances Geochimica et Cosmochimica Acta 553139ndash3148

Eugster O Polnau E Salerno E and Terribilini D 2000 Lunarsurface exposure models for meteorites Elephant Moraine 96008and Dar al Gani 262 from the Moon Meteoritics amp PlanetaryScience 351177ndash1181

Eugster O Terribilini D Polnau E and Kramers J 2001 Theantiquity indicator argon-40argon-36 for lunar samplescalibrated by uranium-235ndashxenon-136 dating Meteoritics ampPlanetary Science 361097ndash1115

Fernandes V A Burgess R and Turner G 2000 Laser argon-40-argon-39 age studies of Dar al Gani 262 meteorite Meteoritics ampPlanetary Science 351355ndash1364

Floss C and Crozaz G 2001 Terrestrial alteration of lunar meteoritesDar al Gani 262 and 400 (abstract 1105) 32nd Lunar andPlanetary Science Conference CD-ROM

Gladman B J Burns J A Duncan M and Levison H 1995 Thedynamical evolution of lunar impact ejecta Icarus 118302ndash321

Grier J A Kring D A and Swindle T D 1995 Impact melts andanorthositic clasts in lunar meteorites QUE 93069 and MAC88105 (abstract) 26th Lunar and Planetary Science Conferencepp 513ndash514

Grier J A McEwen A S Lucey P G Milazzo M and Strom R G2001 Optical maturity of ejecta from large rayed lunar cratersJournal of Geophysical Research 10632847ndash32862

Grinspoon D H 1989 Large impact events and atmosphericevolution on the terrestrial planets PhD thesis The Universityof Arizona Tucson Arizona USA

Hartmann W K 1975 Lunar ldquocataclysmrdquo A misconception Icarus24181ndash187

Hartmann W K 2003 Megaregolith evolution and crateringcataclysm models Lunar cataclysm as a misconception (28 yearslater) Meteoritics amp Planetary Science 38579ndash593

Hartmann W K Ryder G Dones L and Grinspoon D 2000 Thetime-dependent intense bombardment of the primordial Earth-Moon system In Origin of the Earth and Moon edited by CanupR M and Righter K Tucson Arizona The University ofArizona Press pp 493ndash512

Haskin L A 1998 The Imbrium impact event and thoriumdistribution at the lunar highlands surface Journal ofGeophysical Research 1031679ndash1689

Haskin L A Korotev R L Rockow K M and Jolliff B L 1998The case for an Imbrium origin of the Apollo Th-rich impact-melt breccias Meteoritics amp Planetary Science 33959ndash975

Hohenberg C M Marti K Podosek F A Reedy R C and ShirckJ R 1978 Comparisons between observed and predictedcosmogenic noble gases in lunar samples Proceedings 9th Lunarand Planetary Science Conference pp 2311ndash2344

Huneke J C and Smith S P 1976 The realities of recoil 39Ar recoilout of small grains and anomalous age patterns in 39Ar-40Ardating Proceedings 7th Lunar Science Conference pp 1987ndash2008

Jolliff B L Gillis J J Haskin L A Korotev R L and WieczorekM A 2000 Major lunar crustal terranes Surface expressions

776 B A Cohen et al

and crust-mantle origins Journal of Geophysical Research 1054197ndash4216

Jolliff B L Korotev R L and Arnold S A 1999 Electronmicroprobe analyses of Dar al Gani lunar meteorite a sample ofthe feldspathic highlands terrane of the Moon (abstract 2000)30th Lunar and Planetary Science Conference CD-ROM

Jolliff B L Korotev R L and Haskin L A 1991 A ferroan regionof the lunar highlands as recorded in meteorites MAC 88104 andMAC 88105 Geochimica et Cosmochimica Acta 553051ndash3071

Koeberl C Kurat G and Brandstpermiltter F 1991 MAC 88105mdashAregolith breccia from the lunar highlands Mineralogicalpetrological and geochemical studies Geochimica etCosmochimica Acta 553073ndash3087

Koeberl C Kurat G and Brandstpermiltter F 1996 Mineralogy andgeochemistry of lunar meteorite Queen Alexandra Range 93069Meteoritics amp Planetary Science 31897ndash908

Korotev R L 1994 Compositional variation in Apollo 16 impact-melt breccias and inferences for the geology and bombardmenthistory of the Central Highlands of the Moon Geochimica etCosmochimica Acta 583931ndash3969

Korotev R L 2000 The great lunar hot spot and the composition andorigin of the Apollo mafic (ldquoLKFMrdquo) impact-melt brecciasJournal of Geophysical Research 1054317ndash4345

Korotev R L Jolliff B L and Rockow K M 1996 Lunar meteoriteQueen Alexandra Range 93069 and the iron concentration of thelunar highlands surface Meteoritics amp Planetary Science 31909ndash924

Korotev R L Jolliff B L Zeigler R A Gillis J J and HaskinL A 2003 Feldspathic lunar meteorites and their implicationsfor compositional remote sensing of the lunar surface and thecomposition of the lunar crust Geochimica et CosmochimicaActa 674895ndash4923

Kring D A 1995 The dimensions of the Chicxulub impact crater andimpact-melt sheet Journal of Geophysical Research 10016979ndash16986

Kring D A 2000 Impact events and their effect on the originevolution and distribution of life GSA Today 101ndash7

Kring D A and Cohen B A 2001 Cataclysmic bombardmentthroughout the inner solar system 39ndash40 Ga Journal ofGeophysical Research 107 doi1010292001JE001529

Kring D A Cohen B A Swindle T D and Hill D H 2000Regolith breccia (Ourique) with impact-melt clasts and otherdebris from an H chondrite parent body (abstract 1688) 31stLunar and Planetary Science Conference CD-ROM

Kring D A Hill D H and Boynton W V 1995 The geochemistryof a new lunar meteorite QUE 93069 a breccia with highlandaffinities (abstract) 26th Lunar and Planetary ScienceConference pp 801ndash802

Levison H F Dones L Chapman C R Stern S A Duncan M Jand Zahnle K 2001 Could the lunar late heavy bombardmenthave been triggered by the formation of Uranus and NeptuneIcarus 151286ndash306

Lindstrom M M 1989 Antarctic Meteorite Newsletter vol 12HoustonNASA Johnson Space Center

Lindstrom M M 1994 Antarctic Meteorite Newsletter vol 17Houston NASA Johnson Space Center

Lindstrom M M Wentworth S J Martinez R R MittlefehldtD W McKay D S Wang M and Lipschutz M E 1991Geochemistry and petrography of the MacAlpine Hills lunarmeteorites Geochimica et Cosmochimica Acta 553089ndash3103

Lofgren G E 1977 Dynamic crystallization experiments bearing onthe origins of textures in impact-generated liquids Proceedings8th Lunar Science Conference pp 2079ndash2095

Lucey P G Taylor G J and Malaret E 1995 Abundance anddistribution of Fe on the Moon Science 2681150ndash1153

Maher K A and Stevenson D J 1988 Impact frustration of the

origin of life Nature 331612ndash614Mark R K Lee-Hu C and Wetherill G W 1974 Rb-Sr age of lunar

igneous rocks 62295 and 14310 Geochimica et CosmochimicaActa 381643ndash1648

McConville P Kelley S and Turner G 1988 Laser probe 40Ar-39Arstudies of the Peace River shocked L6 chondrite Geochimica etCosmochimica Acta 522487ndash2499

McDougall I and Harrison T M 1999 Geochronology andthermochronology by the 40Ar39Ar method New York OxfordUniversity Press 269 p

McDougall I and Roksandic Z 1974 Total fusion 40Ar39Ar agesusing HIFAR reactor Journal of the Geological Society ofAustralia 2181ndash89

McKay D S Bogard D D Morris R V Korotev R L and JohnsonP 1986 Apollo 16 regolith brecciasmdashCharacterization andevidence for early formation in mega-regolith Journal ofGeophysical Research 91D277ndashD303

McKay G A Wiesmann H Bansal B M and Shih C 1979Petrology chemistry and chronology of Apollo 14 KREEP basalts10th Lunar and Planetary Science Conference pp 181ndash205

Melosh H J 1989 Impact cratering A geologic process New YorkOxford University Press 245 p

Mojzsis S J and Harrison T M 2000 Vestiges of a beginning Cluesto the emergent biosphere recorded in the oldest knownsedimentary rocks GSA Today 101ndash6

Morbidelli A Petit J-M Gladman B and Chambers J 2001 Aplausible cause of the late heavy bombardment Meteoritics ampPlanetary Science 36371ndash380

Napier W M and Clube S V M 1979 A theory of terrestrialcatastrophism Nature 282455ndash459

Neal C R Taylor L A Lui Y-G and Schmitt R A 1991 Pairedlunar meteorites MAC 88104 and MAC 88105 A new ldquoFANrdquo oflunar petrology Geochimica et Cosmochimica Acta 553037ndash3049

Neukum G Ivanov B A and Hartmann W K 2001 Crateringrecords in the inner solar system in relation to the lunar referencesystem In Chronology and evolution of Mars edited byKallenbach R Geiss J and Hartmann W K pp 55ndash86

Nishiizumi K Arnold J R Klein J Fink D Middleton R KubikP W Sharma P Elmore D and Reedy R C 1991 Exposurehistories of lunar meteorites ALHA81005 MAC 88104 MAC88105 and Y-791197 Geochimica et Cosmochimica Acta 553149ndash3155

Nishiizumi K Caffee M W Jull A T J and Reedy R C 1996Exposure history of lunar meteorites Queen Alexandra Range93069 and 94269 Meteoritics amp Planetary Science 31893ndash896

Norman M D Bennett V C and Ryder G 2001 Highly siderophile(Re-PGE) and lithophile element geochemistry of Apollo 17LKFM impact melts (abstract 1418) 32nd Lunar and PlanetaryScience Conference CD-ROM

Papanastassiou D A and Wasserburg G J 1971 Rb-Sr ages ofigneous rocks from Apollo 14 mission and age of Fra Mauroformation Earth and Planetary Science Letters 1236ndash48

Pierazzo E and Chyba C F 1999 Amino acid survival in largecometary impacts Meteoritics amp Planetary Science 34909ndash918

Pierazzo E Vickery A M and Melosh H J 1997 A reevaluationof impact-melt production Icarus 127408ndash423

Pieters C M Head J W III Gaddis L Jolliff B and Duke M2001 Rock types of South PolendashAitken basin and extent ofbasaltic volcanism Journal of Geophysical Research 10628001ndash28022

Podosek F A Huneke J C Gancarz A J and Wasserburg G J1973 The age and petrography of two Luna 20 fragments andinferences for widespread lunar metamorphism Geochimica etCosmochimica Acta 37887ndash904

Polnau E and Eugster O 1998 Cosmic-ray-produced radiogenic


and solar noble gases in lunar meteorites Queen AlexandraRange 94269 and 94281 Meteoritics amp Planetary Science 33313ndash319

Renne P R Swisher C C Deine A L Karner D B Owens T Land Depaolo D J 1998 Intercalibration of standards absoluteages and uncertainties in 40Ar39Ar dating Chemical Geology145117ndash152

Ryder G 1990 Lunar samples lunar accretion and the earlybombardment of the Moon Eos 71313 322ndash323

Scherer P Ppermiltsch M and Schultz L 1998 Noble gas study of thenew lunar highland meteorite Dar al Gani 400 Meteoritics ampPlanetary Science 33A135ndashA136

Schoenberg R Kamber B S Collerson K D and Moorbath S2002 Tungsten isotope evidence from sim38-Gyr metamorphosedsediments for early meteorite bombardment of the Earth Nature418403ndash405

Semenova A S Nazarov M A Kononkova N N Patchen A andTaylor L A 2000 Mineral chemistry of lunar meteorite Dar alGani 400 (1252) 31st Lunar and Planetary Science ConferenceCD-ROM

Sleep N H and Zahnle K 1998 Refugia from asteroid impacts onearly Mars and the early Earth Journal of Geophysical Research10328529ndash28544

Stettler A Eberhardt P Geiss J Grogler N and Maurer P 1973Ar39-Ar40 ages and Ar37-Ar38 exposure ages of lunar rocksProceedings 4th Lunar Science Conference pp 1865ndash1888

Stˆffler D and Ryder G 2001 Stratigraphy and isotope ages of lunargeologic units Chronology and standard for the inner solarsystem Space Science Reviews 969ndash54

Stˆffler D A Bischoff A Borchardt R Burghele A Deutsch AJeflberger E K Ostertag R Palme H Spettel B Reimold WU Wacker K and Wpermilnke H 1985 Composition and evolutionof the lunar crust in the Descartes Highlands Apollo 16Proceedings 15th Lunar and Planetary Science Conference pp449ndash506

Swindle T D and Kring D A 2001 Cataclysm + cold comets = lotsof asteroid impacts (abstract 1466) 32nd Lunar and PlanetaryScience Conference CD-ROM

Swindle T D Spudis P D Taylor G J Korotev R L and NicholsR H Jr 1991 Searching for Crisium Basin ejectamdashChemistryand ages of Luna 20 impact melts Proceedings 21st Lunar andPlanetary Science Conference pp 167ndash181

Taylor G J 1991 Impact melts in the MAC 88105 lunar meteoriteInferences for the lunar magma ocean hypothesis and thediversity of basaltic impact melts Geochimica et CosmochimicaActa 553031ndash3036

Tera F Papanstassiou D A and Wasserburg G J 1974 Isotopicevidence for a terminal lunar cataclysm Earth and PlanetaryScience Letters 221ndash21

Thalmann C Eugster O Herzog G F Klein J Krahenbcedilhl UVogt S and Xue S 1996 History of lunar meteorites QueenAlexandra Range 93069 Asuka-881757 and Yamato-793169based on noble gas isotopic abundances radionuclideconcentration and chemical composition Meteoritics ampPlanetary Science 31857ndash868

Turner G 1971 40Ar-39Ar ages from the lunar maria Earth andPlanetary Science Letters 11169ndash191

Turner G 1972 40Ar-39Ar age and cosmic ray irradiation history ofthe Apollo 15 anorthosite 15415 Earth and Planetary ScienceLetters 14169ndash175

Turner G and Cadogan P H 1974 Possible effects of 39Ar recoil on40Ar-39Ar dating Proceedings 5th Lunar Science Conferencepp 1601ndash1615

Turner G Cadogan P H and Yonge C J 1973 Argonselenochronology Proceedings 4th Lunar Science Conferencepp 1889ndash1914

Turner G Huneke J C Podosek F A and Wasserburg G J 197140Ar-39Ar ages and cosmic ray exposure ages of Apollo 14samples Earth and Planetary Science Letters 1219ndash35

Warren P H 1994 Lunar and Martian meteorite delivery servicesIcarus 111338ndash363

Warren P H 1996 Global inventory of lunar impact melt as afunction of parent crater size Lunar and Planetary Science 271379ndash1380

Warren P H 1997 The unequal host-phase density effect in electronprobe defocused beam analysis An easily correctable problem(abstract) 28th Lunar and Planetary Science Conference pp1497ndash1498

Warren P H and Kallemeyn G W 1991 The MacAlpine Hills lunarmeteorite and implications of the lunar meteorites collectively forthe composition and origin of the Moon Geochimica etCosmochimica Acta 553123ndash3138

Wetherill G W 1975 Late heavy bombardment of the moon andterrestrial planets Proceedings 6th Lunar Science Conferencepp 1539ndash1561

Wilhelms D E 1987 The geologic history of the Moon USGSProfessional Paper 1348

York D Kenyon W J and Doyle R J 1972 40Ar-39Ar ages ofApollo 14 and 15 samples Proceedings 3rd Lunar ScienceConference pp 1613ndash1622

Zappala V Cellino A Gladman B J Manley S and Migliorini F1998 Asteroid showers on Earth after family breakup eventsIcarus 134176ndash179

Zipfel J Spettel B Palme H Wolf D Franchi I Sexton A SPillinger C T and Bischoff A 1998 Dar al Gani 400 Chemistryand petrology of the largest lunar meteorite (abstract)Meteoritics amp Planetary Science 33A171

756 B A Cohen et al

are so rapidly quenched that 40Ar often does not have time toescape leading to irregular degassing profiles anduninterpretable ages (McConville et al 1988 Cohen et al2001) Passage of the shock front and subsequent cooling isalso rapid preventing total degassing in shocked samples(Deutsch and Schpermilrer 1994 Bogard et al 1995) At the otherextreme rocks exposed to a small elevation in temperature fora long time often show characteristic signs of 40Ar diffusiveloss without complete resetting such as granulitic impactites(Cushing et al 1999) and rocks exposed to daily thermalcycling on the lunar surface (Turner 1971 Turner et al 1971)Crystalline impact-melt rocks which have been completelymelted and slowly cooled are the most useful in revealingabsolute impact datesmdashcritical information in determining thelarge-scale bombardment history of the Moon

Crystalline clast-free impact-melt rocks occur in thereturned Apollo and Luna sample collections as rocks clastsin breccias and soil fragments They can be distinguishedfrom lunar igneous rocks by their unusual chemicalcompositions (combinations of target materials unable to bederived from igneous fractionation processes) and uniquetextures (haystack poikilitic) The largest example 14310(3439 g) has nearly identical 40Ar-39Ar and internal Rb-Srisochron ages of 388 Ga (York et al 1972 Mark et al 1974McKay et al 1979) Melt rocks 684156 (544 g total) also havea well-constrained Rb-Sr internal isochron of 384 plusmn 001 Ga(Papanastassiou and Wasserburg 1971) and a concurrent 40Ar-39Ar age (Stettler et al 1973) demonstrating the ability of the40Ar-39Ar technique to accurately date impact-melt rocksSmaller rocks and breccia clasts are not large enough to permitmineral separation and the majority of impact ages have beenobtained using only 40Ar-39Ar systematics The availability oflaser heating in 40Ar-39Ar work inspired new very precise agedeterminations of Apollo 15 and 17 impact-melt samples(Dalrymple and Ryder 1993 1996) These samples wereinferred to have been formed in the Serenitatis basin at 389 Gaand give an upper limit on the age of the Imbrium basin of 385Ga Formation of the Crisium basin at 389 Ga has beeninferred by dating melt rocks in the Luna 20 collection(Podosek et al 1973 Swindle et al 1991) Together with theinferred radiometric age of Nectaris at sim392 Ga and craterdensity age of Orientale at le375 Ga (Wilhelms 1987 Stˆfflerand Ryder 2001) five large basins were formed on the Moonwithin 200 Ma though subtle variations in age and traceelement chemistry among the dated samples may argue formore or fewer impact events creating all the samples(Dalrymple and Ryder 1993 Korotev 1994 Haskin et al1998)

The preponderance of 39 Ga impact-melt rock ages andthe lack of older samples of unequivocal impact origin arehighly suggestive of an extraordinary bombardment event atsim39 Ga However the Apollo and Luna sample sites were allin or near an area called the Procellarum KREEP terrain(PKT) identified by the surface expression of elevated

thorium levels and by inference other incompatible elementssuch as K P and rare earths (KREEP) (Jolliff et al 2000) Theimpact-melt rocks that have been dated from these collectionsare predominantly mafic KREEP-rich samples which affordenough radiogenic elements to be feasibly dated but whichmay be dominated by the large amount of melt created in thenearside basins Nectaris Crisium Serenitatis and ImbriumHaskin et al (1998) and Korotev (2000) have argued that themafic KREEP-bearing impact-melt rocks in the Apollo 14ndash17samples are a special product of impacts into the PKT and notproduced by impact melting of the lower crust under typicalfeldspathic highlands These samples may not includeevidence of earlier events and thus the age distributionobtained from these samples may not reflect the global impactrecord on the Moon Lunar meteorites which were onlyrecognized after the Apollo missions provide a newopportunity to test the hypothesis These are samples of theMoon randomly ejected from the lunar surface (Warren andKallemeyn 1991) without enough velocity to escape the Earth-Moon system They experienced only mild shock duringlaunch (Warren 1994) and typically landed on Earth afterle1 Ma in space (Warren 1994 Gladman et al 1995)Meteorites that are chemically distinct from the Apollo maficimpact-melt samples may not have been affected by nearsideequatorial basin impacts Instead impact-melt clasts withinthese meteorites may have been formed in large impact eventsin other regions of the Moon as suggested by Taylor (1991)for MacAlpine Hills (MAC) 88105

There are two objectives in this study The first is toidentify impact-melt clasts in lunar highland meteorites thatare chemically different from the mafic KREEP-rich Apolloand Luna impact-melt rocks Candidate samples have texturessimilar to well-known rocks of impact origin establishingtheir origins in slowly cooled impact-melt sheets but arefeldspathic and lacking in basaltic or KREEPy contributionsindicating their formation either before the formation of thePKT or far from the Apollo sampling sites The secondobjective is to determine the ages of the impact-melt clastsusing 40Ar-39Ar techniques The impact-melt samples occur assmall (generally lt500 microm) clasts in the meteorite breccias sospecial handling techniques for extracting and analyzing thesamples had to be developed for this task These techniquesallow us to date a large number of clasts within a singlemeteorite looking for multiple impact events in a single rockand contributing 31 new impact-melt sample ages astatistically significant number to test the lunar cataclysmhypothesis If the Apollo and Luna samples are biased becauseof their proximity to one or more large nearside basins wewould expect to see a more random distribution of lunarimpact-melt clast ages from the feldspathic breccia meteoritesWe gave a brief report on these sample ages in Cohen et al(2000) In this paper we give a full treatment of thegeochemical and geochronological data that led to theconclusions in that paper For more details see Cohen (2000)


METEORITES








GEOCHEMISTRY

Technique




758 B A Cohen et al






Results





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GEOCHRONOLOGY

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ble

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of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al











































































METEORITES








GEOCHEMISTRY

Technique




758 B A Cohen et al






Results





760 B A Cohen et al








GEOCHRONOLOGY

Technique










ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

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C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

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022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

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C H

4344

30

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40

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890

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05

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1544

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680

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B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al










































































758 B A Cohen et al






Results





760 B A Cohen et al








GEOCHRONOLOGY

Technique










ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al












































































Results





760 B A Cohen et al








GEOCHRONOLOGY

Technique










ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al










































































760 B A Cohen et al








GEOCHRONOLOGY

Technique










ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al













































































GEOCHRONOLOGY

Technique










ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al











































































ble

3 M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

MA

C A

1244

60

1528

50

07n

d4

820

074

9016

50

380

040

0310

01

MA

C B

3344

50

1432

70

06n

d2

220

042

2018

30

350

020

0410

06

MA

C C

2044

00

0834

70

040

031

210

031

0918

90

33n

d0

0310

04

MA

C D

1544

40

1233

30

040

031

370

021

5618

70

350

060

0410

00

MA

C E

1144

00

0634

20

030

031

320

021

5918

90

360

020

0310

06

MA

C F

1144

60

1332

30

030

022

670

052

1018

20

34n

d0

0410

05

MA

C G

744

70

1930

90

040

033

320

053

1717

90

350

020

0410

07

MA

C H

4344

30

1233

40

030

031

890

031

5918

60

380

030

0510

05

QU

E A

1544

00

2129

70

090

024

680

073

4717

30

42n

d0

0310

00

QU

E B

3444

10

2031

50

060

023

330

063

3517

70

330

020

0310

07

QU

E D

4643

70

1531

00

070

023

630

063

1117

50

320

020

0399

6Q

UE

F212

443

007

310

007

nd

288

007

207

185

039

002

003

994

QU

E F

2844

20

3027

70

120

035

610

095

3315

80

380

050

1299

9Q

UE

G15

447

020

259

009

002

617

011

554

171

024

nd

003

100

1Q

UE

I13

439

009

324

004

002

240

003

238

180

035

nd

002

997

262

A3

443

043

217

020

002

915

013

103

213

20

290

070

0899

926

2 E

546

90

3524

80

140

045

500

117

8114

90

310

040

0510

10

262

F3

439

018

294

008

003

468

007

588

160

033

005

010

100

626

2 H

445

40

2631

20

10n

d2

620

053

0217

80

400

060

0610

09

262

I4

444

024

293

003

002

367

005

414

169

043

009

010

993

262

J4

435

016

322

005

003

176

004

236

184

036

005

008

990

262

M4

482

052

227

021

002

788

013

522

141

045

016

011

996

262

N5

439

006

322

002

003

294

004

284

183

034

003

006

100

726

2 O

441

60

0426

70

030

035

510

0711

49

140

026

002

004

998

262

P5

443

021

259

013

018

759

010

757

149

038

004

005

101

326

2 Q

210

447

032

289

009

002

406

005

424

169

039

006

013

999

262

R5

442

017

314

007

002

247

006

335

178

039

004

009

100

140

0 A

843

40

1828

80

100

054

600

075

3916

20

380

030

1199

240

0 A

24

440

033

282

007

nd

497

007

598

156

034

005

004

998

400

B8

434

017

272

012

004

604

007

605

158

036

007

005

994

400

C14

438

008

333

004

004

187

004

211

184

036

003

005

100

240

0 D

343

70

1926

10

070

055

900

096

3616

40

390

060

1199

540

0 E

543

90

1232

30

080

022

480

022

9617

80

380

040

0510

01

400

Eb29

429

014

294

006

002

434

007

468

168

035

004

005

989

400

F6

435

018

296

009

004

409

006

462

164

040

006

007

990

400

G5

429

017

283

006

002

531

007

574

158

041

006

008

990

400

H3

433

020

301

006

nd

348

006

421

174

035

003

009

994

400

I5

435

019

295

004

002

391

008

451

170

042

004

011

993

400

J3

435

012

313

004

nd

289

007

336

178

039

005

007

995

400

K6

435

013

316

005

002

256

002

309

180

036

004

006

993

400

L8

446

016

291

009

005

372

006

454

174

038

005

006

100

240

0 L1

543

40

0335

40

040

040

26n

d0

6619

10

350

030

1499

440

0 P

343

60

2029

40

08n

d3

850

064

8316

60

290

060

0799

140

0 Q

643

10

1330

50

08n

d3

630

053

8817

40

390

050

0799

440

0 Q

b17

431

027

301

011

003

349

005

412

171

035

005

009

989


400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al











































































400

R4

437

011

340

003

003

116

003

165

185

040

005

003

997

400

T5

431

014

286

011

003

445

008

550

167

035

005

030

995

400

Tb34

433

018

297

008

004

379

005

446

169

034

004

014

991

400

T29

439

014

307

004

nd

315

004

391

176

036

005

007

100

040

0 U

744

20

2030

10

070

033

560

073

9717

10

450

050

0599

840

0 V

443

80

2328

90

100

033

750

074

5317

30

410

080

0699

340

0 V

23

441

014

313

008

nd

321

005

380

174

036

005

008

100

540

0 V

2b25

432

017

284

008

012

461

006

608

162

034

006

005

994

400

AA

443

40

1731

70

07n

d2

470

033

0218

00

400

070

0599

440

0 B

B4

436

020

286

011

nd

403

006

504

165

042

006

012

987

400

EE3

425

nd

354

002

003

019

nd

032

204

029

nd

002

992

400

HH

1043

30

2528

50

100

034

400

075

0816

30

390

080

2598

740

0 JJ

343

50

2527

60

09n

d4

220

065

6416

70

420

090

1298

740

0 JJ

b19

434

021

291

011

003

412

007

481

167

036

005

010

990

σc0

20

020

20

070

080

120

002

800

20

020

060

13M

ean

sdd

10

012

37

006

003

246

004

271

18

006

002

006

Ran

ge

sdd

02ndash

44

001

ndash03

90

2ndash15

80

01ndash0

19

001

ndash04

90

03ndash1

08

001

ndash01

40

05 ndash

888

01ndash

57

001

ndash01

60

01ndash

007

001

ndash03

4

a The

num

ber o

f sep

arat

e d

efoc

used

-bea

m a

naly

ses (

95

lt to

tal lt

105

) a

vera

ged

toge

ther

b T

wo

diffe

rent

grid

s ana

lyze

d on

thes

e cl

asts

to a

scer

tain

repr

oduc

ibili

ty o

f tec

hniq

ue

c Abs

olut

e an

alyt

ical

unc

erta

inty

ass

ocia

ted

with

eac

h el

emen

t (co

untin

g st

atis

tics)

d M

ean

and

rang

e in

stan

dard

dev

iatio

n am

ong

anal

yses

of e

ach

clas

tn

d =

not

det

ecte

d

Tabl

e 3

Con

tinue

d M

ajor

ele

men

t com

posi

tion

of im

pact

-mel

t cla

sts

Cla

stN

oaSi

O2

TiO

2A

l 2O

3C

r 2O

3N

iOFe

OM

nOM

gOC

aON

a 2O

K2O

P 2O

5To

tal

764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al










































































764 B A Cohen et al








of 39Ar in plateau










766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al
















































































766 B A Cohen et al




768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al












































































768 B A Cohen et al







Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al












































































Results









770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al










































































770 B A Cohen et al













772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al
















































































772 B A Cohen et al


DISCUSSION














774 B A Cohen et al








REFERENCES















































776 B A Cohen et al

















































































774 B A Cohen et al








REFERENCES















































776 B A Cohen et al




















































































































776 B A Cohen et al










































































Geochemistry and Ar- Ar geochronology of impact-melt ...bcohen/publications/Cohen_MAPS2005.pdf ·...

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