distinguishing between basalts produced by endogenic volcanism and impact processes a non destructive method using quantitative petrography of lunar basaltic samples abstract impact processes play an important role in shaping and reshaping the surfaces of airless planetary bodies such processes produce regoliths and generate melts that crystallize and record the homogenization of the geology at the impact site if the volume of melt is substantial the resultant crystallized product has an igneous texture and may be free of xenolithic clasts making it difficult to distinguish from melts produced by endogenic magmatic processes this has been clearly demonstrated during the return of the apollo samples from the moon where apollo 14 basalt 14310 was initially described as a mare basalt and was only subsequently reclassified as an impact melt following detailed and time consuming crystallization experiments another way of distinguishing lunar impact melts from endogenically derived mare basalts is through the quantification of the highly siderophile elements hse pd rh ru ir pt os which have relatively low abundances in pristine lunar samples but are high in meteorites and therefore may be enriched in impact melts however these analyses consume relatively large quantities of valuable sample and because of mass constraints cannot be performed on many lunar samples in this paper we present a quantitative petrographic method that has the potential to distinguish lunar impact melts from endogenically derived mare basalts using plagioclase and olivine crystal size distributions csds the slopes and intercepts of these csds are used to show that olivine from impact melts displays a steeper csd relative to olivine from mare basalts for plagioclase generally impacts melts display csds with shallower gradients than those from endogenous mare basalts and as for olivines plot in a distinct field on a csd slope vs csd intercept plot using just a thin section to distinguish impact melts from mare basalts enables the goals of future robotic sample return missions to determine the age of the south pole aitken basin in the moon because such missions will potentially only return small 2 4 mm rocklets for analysis obviating hse analyses for impact melt identification

the behavior of osmium and other siderophile elements during impacts insights from the ries impact structure and central european tektites abstract impact processes are natural phenomena that contribute to a variety of physico chemical mechanisms over an extreme range of shock pressures and temperatures otherwise seldomly achieved in the earth s crust through other processes under these extreme conditions with transient temperatures and pressures 3000 k and 100 gpa followed by their rapid decrease the behavior of elements has remained poorly understood distal glassy ejecta tektites were produced in early phases of contact between the earth s surface and an impacting body here we provide evidence for a complex behavior of os and other highly siderophile elements hse ir ru pt pd and re during tektite production related to a hyper velocity impact that formed the ries structure in germany instead of simple mixing between the surface materials which are thought to form the major source of central european tektites moldavites and impactor matter the patterns of hse contents and 187 re 188 os 187 os 188 os ratios in moldavites target sediments and impact related breccias suevites can be explained by several sequential and or contemporary processes these involve i evaporative loss of partially oxidized hse from the overheated tektite melt ii mixing of target derived and impactor derived hse vapor plasma phases and iii early high temperature condensation of a part of the mixed vapor phase back to silicate melt droplets an almost complete loss of terrestrial os from the tektite melt and its replacement with extra terrestrial os are indicated by low 187 os 188 os ratios in tektites 0 69 this is paralleled by a co variation between os and ni contents in tektites but not in suevites formed later in the impact process

argon 40 argon 39 age spectra of apollo 17 highlands breccia samples by laser step heating and the age of the serenitatis basin we have obtained high resolution 21 63 steps 40ar 39ar age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from apollo 17 samples collected from the pre imbrian highlands in the easternmost part of the serenitatis basin the samples include poikilitic melt rocks inferred to have been formed in the serenitatis basin forming impact aphanitic melt rock whose compositions vary and whose provenance is uncertain and granulite gabbro and melt clasts three of the poikilitic melts have similar age spectrum plateau ages 72395 96 3893 16 ma 2 72535 7 3887 16 ma 76315 150 3900 16 ma with a weighted mean age of 3893 9 ma which we interpret as the best age for the serenitatis basin forming impact published 40ar 39ar age spectrum ages of apollo 17 poikilitic melts are consistent with our new age but are much less precise two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of 3869 ma 72558 7 and 3743 ma 77135 178 plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 16 ma to 3937 16 ma and have probably been affected by the ubiquitous older clasts and by post formation heating impact events plateau ages from two of the aphanitic melt blobs and two granulites in sample 72255 fall in the narrow range of 3850 16 ma to 3869 16 ma with a weighted mean of 3862 8 ma two of the aphanitic melt blobs from 72255 have ages of 3883 16 ma and 3894 ma whereas a poikilitic melt clast of different composition from the serenitatis melts has an age of 3835 16 ma which is the upper limit for the accretion of 72255 these data suggest that either the aphanitic melts vary in age as is also suggested by their varying chemical compositions or they formed in the 72255 accretionary event about 3 84 3 85 ga and older relict material is responsible for the dispersion of ages in any case the aphanitic melts do not appear to be serenitatis products our age for the serenitatis impact shows on the basis of the isotopic age evidence alone that serenitatis is 20 25 ma and probably 55 60 ma older than imbrium 3870 ma and probably 3836 ma dalrymple and ryder 1993 noritic granulite sample 78527 has a plateau age of 4146 17 ma representing a minimum age for cooling of this sample in the early lunar crust so far there is no convincing evidence in the lunar melt rock record for basin forming impacts significantly older than 3 9 ga

os pb nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the volotsk suite se baltic shield abstract highly siderophile element hse ru pd re os ir and pt abundances and re os isotope systematics in combination with lithophile trace element and sm nd and pb pb isotopic data are reported for komatiites and basalts from the volotsk suite in the se baltic fennoscandian shield the new sm nd and pb pb whole rock isochron ages of 2850 84 and 2861 150 ma respectively define the emplacement age of the volcanic sequence this age is statistically indistinguishable from the emplacement age of the lower mafic ultramafic sequences of the adjacent sumozero kenozero greenstone belt the re os internal isochron derived from the analysis of a whole rock b2 4 cumulate and three chromite separates yields an age of 2878 81 ma which is consistent with the ages obtained from the lithophile element isotopic systems the komatiites of the volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements with the magnitude of the time integrated lree depletion initial nd 2 7 0 2 typical of this period in the earth s mantle history yet this source had an initial 187os 0 4 0 4 and thus evolved with a time integrated re os that was identical within uncertainty to that of the carbonaceous chondritic reference in addition the calculated relative abundances of os ir ru and pt in the komatiite source are similar to chondrites and may be explained by late post core segregation accretion to the earth of materials with high absolute and chondritic relative abundances of hse or by re entrainment of the core if the former interpretation is correct the late accreted materials were well homogenized within the upper mantle by 2 9 ga like many other archean komatiites the studied rocks have radiogenic nd isotopic composition indicative of their long term relative depletions in highly incompatible lithophile trace elements yet the long term re os in their source remained near chondritic modeling indicates that the komatiite source must have experienced only a small degree of partial melting 2 3 and extraction of basaltic melts ca 150 ma prior to emplacement of the komatiitic sequences larger degrees of melting would otherwise have led to loss of a substantial fractionation of re in the source which would then have evolved to a subchondritic 187os 188os by the time of komatiite formation

the meteoritical bulletin no 85 2001 september meteoritical bulletin no 85 lists information for 1376 newly classified meteorites comprising 658 from antarctica 409 from africa 265 from asia 262 of which are from oman 31 from north america 7 from south america 3 from australia and 3 from europe information is provided for 11 falls dergaon dunbogan gujba independence itqiy moravka oued el hadjar sayama sologne valera and worden noteworthy non antarctic specimens include 5 martian meteorites dar al gani 876 northwest africa 480 and 817 and sayh al uhaymir 051 and 094 6 lunar meteorites dhofar 081 280 and 287 and northwest africa 479 482 and 773 an ungrouped enstatite rich meteorite itqiy a bencubbin like meteorite gujba 9 iron meteorites and a wide variety of other interesting stony meteorites including ch ck cm co cr cv r enstatite and unequilibrated ordinary chondrites primitive achondrites hed achondrites and ureilites

feldspathic lunar meteorites pecora escarpment 02007 and dhofar 489 contamination of the surface of the lunar highlands by post basin impacts abstract pca pecora escarpment 02007 and dhofar 489 are both meteorites from the feldspathic highlands of the moon pca 02007 is a feldspathic breccia consisting of lithified regolith from the lunar surface it has concentrations of both incompatible and siderophile elements that are at the high end of the ranges for feldspathic lunar meteorites dhofar 489 is a feldspathic breccia composed mainly of impact melted material from an unknown depth beneath the regolith concentrations of incompatible and siderophile elements are the lowest among brecciated lunar meteorites among 19 known feldspathic lunar meteorites all of which presumably originate from random locations in the highlands concentrations of incompatible elements like sm and th tend to increase with those of siderophile elements like ir feldspathic meteorites with high concentrations of both suites of elements are usually regolith breccias iridium derives mainly from micrometeorites that accumulate in the regolith with duration of surface exposure micrometeorites have low concentrations of incompatible elements however so the correlation must reflect a three component system we postulate that the correlation between sm and ir occurs because the surface of the feldspathic highlands terrane has become increasingly contaminated with time in sm rich material from the procellarum kreep terrane that has been redistributed across the lunar surface by impacts of moderate sized post basin impacts the most sm rich regolith breccias among feldspathic lunar meteorites are about 3 enriched compared to the most sm poor breccias but this level of enrichment requires only a few percent sm rich material typical of the procellarum kreep terrane the meteorite data suggest that nowhere in the feldspathic highlands are the concentrations of k rare earths and th measured by the lunar prospector mission at the surface representative of the underlying bedrock all surfaces covered by old regolith as opposed to fresh ejecta are at least slightly contaminated dhofar 489 is one of 15 paired lunar meteorite stones from oman total mass of meteorite 1037 g on the basis of its unusually high mg fe ratio the meteorite is likely to have originated from northern feldspathic highlands

the case for an imbrium origin of the apollo thorium rich impact melt breccias abstract mafic th rich impact melt breccias most of which are identified with the composition known as low k fra mauro lkfm are the most common rock type in the nonmare regoliths of the apollo lunar landing sites the origin of mafic impact melt breccias bears on many lunar problems the nature of the late meteoroid bombardment cataclysm the spatial distribution of kreep both near the surface and at depth the ages of the major basins and the composition of the early crust of the nearside lunar highlands thus it is crucial that the origin of mafic impact melt breccias be accurately understood because of both intra and intersite differences in compositions of mafic impact melt breccia samples apparent differences in crystallization age and differences in siderophile element ratios previous studies have argued that either 1 most mafic impact melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that 2 they are all from the imbrium apollos 14 and 15 nectaris apollo 16 and serenitatis apollo 17 basins here we reconsider the hypothesis that virtually all of the th rich mafic impact melt breccias from the apollo missions are products of the imbrium impact ejecta deposit modeling based on modern crater scaling indicates that the imbrium event produced ejecta deposits that average hundreds of meters thick or more at all apollo highland sites which is thicker than some previous estimates substantial amounts of imbrium ejecta should have been sampled at every apollo highland site we suggest that the mafic impact melt breccias may be the principal form of those ejecta the imbrium projectile impacted into th rich material that we regard as part of a unique mafic lunar geochemical province we call the high th oval region based on the surface distribution of th only basins within the high th oval region excavated th rich material the th concentrations of the highlands as observed by the apollo orbiting ray experiments are consistent with the estimates from ejecta modeling of the younger basin forming impacts only imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact melt breccias in the apollo sampled regolith the high th oval region still may have been molten or hot at shallow depths 4 ga ago when the imbrium projectile struck we reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances we argue that siderophile element fingerprints of mafic impact melt breccias are not inconsistent with production of all common types by a single projectile we suggest that the narrow range of ages of 3 7 4 0 ga for all successfully dated mafic impact melt breccias may reflect a single event whose age is difficult to measure precisely rather than a number of discrete impact events closely spaced in time such that reported age variations among mafic impact melt breccias reflect the ability to measure 40ar 39ar ages with greater precision than the accuracy with which measured portions of mafic impact melt breccias have recorded the time of their formation

impact induced perturbations of atmospheric sulfur abstract asteroids and comets that are vaporized during hypervelocity impact events can inject large masses of s into the stratosphere where it can potentially affect the radiation budget of the earth alter the chemistry of the ozone layer and eventually be converted to sulfuric acid rain relatively small carbonaceous asteroids 0 3 km in diameter contain 5 times more s than the entire modern stratosphere and these objects hit the earth at an average rate of 1 per 10 000 years larger impact events capable of injecting 10 15 g of s into the stratosphere occur at an average rate of 1 per 1 million years calculations indicate there is sufficient o and h in the vapor plumes of most impact events to convert the s to sulfuric acid aerosols if this conversion occurs then the larger impact events could depress mean surface temperatures by more than 2 c for 3 years or longer

highly siderophile element constraints on accretion and differentiation of the earth moon system a new combined rhenium osmium and platinum group element data set for basalts from the moon establishes that the basalts have uniformly low abundances of highly siderophile elements the data set indicates a lunar mantle with long term chondritic highly siderophile element ratios but with absolute abundances that are over 20 times lower than those in earth s mantle the results are consistent with silicate metal equilibrium during a giant impact and core formation in both bodies followed by post core formation late accretion that replenished their mantles with highly siderophile elements the lunar mantle experienced late accretion that was similar in composition to that of earth but volumetrically less than approximately 0 02 lunar mass and terminated earlier than for earth

targeting the impactors siderophile element signatures of lunar impact melts from serenitatis abstract highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner solar system melt breccias associated with the 3 89 ga serenitatis impact basin resolve at least two separate impact events kreep rich melt breccias representing the apollo 17 poikilitic suite are enriched in highly siderophile elements 3 6 15 8 ppb ir with ci normalized patterns that are elevated in re ru and pd relative to ir and pt the restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an eh chondrite asteroid probably as melt sheet deposits from the serenitatis basin one exceptional sample a split from melt breccia 77035 has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites indicating a discrete impact event the recognition of multiple impact events and the clear signatures of specific types of meteoritic impactors in the apollo 17 melt breccias shows that the lunar crust was not comprehensively reworked by prior impacts from 3 9 to 4 5 ga an observation more consistent with a late cataclysm than a smoothly declining accretionary flux late accretion of enstatite chondrites during a 3 8 4 0 ga cataclysm may have contributed to siderophile element heterogeneity on the earth but would not have made a significant contribution to the volatile budget of the earth or oxidation of the terrestrial mantle siderophile element patterns of apollo 17 poikilitic breccias become more fractionated with decreasing concentrations trending away from known meteorite compositions to higher re ir and pd pt ratios the compositions of these breccias may be explained by a two stage impact melting process involving 1 deep penetration of the serenitatis impactor into meteorite free lower crust followed by 2 incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet trends to higher re ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component or a non chondritic late accretionary veneer in the pre serenitatis upper crust

northwest africa 482 a crystalline impact melt breccia from the lunar highlands northwest africa 482 nwa 482 is a crystalline impact melt breccia from the moon with highlands affinities the recrystallized matrix and the clast population are both highly anorthositic clasts are all related to the ferroan anorthosite suite and include isolated plagioclase crystals and lithic anorthosites troctolites and spinel troctolites potassium rare earth element and phosphorus bearing kreep and mare lithologies are both absent constraining the source area ofthis meteorite to a highland terrain with little to no kreep component most likely on the far side of the moon glass is present in shock veins cutting through the sample and in several large melt pockets indicating a second impact event there are two separate events recorded in the 40ar 39ar system one at 3750 ma which completely reset the k ar system and one at 2400 ma which caused onlypartial degassing these events could represent respectively crystallization ofthe impact melt breccia and later formation of the glass or the formation of the glass and a later thermal event the terrestrial age of the meteorite is 8 6 1 3 ka this age corresponds well with the modest amount ofweathering in the rock in the form ofsecondaryphyllosilicates and carbonates based on terrestrial age and location lithology and chemistry nwa 482 is unique among known lunar meteorites

support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages lunar meteorites represent a more random sampling of lunar material than the apollo or luna collections and as such lunar meteorite impact melt ages are the most important data in nearly 30 years with which to reexamine the lunar cataclysm hypothesis within the lunar meteorite breccias mac 88105 que 93069 dag 262 and dag 400 seven to nine different impact events are represented with 40ar 39ar ages between 2 76 and 3 92 billion years ago ga the lack of impact melt older than 3 92 ga supports the concept of a short intense period of bombardment in the earth moon system at approximately 3 9 ga this was an anomalous spike of impact activity on the otherwise declining impact frequency curve

lunar and martian meteorite delivery services launch mechanisms for lunar and martian meteorites have been investigated by integrating physical modeling constraints geochemical cosmic ray exposure cre constraints and petrologic constraints the potential source region for lunar meteorites is remarkably small compared to the final crater volume cre constraints indicate that most launches start at depths of 3 2 m and cratering theory implies derivation of suitably accelerated objects from a subvolume with diameter only about 0 3 the final crater diameter the shallow depth provenance is probably related to shock wave interference enhanced by the lunar regolith s extremely low compressional wave velocity cre constraints alone imply that four to five separate launch events are represented among the eight well studied lunar meteorites most of the lunar meteorites are regolith breccias which tend to show only limited compositional diversity within any kilometer scale region of the moon several others are polymict breccias which also show relatively subdued compositional diversity compared to igneous rocks the observed diversity among these samples in terms of abundances of mare basalt and kreep and in mg mg fe ratio implies that among eight well studied lunar meteorites only two potential source crater pairings are plausible between asuka 881757 y 793169 most probable and between y 793274 eet875721 altogether these eight lunar meteorites apparently represent at least six separate source craters including three in the past 105 years and five in the past 106 years cre constraints imply that snc meteorites are launched from systematically greater depths than lunar meteorites sncs are also systematically bigger and all nine well studied sncs are uncommonly young by martian standards mafic igneous rocks comparison between viking and apollo results reveals that rocks the size of common meteorites are remarkably scarce in the martian regolith probably due to pervasive weathering a plausible explanation for these trends is that most old small and shallow rocks on mars have been weakened by a two stage process of brecciation followed by pervasive weathering to the point where they seldom survive the stresses of spallation off the planet the scarcity of source crater pairing among the lunar meteorites implies that these objects can be launched from craters much smaller than previously estimated and tends to support the suggestion of rabinowitz 1993 astrophys j 407 412 427 that present day cratering rates for the earth moon region may be higher than previously estimated

the isotopic record of lunar volcanism abstract the timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon a late heavy bombardment produced most of the observable lunar basins easily recognized stratigraphic markers and also may have influenced lunar volcanism most of the nearside basins seem to have been produced 3 8 4 0 ga ago during a period often referred to as the lunar cataclysm the pre cataclysm period was characterized by formation of the lunar crust plutonic rocks within the crust and pre mare volcanism isotopic analysis of breccia clasts pre dating the cataclysm has yielded an outline of this period the isotopic record of post cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking specifically the isotopic data show that some lunar anorthosites formed 4 45 ga ago consistent with early formation of an anorthositic lunar crust some mg suite norites troctolites and dunites also formed approximately contemporaneously with the oldest anorthosites but others are up to 200 300 ma younger more evolved rocks granites and quartz monzodiorites also formed between 4 4 and 4 0 ga ago the earliest known pre mare volcanism at 4 25 ga ago included aluminous mare like basalts and an olivine gabbronorite similar to apollo 12 basalts kreep basalt volcanism began at least 4 1 ga ago approximately contemporaneously with that of the ancient mare like basalts and continued during the cataclysm recorded instances of high ti mare basalt volcanism began during or immediately after the cataclysm low ti mare basalt volcanism is the most recent for which samples are available the isotopic data record an early primary lunar differentiation during which mare basalt sources formed probably by crystallization of cumulus minerals from a lunar magma ocean the isotopic record of post cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources but more complex petrogenesis also has been suggested more complex petrogenetic processes including assimilation and assimilation fractional crystallization afc seem to be required for pre mare volcanics the isotopic data contain a hint that petrogenesis of the oldest pre mare volcanics may be linked to formation of the so called procellarum basin possibly the oldest and largest lunar basin such a basin would provide a geologic context for the pre mare volcanics and its formation may have been a defining moment in lunar history

chromium isotopic homogeneity between the moon the earth and enstatite chondrites abstract among the elements exhibiting non mass dependent isotopic variations in meteorites chromium cr has been central in arguing for an isotopic homogeneity between the earth and the moon thus questioning physical models of moon formation however the cr isotopic composition of the moon relies on two samples only which define an average value that is slightly different from the terrestrial standard here by determining the cr isotopic composition of 17 lunar 9 terrestrial and 5 enstatite chondrite samples we re assess the isotopic similarity between these different planetary bodies and provide the first robust estimate for the moon in average terrestrial and enstatite samples show similar e 54 cr on the other hand lunar samples show variables excesses of 53cr and 54cr compared to terrestrial and enstatite chondrites samples with correlated e 53 cr and e 54 cr per 10 000 deviation of the 53cr 52cr and 54cr 52cr ratios normalized to the 50cr 52cr ratio from the nist srm 3112a cr standard unlike previous suggestions we show for the first time that cosmic irradiation can affect significantly the cr isotopic composition of lunar materials moreover we also suggest that rather than spallation reactions neutron capture effects are the dominant process controlling the cr isotope composition of lunar igneous rocks this is supported by the correlation between e 53 cr and e 54 cr and 150sm 152sm ratios after correction of these effects the average e 54 cr of the moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an earth moon enstatite chondrite system homogeneity this is compatible with the most recent scenarios of moon formation suggesting an efficient physical homogenization after a high energy impact on a fast spinning earth and or with an impactor originating from the same reservoir in the inner proto planetary disk as the earth and enstatite chondrites and having similar composition

osmium isotope and highly siderophile element systematics of lunar impact melt breccias implications for the late accretion history of the moon and earth abstract to characterize the compositions of materials accreted to the earth moon system between about 4 5 and 3 8 ga we have determined os isotopic compositions and some highly siderophile element hse re os ir ru pt and pd abundances in 48 subsamples of six lunar breccias these are apollo 17 poikilitic melt breccias 72395 and 76215 apollo 17 aphanitic melt breccias 73215 and 73255 apollo 14 polymict breccia 14321 and lunar meteorite nwa482 a crystallized impact melt plots of ir versus other hse define excellent linear correlations indicating that all data sets likely represent dominantly two component mixtures of a low hse target presumably endogenous component and a high hse presumably exogenous component linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all hse except for ru and pd in two samples the slopes of the linear regressions are insensitive to target rock contributions of ru and pd of the magnitude observed thus the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks the 187 os 188 os and regression derived elemental ratios for the apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites the hse in the apollo 17 aphanitic melt breccias however might partially or entirely reflect the hse characteristics of hse rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation in this case the hse characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias the impactor components in the apollo 17 poikilitic melt breccias and in the apollo 14 breccia have higher 187 os 188 os pt ir and ru ir and lower os ir than most chondrites these compositions suggest that the impactors they represent were chemically distinct from known chondrite types and possibly represent a type of primitive material not currently delivered to earth as meteorites

the meteorite component of apollo 16 noritic impact melt breccias noritic melt rocks alias lkfm and vha from apollo 16 have concentrations of ni approximately three to five times those of impact melt rocks of similar composition from other lunar landing sites the melt rock associated with apollo 16 dimict breccias has the highest mean concentration of ni of any lunar rock type 1150 g g in 11 samples the ni is contained in fe ni metal that is dissimilar in composition to metal from ordinary chondrites in having a lower ni concentration 6 and lower ni co ratio 17 it also has ir ni and ir au ratios of about one half and one third those of chondrites the metal in the noritic impact melts is the carrier of the siderophile element signature of ancient meteorite group 1h the composition of the fe ni metal is dissimilar to that of chondrites because it derives from metal rich meteorites probably irons that created the impact melts 3 9 ga ago the metal from these iron meteorites dominates the siderophile element concentrations of polymict samples at apollo 16 models that attempt to estimate indigenous concentrations of ni and co in apollo 16 materials overestimate their abundance by assuming that the meteoritic component has chondritic ratios of ir to ni and co the type 2 melt vha associated with the dimict breccias and the type 1 melt apollo 16 lkfm were most likely produced by impact of two related iron meteoroids that impacted near the apollo 16 site the type 2 melt found in samples from station 7 is probably from the same impact as that producing the dimict breccias although it could represent the impact of a third related iron

platinum group elements in the morokweng impact structure south africa evidence for the impact of a large ordinary chondrite projectile at the jurassic cretaceous boundary radiometric dating of melt rocks at impact craters has revealed that some giant impacts appear to overlap in time with major boundaries in earth history e g the cretaceous tertiary k t and jurassic cretaceous j k boundaries the morokweng impact crater in south africa is coincident in age with the j k boundary however the types of objects that generate large craters are poorly known because it is difficult to unambiguously identify the projectile from the signature it imparts into the impact rocks meteorites are highly enriched in the platinum group elements pge which have been widely used as a tool for identifying the presence of a meteorite signature here we present new pge analyses from the morokweng impact melt sheet our data reveal high pge concentrations and high degree of pge correlation through the melt sheet regression analysis was used to determine the projectile pge signature and constrain input from the terrestrial target rocks the closest match to morokweng is the pge signature of ordinary l or ll chondrite meteorites which is broadly in agreement with the results of an earlier cr isotope study the results of these independent studies provide strong evidence that a large ordinary chondrite projectile struck the area of morokweng in the late jurassic

an ordinary chondrite impactor for the popigai crater siberia with a diameter of 100 km popigai in northern siberia is the largest crater known in the cenozoic the concentrations in platinum group elements pge were analyzed in twenty samples of homogeneous impact melt collected in the northwestern flank of the crater to identify the composition of the projectile the method selected was preconcentration by nis fire assay followed by inductively coupled plasma mass spectrometry icp ms this technique measures all the pge except os and by using aliquots 10g the results are highly reproducible the major and trace element composition of the impact melt resembles that of gneissic lithologies of the anabar shield which are representative of the target rock the pge are enriched in the melt by factors of 3 to 14 compared to the main target lithology but the meteoritic contamination is only around 0 2 wt using plots of elemental ratios such as ru rh vs pt pd or ru rh vs pd ir the popigai impactor is clearly identified as an ordinary chondrite and most likely l chondrite this study indicates that pge elemental ratios allow discrimination of the type of impactor even in the case of low meteoritic contamination this study confirms that a significant fraction of the crater forming projectiles presently documented could have an ordinary chondrite composition their probable source the s type asteroids appears to form the majority of the bodies in the main asteroid belt and among near earth objects neos the ordinary chondrite origin of the popigai projectile supports an asteroidal origin for the late eocene impacts as a plausible alternative to the comet shower scenario proposed by farley et al 1998 copyright 2005 elsevier ltd

compositional variation in apollo 16 impact melt breccias and inferences for the geology and bombardment history of the central highlands of the moon abstract high precision data for the concentrations of a number of lithophile and siderophile elements were obtained on multiple subsamples from 109 impact melt rocks and breccias mostly crystalline from the apollo 16 site compositions of nearly all apollo 16 melt rocks fall on one of two trends of increasing sm concentration with increasing sc concentration the eastern trend lower sm sc mg fe and sm yb ratios consists of compositional groups 3 and 4 of previous classification schemes these melt rocks are feldspathic poor in incompatible and siderophile elements and appear to have provenance in the descartes formation to the east of the site the western trend higher sm sc mg fe and sm yb ratios consists of compositional groups 1 and 2 these relatively mafic kreep bearing breccias are a major component 35 of the cayley plains west of the site and are unusual compared to otherwise similar melt breccias from other sites in having high concentrations of fe ni metal 1 2 the metal is the carrier of the low ir au 0 3 chondritic siderophile element signature that is characteristic of the apollo 16 site four compositionally distinct groups 1m 1f 2db and 2nr of western trend melt breccias occur that are each represented by at least six samples compositional group 1 of previous classification schemes the poikilitic or lkfm melt breccias can be subdivided into two groups group 1m represented by six samples including 60315 is characterized by lower al2o3 concentrations higher mgo and alkali concentrations and higher mg fe and cr sc ratios than group 1f represented by fifteen samples including 65015 group 1m also has siderophile element concentrations averaging about twice those of group 1f and ir au and ir ni ratios that are even lower than those of other western trend melt rocks ir au 0 24 0 03 ci normalized at the mafic extreme of group 2 vha melt breccias the melt lithology occurring as clasts in feldspathic fragmental breccias from north ray crater group 2nr is compositionally distinct from the melt lithology of dimict breccias from the cayley plains group 2db in having higher concentrations of sc cr and heavy rare earth elements and lower concentrations of siderophile elements the distinct siderophile element signature high absolute abundances low ir au ratio suggest that the four groups of mafic melt breccia are all somehow related ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt breccias from other apollo sites however none of the breccia compositions can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials thus the origin of the four groups of mafic melt breccia is enigmatic if they were produced in only one or two impacts then a mechanism exists for generating regimes of impact melt breccia in a single impact that are substantially different from each other in composition for various reasons including the problem of delivering large volumes of four different types of melt to the apollo 16 site it is unlikely that any of these breccias were produced in basin forming impacts if they were produced in as many as four crater forming impacts then the unusual siderophile element signature is difficult to explain possible explanations are 1 1 the four groups of melt breccia all contain metal from a single earlier impact 2 2 they were each formed by related metal rich meteoroids or 3 3 some common postimpact process has resulted in metal of similar composition in each of four melt pools within a compositional group most intrasample and intersample variation in lithophile element concentrations is caused by differences among samples in the proportion of a component of normative anorthosite or noritic anorthosite in most cases this compositional variation probably reflects variation in clast abundance for group 2db and probably 2nr differences in abundance of a component of ferroan anorthosite estimated al2o3 32 accounts for the compositional variation for groups 1m and 1f the anorthositic component is more mafic estimated al2o3 26 some group 2 samples may be related by a troctolitic component of varying abundance

chronology and sources of lunar impact bombardment abstract the moon has suffered intense impact bombardment ending at 3 9 gyr ago and this bombardment probably affected all of the inner solar system basin magnetization signatures and lunar crater size distributions indicate that the last episode of bombardment at about 3 85 gyr ago was less extensive than previously thought we explore the contribution of the primordial mars crosser population to early lunar bombardment we find that mars crosser population initially decays with a 80 myr half life with the long tail of survivors clustering on temporarily non mars crossing orbits between 1 8 and 2 au these survivors decay with half life of about 600 myr and are progenitors of the extant hungaria asteroid group in the same region we estimate the primordial mars crosser population contained about 0 01 0 02 earth masses such initial population is consistent with no lunar basins forming after 3 8 gya and the amount of mass in the hungaria group as they survive longer and in greater numbers than other primordial populations mars crossers are the best candidate for forming the majority of lunar craters and basins including most of the nectarian system however this remnant population cannot produce imbrium and orientale basins which formed too late and are too large to be part of a smooth bombardment we propose that the imbrian basins and craters formed in a discrete event consistent with the basin magnetization signatures and crater size distributions this late impactor shower would be triggered by a collisional disruption of a vesta sized body from this primordial mars crossing population wetherill g w 1975 proc lunar sci conf 6 1539 1561 that was still comparable to the present day asteroid belt a 3 9 gya this tidal disruption lead to a short lived spike in bombardment by non chondritic impactors with a non asteroidal size frequency distribution in agreement with available evidence this body wetherill s object also uniquely matches the constraints for the parent body of mesosiderite meteorites we propose that the present day sources of mesosiderites are multi km sized asteroids residing in the hungaria group that have been implanted there soon after the original disruption of their parent 3 9 gyr ago

manganese precipitates in the karst terrain of grand cayman british west indies ten morphologically distinct forms of mn precipitates probably birnessite occur in a wide variety of paleokarst features that characterize dolostones of the bluff formation on grand cayman these

low abundances of highly siderophile elements in the lunar mantle evidence for prolonged late accretion abundances of the highly siderophile elements hse re ir os ru pt and pd were determined for etchates and residues of highly purified lunar orange 74220 and 74001 1217 and 1218 and green 15421 65 and 15426 164 volcanic glasses most etchates have 100 pg g os and ir and in some instances have hse concentrations that approach chondritic relative abundances some etchates also have 187 os 188 os within the range of chondritic meteorites in contrast most residues have os concentrations of 50 pg g and have considerably suprachondritic 187 os 188 os ratios the disparities in os isotopic compositions between etchates and residues reflect the presence of at least two components in each of the glass fractions the components are interpreted to be the indigenous lunar glasses and a meteoritic contaminant mixing models suggest the indigenous components in orange and green glasses have os concentrations v25 pg g compared to terrestrial rocks with comparable mgo contents the concentration estimates suggest the lunar mantle sources of the volcanic glasses were depleted in hse by at least a factor of 20 relative to the terrestrial mantle consistent with os and ir concentrations in the lunar mantle of v0 15 ng g the hse depletion in the lunar mantle indicates that it may be missing a substantial share of late accreted materials the depletion is best explained as a consequence of the early formation of the lunar crust and subsequent isolation of the lunar mantle from late accreted materials given that the lunar crust was largely formed by 4 4 ga this suggests a preponderance of late accreted materials were added to the earth moon system subsequent to 4 4 ga and requires a substantial influx of mass into the inner solar system more than 150 ma after the formation of the solar system d 2004 elsevier b v all rights reserved

the nature of the meteoritic components of apollo 16 soil as inferred from correlations of iron cobalt iridium and gold with nickel concentrations of co ni ir and au in small samples of submature and mature soil from apollo 16 are highly variable correlations of fe co ir and au with ni indicate that the variation in siderophile element concentrations results from variation in the concentration of fe ni metal from these correlations the mean composition of the metal is inferred to be 5 6 ni and 0 36 co for a ni co ratio of 15 5 0 7 i e dissimilar to the chondritic ratio of 20 21 the fe ni metal is also characterized by low ir ni and ir au ratios compared to metal in ordinary chondrites metal with a similar composition is also found in metal rich noritic impact melt breccias that occur as discrete rocks as well as small clasts in ancient regolith breccias at apollo 16 mass balance models using lithophile element concentrations indicate that about 35 of the soil is noritic impact melt the fe ni metal associated with this component is sufficient to supply virtually all the ancient fe ni metal found in the typical apollo 16 soil 0 4 0 5 i e no other source is required the soil also contains 1 2 carbonaceous chondrite the concentration of this component appears to be relatively constant from sample to sample and does not contribute significantly to the variation in siderophile element abundances these two meteoritic components contribute nearly equal concentrations of ni to the typical apollo 16 soil

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the