metallographic cooling rates and origin of iva iron meteorites we have determined the metallographic cooling rates for 13 iva irons using the most recent and most accurate metallographic cooling rate model group iva irons have cooling rates that vary from 6600 c myr at the low ni end of the group to 100 c myr at the high ni end of the group this large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate thermal and fractional crystallization models have been used to describe the cooling and solidification of the iva asteroid the thermal model indicates that a metallic body of 150 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in

apollo 16 regolith breccias characterization and evidence for early formation in the mega regolith all of the apollo 16 regolith breccias 8 specimens have been characterized in terms of their petrography grain size distribution porosity major and trace element composition noble gas contents and ferromagnetie resonance properties these breccias vary signkqcantly with respect to their density and porosity with the more dense breccias showing significant shock damage the regolith breccias resemble the soils in grain size distribution and in the relative proportions of major petrological components except agglutinates many of the breecias are compositionally different from the apollo 16 soils in that they lack an important marie component present in the soils although some groupings occur the petrologic and chemical compositions of the regolith breedas do not correlate with the station location of the samples all but one of the breceias show some evidence of irradiation at the lunar surface solar gases measurable fmr agglutinates and analyses made on grain size separates from two disaggregated breedas indicate that this irradiation occurred before compaction when the breccia material was finely disseminated on the surface however the concentrations of surface irradiation parameters solar gases fmr agglutinates for most breedas are far less than seen in any lunar soils or in regolith breccias from other apollo missions several breedas also contain unusually high trapped 4oar 36ar ratios of 8 12 and a significant fission xe component in excess of that expected from in situ production these observations suggest that the surface irradiation of these breedas occurred as early as 4 x 109 years ago we conclude that most of the apollo 16 regolith breedas were not formed from any known apollo 16 soil they appear to be well comminuted material that contains ancient regolith developed during the late stage heavy bombardment of the moon when large impacts were much more common relative to small impacts so that regoliths did not have time to significantly mature before being diluted by fresh ejecta and buffed this ancient megaregolith is significantly different from more recent lunar regolith but may be similar to asteroidal regoliths from which some brecciated meteorites have formed

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

apollo 16 impact melt sheets contrasting nature of the cayley plains and descartes mountains and geologic history apollo 16 stations 4 and 5 rake samples were examined petrographically and by electron microprobe and inaa results were combined with published information major and trace element compositions petrologic information and age determinations for nonrake samples from stations 4 and 5 and some other apollo 16 sample localities lithologic abundances support the idea korotev 1981 that the variation of soil compositions at apollo 16 is a result of mixing between a component represented by station 5 soil and components much like either the dimict breccias station 4 soils or feldspathic fragmental breccias stations 1 2 and 11 in composition dimict breccias because of their restricted occurrence are apparently not a constituent of either the cayley or descartes formations they were contributed to the surface materials by south ray crater and were probably excavated from beneath both formations by this event pyroxene olivine and coexisting plagioclase compositions from within the anorthosite portions of dimict breccias bridge the gap between the mg rich and ferroan anorthosite fields analyses from associated cumulate and granulitic clasts indicate that they are the source of the intermediate material dimict breccia anorthosites are thus polymict rocks not pristine ferroan anorthosite compositions of apollo 16 impact melt rocks suggest a number of compositional clusters discriminant and cluster analyses of compositional data confirm the existence and strength of these clusters as do age determinations and ir au ratios within each group each of three clusters represents an impact event in the apollo 16 region petrologic information and age determinations indicate that dimict breccias formed approximately 3 92 b y ago the nectaris event occured between 3 84 and 3 92 b y ago and the cayley plains were deposited as a result of the imbrium event sometime later than 3 84 b y

late quaternary history of the marmara sea and black sea from high resolution seismic and gravity core studies abstract lithologic and multi proxy paleoenvironmental data from 21 dated cores have been used to define three allostratigraphic units allounits within the late quaternary successions of the marmara sea and black sea allounits are bounded by unconformities and their correlative conformities in both regions allounit a extends from the seafloor downward to a 12 11 ka sequence boundary which is a major shelf crossing unconformity in water depths less than 100 110 m in deep basins of the marmara sea the lower part of allounit a designated subunit a2 is a laminated sapropel m1 on the shelf subunit a2 consists of backstepping delta lobes and early transgressive barrier islands and sand sheets allounit b has only been recovered in marmara sea cores collected at water depths greater than 90 m and represents basinal or prodeltaic deposition during the 23 12 ka late pleistocene lowstand during the last glacial maximum the shelves surrounding the marmara sea were subaerially exposed and deltas of allounit b accumulated along the present day shelf edge following the post glacial rise of global sea level to 75 m at 12 ka the marmara sea quickly became inundated and thereafter rose in synchroneity with the mediterranean by 10 ka the black sea rose to start spilling into the marmara sea leading to establishment of a brackish water lid that has persisted to the modern day the strongest black sea outflow began at 10 ka and persisted to 6 ka promoting the accumulation of sapropel m1 in the deep marmara sea and progradation of an overflow delta just south of the exit from the bosphorus strait allounit c is a laminated sapropel m2 in basinal cores dated at 30 23 ka like m1 it is believed that m2 accumulated during a period of increased brackish water input into the marmara sea mainly from the black sea in the black sea wave erosion kept the shelf stripped of unconsolidated sediments during the falling sea level associated with the last glaciation and subsequent early stages of the post glacial holocene transgression this erosion created a major unconformity shelf edge deltas of allounit b received their sediment during the last lowstand from small rivers that likely coalesced into a single system toward the shelf edge at modern water depths of 100 to 110 m these deltas were active until 11 10 5 ka subsequently sea level in the black sea rose to 40 m by 10 ka and a set of backstepping barrier islands developed on the shelf as part of the associated transgressive systems tract once water level reached 40 m continued sea level rise stalled until 9 ka as the black sea began to spill across the bosphorus strait into the marmara sea

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

direct detection of projectile relics from the end of the lunar basin forming epoch the lunar surface a key proxy for the early earth contains relics of asteroids and comets that have pummeled terrestrial planetary surfaces surviving fragments of projectiles in the lunar regolith provide a direct measure of the types and thus the sources of exogenous material delivered to the earth moon system in ancient 3 4 billion years ago ga regolith breccias from the apollo 16 landing site we located mineral and lithologic relics of magnesian chondrules from chondritic impactors these ancient impactor fragments are not nearly as diverse as those found in younger 3 4 ga to today regolith breccias and soils from the moon or that presently fall as meteorites to earth this suggests that primitive chondritic asteroids originating from a similar source region were common earth moon crossing impactors during the latter stages of the basin forming epoch

isotope analysis of crystalline impact melt rocks from apollo 16 stations 11 and 13 north ray crater rubidium strontium and sm nd isotopic data were obtained for lunar impact melt rocks from apollo 16 stations 11 and 13 as part of the north ray crater target rock consortium study rubidium strontium whole rock data define an isochron age t 3 69 0 16 b y 0 0142 b y 1 and initial 87sr 86sr isr 0 69918 5 for feldspathic microporphyritic fm samples internal rb sr isochrons of t 3 86 0 05 b y isr 0 69952 10 and t 3 76 0 04 b y isr 0 69916 4 were determined for very high alumina vha melt rock 67747 and anorthositic noritic melt rock anmr 67559 respectively t isr for these and other lunar melt rocks were examined to infer the nature of the protolith for the rocks isr values for the an mr and fm rocks are consistent with an anorthositic protolith probably a mixture of nonpristine anorthositic gabbros and pristine anorthosites anorthositic norites norites troctolites and possibly dunites the source protolith of the vha rocks apparently had a significant kreep component and the protolith for apollo 16 kreep must have had nearly the same rb sr ratio as that of apollo 14 kreep two stage model calculations using the t isr parameters suggest that some of the melt rocks were open rb sr systems during impact melting rubidium strontium model ages for the fm and vha rocks show a smoothly monotonic decrease with increasing rb content model ages of the anmr are uniformly low relative to 4 56 b y it is suggested that these effects are due to formation of these rocks by impact partial melting rubidium strontium model ages of the kreep rocks do not vary with rb content suggesting that they were closed rb sr systems during impact melting samarium neodymium data for mineral separates of 67747 showed only a small variation in sm nd ratio and yield an imprecisely defined isochron of 3 6 0 4 b y plagioclase separates of 67559 appear not to have reached nd isotopic equilibrium during impact melting consequently the internal isochron age is also imprecisely defined at 4 0 0 4 b y nd values for the melt rocks are about 1 to 2 at their rb sr or 40ar 39ar ages are consistent with the protolith compositions inferred from the rb sr data and show that lunar crustal sm nd evolution departed from chondritic evolution early in lunar history the average chondritic uniform reservoir chur model age of those anmr and fm rocks that apparently remained closed rb sr and sm nd systems during impact melting is 4 37 0 10 b y consistent with earlier suggestions that the lunar crust formed at about that time the average chur model age of two vha rocks is 4 10 0 15 b y suggesting that these rocks were open sm nd systems as well as open rb sr systems during impact partial melting current hypotheses for the site geology stratigraphy and radiometric age data lead us to suggest that the nectaris basin and the descartes formation were formed about 4 1 b y ago and the imbrium basin and the cayley formation were formed about 3 81 3 86 b y ago the ages of some individual melt rocks probably date local cratering events however

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

high resolution u pb ages of ca phosphates in apollo 14 breccias implications for the age of the imbrium impact abstract previous age estimates of the imbrium impact range from 3770 to 3920 ma with the latter being the most commonly accepted age of this basin forming event the occurrence of ca phosphates i

rhodium gold and other highly siderophile element abundances in chondritic meteorites abstract the abundances of the highly siderophile elements hse re os ir ru pt rh pd and au and 187os 188os isotope ratios have been determined for a set of carbonaceous ordinary enstatite and rumuruti chondrites using an analytical technique that permits the precise and accurate measurement of all hse from the same digestion aliquot concentrations of re os ir ru pt and pd were determined by isotope dilution icp ms and n tims analysis the monoisotopic elements rh and au were quantified relative to the abundance of ir differences in hse abundances and ratios such as re os 187os 188os pd ir and au ir between different chondrite classes are further substantiated with new data and additional rh and au data including new data for ci chondrites systematically different relative abundances of rh between different chondrite classes are reminiscent of the behaviour of re carbonaceous chondrites are characterized by low average rh ir of 0 27 0 03 1s which is about 20 lower than the ratio for ordinary 0 34 0 02 and enstatite chondrites eh 0 33 0 01 el 0 32 0 01 r chondrites show higher and somewhat variable rh ir of 0 37 0 07 well defined linear correlations of hse in particular for bulk samples of ordinary and el chondrites are explained by binary mixing and or dilution by silicates the hse carriers responsible for these correlations have a uniform chemical composition indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components excepting rumuruti chondrites and au in carbonaceous chondrites these correlations also suggest that metamorphism alteration and igneous processes had negligible influence on the hse distribution on the bulk sample scale depletion patterns for rh pd and au in carbonaceous chondrites other than ci are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites fractionated hse abundance patterns of ordinary enstatite and rumuruti chondrites however are more difficult to explain fractional condensation combined with the removal of metal phases at various times and later mixing of early and late formed metal phases may provide a viable explanation planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the hse abundance patterns of chondrite groups hse abundances of some but not all rumuruti chondrites may be consistent with solid sulphide liquid sulphide fractionation processes during impact induced melting

characterization of the dominant impactor signature for apollo 17 impact melt rocks abstract concentrations of the highly siderophile elements hse re os ir ru pt and pd and 187 os 188 os isotopic compositions are reported for seven apollo 17 impact melt rocks these data are used to examine the dominant chemical signature of the impactor that formed the melts six of the samples 72355 72435 72535 76035 76055 and 76135 have poikilitic textures one sample 73235 has an aphanitic texture data for the samples define linear correlations when ir is plotted versus other hse concentrations with y intercepts indistinguishable from zero for most hse in most rocks scatter about some of the trends and occasional trends with positive y intercepts indicate either mixing of additional components that are heterogeneously distributed within several rocks or modest fractionation of some hse by volatilization crystal fractionation or other processes during formation and evolution of the melt sheet there is no statistical difference between the aphanitic and poikilitic samples in terms of hse ratios after visible granulitic clasts were removed from aphanite 73235 hence earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data most apollo 17 samples examined here and in prior studies are characterized by very similar hse signatures consistent with a common impactor these samples are characterized by elevated ru ir pd ir and re os relative to most chondrites collectively the data indicate that the impactor was characterized by the following hse ratios 2 re ir 0 093 0 020 os ir 1 03 0 28 ru ir 1 87 0 30 pt ir 2 36 0 31 pd ir 1 85 0 41 and present day 187 os 188 os of 0 1322 0 0013 the results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on earth

geochemistry and 40ar 39ar geochronology of impact melt clasts in feldspathic lunar meteorites implications for lunar bombardment history we 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 for texture chemical composition and or chronology although the textures are similar to the impactmelt clasts identified in mafic apollo and luna samples the meteorite clasts are chemically distinct from 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 of the apollo and luna sampling sites range from 4 to 2 5 ga we interpret these samples to have been created in at least six and possibly nine or more different impact events one inferred impact event may be consistent with the apollo impact melt rock age cluster at 3 9 ga but the meteorite impact melt clasts with this age are different in chemistry from the apollo samples suggesting that the mechanism responsible for the 3 9 ga peak in lunar impact melt clast ages is a lunar wide phenomenon no meteorite impact melts have ages more than 1 older than 4 0 ga this observation is consistent with but does not require a lunar cataclysm

osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks abstract the concentrations of highly siderophile elements hse re os ir ru pt rh pd au and 187os 188os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the apollo 14 16 and 17 landing sites and the lunar meteorite dar al gani dag 400 using inductively coupled plasma mass spectrometry icp ms and negative thermal ionization mass spectrometry n tims we report the first re os isochron age on a lunar impact melt rock 187re 187os isotope systematics for apollo 16 sample 67935 define an isochron age of 4 21 0 13 ga mswd 1 5 which is interpreted to reflect localized partitioning processes between solid metal liquid metal as this rock melted the new age adds further constraints on the significance of pre 4 0 ga basin forming impacts on the moon and possible mixing of ancient impactor compositions in lunar impact rocks linear correlations displayed by subsamples of a given impact rock in plots of hse versus ir concentrations are explained by dilution processes through essentially hse free anorthositic lunar crustal target rocks or binary mixing between a high hse meteoritic end member and a low hse end member composition slope derived hse ratios and 187os 188os of the meteoritic component in granulitic impactites 67915 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites suprachondritic ratios of ru ir pt ir rh ir and pd ir for apollo 14 impact melt rock 14310 are similar to ratios observed for other apollo 14 samples and apollo 17 poikilitic impact melt rocks apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic os ir and suprachondritic ratios of 187os 188os ru ir pt ir rh ir pd ir and au ir their strongly fractionated hse compositions are similar to some members of the iva iron meteorite group and provide further evidence for an iron meteorite impactor component in apollo 16 impact melt rocks the range of chondritic to suprachondritic ratios of 187os 188os ru ir pt ir pd ir and chondritic to slightly subchondritic os ir of apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite like hse composition as in granulitic impactites with a suprachondritic iron meteorite like impactor composition as inferred from 67935 and 60315 and subsequent homogenization by younger impact events the inferred dispersal of these two components across the lunar nearside and their antiquity 4 2 ga suggest at least two basin forming impacts that predate the 3 9 3 8 ga basin forming era the hse composition of the silicate earth can be matched by a late veneer comprised of 80 volatile depleted carbonaceous chondrite like material and 20 differentiated meteoritic metal similar to what has been identified in ancient lunar impact rocks

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

a 4 2 billion year old impact basin on the moon u pb dating of zirconolite and apatite in lunar melt rock 67955 abstract a sharp rise in the flux of asteroid size bodies traversing the inner solar system at 3 9 ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments the prevalence of 3 9 ga crystallization ages for lunar impact melt breccias and u pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short lived cataclysmic episode of late heavy bombardment at that time here we report u pb isotopic compositions of zirconolite and apatite in coarse grained lunar melt rock 67955 measured by ion microprobe that date a basin scale impact melting event on the moon at 4 22 0 01 ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later significant impacts prior to 3 9 ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish other isotopic evidence such as 40 ar 39 ar ages of granulitic lunar breccias regolith fragments and clasts extracted from fragmental breccias and re os isotopic compositions of lunar metal is also suggestive of impact related thermal events in the lunar crust during the period 4 1 4 3 ga we conclude that numerous large impactors hit the moon prior to the canonical 3 9 ga cataclysm that some of those pre cataclysm impacts were similar in size to the younger lunar basins and that the oldest preserved lunar basins are likely to be significantly older than 3 9 ga this provides sample based support for dynamical models capable of producing older basins on the moon and discrete populations of impactors an extended period of basin formation implies a less intense cataclysm at 3 9 ga and therefore a better opportunity for preservation of early habitable niches and hadean crust on the earth a diminished cataclysm at 3 9 ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance

ages of pristine noritic clasts from lunar breccias 15445 and 15455 abstract rb sr and sm nd isotopic studies were undertaken of two apollo 15 breccias 15445 and 15455 collected near spur crater on the apennine front seven pristine lithic clasts including four norites one anorthositic norite one troctolitic anorthosite and one spinel troctolite as well as a matrix sample of 15445 were analysed in addition a k ar age determination was also made for a plagioclase separate from a different anorthositic norite clast sm nd isotopic data of a pristine mg norite sample 15445 17 from the large white clast b of breccia 15445 yielded a precise internal isochron age of 4 46 0 07 ga for 147 sm 0 00654 ga 1 and an initial 143 nd 144 nd of 0 506058 0 000078 normalized to 148 nd 144 nd 0 24308 corresponding to an initial nd of 0 71 however sm nd isotopic results of another mg norite sample 15445 247 chipped from the same clast b about 1 cm away from 15445 17 yielded a significantly younger isochron age of 4 28 0 03 ga and a higher initial 143 nd 144 nd of 0 506246 0 000037 corresponding to an initial nd of 0 35 the sm nd data indicate that clast b is heterogeneous and contains at least two similar lithologies the rb sr isotopic systems were severely disturbed for both norites no useful isochron ages were observed the sm nd age and nd data for a 15 mg norites 15445 17 and 15445 247 a 17 mgnorite 78236 and two mg gabbronorites 73255 27 45 and 67667 are clearly resolved from each other four distinct parental magmas are required for derivation of these two major groups of mg suite noritic rocks the rb sr isotopic system of an anorthositic norite clast 15455 228 was slightly disturbed and isotopic data defined an isochron age of 4 59 0 13 ga for 87 rb 0 0139 ga 1 or 4 55 0 13 ga for 87 rb 0 01402 ga 1 the corresponding initial 87 sr 86 sr for the rock is 0 69899 0 00006 and is in satisfactory agreement with the luni value of nyquist et al 1974 the sm nd system is more disturbed than the rb sr system for this rock a tie line between whole rock samples and a pyroxene separate gives a sm nd age of 4 53 0 29 ga which is in nominal agreement with the rb sr isochron age the 39ar40ar age of a plagioclase separate of the rock has a weighted average value of 3 83 ga suggesting that the age was reset by the imbrium impact event probably the sm nd and rb sr systems of the clast were also partly reset by this event our age results indicate that some mg suite rocks are as old as ferroan anorthosite suite rocks furthermore age data of three major crustal rocks a mg suite a ferroan anorthosite suite and an evolved suite show that they all have variable ages these observations are difficult to explain by the magma ocean hypothesis alone petrogenetic processes such as production of mg suite rocks by serial magmatism walker 1983 of ferroan anorthosite suite rocks by multiple diapiric intrusions longhi and ashwal 1985 and evolved suite rocks by continuous magmatism meyer et al 1988 1989 probably all played a role these processes seem to be able to accommodate the wide ranges of ages and different origins observed for lunar crustal rocks

high velocity collisions from the lunar cataclysm recorded in asteroidal meteorites lunar samples suggest that the inner solar system was bombarded by asteroids about 4 gyr ago radiometric ages of meteorites suggest an unusual number of high velocity asteroids at this time consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits

identifying impact events within the lunar cataclysm from 40ar 39ar ages and compositions of apollo 16 impact melt rocks abstract lunar impact melt breccias provide a unique record of the timing and frequency of collisional events during the early history of the inner solar system prior to the development of a significant rock record on earth the predominance of ages clustering between 3 8 and 4 0 ga was a major unexpected discovery obtained from geochronological studies of lunar impact melts and is the basis of the concept that a cataclysmic bombardment of large planetesimals struck the earth and moon and possibly the entire inner solar system about 3 85 0 10 billion years ago as a test of the cataclysm hypothesis we measured high resolution 20 50 steps 40 ar 39 ar age spectra on 25 samples of apollo 16 impact melt breccias using a continuous laser heating system on sub milligram fragments twenty one of these 25 breccias produced multi step plateaus that we interpret as crystallization ages with 20 of these ages falling in the range 3 75 3 96 ga we propose that at least four different melt producing impact events can be distinguished based on the ages bulk compositions and petrographic characteristics of apollo 16 melt breccias the recognition of multiple impact events within the apollo 16 melt breccia suite shows that numerous impact events occurred on the lunar surface within a relatively narrow time interval providing additional evidence of a heavy bombardment of the moon during the early archean

composition and evolution of the lunar crust in the descartes highlands apollo 16 samples from the north ray crater ejecta blanket apollo 16 were investigated by petrographic microscope electron microprobe instrumental neutron activation and xray fluorescence analyses and 40ar 39ar and rb sr dating techniques nine major groups of monomict and polymict breccias were defined on the basis of microscopic texture and these were further subdivided into chemical subgroups on the basis of characteristic elements such as al mg fe cr ree ni and co the polymict breccias fragmental breccias granulitic breccias and impact melt breccias are the result of multiple impact induced mechanical mixing and melting and of thermal and impact metamorphism of rock and mineral clasts derived from primordial igneous crustal rocks for calculations of mixing models it was found that end members consisting of the pristine igneous rock components present as discrete samples at the apollo 16 site and supplemented by kreep dunite and a meteoritic component yield the best fits for the composition of polymict breccias the end member rocks a re ferroan anorthosite various magnesian gabbronorites including sodic ferrogabbro and feldspathic lherzolite and spinei troctolite the following model is proposed for the composition and stratigraphy of the target for north ray crater the lower section of the stratigraphy is composed of a megabreccia with clasts of highly feldspathic polymict breccias kreep free old eastern highland rock suite interpreted as nectaris ejecta descartes formation the top section contains kreep bearing polymict breccias kreep bearing young western highland rock suite and appears to be similar to the lithologies found in the cayley plains this material interpreted as cayley formation may be a distant facies of imbrium basin ejecta deposits of the imbrium basin the petrographic differences between these two major selenographic units the descartes and the cayley formations in the apollo 16 area are more distinct than the chemical differences the petrographic and chemical composition of the primordial igneous upper crust in the regions of the nectaris and imbrium basins has been calculated by subtraction of the kreep and meteoritic components from the bulk composition of the descartes and cayley materials the nectaris crust which is better constrained consists of 86 87 a northosite 4 sodic ferrogabbro 0 5 1 3 feldspathic lherzolite 6 8 regular magnesian gabbronorite 1 8 2 8 dunite and 0 1 spinel troctolite a model for the evolution of the upper lunar crust in the descartes highlands is proposed on the basis of isotope ages and clast matrix relationships of polymict breccias essential features of this model in sequenial order are 1 development of multiple layers of kreep free early feldspathic fragmental megabreccias and impact melt sheets on the primordial crust in the time period from 4 4 aeons to the time of the nectaris impact which could have occurred as late as 3 85 aeons ago 2 excavation of these megabreccias by the nectaris event and deposition of a nectarian feldspathic fragmental megabreccia in the descartes area 3 introduction of kreep basaltic material into the upper crust mainly west and northwest of the descartes site and formation of kreepy impact melt sheets most probably from 4 05 to 3 80 aeons ago 4 imbrium impact at 3 8 aeons ago and deposition of kreep bearing imbrian feldspathic fragmental breccias at the descartes site which form the cayley formation and 5 redeposition mixing and melting of all preexisting breccias yielding kreep bearing and kreep poor post cayley fragmental breccias and melt ejecta glass bombs as the result of small impacts craters 1 5 km in diameter in the cayley plains and the descartes mountains respectively

evidence for homogeneous distribution of osmium in the protosolar nebula separate s r and possibly p process enriched and depleted components have been shown to host os in low metamorphic grade chondrites although no measureable os isotopic anomalies have yet been discovered for bulk chondrites here iron meteorites from groups iab iiab iiiab iva and ivb as well as the main group pallasites are examined many of these meteorites show well resolved anomalies in e190os e189os and e186osi the anomalies however differ from those observed in chemically extracted components from chondrites and are interpreted to reflect long term exposure of the meteorites to cosmic rays rather than nucleosynthetic effects a neutron capture model is presented that can well account for observed isotopic variations in 190os and 189os the same model predicts greater enrichment in 186osi than is observed for at least one iron suggesting as yet unaccounted for effects or failings of the model despite the variable anomalies resulting from cosmic ray exposure each of the major meteorite groups examined contains at one member with normal os isotopic compositions that are unresolved from chondritic compositions this indicates that some domains within these meteorites were little affected by cosmic rays these domains are excellent candidates for application of the 182hf 182w system for dating metal silicate segregation on their parent bodies the normal os also implies that os was homogeneously distributed throughout the protosolar nebula on the scale of planetesimal accretion within the current level of analytical resolution the homogeneity in os contrasts with isotopic heterogeneity present for other siderophile elements including mo ru and w the contrast in the scale of anomalies may reflect a late stage injection of s and p process rich material into the coalescing nebula alternately nebular thermal processing and destruction of some presolar host phases of mo ru and w may also be responsible

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

complex history of a zircon aggregate from lunar breccia 73235 abstract results are reported of an investigation of the age and origin of the exceptional zircon aggregate in an anorthositic clast from lunar breccia 73235 cathodoluminescence and birefringence images show that the aggregate consists of numerous angular fragments of sector zoned primary zircon in a matrix of secondary zircon with an overall texture that resembles a pseudotachylite sims u pb analyses of the primary fragments and the matrix yielded two clearly defined ages an age of 4 315 0 015 ga and initial th u ratio of 0 21 0 35 for the primary zircon and an age of 4 187 0 011 ga and th u of 0 04 0 17 for the secondary zircon matrix a raman spectroscopic study the secondary matrix zircon was undertaken to investigate its structure results showed that the matrix has a zircon structure but there is also evidence for the presence of an amorphous component implications of the structural and u pb age data are discussed in terms of the origin and evolution of the aggregate and the history of lunar events it is proposed that an original single millimetre sized sector zoned zircon formed at 4 31 ga was subjected to a severe shock event at 4 18 ga this event resulted in the fracturing of the zircon the displacement and rotation of fragments the compression of the aggregate to a lensoid shape and the shock reduction of zircon to sub micron sized and amorphous granules in crush zones in the mosaic of fractures volatilisation loss of pb and the addition of u to the secondary zircon is attributed to processes activated by the extreme thermal pulse which accompanied the 4 18 ga shock event shock effects are seen in some of the primary fragments but raman spectra of the primary and secondary zircon show no evidence for pressure induced transformation of zircon to a scheelite structure the zircon u pb system has not been affected by the ca 3 95 ga thermal pulse that accompanied formation of the host breccia although this event has largely reset the k ar systems

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

pinpointing the source of a lunar meteorite implications for the evolution of the moon the lunar meteorite sayh al uhaymir 169 consists of an impact melt breccia extremely enriched with potassium rare earth elements and phosphorus thorium 32 7 parts per million ppm uranium 8 6 ppm potassium oxide 0 54 weight percent and adherent regolith the isotope systematics of the meteorite record four lunar impact events at 3909 13 million years ago ma 2800 ma 200 ma and 0 34 ma and collision with earth sometime after 9 7 1 3 thousand years ago with these data we can link the impact melt breccia to imbrium and pinpoint the source region of the meteorite to the lalande impact crater

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

a sawtooth like timeline for the first billion years of lunar bombardment abstract we revisit the early evolution of the moon s bombardment our work combines modeling based on plausible projectile sources and their dynamical decay rates with constraints from the lunar crater record radiometric ages of the youngest lunar basins and the abundance of highly siderophile elements in the lunar crust and mantle we deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history but also there was no prominent and narrow impact spike 3 9 gy ago unlike that typically envisioned in the lunar cataclysm scenario instead we show the timeline of the lunar bombardment has a sawtooth like profile with an uptick in the impact flux near 4 1 gy ago the impact flux at the beginning of this weaker cataclysm was 5 10 times higher than the immediately preceding period the nectaris basin should have been one of the first basins formed at the sawtooth we predict the bombardment rate since 4 1 gy ago declined slowly and adhered relatively close to classic crater chronology models neukum and ivanov 1994 overall we expect that the sawtooth event accounted for about one fourth of the total bombardment suffered by the moon since its formation consequently considering that 12 14 basins formed during the sawtooth event we expect that the net number of basins formed on the moon was 45 50 from our expected bombardment timeline we derived a new and improved lunar chronology suitable for use on pre nectarian surface units according to this chronology a significant portion of the oldest lunar cratered terrains has an age of 4 38 4 42 gyr moreover the largest lunar basin south pole aitken is older than 4 3 gy and therefore was not produced during the lunar cataclysm

pieces of the ancient lunar crust ages and composition of clasts in consortium breccia 67915 we discuss the composition and chronology of clasts representing three minor lithologies in consortium breccia 67915 the lithologies studied are sodic ferrogabbro pristine troctolitic anorthosite 67915 26 and granulitic troctolitic anorthosite 67915 67 these lithologies were presumed to represent samples of the ancient lunar crust the pristine troctolitic anorthosite a typical member of the pristine anorthosite suite contains little or no trapped liquid consistent with its two phase mineralogy the plagioclase separates have 39ar 40ar plateau ages of 4 10 0 06 b y and a final rise in the age pattern may represent a memory of the earlier evolution the granulitic troctolitic anorthosite which has 13 times larger ree abundances displays a continuous rise of the 40ar 39ar ratio the last 25 of the 39ar release data establish a minimum age of 3 5 b y but the 244pu nd chronology suggests an age of 4 3 b y the sodic ferrogabbro samples which appear to be members of the mg gabbronorite group of pristine rocks show large ar losses but indicate plateaus in the low temperature data suggesting a metamorphic event more recently than 260 m y ago a time that is consistent with the north ray impact event 50 m y ago

re examination of the formation ages of the apollo 16 regolith breccias abstract the lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids comets and inter planetary dust fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the moon constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner solar system here we use a revised antiquity calibration after eugster et al 2001 that utilises the ratio of trapped 40 ar 36 ar parentless 40 ar derived from radioactive decay of 40 k against solar wind derived 36 ar to semi quantitatively calculate the timing of the assembly of the apollo 16 regolith breccias we use the trapped 40 ar 36 ar ratios reported by mckay et al 1986 our model indicates that the apollo 16 ancient regolith breccia population was formed between 3 8 and 3 4 ga consistent with regoliths developed and assembled after the imbrium basin forming event at 3 85 ga and during a time of declining basin forming impacts the material contained within the ancient samples potentially provides evidence of impactors delivered to the moon in the late imbrian epoch we also find that a young regolith population was assembled probably by local impacts in the apollo 16 area in the eratosthenian period between 2 5 and 2 2 ga providing insights to the sources of post basin bombardment the soil like regolith breccia population and the majority of local apollo 16 soils were likely closed in the last 2 ga and therefore potentially provide an archive of projectile types in the eratosthenian and copernican periods

the great lunar hot spot and the composition and origin of the apollo mafic lkfm impact melt breccias thorium rich mafic impact melt breccias from the apollo 14 17 missions that is those breccias identified with the composition known as lkfm are regarded largely as products of basin forming impacts that penetrated the feldspathic crust and sampled underlying mafic material and magma ocean residuum carrying the compositional signature of kreep potassium rare earth elements phosphorous despite considerable compositional variation among such breccias compositions of all of them correspond to mixtures of only four components 1 a norite with composition generally similar to that of apollo 15 basalt mean abundance 58 range 30 95 2 fo 90 dunite mean 13 range 1 27 3 feldspathic upper crust mean 29 range 4 50 and feni metal 0 1 1 7 petrographic evidence has shown that much of the feldspathic component but none of the kreep component is clastic this observation and the high proportion of kreep norite component in the breccias suggest that the melt zone of the impact or impacts forming the breccias contained little feldspathic material but consisted predominantly of material with the average composition of kreep norite the dunite component probably derives ultimately from the upper mantle these conclusions support the hypothesis that the breccias were not formed in typical feldspathic crust but instead by one or more impacts into what is designated here the great lunar hot spot that is the anomalous th rich terrane in the imbrium procellarum area identified by the apollo and lunar prospector gamma ray spectrometers the lkfm composition is a special product of the great lunar hot spot and is not the average composition of the lower crust in typical feldspathic highlands similarly mg suite and alkali suite plutonic rocks of the apollo collection are likely all differentiation products of the hot spot not of plutons that might occur in typical feldspathic crust

diverse impactors in apollo 15 and 16 impact melt rocks evidence from osmium isotopes and highly siderophile elements abstract concentrations of highly siderophile elements hse and 187 os 188 os isotopic compositions for eleven impact related rocks from the apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins data for the samples all define linear trends on plots of hse versus ir concentrations whose slopes likely reflect the relative hse compositions of the dominant impactors that formed the rocks the inferred imbrium basin impactor that generated apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187 os 188 os ru ir pt ir and pd ir ratios diverse impactor components are revealed in the apollo 16 impact melt rocks the 187 os 188 os and hse ir ratios of the impactor components in melt rocks 60635 63595 and 68416 with reported ages 4 0 ga by contrast the impactor components in melt rocks 60235 62295 and 67095 with reported ages of 3 9 ga are characterized by suprachondritic 187 os 188 os and hse ir ratios similar to the apollo 15 impact melt rocks and may also sample the imbrium impactor three lithic clasts from regolith breccias 60016 and 65095 also with 3 9 ga ages contain multiple impactor components of which the dominant composition is considerably more suprachondritic than those implicated for imbrium and serenitatis apollo 17 impactors the dominant composition recorded in these rocks was most likely inherited from a pre imbrium impactor consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend virtually all lunar impact melt rocks sampled by the apollo missions as well as meteorites are characterized by 187 os 188 os and hse ir ratios that when collectively plotted define linear trends ranging from chondritic to fractionated compositions the impact melt rocks with hse signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had hse compositions similar to known chondrite groups by contrast the impact melt rocks with non chondritic relative hse concentrations could not have been made by mixing of known chondritic impactors these signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections alternately they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core pre 4 0 ga ages for at least some impactor components with both chondritic and fractionated hse raise the possibility that the bulk of the hse were added to the lunar crust prior to the later stage basin forming impacts such as imbrium and serenitatis as proposed by fischer godde and becker 2012 for this scenario the later stage basin forming impacts were more important with respect to mixing prior impactor components into melt rocks rather than contributing much to the hse budgets of the rocks themselves

the typical structure of tornado proximity soundings an objective scheme based on empirical orthogonal function analysis to detect patterns in a single or multivariate data set is developed and applied to rawinsonde observations taken in the near tornado environment if only temperature data are considered two distinct categories differentiated chiefly by the tropopause height are found when moisture observations are included with the temperatures the separation between categories becomes less distinct however it is noted that within the near tornado environment there is an inverse relationship between the degree of observed convective and conditional instability analysis of only the winds shows that a low level veering with height is the rule however the strength of the veering can vary considerably when the temperature moisture and winds are treated in concert two categories again appear one group occurs with strong winds and a low tropopause while the other group features weak winds and a high tropopause these groups correspond to springtime and summertime synoptic situations respectively comparisons of the various analyses indicate that the near tornado environment typically features a balance between the strength of the veering of the winds and the amount of conditional instability present summer tornadoes feature strong conditional instability and weak winds while springtime tornadoes occur with stronger veering and convective instability the strongest tornadoes occur with springtime type conditions

rb sr analyses of apollo 16 melt rocks and a new age estimate for the imbrium basin lunar basin chronology and the early heavy bombardment of the moon rb sr model ages on 7 impact glass bombs and internal rb sr isochrons for two crystalline impact melt rocks from the apollo 16 collection have been determined the post cayley glass bombs with model ages between 4 75 0 45 ae and 3 97 0 08 ae can be classified according to their calculated single stage 87rb86sr i ratios 67728 67946 and 67627 8 point to a kreep free precursor terrain the descartes highlands whereas 63566 67567 67627 10 and 67629 are derived from the more heterogeneous cayley plains the very feldspar rich impact melt rock 65795 which is compositionally similar to the group of feldspathic microporphyritic melt breccias fm suite yields a crystallization age of 3 81 0 04 ae 2 87rb 1 42 11yr and isr of 69929 3 in the subophitic impact melt rock 60635 5 two ages were recorded one isochron with 3 87 0 02ae and isr of 0 69920 3 gives the crystallization age of a fine grained internal clast at 3 75 0 03 ae this fragment was incorporated into the host lithology of 60635 which is a member of the anorthositic noritic melt rock anmr group this group has a crystallization age of about 3 75 ae the anmr rocks and several samples of apollo 14 crystalline impact melt rocks belong to a suite of young crystalline impact melt rocks ranging in age from 3 71 to 3 81 ae these ages are in conflict with the canonical age of the imbrium event 3 85 ae it is argued that the young apollo 14 and 16 melt rocks cannot represent solid exotic fragments or melt splashes ejected from post imbrium local or distant craters instead these rocks are derived from coherent impact melt sheets of pre imbrian craters with diameters larger than 5 7 km they are interpreted as clasts of the fra mauro and cayley formation breccia deposits and therefore their ages are used as an upper limit for the age of the imbrium event we suggest that the imbrium basin and the related fra mauro and cayley formations were formed 3 77 0 02 ae ago and could be even as young as 3 75 ae as a consequence we adopt 3 92 0 03 ae 3 87 0 03 ae and 3 84 0 04 ae as ages for the nectaris serenitatis and crisium basins respectively in agreement with the relative crater densities measured on the ejecta blankets of these basins wilhelms 1984 the proposed age sequence leads to an average formation interval for the observed 12 13 nectarian basins of 7 to 14 m y leaving 30 pre nectarian basins of unknown age these facts suggest that there is no late terminal lunar cataclysm in the sense of a culmination of the lunar impact rate at 3 8 ae ago rather the observations are compatible with a steeply and steadily decreasing flux of impactors hartmann 1980 in the sense of an early heavy bombardment which started at the time of the moon s accretion and terminated around 3 75 ae ago

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

40ar 39ar age spectra of apollo 15 impact melt rocks by laser step heating and their bearing on the history of lunar basin formation we have obtained 26 high resolution 16 51 steps 40ar 39ar age spectra using a continuous laser system on submilligram fragments of recrystallized melt and single crystal plagioclase clasts from 12 apollo 15 impact melt rocks collected at the apennine front where the imbrium and serenitatis basins intersect these melt rocks represent a wide range of compositions and at least half a dozen different impacts six of the melt rocks have reproducible intermediate temperature plateaus over 40 or more of the 39ar released the plateaus are interpreted as crystallization impact ages and much of the non plateau behavior is attributable to recoil samples 15294 6 21 15304 7 69 15314 26 156 15357 15 and 15359 12 have mean 40ar 39ar plateau ages that are statistically indistinguishable and fall within the narrow range 3852 14 2 ma to 3870 12 ma with a weighted mean of 3865 5 ma sample 15356 9 has a mean plateau age of 3836 11 ma and may represent a distinctly younger impact a seventh sample 15314 30 158 has a peculiar but reproducible double plateau a low t one at 3873 9 ma which we think records the crystallization age and a high t one of 3831 10 ma which we interpret as an experimental 39ar recoil artifact four of the remaining melt rocks 15308 9 15414 2 37 15436 2 15445 253 have complex 40ar 39ar age spectra that indicate that they either formed in or were disturbed by impacts that occurred 3850 ma but did not completely reset the k ar isotopic system sample 15414 3 36 is different its spectrum may represent release mainly from clasts that were not well degassed in a melt event at 3870 ma the melt phase contains little potassium because most of the apennine front material must be coeval with or predate formation of the imbrium basin it seems likely that the imbrium impact is no older than 3870 ma and probably no older than 3836 ma so far there is no convincing evidence in the lunar record for melt producing impacts such as basin formation older than about 3 9 ga

group iva irons new constraints on the crystallization and cooling history of an asteroidal core with a complex history we report analyses of 14 group iva iron meteorites and the ungrouped but possibly related elephant moraine eet 83230 for siderophile elements by laser ablation icp ms and isotope dilution eet was also analyzed for oxygen isotopic composition and metallographic structure and fuzzy creek currently the iva with the highest ni concentration was analyzed for metallographic structure highly siderophile elements hse re os and ir concentrations vary by nearly three orders of magnitude over the entire range of iva irons while ru pt and pd vary by less than factors of five chondrite normalized abundances of hse form nested patterns consistent with progressive crystal liquid fractionation attempts to collectively model the hse abundances resulting from fractional crystallization achieved best results for 3 wt s compared to 0 5 or 9 wt s consistent with prior studies concentrations of hse and other refractory siderophile elements estimated for the bulk iva core and its parent body are in generally chondritic proportions projected abundances of pd and au relative to more refractory hse are slightly elevated and modestly differ from l ll chondrites which some have linked with group iva based on oxygen isotope similarities abundance trends for the moderately volatile and siderophile element ga cannot be adequately modeled for any s concentration the cause of which remains enigmatic further concentrations of some moderately volatile and siderophile elements indicate marked progressive depletions in the iva system however if the iva core began crystallization with 3 wt s depletions of more volatile elements cannot be explained as a result of prior volatilization condensation processes the initial iva core had an approximately chondritic ni co ratio but a fractionated fe ni ratio of 10 indicates an fedepleted core this composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron consistent with an oxidized parent body the depletions in ga may reflect decreased siderophilic behavior in a relatively oxidized body and more favorable partitioning into the silicate portion of the parent body phosphate inclusions in eet show d 17 o values within the range measured for silicates in iva iron meteorites eet has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite chemical and isotopic evidence for a genetic relation between eet and group iva is strong but the high ni content and the newly determined rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped previously reported metallographic cooling rates for iva iron meteorites have been interpreted to indicate an inwardly crystallizing 150 km radius metallic body with little

comparative zircon u pb geochronology of impact melt breccias from apollo 12 and lunar meteorite sau 169 and implications for the age of the imbrium impact abstract the ages of zircons from high th impact melt breccias imbs from meteorite sayh al uhaymir sau 169 and from rock fragments in soil samples from apollo 12 have been determined using the shrimp ii ion microprobe the imbs are very similar to each other in chemistry mineralogy and texture and the zircons from the kreep rich high th crystalline impact melt have similar u and th contents and identical ages within uncertainties of 3920 13 2 ma sau 169 and 3914 7 2 ma apollo 12 the age results support the idea that the high th imbs apollo 12 and sau 169 formed in the same impact event the similarity of composition and age suggest that sau 169 and the high th imb fragments of apollo 12 originated from the same area of the procellarum kreep terrane we interpret the age of zircon grains in the apollo 12 high th imb as a precise and direct determination of the age of the imbrium impact this age is significantly older than the commonly cited age of 3 85 ga but is similar to recent determinations from sims u pb dating of apollo 14 apatite grains and with anticipated revision of ages by 40 ar 39 ar and 87 rb 86 sr the present zircon 207 pb 206 pb age is the first direct zircon age determination of the imbrium impact event from an apollo sample previous measurements of zircon ages of apollo imbs have recorded events pre dating the imbrium basin forming event

the bombardment history of the moon as recorded by 40 ar 39 ar chronology new petrography and 40ar 39ar ages have been obtained for 1 3 mm sized rock fragments from apollo 16 station 13 soil 63503 north ray crater ejecta and chips from three rocks collected by apollo 16 and apollo 17 missions selection of these samples was aimed at the old 40ar 39ar ages to understand the early history of the lunar magnetic field and impact flux fifteen samples were studied including crustal material polymict feldspathic fragmental breccias and impact melts the impact ages obtained range between approximately 3 3 and 4 3 billion years ga polymict fragmental breccia 63503 1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4 547 0 027 the plateau age of 4 293 0 044 ga obtained for impact melt rock 63503 13 represents the oldest known age for such a lithology possibly this age represents the minimum age for the south pole aitken spa basin in agreement with literature data these results show that impact ages 3 9 ga are found in lunar rocks especially within soil 63503 impact exhumation of deep seated warm crustal material onto the lunar surface is considered to explain the common 4 2 ga ages obtained for weakly shocked samples from soil 63503 and apollo 17 this would directly imply that one or more basin forming events occurred at that time some rock fragments showing none to limited petrologic features indicate thermal annealing these rocks may have lost ar while resident within the hot ejecta of a large basin concurrent with previous studies these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1 3 ga