the lappajarvi meteorite crater finland petrography rb sr major and trace element geochemistry of the impact melt and basement rocks abstract impact melt lithologies of the 77 m y old finnish meteorite crater lappajarvi as well as the precambrian target rocks have been studied in detail to identify and characterize different impact melt types clast poor clast rich suevitic melt and to study their chemical major and trace elements and isotopic rb sr compositions in comparison to the composition of the target rocks the rb sr system of the whole melt body including the suevitic melt is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks chemical interactions exchange of alkali elements 87sr redistribution between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt exchange reactions between clasts and melt are determined by thermal activation but the degree of shock metamorphism in the clasts plays an important role too major and trace element distributions in impact melt and basement rocks indicate that the lappajarvi melt body chemically is extremely homogeneous even volatile elements such as zn and cu were not strongly fractionated comparison of the abundances of siderophile elements in the impact melt e g 118 177 ppm cr 195 340 ppm ni 6 12 ppb ir and calculated target rock mixture 79 mica schist 11 granite pegmatite 10 amphibolite e g 85 6 ppm cr 54 8 ppm ni 0 5 ppb ir revealed the chondritic nature c or h chondritic of the meteoritic projectile less than 2 of the meteorite can be detected in the coherent melt whereas the suevitic melt is uncontaminated by the projectile

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

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

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

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

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

disequilibrium features in experimentally shocked mixtures of olivine plus silica glass powders seventeen shock recovery experiments were performed on powder mixtures of one part by weight olivine st john s forsterite plus two parts silica glass pure vitreous silica in order to characterize the physical and chemical interaction of two chemically incompatible components during shock powders of 45 m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6 2 to 64 2 gpa 1 gpa 10kbar

crosscutting relations and relative ages of ridges and faults in the tharsis region of mars abstract observations of ridge fault crosscutting relationships on the ridged plains units surrounding the tharsis region of mars have led to the development of a classification scheme involving three distinct types of intersections ridges crosscut by faults are designated type c and account for 81 of the observed intersections ridges terminated at one end by a fault type t as well as those superposed on grabens type s are less numerous interpretation of the morphology of these intersections and the angles of intersection between ridges and faults with radial trends to major topographic features in the tharsis region have led to the following conclusions 1 the major ridge forming events in the tharsis region were roughly coincident with and in some cases possibly prior to the extensional events that produced the faulting of the tempe and mareotis regions the coprates and memnonia regions and the rifting of valles marinrris 2 the compressional events that formed most of the ridges are restricted in time both by the irrelationship to regional extensional events and by the age of the units on which they formed the suggestion that compressional ridges are a result of a single long term viscoelastic response of the lithosphere to loading of the crust is not supported by this study a model involving one or more isostatically compensated uplifts and subsequent relaxation of the crust after the emplacement of the ridged plains volcanic units is favored

strontium and neodymium isotopic characteristics of a heterolithic breccia in the basement of the sudbury impact structure canada abstract rb sr and sm nd measurements were undertaken on footwall breccia samples originating from levack township in the north range of the sudbury structure the granitic to quartzdioritic matrix of this heterolithic breccia which contains shocked rock and mineral fragments yields a rb sr isochron age of 1 825 0 021 ga isr 0 70356 0 00005 2 errors based on the analysis of 3 6 mm thick adjacent whole rock slabs from a large hand specimen this age is interpreted as a cooling age after the thermal metamorphism of the footwall breccia by the 1 85 ga old sudbury igneous complex which had caused temperatures in excess of 1000 c in the matrix near the contact mafic rock fragments in this breccia were excluded from the sr isotope exchange at that time the sm nd system of the identical small whole rock samples was only partly disturbed by the sudbury igneous complex the data points scatter around a 2 05 0 49 ga regression line without age significance plagioclase k feldspar and mafic mineral phases of the breccia matrix combined with the corresponding whole rocks display a range in rb sr ages from 1 67 0 08 ga to 1 430 0 015 ga obviously associated with metasomatic processes in the footwall breccia during the declining penokean metamorphism strontium and neodymium model parameters of the footwall breccia matrix near levack show little variation esrt 16 33 to 18 07 endt 10 69 to 11 79 t 1 86 ga and mean tnddm 2 74 0 04 ga based on these results bedrocks exposed in the south range and rocks now forming the sedimentary filling of the sudbury basin can be excluded as major components in the matrix of the analyzed footwall breccia the convergence of neodymium model ages with u pb ages on zircons from the levack gneiss which forms the crystalline basement adjacent to the footwall breccia in the levack area support an exclusively local origin for matrix and fragments of this breccia it is proposed that the footwall breccia is part of the uplifted floor of the sudbury crater

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

ages of mare basalts on the lunar nearside the chronology of lunar volcanism is based on radiometric ages determined from apollo and luna landing site samples regional stratigraphic relationships and crater degradation and size frequency distribution data for units largely defined prior to the end of the apollo program here we report on new crater size frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins australe tranquillitatis humboldtianum humorum serenitatis and imbrium crater size frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the moon compared to crater degradation ages crater size frequency ages performed on spectrally defined units offer significant improvements in accuracy our investigation showed that 1 in the investigated basins lunar volcanism was active for at least 1 5 2 0 b y starting at about 3 9 4 0 b y and ceasing at 2 0 b y 2 most basalts erupted during the late imbrian period at about 3 6 3 8 b y 3 significantly fewer basalts were emplaced during the eratosthenian period 4 basalts of copernican age were not found in any of the investigated basins 5 lunar basin filling volcanism probably started within 100 m y after the formation of the individual basins we also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins we found that 1 in all pre nectarian basins australe and tranquillitatis as well as in the humboldtianum basin which is of nectarian age the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3 6 3 8 b y 2 in the younger basins humorum serenitatis and imbrium a second peak at 3 3 3 5 b y is observed 3 basalt eruptions younger than 2 6 b y occur only intermittently and 4 in the youngest basins serenitatis and imbrium we see an extended period of active basin filling volcanism 1 5 1 6 b y which is 500 m y longer than in the australe and humorum and even 1 0 b y longer than in tranquillitatis and humboldtianum

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

petrology of the apollo 11 highland component new data for 38 highland fragments hand picked from apollo 11 coarse fines are reported petrographic mineralogic and bulk chemical data show that 1 the apollo 11 highland component is non kreepy and like that of apollo 16 2 poikilitic rocks granulitic breccias and the anorthosite norite troctolite suite are the most abundant rock types followed by polymict breccias and glasses 3 both the ferroan anorthosite and mg rich plutonic suites are represented in the apollo 11 highland component 4 except for one sample the intermediate k fra mauro and high k fra mauro groups are not represented the data and observations are consistent with local derivation of the highland material from beneath relatively thin basalt flows and addition to the regolith via vertical mixing

apollo 16 site geology and impact melts implications for the geologic history of the lunar highlands the geology of the apollo 16 site is reconsidered on the basis of data from photogeology geochemical remote sensing and lunar samples the site possesses an upper surface of anorthositic gabbro and related rocks mafic components were deposited as basin ejecta the events involved in its geological evolution were the nectaris impact and the imbrium impact the role of large local craters in the history of the region was to serve as topographic depressions to accumulate basin ejecta the most abundant melt composition at apollo 16 is an aluminous variety of lkfm basalt supplied by the nectaris impact as ejected basin impact melt the mafic lkfm melt may have been supplied by the imbrium impact more aluminous melt groups are probably derived from local small craters the remainder of the deposits in the region are composed of anorthositic clastic debris derived from the nectaris basin the local crustal substrate and imbrium and other basins

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

flexure and gravity anomalies of the oceanic lithosphere beneath the louisville seamount abstract we have calculated the elastic thickness te flexural deflection and gravity anomaly of the oceanic crust beneath the louisville seamount lsc 03 near the kermadec trench a regional residual separation of the bathymetry was performed to remove the effect of other geologic features e g the trench we used the uniform density and dense core models to approximate the total mass of the seamount which was defined as the surface load required for flexural deformation from the flexure modeling results we found that more flexural depression was predicted by the uniform density model than by the dense core model however the uniform density model predicted a significantly smaller gravity anomaly than observed whereas the dense core model minimized the prediction misfits reasonably the best flexure model was found with a te of 16 km for the uniform density model and 6 km for the dense core model the flexure computed with the dense core model was consistent with the seismically detected moho the flexure modeling for lsc 03 thus indicates that the dense core model better approximates the inner structure of the lsc 03 based on the crustal age and geochronology of the given seamount the age of the oceanic crust at the time of seamount formation t is 20 ma if this is the case however the te estimates from both flexure models require some degree of lithospheric reheating by louisville hotspot activity alternatively considering the tectonic plate motion of the osbourn trough t becomes approximately 4 ma this younger lithosphere model is more consistent with the observed flexural deformation and the te estimate from the dense core model therefore the time that the seamount induced lithospheric deformation occurred may be far earlier than the age dated volcanism

the petrogenesis of impact basin melt rocks in lunar meteorite shisr 161 this study explores the petrogenesis of shisr 161 an immature lunar regolith breccia meteorite with low abundances of incompatible elements a feldspathic affinity and a significant magnesian component our approach was to identify all clasts 0 5 mm in size in a thin section characterize their mineral and melt components and reconstruct their bulk major and minor element compositions trace element concentrations in representative clasts of different textural and compositional types indicate that the clast inventory of shisr 161 is dominated by impact melts that include slowly cooled cumulate melt rocks with mafic magnesian mineral assemblages minor exotic components are incompatible element rich melt spherules and glass fragments and a gas associated spheroidal precipitate our hypothesis for the petrologic setting of shisr 161 is that the crystallized melt clasts originate from the upper 1 km of the melt sheet of a 300 to 500 km diameter lunar impact basin in the moon s feldspathic highlands this hypothesis is based on size requirements for cumulate impact melts and the incorporation of magnesian components that we interpret to be mantle derived the glassy melts likely formed during the excavation of the melt sheet assemblage by an impact that produced a 15 km diameter crater the assembly of shisr 161 occurred in a proximal ejecta deposit of this excavation event a later impact into this ejecta deposit then launched shisr 161 from the moon our geochemical modeling of remote sensing data combined with the petrographic and chemical characterization of shisr 161 reveals a preferred provenance on the moon s surface that is close to pre nectarian riemann fabry basin

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

isotopic studies of ferroan anorthosite 62236 a young lunar crustal rock from a light rare earth element depleted source isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a sm nd isochron with an age of 4 29 0 06 ga and an initial end143 value of 3 1 0 9 we have also measured end142 anomalies of 0 25 on two fractions of 62236 these values are higher than the value of 0 1 predicted if 62236 was derived from a chondritic source at 4 29 ga but are consistent with the positive initial end143 value the sm nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the 147sm 144nd 143nd 144nd and 142nd 144nd ratios measured on the mineral fractions and whole rocks must be corrected the corrections do not significantly alter the sm nd isotopic results determined on 62236 despite the fact that the ar ar and rb sr isotopic systematics of 62236 have been reset by impact metamorphism at 3 93 0 04 ga the sm nd systematics appear to have been unaffected the sm nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization first the relatively young age of 62236 as well as the old ages determined on several crustal plutonic rocks of the mg suite require multiple sources of magmas on the moon very early in its history second positive end143 values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements and third models based on initial end143 and present day end142 values suggest that the source of 62236 was depleted in light rare earth elements at 4 46 ga in order to reconcile these observations with the lunar magma ocean model 1 the magma ocean must have existed for a very short period of time and may have had a sub chondritic nd sm ratio and 2 the youngest ferroan anorthosites such as 62236 cannot be cumulates from the magma ocean but must form by other processes

an extremely low source in the moon uthpb smnd rbsr and isotopic systematics and age of lunar meteorite asuka 881757 abstract we have undertaken u th pb sm nd rb sr and 40 ar 39 ar isotopic studies on asuka 881757 a coarse grained basaltic lunar meteorite whose chemical composition is close to low ti and very low ti vlt mare basalts the pb pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 28 ma which is similar to the u pb 3850 150 ma and th pb 3820 290 ma internal isochron ages the sm nd data for the mineral separates yield an internal isochron age of 3871 57 ma and an initial 143 nd 144 nd value of 0 50797 10 the rb sr data yield an internal isochron age of 3840 32 ma 87 rb 1 42 10 11 yr 1 and a low initial 87 sr 86 sr ratio of 0 69910 2 the 40 ar 39 ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 12 ma we interpret these ages to indicate that the basalt crystallized from a melt 3 87 ga ago the sm nd age and an impact event disturbed the rb sr system and completely reset the k ar system at 3 80 ga the slightly higher pb pb age compared to the sm nd age could be due to the secondary pb from terrestrial and or lunar surface pb contamination that remained in the residues after acid leaching alternatively the following interpretation is also possible the meteorite crystallized at 3 94 ga the pb pb age and the sm nd rb sr and k ar systems were disturbed by an impact event at 3 80 ga the crystallization age obtained here is older than those reported for low ti basalts 3 2 3 5 ga and for vlt basalts 3 4 ga but similar to ages of some mare basalts indicating that the basalt may have formed from a magma related to a basin forming event imbrium the age span for vlt basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least 500 million years the impact event that thermally reset the k ar system of asuka 881757 must have been post imbrium perhaps orientale in age the lead isotopic composition of asuka 881757 is nonradiogenic compared with typical apollo mare basalts and the estimated 238 u 204 pb value for the basalt source is 10 3 this source value is the lowest so far measured for lunar rocks a large positive nd value 7 4 0 5 and the time averaged 147 sm 144 nd ratio for the basalt source are similar to those for some apollo 12 15 and 17 basalts suggesting a lree depleted mantle which is consistent with the global magma ocean hypothesis the u th pb sm nd and rb sr data on asuka 881757 suggest that the basalt was derived from a low u pb low rb sr and high sm nd source region mainly composed of olivine and orthopyroxene with minor amounts of plagioclase or clinopyroxene and with sulfides enriched in volatile chalcophile elements the basalt source may be deep in origin and different in chemistry from those previously estimated from studies of apollo and luna mare basalts indicating heterogeneous sources for mare basalts

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

homogeneous impact melts produced by a heterogeneous target sr nd isotopic evidence from the popigai crater russia the 35 7 0 2 ma old popigai crater siberia with a diameter of about 100 km is one of the best preserved large terrestrial impact structures the heterogeneous target at the impact site consists of archean to lower proterozoic metamorphic rocks of the crystalline basement upper proterozoic quartzites and other clastic deposits as well as cambrian to cretaceous clastic sediments and sedimentary rocks including carbonate rocks moreover proterozoic and permo triassic dolerite dykes are found in the target area we report major element sr and nd isotope data for 13 of these target rocks and for various types of impactites the 15 analysed impactite samples include tagamites impact melt rocks suevites and impact glass from small veins furthermore two impact breccias and two impact glass coated gneiss bombs were analysed we discuss the relation of these impactites to the target lithologies and evaluate on the basis of literature data the relation of microkrystites and associated microtektites in upper eocene sediments to the popigai event the impactites have sio2 abundances ranging from 59 to 66 wt and show significant variations in the content of fe ca and ti they have present day 87sr 86sr ratios between 0 7191 and 0 7369 their sr model ages tsrur range from 1 9 to 2 3 ga the 143nd 144nd ratios for the impactite samples cluster between 0 5113 and 0 5115 the nd model ages tndchur range from 1 9 to 2 1 ga in an echur nd eur sr diagram the impactites and upper eocene microkrystites and associated microtektites plot in a field delimited by popigai target lithologies the impactites are restricted to the field of crystalline basement rocks and upper proterozoic quartzites but they show different isotopic signatures in different crater sectors impactites and upper eocene microkrystites plot in different only partly overlapping clusters the leucocratic microkrystites and microtektites have a higher affinity to the post proterozoic rocks in the target area than the impactites seemingly the melanocratic microkrystites originated mostly from crystalline basement this data alignment supports the assumption that popigai is the source crater for all three types of ejecta for the first time clear relations are established of the geochemically variable upper eocene microkrystites and associated microtektites to specific target lithologies at popigai crater finally the observed range in sr and nd isotope parameters determined for impact melt lithologies that originated during the popigai event show a much higher variability than known from other craters this result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias was incomplete at the time of ejecta dispersal