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

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

a ferroan region of the lunar highlands as recorded in meteorites mac88104 and mac88105 macalpine hills 88104 and 88105 mac88104 5 are paired meteorites of noritic anorthosite composition from the lunar highlands mac88105 is a breccia composed mainly of melt breccia clasts in a fine grained fragmental and partly glassy matrix the most abundant melt lithologies are feldspathic and are similar in composition to the bulk meteorite other melt lithologies include feldspathic melt rocks mafic melt breccias and a rare melt breccia relatively enriched in incompatible trace elements subordinate lithic clasts are granulitic breccias and ferroan relatively low mg mg fe igneous lithologies including troctolitic anorthosite anorthositic norite gabbronorite and anorthosite igneous clasts having mafic mineral compositions more magnesian than fo55 and en60 were not observed rare fragments of glass spheres and shards as well as glass clasts indicate that the meteorite was derived from an immature regolith the bulk composition of mac88105 is characterized by a molar mg mg fe ratio of 0 62 at the extreme low end of the range for meteorites from the lunar highlands its low concentrations of incompatible trace elements and feldspathic bulk composition 29 al2o3 suggest that it like the other lunar meteorites formed at a site far removed from the areas sampled by the apollo missions similarities in mineral compositions among the different lithologies of the breccia melt breccia clasts granulitic breccias igneous fragments and the distribution of mineral fragments suggest that most components of the meteorite were derived from a crustal section dominated by material with a noritic anorthosite composition and an affinity to the ferroan suite of plutonic rocks the mafic melt breccias have roughly peritectic melt compositions similar to many low k fra mauro melt breccias in the apollo collection but much more ferroan and represent a source material distinct from the noritic anorthosite moderate ce anomalies both positive and negative occur in some separated clasts and to a lesser extent in some small chips dominated by matrix but not in powder prepared from 20 g of bulk meteorite these anomalies are almost certainly terrestrial weathering effects one granulitic breccia clast is rich in siderophile elements as a result of a large amount of fe ni metal the auir ratio of the clast 0 64 ci normalized is distinctly different from that of metal rich impact melts from apollo 16 3 ci normalized the compositional range observed among the nonmare lunar meteorites alha81005 y791197 y82192 3 y86032 and mac88104 5 is equivalent to that observed among representative samples of 1 mm fines from apollo 16 thus compositional data provide little constraint on the number of impacts required to eject these meteorites from the moon

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

composition of the cayley formation at apollo 16 as inferred from impact melt splashes chemical data for impact melt splashes ims collected at the apollo 16 landing site show that ni concentrations and feo ni cr ni and co ni correlations are consistent with a chondritic meteoritic component present at levels between approximately 1 5 and 11 wt like apollo 16 soils the ims are intermediate in composition to apollo 16 rocks the chemical data for the ims are consistent with two main compositional groups the group a ims 28 samples are chemically distinct from local apollo 16 soils and are very similar to mixtures of anorthosite and low sc vha lscvha impact melt rocks calculated to a meteorite free basis their average sc concentration la sc mgo sc and tio2 mgo ratios and mg value are 6 1 ug g 1 94 9800 0 066 and 0 74 respectively the group b ims seven samples have chemical equivalents among the soils five samples are chemically very similar to mature apollo 16 soils from the central and southern parts of the landing site two are more similar to soils 67601 and 67941 from north ray crater generally the group b ims and soils are enriched in a composition similar to anorthositic norite 67215 or mixtures thereof with anorthosite as compared to the group a ims the average sc concentration la sc mgo and tio2 mgo ratios and mg value for the subgroup of five are 9 3 ug g 1 41 5780 0 124 and 0 66 respectively the corresponding values for the subgroup of two are 7 5 ug g 1 04 7400 0 090 and 0 71 respectively the group b ims and most soils have a minor high ti component not present in the group a ims if this component is high ti mare basalt its average concentration is approximately 1 6 wt the seven group a ims that have been dated by rare gas methods all have approximately 2 m y exposure ages an age generally associated with south ray crater these seven and potentially most group a ims are likely quenched samples of the melt volume produced by the south ray impact the chemical signature of the impacting projectile is chondritic and its average concentration for the group a ims is 7 3 wt like large coherent melt sheets from terrestrial impacts the impact melt volume of the south ray impact as represented by the group a ims is homogeneous with respect to target lithologies target lithologies at south ray crater which are represented by the host rocks for the ims include anorthosite lscvha impact melt rocks hscvha impact melt rocks and dimict breccias with the exception of 61016 the melt portion of analyzed dimict breccias has the hscvha composition implying that the dimensional scales of lscvha type of rocks and anorthosite in the south ray target do not extend to the 1 10 cm scale observed for the hscvha dimict breccias the compositional difference between group a ims and surface soils from the southern part of the landing site implies stratigraphic variation in the dominant type of vha impact melt rock within the cayley formation chemical data and mixing model results are consistent with a model in which hscvha rocks dominate lscvha rocks in the surface layer the lscvha rocks dominate hscvha rocks in the layer beneath the south ray impact penetrated both layers as both types of vha impact melt rocks are traceable as ejecta material its impact melt volume was formed primarily from the lower layer to give the observed composition of the group a ims that the north ray impact also sampled the lower layer is indicated by the chemical affinities of soils 67601 and 67941 with the lscvha composition away from north ray crater most soils have chemical affinities with the hscvha composition reflecting soil development primarily from the surface layer estimated average compositions give tio2 mgo sc and la concentrations of 0 69 and 7 1 wt and 10 5 and 21 2 ug g respectively for the surface cayley layer and 0 38 and 5 9 wt and 6 1 and 11 8 ug g respectively for the subsurface cayley layer an average composition estimated for the descartes formation from soil compositions has tio2 mgo sc and la concentrations of 0 25 and 3 5 wt and 7 7 and 2 2 ug g respectively the cayley formation is more kreepy more magnesian and less aluminous than the descartes formation

lunar meteorites siderophile element contents and implications for the composition and origin of the moon abstract six lunar meteorites have thus far been discovered in antarctica although these meteorites are of undocumented lunar geographic provenance all other available lunar samples obtained from the apollo and luna programs came from a relatively minuscule region of the central nearside around which a polygon could be drawn covering just 4 7 of the lunar surface the five lunar meteorites that are now well studied represent at least two but probably three separate lunar source craters alha81005 and y791197 are probably from separate craters and compound meteorite y82192 6032 is certainly from a crater of its own at least alha81005 and y791197 and probably also at least in a loose sense y82192 6032 are highlands regolith breccias thus their compositions may be appropriately compared with compositions of apollo and luna highlands regolith samples compared with average highlands regolith samples from the central nearside apollo sites lunar meteorites have lower concentrations of siderophile elements in general and especially au ge and ni among anorthositic apollo samples i e mainly those from apollo 16 au ir and to a lesser extent ge ir ratios tend to be hyperchondritic most previous models for lunar regolith siderophile geochemistry assumed that the apollo 16 samples are representative of the overall lunar highlands such models tended to conclude that the hyperchondritic au ir and ge ir ratios were consequences of variations in the compositions of moon striking projectiles as a function of time it was even speculated that these temporal variations were consequences of the heterogeneous accretion of a moon with a general enrichment in refractory elements the lunar meteorites suggest that hyperchondritic au ir and ge ir are not general features of the highlands but peculiarities of the apollo 16 region or more generally the central nearside the high ni contents of the apollo 16 megaregolith substantially in excess of the ni implied by a model in which all meteoritic debris has an h chondritic ni ir ratio have been interpreted to imply that the mg rich fraction of the lunar crust is extremely ni rich and thus similar in siderophile geochemistry to terrestrial komatiites this coincidence has been interpreted as confirmation that the moon formed by modified fission from the earth s mantle after core formation plus late accretion effects had established the final siderophile pattern of the earth s mantle however the lunar meteorites and a survey of data for the luna 20 highlands regolith sample suggest that the high ni content of the apollo 16 regolith is strictly a peculiarity of the apollo 16 region and not a general feature of the moon s crust the anomalous siderophile composition of the central nearside regolith probably stems from a combination of regional heterogeneity in cratering projectile compositions plus regional enrichments in relatively labile siderophile elements most notably au ge and perhaps to a certain extent ni which might be loosely correlated with the anomalously high concentration of kreep over the central nearside

composition structure and age of the apollo 16 subregolith basement as deduced from the chemistry of post imbrium melt bombs irregularly shaped slag like impact melt bombs with variolitic crystallization texture mostly 1 to 10 cm in size occur throughout the regolith of the apollo 16 landing site we have analyzed 16 samples from stations 11 and 13 near north ray crater using the microscope electron microscope microprobe neutron activation and 40ar 39ar age dating techniques we interpret the melt rocks as rapidly quenched high velocity ejecta glass bombs from impact craters less than 1 5 km in diameter located in the vicinity of the apollo 16 landing site a comparison between glass bombs and typical regolith samples from various sampling stations reveals differences in their bulk chemical compositions more than 80 of the glass bombs cannot be derived from the melting of the regolith but can be produced by melting the subregolith basement the differences in chemical composition of metal particles in glass bombs and regolith samples are consistent with these findings mixing calculations show that the glass bombs were formed by melting of two or more of the most abundant lithologies granulites feldspathic melt breccias anorthosites and various mafic melt breccias of vha and lkfm affinity that constitute the subregolith basement from these results we conclude that the subregolith basement represents a polymict megabreccia target the glass bombs are post cayley in age they were produced by several different impact events more than 370 ma ago by which they were emplaced in proximity to the north ray crater area they were buried under a younger regolith layer until the north ray impact event 50 ma ago brought them up to the lunar surface again and since then they have been exposed to cosmic radiation

chemical composition and origin of apollo 15 impact melts to better understand the apennine front we have analyzed 14 apollo 15 impact melt rocks including hand samples rake samples and coarse fines for major fused bead microprobe and trace element inaa abundances the melt rocks show a remarkable diversity of chemical composition although all but one have typical kreep incompatible element patterns they appear to form five chemical groups all but the most kreep rich of which may represent single impacts three groups are of low k fra mauro composition none correspond to very high alumina vha compositions the compositions are unlike nearly all other impact melts and regoliths from highland sites indicating a distinct provenance for the apollo 15 samples serenitatis basin impact melt compositions apollo 17 poikilitic boulders are not duplicated although one apollo 17 aphanitic melt sample 73215 is similar to one of the apollo 15 groups the apollo 15 group consisting only of 15445 and 15455 remains the best candidate for imbrium basin impact melt while the most kreep rich group probably consists of melts produced in varied post imbrium impacts into the apennine bench formation apollo 15 kreep basalt flows the most populated group was previously unrecognized and could be either pre or post imbrium basin it is possible that it not 15445 15455 represents imbrium melt one unique sample has a non kreep incompatible element pattern with low abundances and a positive eu anomaly its target was rather primitive and would not appear to be of regional extent or present in the area in post imbrium times it is either a pre imbrian melt or exotic the apennine front does not consist of a monolithic lkfm composition but shows a much wider diversity of compositions than was previously recognized

rock 67015 a feldspathic fragmental breccia with kreep rich melt clasts breccia 67015 one of the feldspathic fragmental breccias ffbs from the rim of north ray crater contains a wider range of clast lithologies than has been described previously in ffb s our petrologic and inaa studies have revealed two components of major petrologic and geologic significance one of these is a dark vesicular kreep rich melt rock of vha very high alumina basalt composition which occurs in clasts up to 6 cm long melt rocks of this type are common at the apollo 16 site but in previous years had not been discovered as prominent constituents of the feldspathic fragmental breccias their apparent absence led to hypotheses no longer valid that the kreepy melt rocks were younger than the anorthositic breccia matrices and that the two lithologies might represent imbrian and nectaris ejecta respectively the second lithology we discovered is a large clast of medium grained ferroan anorthositic norite with a composition that falls among the most magnesian members of the ferroan anorthosite suite of pristine cumulate rocks this material constitutes a previously missing member of certain mixing models designed to account for the north ray crater melt rocks and bulk breccias

composition and maturity of apollo 16 regolith core 60013 14 abstract samples from every half centimeter dissection interval of double drive tube 60013 14 sections 60013 and 60014 were analyzed by magnetic techniques for fe concentration and surface maturity parameter i s feo and by neutron activation for concentrations of 25 lithophile and siderophile elements core 60013 14 is one of three regolith cores taken in a triangular array 40 50 m apart on the cayley plains during apollo 16 mission to the moon the core can be divided into three zones based both on i s feo and composition unit a 0 44 cm depth is compositionally similar to other soils from the surface of the central region of the site and is mature throughout although maturity decreases with depth unit b 44 59 cm is submature and compositionally more feldspathic than unit a regions of lowest maturity in unit b are characterized by lower sm sc ratios than any soil obtained from the cayley plains as a result of some unidentified lithologic component with low surface maturity the component is probably some type of mafic anorthosite that does not occur in such high abundance in any of the other returned soils unit c 59 62 cm is more mature than unit b and compositionally equivalent to an 87 13 mixture of soil such as that from unit a and plagioclase such as found in ferroan anorthosite similar soils but containing greater abundances of anorthosite plagioclase are found at depth in the other two cores of the array these units of immature to submature soil enriched to varying degrees compared to the mature surface soil in ferroan anorthosite consisting of 99 plagioclase are the only compositionally distinct subsurface similarities among the three cores each of the cores contains other units that are compositionally dissimilar to any soil unit in the other two cores these compositionally distinct units probably derive from local subsurface blocks deposited by the event s that formed the cayley plains the ferroan anorthosite with 99 plagioclase however must represent some subsurface lithology that is significant on the scale of tens of meters the compositional uniformity of the surface soil 0 10 cm depth over distances of kilometers reflects the large scale uniformity of the plains deposits the fine structure reflects small scale nonuniformity and the inefficiency of the impact mixing process at depths as shallow as even one meter

the composition and history of breccia 67015 from north ray crater we have examined feldspathic fragmental breccia 67015 from the rim of north ray crater and found it to be the most diverse and complex of the apollo 16 ffbs it contains a wider range of clast types than any of the others and is not dominated by any single lithology we have identified several crystalline lithologies occurring in minor amounts that we believe represent highland plutonic rocks plagioclase fragments are the most abundant breccia constituent followed by clasts of impact melt rocks and granulites the impact melts are of two main types feldspathic fragment laden melts and mafic kreepy melt rocks of vha basalt composition textural evidence indicates that some of the vha melts were hot and plastic when they accumulated into the breccia 40ar 39ar dates show that both types of melts solidified about 3 9 109 years ago magnetic measurements on clasts show a scatter of nmr directions making it impossible to read a consistent imprint of an ancient dipolar magnetic field previous studies of apollo 16 stratigraphy suggested that kreepy melt rocks were introduced to the site as imbrium impact ejecta about 3 8 106 years ago our results indicate that the highlands crust contained kreep components as early as 3 9 109 years ago

alkali gabbronorite ultra kreepy melt rock and the diverse suite of clasts in north ray crater feldspathic fragmental breccia 67975 lunar breccia 67975 is a feldspathic fragmental breccia from north ray crater apollo 16 it contains clasts of alkali gabbronorite and ultra kreepy mafic fragment laden melt breccias which are unique among apollo 16 samples both are alkali and iron rich rocks with moderate to high ree concentrations they more strongly resemble apollo 14 gabbronorites and alkali anorthosites and kreep rich rocks than they do other apollo 16 samples the other clasts in 67975 are the ferroan anorthosites feldspathic melt rocks and magnesian granulites which are typical of other feldspathic fragmental breccias examination of bulk and mineral compositions of other breccias and melt rocks suggests that alkali gabbronorite may be a minor component in other north ray crater breccias and feldspathic melt rocks this implies that alkali gabbronorite was a fairly early 4 0 b y crustal component in the north ray crater region

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

chemistry of the apollo 11 highland component chemical data 28 major minor and trace elements were obtained by instrumental neutron activation analysis inaa for 38 hand picked highland fragments 1 20 mg from coarse fines 10085 the apollo 11 highland chemical groups are represented by a range of petrographic types six major chemical highland groups are identified in the apollo 11 fragments these are 1 high k kreep 2 anorthosite e g 15415 60015 with 10 chondrite rare earth elements ree abundances a positive eu anomaly anorthosite with 30 chondrite abundances and a positive eu anomaly 3 anorthosite norite troctolite ant 67075 4 low k fra mauro lkfm 77135 5 anorthositic gabbro 68415 with no eu anomaly anorthositic gabbro 65055 67955 with a positive eu anomaly and anorthositic gabbro 68516 60015 with a negative eu anomaly and 6 apollo 11 dominant highland component 2 10 chondrite ree abundances with a positive 10 14 chondrite eu anomaly there are three newly recognized groups in the apollo 11 highland suite based on the ree patterns these are a ant with 5 chondrite la 22 chondrite abundances and a positive eu anomaly b 10 chondrite flat pattern with 14 chondrite positive eu anomaly and c 2 3 chondrite flat pattern with 10 chondrite positive eu anomaly the apollo 11 highland suite is very low in potassium non kreepy overall the apollo 11 highland suite is quite similar to the apollo 16 suite most previously recognized highland chemical groups are represented in the apollo 11 highland component

on the co seismic ionospheric disturbances cids in the rapid run ionosonde observations over allahabad following mw 7 8 nepal earthquake on april 25 2015

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

cobalt and nickel concentrations in the komatiite component of apollo 16 polymict samples reply to r l korotev abstract we agree with korotev that the abundances of siderophiles in apollo 16 breccias are anomalously large but do not accept his hypothesis that they were supplied by the impact of a hitherto unknown type of meteorite possessing a grossly non chondritic composition co ni and other siderophiles in apollo 16 breccias display systematic relationships which imply that a substantial proportion of these siderophiles were present as oxidised species in the lunar crust prior to the impacts of chondritic type meteorites with the lunar surface these pre existing indigenous lunar siderophiles were reduced to the metallic state to varying degrees during impact bombardment and became alloyed with chondritic meteoritic metal in this manner the metallic particles of complex and unusual compositions now present at the a16 site were formed the indigenous siderophiles present in the a16 crust prior to meteoritic bombardment may themselves have been derived from two distinct sources one endogenous and the other exogenous the first corresponds to basic and ultrabasic magmas formed by partial melting of the moon s deep interior which were erupted and then and intimately mixed into the lunar crust the latter represents basic and ultrabasic material ejected from the earth s primitive crust by early meteorite bombardment which then fell upon the lunar surface the siderophiles present in each of these components display terrestrial like abundance patterns because they are both ultimately of terrestrial origin the anomalously high abundances of siderophiles at the a16 site may reflect an important contribution from ejected terrestrial material overall the siderophile abundance patterns present in lunar highland breccias represent mixtures of terrestrial and chondritic components

a komatiite component in apollo 16 highland breccias implications for the nickel cobalt systematics and bulk composition of the moon the lunar crust at the apollo 16 landing site contains substantial amounts of a primitive component in which the ferromagnesian group of elements is concentrated the composition of this component can be retrieved via an analysis of mixing relationships displayed by lunar breccias it is found to be a komatiite which is compositionally similar to terrestrial komatiites both in major and minor elements the komatiite component of the lunar crust is believed to have formed by extensive degrees of melting of the lunar interior at depths greater than were involved in the formation of the lunar magma ocean which was parental to the crust after formation of the anorthositic crust it was invaded by extensive flows and intrusions of komatiite magma from these deeper source regions the komatiites became intimately mixed with the anorthosite by intensive meteoroid impacts about 4 5 b y ago thereby accounting for the observed mixing relationships displayed by the crust the compositional similarity between lunar and terrestrial komatiites strongly implies a corresponding similarity between the compositions of their source regions in the lunar interior and the earth s upper mantle the composition of the lunar interior can be modelled more specifically by combining the komatiite composition with its liquidus olivine composition as determined experimentally in proportions chosen so as to produce a cosmochemically acceptable range of mg si ratios for the bulk moon except for higher feo and lower na2o the range of compositions thereby obtained for the bulk moon is very similar to the composition of the earth s upper mantle the effects of meteoritic contamination on the abundances of cobalt and nickel in lunar highland breccias were subtracted on the assumption that the contaminating projectiles were chondritic the cobalt and nickel residuals thereby obtained were found to correlate strongly with the mg fe content of the breccias demonstrating that the co and ni are associated with the ferromagnesian component of the breccias and are genuinely indigenous to the moon the lunar highland co and ni residuals also display striking ni co versus ni correlations which follow a similar trend to those displayed by terrestrial basalts picrites and komatiites the lunar trends provide further decisive evidence of the indigenous nature of the co and ni residuals and suggest the operation of extensive fractionation controlled by olivine liquid equilibria in producing the primitive component of the lunar breccias indigenous nickel abundances at the apollo 14 15 and 17 sites are much lower than at the apollo 16 site although rocks from all sites follow the same ni co versus ni trends it is suggested that the primitive component at the apollo 14 15 and 17 sites was generally of basaltic composition in contrast to the komatiitic nature of the apollo 16 primitive component

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

geochemical studies of feldspathic fragmental breccias and the nature of north ray crater ejecta major and trace element analyses of clasts and matrix from feldspathic fragmental breccias show that these breccias are not all alike but vary widely in their bulk compositions and clast assemblages these studies together with analyses of other north ray crater large rocks reveal the major components to be anorthosites ferroan anorthositic norites magnesian granulites feldspathic melt rocks and vha impact melts the ferroan anorthositic norites are important lunar crustal rocks identified here for the first time the kreepy vha melts are found to be much more important in north ray crater ejecta than was previously assumed sodic ferrogabbro is an important minor component the compositional characteristics of bulk breccias and feldspathic melt rocks are modeled quantitatively as mixtures of two to five components anorthosites ferroan anorthositic norites magnesian granulites sodic ferrogabbros and vha melt rocks breccia 67455 is unique in consisting wholly of ferroan kreep free components

petrology and geochemistry of lunar dimict breccia 61015 dimict breccia 61015 consists dominantly of granulated anorthosite and impact melt rock of vha very high alumina basalt composition the anorthosite is a typical apollo 16 ferroan anorthosite except that it is slightly more augite rich than most other ferroan anorthosites the breccia probably formed in the floor of an expanding crater cavity by the following process impact melt generated within the cavity was injected into underlying anorthositic bedrock and solidified rapidly and the anorthosite and melt rock were then fragmented by deformation in the later stages of the impact all the apollo 16 dimict breccias except perhaps 61016 appear to have formed in the same impact the source crater cannot yet be identified but it may be an intermediate sized 50 150 km diameter crater within a few hundred kilometers of the apollo 16 site in the direction of the imbrium basin of either pre nectarian or post nectarian pre imbrian age the breccia has been affected by at least one and perhaps as many as four recent small impacts the most recent was probably the south ray impact the recent impact s produced internal shock induced melting of the breccia and injected and coated the rock with externally derived impact melts the coating glass is heterogeneous its average composition is similar to the compositions of other apollo 16 coating glasses except that its tio2 content is lower the vein glass is unusually mafic and has very low tio2 for its al2o3 content the tio2 contents of these glasses establish that they cannot be melted soils their compositions require a mafic low tio2 protolith component that has not yet been identified among the large rock samples perhaps a very mafic member of the ferroan anorthosite suite

cobalt and nickel concentrations in the komatiite component of apollo 16 polymict samples abstract some estimates for the concentrations of co and ni in the primitive mafic component of apollo 16 breccias and soils are large comparable to concentrations in terrestrial komatiites these estimates may be erroneously high because the contribution from meteoritic contamination of the samples has been underestimated however even if the correction for meteoritic co and ni is valid and the calculated residual ni is not of meteoritic origin the ni is presently carried by fe ni metal while the fe and mg are carried by mafic silicates if any large scale separation of metal and silicate phases has occurred the concentration ratio of ni to fe mg in the mafic silicates of the ancient crust in the vicinity of the apollo 16 site has not been preserved by apollo 16 rocks apollo 16 polymict breccias and soils contain much higher concentrations of siderophile elements and fe ni metal than other nonmare samples from the moon thus it is likely that the ni mg fe ratio estimated for the komatiite component from apollo 16 samples is high compared to the actual ratio for the lunar crust

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

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