the age of the solar system redefined by the oldest pb pb age of a meteoritic inclusion the age of the solar system is defined by the formation of the first solid grains in the solar nebula pb pb age dating of these solids which were later trapped in a meteorite indicates that the solar system is 0 34 1 91 million years older than previously thought the age of the solar system can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto sun this age is estimated by dating calcium aluminium rich inclusions in meteorites these inclusions are considered as the earliest formed solids in the solar nebula their formation marks the beginning for several long and short lived radiogenic clocks that are used to precisely define the timescales of solar system events such as the formation and evolution of planetary bodies1 2 3 here we present the 207pb 206pb isotope systematics in a calcium aluminium rich inclusion from the northwest africa 2364 cv3 group chondritic meteorite which indicate that the inclusion formed 4 568 2 million years ago this age is between 0 3 refs 4 5 and 1 9 refs 1 6 million years older than previous estimates and is the oldest age obtained for any solar system object so far we also determined the 26al 26mg model age of this inclusion and find that it is identical to its absolute pb pb age implying that the short lived radionuclide 26al was homogeneously distributed in the nebular disc surrounding the proto sun from the consistently old ages in the studied inclusion we conclude that the proto sun and the nebular disc formed earlier than previously thought

the longevity of lunar volcanism implications of thermal evolution calculations with 2d and 3d mantle convection models abstract lunar volcanism as is generally accepted started well before the emplacement of the mare fill at 3 1 3 9 ga b p and likely extended well into the eratosthenian 3 1 1 1 ga b p the early volcanism is relatively easily understood the extension of the activity to possibly 1 5 ga b p albeit at a decreasing rate poses a problem since a relatively small body such as the moon may be expected to cool rapidly and freeze a partially molten mantle layer quickly we present thermal history models in which a region of partial melt forms in the mantle underneath the lithosphere almost immediately after the start of the model at depths between 300 and 700 km depending on the chosen initial depth of the magma ocean to calculate the thermal histories we use axisymmetric 2d and fully 3d spherical shell convection codes with viscosity depending on the azimuthally averaged temperature the thermal evolution of the moon is found to be characterized by the growth of a massive 700 to 800 km thick lithosphere while the lower mantle and core cool only by 100 200 k the partial melt zone decreases in thickness with time from top to bottom until it vanishes at times between 3 4 and 2 2 ga b p the maximum degree of partial melting is between roughly 10 and 20 the partial melt layer is initially global and is disrupted by cold downwellings as cooling proceeds the melt zone freezes from above due to the thickening of the lithosphere which implies that the source region of volcanic rock proceeds to increasing depth with time this corresponds nicely with the variation of titanium with age of lunar mare basalts mixing of the melt zone with the underlying mantle chemically rejuvenates the melt zone to some degree and thus the source region of the mare basalts

sm nd age of lherzolite 67667 implications for the processes involved in lunar crustal formation abstract pristine samples from the lunar highlands potentially offer important information bearing on the nature of early crustal development on all the terrestrial planets one apparently unique sample of this group of lunar crustal rocks the feldspathic lherzolite 67667 was studied utilizing the sm nd radiometric system in an attempt to define its age and the implications of that age for the evolution of the lunar highlands data for 67667 precisely define an isochron corresponding to an age of 4 18 0 07 ae the observed lack of disturbance of the sm nd system of this sample may suggest that this time marks its crystallization at shallow depth in the lunar crust however the possibility that this age as well as those of other highland rocks indicate the time of their impact induced excavation from regions deep enough in the lunar crust to allow subsolidus isotopic equilibrium to be produced or maintained between their constituent minerals is also considered taken together bulk rock sm nd data for four high mg rocks including 67667 indicate that the chemical characteristics of all their source materials were established 4 33 0 08 ae ago and were intimately associated with the parent materials of kreep this finding provides more support for the concept of a large scale differentiation episode early in lunar history the possible roles of the crystallization of a global magma ocean endogenous igneous activity and of planetesimal impact in producing the observed geochemical and chronological aspects of lunar highland rocks are discussed

39ar 40ar evidence for early impact events on the ll parent body abstract we determined 39ar 40ar ages of eight ll chondrites and one igneous inclusion from an ll chondrite with the object of understanding the thermal history of the ll chondrite parent body the meteorites in this study have a range of petrographic types from ll3 3 to ll6 and shock stages from s1 to s4 these meteorites reveal a range of k ar ages from 3 66 to 4 50 ga and peak ages from 3 74 to 4 55 ga significantly three of the eight chondrites ll4 5 6 have k ar ages of 4 27 ga one of these mil99301 preserves an 39ar 40ar age of 4 23 0 03 ga from low temperature extractions and an older age of 4 52 0 08 ga from the highest temperature extractions in addition an igneous textured impact melt dom85505 22 has a peak 39ar 40ar age of 4 27 ga we interpret these results as evidence for impact events that occurred at about 4 27 ga on the ll parent body that produced local impact melts reset the 39ar 40ar ages of some meteorites and exhumed or interred others resulting in a range of cooling ages the somewhat younger peak age of 3 74 ga from gro95658 ll3 3 suggests an additional impact event close to timing of impact reset ages of some other ordinary chondrites between 3 6 3 8 ga the results from mil99301 suggest that some apparently unshocked s1 chondrites may have substantially reset 39ar 40ar ages a previous petrographic investigation of mil99301 suggested that reheating to temperatures less than or equal to type 4 petrographic conditions 600 c caused fractures in olivine to anneal resulting in a low apparent shock stage of s1 unshocked the 39ar 40ar age spectrum of mil99301 is consistent with this interpretation older ages from high t extractions may date an earlier impact event at 4 52 0 08 ga whereas younger ages from lower t extractions date a later impact event at 4 23 0 03 ga that may have caused annealing of feldspar and olivine

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

time and duration of lunar highlands crust formation abstract all radiometric systems indicate that crust mantle differentiation on the moon is dominated by events which occurred very early in lunar history however due to remaining uncertainties in model parameters and assumptions in the calculation of model ages it is not yet possible to resolve the precise times of occurrence of these events nor the duration of the formation of the highlands crust the strongest time constraints are offered by direct radiometric ages of samples formed during this earliest period two possible candidates for this material norites 78236 and 73255 27 45 were examined utilizing the sm nd radiometric system sm nd systematics of 78236 show post crystallization disturbance but indicate that this norite crystallized in the lunar crust about 4 34 ae ago data for 73255 27 45 define an isochron and yield a crystallization age of 4 23 0 05 ae the initial nd isotopic composition of both norites is within uncertainty of a chondritic reference reservoir at the time of their respective crystallizations the implications for lunar crustal formation persisting over a time span of close to 350 m y are discussed

volcanism in mare fecunditatis and mare crisium ar ar age studies the laser 40ar 39ar dating technique has been applied to five luna 16 basalt fragments and one impact glass and nine luna 24 basalt fragments and one breccia the textures of these basalts are fine grained ophitic and coarse grained basalts the samples contain high levels of solar and lunar atmospheric argon acquired during their residence on the lunar surface these trapped argon components are predominantly released at low temperature steps and can be distinguished from radiogenic and cosmogenic released at intermediate and high temperature steps the apparent ages obtained for luna 16 samples span a narrow range of 3 29 to 3 38 ga a young age of 0 988 ga was obtained for a basaltic impact glass indicating the age of an impact event in the vicinity of luna 16 landing site the ages obtained for luna 24 samples suggest the existence of at least three periods of volcanism occurring over a protracted interval of between 3 45 and 2 52 ga the long period of volcanism suggested for the mare crisium was likely due to a combination of geophysical and geochemical features in the surrounding and underlying areas of the crisium basin attempts at dating three luna 20 samples were inconclusive due to their high trapped argon contents

sims u pb study of zircon from apollo 14 and 17 breccias implications for the evolution of lunar kreep abstract we report the results of a sims u pb study of 112 zircons from breccia samples from the apollo 14 and 17 landing sites zircon occurs in the breccia matrices as rounded irregular shaped broken and rarely euhedral grains and as constituent minerals in a variety of lithic clasts ranging in composition from ultra mafic and mafic rocks to highly evolved granophyres crystallisation of zircon in magmatic rocks is governed by the zirconium saturation in the melt as a consequence the presence of zircon in mafic rocks on the moon implies enrichment of their parent melts in the kreep component our sims results show that the ages of zircons from mafic to ultramafic clasts range from ca 4 35 ga to ca 4 00 ga demonstrating multiple generations of kreepy mafic and ultramafic magmas over this time period individual zircon clasts in breccia matrices have a similar age range to zircons in igneous clasts and all represent zircons that have been incorporated into the breccia from older parents the age distributions of zircons from breccias from both the apollo 14 and apollo 17 landing sites are essentially identical in the range 4 35 4 20 ga however whereas apollo 14 zircons additionally show ages from 4 20 to 3 90 ga no zircons from apollo 17 samples have primary ages less than ca 4 20 ga also in contrast to previous suggestions that the magmatism in the lunar crust is continuous our results show that the zircon age distribution is uneven with distinct peaks of magmatic activity at ca 4 35 ga ca 4 20 ga in apollo 14 and 17 and a possible third peak in zircons from apollo 14 at ca 4 00 ga to explain the differences in the zircon age distributions between the apollo 14 and 17 landing sites we propose that episodes of kreep magmatism were generated from a primary reservoir and that this reservoir contracted over time towards the centre of procellarum kreep terrane we attribute the peaks in kreep magmatism to impact induced emplacement of kreep magma from a primary mantle source or to a progressive thermal build up in the mantle source until the temperature exceeds the threshold for generation of kreep magma which is transported into the crust by an unspecified possibly plume like process

a 4 gyr shock age for a martian meteorite and implications for the cratering history of mars analyses of meteorites that originated on mars provide important insights into the geological and atmospheric evolution of the planet such analyses have hitherto been restricted to relatively young martian rocks1 the oldest martian meteorites have an age of approximately 1 3 billion years but the recently recognized2 martian meteorite allan hills 84001 which is distinct from the other martian meteorites2 4 shows evidence for a much older age5 6 here we report an analysis of the shock alteration history of this meteorite based on argon isotope dating from which we derive a shock age of 4 0 0 1 billion years the age and geological history of this meteorite suggest that it came from the heavily cratered noachian age terrains of mars s southern hemisphere and it may thus provide an absolute chronology for this region of the planet independent of that inferred from the cratering record the shock age of the meteorite also coincides with that of the so called lunar cataclysm a relatively short period during which many of the craters on the moon are believed to have formed supporting the idea7 that intense bombardment was widespread throughout the inner solar system between 3 9 and 4 1 billion years ago

petrogenesis and chronology of lunar meteorite northwest africa 4472 a kreepy regolith breccia from the moon northwest africa nwa 4472 is a polymict lunar regolith meteorite the sample is kreep rich high concentrations of potassium rare earth elements and phosphorus and comprises a heterogeneous array of lithic and mineral fragments these clasts and mineral fragments were sourced from a range of lunar rock types including the lunar high magnesian suite the high alkali suite kreep basalts mare basalts and a variety of impact crater environments the kreep rich nature of nwa 4472 indicates that the sample was ejected from regolith on the nearside of the moon in the procellarum kreep terrane and we have used lunar prospector gamma ray remote sensing data to show that the meteorite is most similar to and most likely sourced from regoliths adjacent to the imbrium impact basin u pb and pb pb age dates of nwa 4472 phosphate phases reveal that the breccia has sampled pre nectarian 4 35 ga rocks related to early episodes of kreep driven magmatism some younger phosphate u pb and pb pb age dates are likely indicative of impact resetting events at 3 9 4 ga consistent with the suggested timing of basin formation on the moon our study also shows that nwa 4472 has sampled impact melts and glass with an alkali depleted incompatible trace element rich high sc low rb th ratios low k compositional signature not related to typical apollo high k kreep or that sampled by kreepy lunar meteorite sayh al uhaymir sau 169 this provides evidence that there are numerous sources of kreep rich protoliths on the moon

ar ar ages and thermal histories of enstatite meteorites abstract compared with ordinary chondrites there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies in this study we report 39ar 40ar dating results for five el chondrites khairpur pillistfer hvittis blithfield and forrest five eh chondrites parsa saint marks indarch bethune and reckling peak 80259 three igneous textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies zaklodzie queen alexandra range 97348 and queen alexandra range 97289 and three aubrites norton county bishopville and cumberland falls several ar ar age spectra show unusual 39ar recoil effects possibly the result of some of the k residing in unusual sulfide minerals such as djerfisherite and rodderite and other age spectra show 40ar diffusion loss few additional ar ar ages for enstatite meteorites are available in the literature when all available ar ar data on enstatite meteorites are considered preferred ages of nine chondrites and one aubrite show a range of 4 50 4 54 ga whereas five other meteorites show only lower age limits over 4 35 4 46 ga ar ar ages of several enstatite chondrites are as old or older as the oldest ar ar ages of ordinary chondrites which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies or were metamorphosed to lower temperatures compared to other chondrite types many enstatite meteorites are brecciated and or shocked and some of the younger ar ar ages may record these impact events although impact heating of ordinary chondrites within the last 1 ga is relatively common for ordinary chondrites only bethune gives any significant evidence for such a young event

lunar surface geochemistry global concentrations of th k and feo as derived from lunar prospector and clementine data abstract accurate estimates of global concentrations of th k and feo have an important bearing on understanding the bulk chemistry and geologic evolution of the moon we present empirical ground truth calibrations transformations for lunar prospector gamma ray spectrometer data k and th and a modified algorithm for deriving feo concentrations from clementine spectral reflectance data that incorporates an adjustment for tio 2 content the average composition of soil samples for individual landing sites is used as ground truth for remotely sensed data among the apollo and luna sites apollo 12 and 14 provide controls for the incompatible element rich compositions apollo 16 and luna 20 provide controls for the feldspathic and incompatible element poor compositions and apollo 11 15 and 17 and luna 16 and 24 provide controls for fe rich compositions in addition to these apollo and luna sample data we include the composition of the feldspathic lunar meteorites as a proxy for the northern farside highlands to extend the range of the calibration points thus providing an additional anorthositic end member composition these empirical ground truth calibrations for lunar prospector th and k provide self consistency between the existing derived data and lunar sample data clementine spectral reflectance data are used to construct a tio 2 sensitive feo calibration that yields higher feo concentrations in areas of high fe low ti mare basalt rich surfaces than previous feo algorithms the data set so derived is consistent with known sample compositions and regolith mixing relationships

the sculptured hills of the taurus highlands implications for the relative age of serenitatis basin chronologies and the cratering history of the moon 1 new images from the lunar reconnaissance orbiter camera show the distribution and geological relations of the sculptured hills a geological unit widespread in the highlands between the serenitatis and crisium basins the sculptured hills shows knobby undulating radially textured and plains like morphologies and in many places is indistinguishable from the similarly knobby alpes formation a facies of ejecta from the imbrium basin the new lroc image data show that the sculptured hills in the taurus highlands is imbrium ejecta and not directly related to the formation of the serenitatis basin this occurrence and the geological relations of this unit suggests that the apollo 17 impact melts may not be not samples of the serenitatis basin forming impact leaving their provenance undetermined and origin unexplained if the apollo 17 melt rocks are serenitatis impact melt up to half of the basin and large crater population of the moon was created within a 30 ma interval around 3 8 ga in a global impact cataclysm either interpretation significantly changes our view of the impact process and history of the earth moon system

some things we can infer about the moon from the composition of the apollo 16 regolith randy l korotev department of earth and planetary sciences and mcdonnell center for the space sciences washington university st louis missouri 63130 usa author s e mail address rlk levee wustl edu received 1996 september 16 accepted in revised form 1997 may 12 abstract characteristics of the regolith of cayley plains as sampled at the apollo 16 lunar landing site arereviewed and new compositional data are presented for samples of 1 mm fines soils and 1 2 mm rego lith particles as a means of determining which of the many primary igneous and secondary crystalline brec cias lithoiogic components that have been identified in the soil are volumetrically important and providing anestimate of their relative abundances more than 3 x 106 combinations of components representing nearly everylithology that has been observed in the apollo 16 regolith were systematically tested to determine which com binations best account for the composition of the soils conclusions drawn from the modeling include the fol lowing at the site mature soil from the cayley plains consists of 64 5 2 7 components representing prebasin materials anorthosites feldspathic breccias and a small amount 2 6 1 5 of total soil ofnonmare mafic plutonic rocks mostly gabbronorites on average these components are highly feldspathic with average concentrations of 31 32 a1203 and 2 3 feo and a molar mg mg fe ratio of 0 68 theremaining 36 of the regolith is syn and postbasin material 28 8 2 4 mafic impact melt breccias mimbs i e lkfm and vha basalts created at the time of basin formation 6 0 1 4 mare derived material impact and volcanic glass crystalline basalt with an average tio2 concentration of 2 4 and 1 postbasin meteoritic material the mimbs are the principal 80 90 carrier of incompatible traceelements rare earths th etc and the carrier of about one half of the siderophile elements and elementsassociated with mafic mineral phases fe mg mn cr sc most 71 of the fe in the present regolithderives from syn and postbasin sources mimbs mare derived material and meteorites thus althoughthe bulk composition of the apollo 16 regolith is nominally that of noritic anorthosite the noritic part themimbs and the anorthositic part the prebasin components are largely unrelated there is compositionalevidence that 3 4 of the soil is th rich material such as that occurring at the apollo 14 site and one frag ment of this type was found among the small regolith particles studied here if regolith such as that repre sented by the apollo 16 ancient regolith breccias was a protolith of the present regolith such regolith cannotexceed 71 of the present regolith the rest must be material added or redistributed since closure of theancient regolith breccias the postclosure material includes the mare derived material and the apollo 14 like component compositions of all mature surface soils from apollo 16 even those collected 4 km apart on the cayleyplains are very similar which is in stark contrast to the wide compositional range of the lithologies of whichthe soil is composed this uniformity indicates that the ratio of mimbs to feldspathic prebasin componentsis not highly variable in the megaregolith over distances of a few kilometers that there are no large sub surface concentrations of pure mafic impact melt breccia and that the intimate mixing is inherent to thecayley plains at a gross scale thus the mixing ofmafic impact melt breccias and feldspathic prebasin com ponents must have occurred during formation and deposition of the cayley plains such uniformity could nothave been achieved by small postdeposition impacts into a stratified megaregolith using this conclusion asone constraint and the known distribution of th on the lunar surface as another and the assumption that the im brium impact is primarily responsible for formation of the cayley plains arguments are presented that theapollo 16 mimbs derive from the imbrium region and consequently that one fourth of the apollo 16 regolithis primary imbrium ejecta in the form of mafic impact melt breccias

chronological evidence that the moon is either young or did not have a global magma ocean a new age determination for a moon rock collected close to the site of the apollo 16 lander has yielded a surprisingly young age of around 4 360 million years this age would require either that the moon solidified some 200 million years after the formation of the solar system significantly later than is assumed by most lunar formation models or that the long held theory that the moon once had a primordial global magma ocean is flawed in the latter case much of the lunar crust could have been produced by non magma ocean processes such as serial magmatism chemical evolution of planetary bodies ranging from asteroids to the large rocky planets is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth1 2 3 the earth s moon is the archetypical example of this type of differentiation evidence for a lunar magma ocean is derived largely from the widespread distribution compositional and mineralogical characteristics and ancient ages inferred for the ferroan anorthosite fan suite of lunar crustal rocks the fans are considered to be primary lunar flotation cumulate crust that crystallized in the latter stages of magma ocean solidification according to this theory fans represent the oldest lunar crustal rock type2 3 4 attempts to date this rock suite have yielded ambiguous results however because individual isochron measurements are typically incompatible with the geochemical make up of the samples and have not been confirmed by additional isotopic systems5 6 7 8 9 by making improvements to the standard isotopic techniques we report here the age of crystallization of fan 60025 using the 207pb 206pb 147sm 143nd and 146sm 142nd isotopic systems to be 4 360 3 million years this extraordinarily young age requires that either the moon solidified significantly later than most previous estimates or the long held assumption that fans are flotation cumulates of a primordial magma ocean is incorrect if the latter is correct then much of the lunar crust may have been produced by non magma ocean processes such as serial magmatism10

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

argon 39 argon 40 ages and trapped argon in martian shergottites chassigny and allan hills 84001 abstract argon isotopic abundances were measured in neutron irradiated samples of martian meteorites chassigny allan hills alh 84001 alh 77005 elephant moraine eet 79001 yamato y 793605 shergotty zagami and queen alexandra range que 94201 and in unirradiated samples of alh 77005 chassigny gives a 39ar 40ar age of 1 32 0 07 ga which is similar to radiometric ages of the nakhlites argon 39 argon 40 data for alh 84001 indicate ages between 3 9 and 4 3 ga a more precise definition of this age requires detailed characterization of the multiple trapped ar components in alh 84001 and of 39ar recoil distribution all six shergottite samples show apparent 39ar 40ar ages substantially older than the 165 200 ma range in ages given by other isotope dating techniques shergottites appear to contain ubiquitous ar components acquired from the martian atmosphere the martian mantle and commonly terrestrial atmospheric contamination zagami feldspar also suggests inherited radiogenic 40ar these data analyses indicate that the recent martian atmospheric component trapped in shergottites has a 40ar 36ar ratio possibly as low as 1750 and no greater than 1900 these ratios are less than the value of 3000 500 reported by viking the 40ar 36ar ratio for the martian mantle component is probably 500 but is poorly constrained the correlation between trapped 40ar 36ar and 129xe 132xe ratios in shergottite impact glasses and unirradiated samples of alh 77005 shows considerable scatter and suggests that the 36ar 132xe ratio in the martian components may vary resolution of martian atmospheric 40ar 36ar ratio at different time periods i e at 4 0 and 0 2 ga is also difficult without an understanding of the composition of various trapped components

static and dynamic properties of a lightly cemented silicate grouted sand this paper describes the results of an experimental investigation on the behaviour of a lightly grouted loose medium sand under both monotonic and cyclic loading the experimental programme in the present study was carried out on both untreated and treated sand specimens stabilized with a silica based grout and comprises isotropic triaxial compression tests equipped with bender elements drained triaxial monotonic shearing tests and undrained cyclic tests in a simple shear ss apparatus the results highlight that the weak cementation level induced by chemical treatment was sufficient to moderately increase small strain stiffness and stress dilatancy of the grouted sand during drained monotonic shear a small cohesion intercept was observed in the analyzed failure envelope while cyclic liquefaction resistance exhibited a much more significant increase with a different pattern of behaviour between low and high stress levels finally undrained cyclic ss test results evidenced that silicate grout emplo

coupled thermal orbital evolution of the early moon abstract coupled thermal orbital histories of early lunar evolution are considered in a simple model we consider a plagioclase lid overlying a magma ocean overlying a solid mantle tidal dissipation occurs in the plagioclase lid and heat transport is by conduction and melt migration we find that large orbital eccentricities can be obtained in this model we discuss possible consequences of this phase of large eccentricities for the shape of the moon and geochronology of lunar samples we find that the orbit can pass through the shape solution of garrick bethell et al garrick bethell i wisdom j zuber m 2006 science 313 652 but we argue that the shape cannot be maintained against elastic deformation as the orbit continues to evolve

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

global elemental maps of the moon the lunar prospector gamma ray spectrometer lunar prospector gamma ray spectrometer spectra along with counting rate maps of thorium potassium and iron delineate large compositional variations over the lunar surface thorium and potassium are highly concentrated in and around the nearside western maria and less so in the south pole aitken basin counting rate maps of iron gamma rays show a surface iron distribution that is in general agreement with other measurements from clementine and the lunar prospector neutron detectors

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

laser argon 40 argon 39 age determinations of luna 24 mare basalts the ages of seven rock fragments from the soil fraction of the luna 24 core have been determined using a laser 40ar 39ar stepped heating technique the investigated lithologies include fragments of fine grained ophitic basalt coarse grained basalt metabasalts and a regolith breccia most of the samples contain nonradiogenic ar components of variable 36ar 40ar composition these surface correlated trapped components are predominantly released at low temperature and can be distinguished from volume correlated radiogenic and cosmogenic components released at higher temperature during stepped heating binary mixtures of radiogenic and cosmogenic ar components give linear correlations on 36art 40ar 39ar 40ar diagrams from which the age and 36ar 40ar value of trapped ar can be determined the ages obtained span a narrow range between 3 18 3 28 ga with an average of 3 22 0 04 ga this is interpreted as being the age of the basalts at the luna 24 sampling site systematic age differences between lithologies were not detected however a single age of 2 93 ga obtained from a coarse grained basalt hints at the possibility of younger volcanism the results of this work effectively triple the chronological information available for mare crisium and are within the range of radiometric age measurements of luna 24 mare basalts obtained previously

geochemistry petrology and ages of the lunar meteorites kalahari 008 and 009 new constraints on early lunar evolution abstract kalahari 008 and 009 are two lunar meteorites that were found close to each other in botswana kalahari 008 is a typical lunar anorthositic breccia kalahari 009 a monomict breccia with basaltic composition and mineralogy based on minor and trace elements kalahari 009 is classified as vlt very low ti mare basalt with extremely low contents of incompatible elements including the ree the lu hf data define an age of 4286 95 ma indicating that kalahari 009 is one of the oldest known basalt samples from the moon it provides evidence for lunar basalt volcanism prior to 4 1 ga pre nectarian and may represent the first sample from a cryptomare the very radiogenic initial 176hf 177hf ehf 12 9 4 6 the low ree th and ti concentrations indicate that kalahari 009 formed from re melting of mantle material that had undergone strong incompatible trace element depletion early in lunar history this unusually depleted composition points toward a hitherto unsampled basalt source region for the lunar interior that may represent a new depleted endmember source for low ti mare basalt volcanism apparently the moon became chemically very heterogeneous at an early stage in its history and different cumulate sources are responsible for the diverse mare basalt types evidence that kalahari 008 and 009 may be paired includes the similar fayalite content of their olivine the identical initial hf isotope composition the exceptionally low exposure ages of both rocks and the fact that they were found close to each other since cryptomaria are covered by highland ejecta it is possible that these rocks are from the boundary area where basalt deposits are covered by highland ejecta the concentrations of cosmogenic radionuclides and trapped noble gases are unusually low in both rocks although kalahari 008 contains slightly higher concentrations a likely reason for this difference is that kalahari 008 is a polymict breccia containing a briefly exposed regolith while kalahari 009 is a monomict brecciated rock that may never have been at the surface of the moon altogether the compositions of kalahari 008 and 009 permit new insight into early lunar evolution as both meteorites sample lunar reservoirs hitherto unsampled by spacecraft missions the very low th and ree content of kalahari 009 as well as the depletion in sm and the lack of a kreep like signature in kalahari 008 point to a possible source far from the influence of the procellarum kreep terrane possibly the lunar farside

intercalibration of 40ar 39ar age standards nl 25 hb3gr hornblende ga1550 sb 3 hd b1 biotite and bmus 2 muscovite abstract 40 ar 39 ar dating requires precise and accurate age data of standard fluence monitors besides primary calibration of k and 40 ar concentrations i e the ages are calculated with precise k ar dating intercalibration of the different monitors in use provides important cross checks of the validity of standard ages and implicitly reliability and uncertainty of chronological studies that rely on these standards here we report intercalibration of 5 primary international 40 ar 39 ar standards and one internal laboratory monitor a major goal of this study was to check the ages of the nl 25 hornblende standard and of the muscovite laboratory standard bmus 2 against the international standards hd b1 ga1550 sb 3 and hb3gr in replicate measurements of three irradiation series 40 ar 39 ar ages for the fluence monitors were found which agree within 1 error limits with the reported age values of 2660 9 ma for nl 25 hb3gr 1073 6 4 6 ma bmus 2 328 5 1 1 ma sb 3 162 9 0 8 ga1550 98 79 0 54 and 24 21 0 32 for hd b1 for the hornblende nl 25 fluence monitor a new slightly different 40 ar 39 ar age of 2657 4 ma is suggested for the age of the muscovite bmus 2 laboratory standard we recommend 328 5 1 1 ma and for biotite hd b1 a more precise value of 24 18 0 09 ma

40ar 39ar age determinations of lunar basalt meteorites asuka 881757 yamato 793169 miller range 05035 la paz icefield 02205 northwest africa 479 and basaltic breccia elephant moraine 96008 40ar 39ar data are presented for the unbrecciated lunar basaltic meteorites asuka a 881757 yamato y 793169 miller range mil 05035 lapaz icefield lap 02205 northwest africa nwa 479 paired with nwa 032 and basaltic fragmental breccia elephant moraine eet 96008 stepped heating 40ar 39ar analyses of several bulk fragments of related meteorites a 881757 y 793169 and mil 05035 give crystallization ages of 3 763 0 046 ga 3 811 0 098 ga and 3 845 0 014 ga which are comparable with previous age determinations by sm nd u pb th pb pb pb and rb sr methods these three meteorites differ in the degree of secondary 40ar loss with y 793169 showing relatively high ar loss probably during an impact event 200 ma ago lower ar loss in mil 05035 and no loss in a 881757 bulk and impact melt glass bearing samples of lap 02205 gave similar ages 2 985 0 016 ga and 2 874 0 056 ga and are consistent with ages previously determined using other isotope pairs the basaltic portion of eet 96008 gives an age of 2 650 0 086 ga which is considered to be the crystallization age of the basalt in this meteorite the ar release for fragmental basaltic breccia eet 96008 shows evidence of an impact event at 631 20 ma the crystallization age of 2 721 0 040 ga determined for nwa 479 is indistinguishable from the weighted mean age obtained from three samples of nwa 032 supporting the proposal that these meteorites are paired the similarity of 40ar 39ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the k ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime

60025 relict of primitive lunar crust abstract new isotopic analyses are presented for 3 plagioclase rich fractions and one mafic fraction from ferroan anorthosite 60025 the observed206pb 204pb ratios vary between 52 5 and 60 5 all much higher than the ratio for terrestrial contamination in a207pb 206pb 204pb 206pb correlation diagram the plagioclase data define a model pb pb age of 4 520 0 007 ae using meteoritic primordial lead for the non radiogenic component in the concordia diagram the plagioclase data yield intersections at 4 503 0 007 and 0 28 ae the meaning of the lower intercept is obscure the earlier 60025 analysis of tera and wasserburg 1 with an observed206pb 204pb of 23 0 agrees closely with the new plagioclase data in the isotope correlation and concordia diagrams since the apparent age does not correlate with the206pb 204pb ratios and u contents of the samples it does not appear to be controlled by terrestrial lead contamination the time averaged values for the plagioclase leads are exceptionally low 16 55 and agree within factors of 2 with the observed values in the samples these are much lower than the values observed for mafic rocks or their sources showing that the anorthosite lead has never been associated for a substantial length of time with any high source in this way the 60025 data differ substantially from u pb data for two other lunar anorthosites 15415 and 60015 the results suggest that the averaged model pb ages of 4 51 0 01 ae closely approximate the crystallization age for the plagioclase fraction of the anorthosite and that it dates back to an early phase in lunar history one sample from the mafic fraction of 60025 yields a younger model pb age of 4 42 ae the age may have been lowered by post crystallization disturbances or perhaps this fraction is not coeval with the plagioclase fraction

40ar 39ar chronology of lunar meteorites northwest africa 032 and 773 the 40ar 39ar dating technique has been applied to the lunar meteorites northwest africa 032 nwa 032 an unbrecciated mare basalt and northwest africa 773 nwa 773 composed of cumulate and breccia lithologies to determine the crystallization age and timing of shock events these meteorites may have experienced stepped heating analyses of several different samples of nwa 032 gave complex age spectra but indistinguishable total ages with a mean of 2 779 0 014 gyr possible causes of the complex age spectra obtained from nwa 032 include recoil of 39ar or the presence of pre shock 40ar incorporated into shock melt veins the effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins the laser ages show a narrow age distribution between 2 61 2 86 gyr and a mean of 2 73 0 03 gyr identical to the total age of 2 80 gyr obtained for the bulk sample diffusion calculations based on the stepped heating data indicate that ar release can be reconciled by release from feldspar and possibly shock veins at low temperatures followed by pyroxene at higher temperatures the exposure age of nwa 032 is 212 11 myr and it contains low trapped solar ar stepped heating of cumulate and breccia portions of nwa 773 also give a relatively young age of 2 91 gyr the presence of trapped ar in the breccia makes the age determination of this component less precise but release of ar appears to be from the same mineral phase assumed to be plagioclase in both lithologies a marked difference in exposure age between the 2 lithologies also exists with the breccia having spent 81 myr longer at the lunar surface this finding is consistent with the higher trapped ar content of this lithology assuming that 2 80 gyr and 2 91 gyr are the crystallization ages of nwa 032 and nwa 773 respectively these two meteorites are the youngest lunar mare basalts available for study

lunar rare earth element distribution and ramifications for feo and tio2 lunar prospector neutron spectrometer observations lunar prospector neutron spectrometer data have been used to map the global surface distribution of the incompatible rare earth elements gadolinium and samarium from the low altitude 30 15 km mapping orbit these results afford improved surface resolution and detailed views of the potassium rare earth elements and phosphorus kreep distribution within and around mare imbrium and elsewhere the gd and sm results serve as a complementary and independent check of the distribution of kreep on the moon in contrast with the lunar prospector gamma ray spectrometer results for thorium the neutron spectrometer observations reflect the presence of fe and ti as well as gd and sm the contributions of fe and ti are removed using high spatial resolution clementine spectral reflectance determinations of feo and tio2 abundances overall the resulting gd and sm abundance map agrees with the th abundance map determined using the lunar prospector gamma ray spectrometer in general the detailed features of the procellarum imbrium kreep terrane are found in both for example distinct highs in gd sm and th abundances are resolved over the craters mairan aristarchus kepler reinhold lalande and aristillus over the apennine bench and fra mauro regions and over the montes jura and montes carpatus indicating an enhanced abundance of kreep in these locations the neutron observations also provide constraints on feo and tio2 abundances for some high ti locales there is a significant disagreement with tio2 abundances inferred from clementine spectral reflectance

lunar multiring basins and the cratering process abstract numerous studies of the lunar gravity field have concluded that the lunar moho is substantially uplifted beneath the young multiring basins this uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor using a new dual layered crustal thickness model of the moon the excavation cavities of some nearside multiring basins grimaldi and larger and younger than tranquillitatis were reconstructed by restoring the uplifted moho to its preimpact location the farside south pole aitken spa basin was also considered due to its importance in deciphering lunar evolution restoring the moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined using these reconstructed excavation cavities the basin forming process was investigated excavation cavity diameters were generally found to be on the small side of most previous estimates for orientale the modeled excavation cavity lies within the inner rook ring additionally with the exception of the three largest basins serenitatis imbrium and south pole aitken the depth diameter ratios of the excavation cavities were found to be 0 115 0 005 a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size the three largest basins however appear to have significantly shallower depths of excavation compared to this trend it is possible that this may reflect a different physical process of crater formation e g nonproportional scaling special impact conditions or postimpact modification processes the crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust we interpret this thickened crust as representing thick basin ejecta deposits and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small scale experimental studies if multiring basins ever possessed a terraced main crater rim this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim we also show that the interiors of many basins were superisostatic before mare volcanism commenced those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat producing elements

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

laser argon 40 argon 39 age studies of dar al gani 262 lunar meteorite abstract the laser 40ar 39ar dating technique has been applied to the dar al gani dag 262 lunar meteorite a polymict highland regolith breccia to determine the crystallisation age and timing of shock events experienced by this meteorite laser stepped heating analyses of three dominantly feldspathic fragments dag 1 dag 2 and dag 3 revealed the presence of trapped ar mostly released at intermediate and high temperatures with an 40ar 36ar value of 2 8 trapped ar is most likely released from melt glass present as small veins within the fragments the 40ar 39ar ages determined for the three fragments are 3 0 ga for dag 1 and dag 2 and 2 0 ga for dag 3 and probably relate to major impact events laser spot analyses were performed on a feldspathic clast an impact crystalline melt basalt icmb and the matrix in a polished section of dag 262 the feldspathic and icmb clasts have low contents of trapped ar compared with that in the matrix the feldspathic clast shows a wide range of ages from 3 0 to 1 7 ga similar to those obtained by stepped heating the younger age is interpreted as a minimum age for the last major event that assembled this meteorite the icmb shows two age clusters at 3 37 and 3 07 ga where the older age may be that of the impact event that formed the impact melt several cosmic ray exposure cre ages were obtained as expected for a polymict regolith breccia the cre ages are 106 and 141 ma for the feldspathic clast and the icmb respectively one of the feldspathic fragments dag 2 shows a range between 200 400 ma these cre ages which are similar to those determined for returned samples of the lunar regolith indicate that the different components of dag 262 experienced preexposure prior to assemblage of the meteorite

40ar 39ar dating of apollo 12 impact spherules 1 we have used the 40ar 39ar isochron technique to determine ages of 81 lunar spherules from apollo 12 soil sample 12023 most spherules are created in meteoroid impacts and their ages correspond to the timing of the impacts that formed them of the 81 impacts we have dated most occurred in the last 500 million years the abundance of spherules from the most recent 10 of the history of the moon is consistent with an increase in the meteoroid bombardment of the inner solar system but does not require this explanation the soil sample from which we extracted our spherules was from the ejecta of a recent impact our spherule age measurements support models of lunar soil mechanics and impact cratering in which ejecta are stratified and inverted relative to the stratigraphy that was present before the impact

40ar 39ar ages of h chondrite impact melt breccias 40ar 39ar analyses of a total of 26 samples from eight shock darkened impact melt breccias of h chondrite affinity gao guenie lap 02240 lap 03922 lap 031125 lap 031173 lap 031308 nwa 2058 and ourique are reported these appear to record impacts ranging in time from 303 56 ma gao guenie to 4360 120 ma ourique ago three record impacts 300 400 ma ago while two others record impacts 3900 4000 ma ago combining these with other impact ages from h chondrites in the literature it appears that h chondrites record impacts in the first 100 ma of solar system history during the era of the lunar cataclysm and shortly thereafter 3500 4000 ma ago one or more impacts 300 ma ago and perhaps an impact 500 ma ago near the time of the l chondrite parent body disruption records of impacts on the h chondrite parent body are rare or absent between the era of planetary accretion and the lunar cataclysm 4400 4050 ma during the long stretch between heavy bombardment and recent breakup events 3500 1000 ma or at the time of final breakup into meteorite sized bodies 50 ma

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

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

antipodal effects of lunar basin forming impacts initial 3d simulations and comparisons with observations abstract 3d simulations of basin scale lunar impacts are carried out to investigate a the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins and b the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the south pole aitken spa basin the simulations demonstrate that a basin forming impact produces a massive hot partially ionized cloud of vapor and melt that expands thermally around the moon converging near the basin antipode approximately 1 h after the impact for typical impact parameters in agreement with previous work analytic calculations of the interaction of this vapor melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region the time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode the latter produce antipodal shock stresses within the range of 5 25 gpa where stable shock remanent magnetization srm of lunar soils has been found experimentally to occur calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally this suggests that pre existing ejecta materials which would also contain abundant metallic iron remanence carriers may be important anomaly sources a possibility that is consistent with enhanced magnetic anomalies observed peripheral to spa the latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick spa ejecta mantle occurring near the antipodes of the imbrium and serenitatis impacts together with converging seismic compressional waves these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of spa near the imbrium antipode allowing the ascent of magma with enhanced kreep concentrations

megaregolith evolution and cratering cataclysm models lunar cataclysm as a misconception 28 years later the hypothesis of a lunar cataclysmic cratering episode between 3 8 and 3 9 gyr ago lacks proof its strongest form proposes no cratering before about 4 0 gyr followed by catastrophic formation of most lunar craters and basins in 200 myr the premise that zero impact melts implies zero impacts is disproved by data from asteroids on which early collisions clearly occurred but from which early impact melts are scarce plausible cataclysm models imply that any cataclysm should have affected the whole inner solar system but among available lunar and asteroid impact melt and impact age resetting data a narrow strong 3 83 9 gyr spike in ages is seen only in the region sampled by apollo luna reported lunar meteorite data do not show the spike asteroid data show a broader milder peak spreading from about 4 2 to 3 5 gyr these data suggest either that the spike in apollo impact melt ages is associated with unique lunar front side events or that the lunar meteorites data represent different kinds of events than the apollo luna data here we develop an alternate megaregolith evolution hypothesis to explain these data in this hypothesis early impact melts are absent not because there were no impacts but because the high rate of early impacts led to their pulverization the model estimates survival halflives of most lunar impact melts prior to 4 1 gyr at 100 myr after a certain time tcritical 4 0 gyr impact melts began to survive to the present the age distribution differences among impact melts and plutonic rocks are controlled by and hold clues to the history of regolith evolution and the relative depths of sequestration of impact melts versus plutonic rocks both among lunar and asteroidal samples both the zero cratering then cataclysm hypothesis and the megaregolith evolution hypothesis require further testing especially with lunar meteorite impact melt studies

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

lunar anorthosite 60025 the petrogenesis of lunar anorthosites and the composition of the moon systematic variations of the mineral chemistry of ferroan anorthosite 60025 which is probably a mixture of closely related materials suggest that lunar anorthosites formed by strong fractional crystallization and near perfect adcumulate growth without trapping liquid the parent liquid for the most primitive samples was saturated with olivine plagioclase pigeonite and chromite and evolved to one saturated with plagioclase pigeonite high ca pyroxene and ilmenite the parent liquid also had a very low na2o content and combined with strong fractional crystallization this explains the steep trend of anorthosites on an mg atomic 100 mg mg fe v an diagram the mineral and chemical data for other anorthosites are consistent with such a model near perfect adcumulation can occur if growth takes place at the crystal liquid interface without the physical accumulation of crystals grown elsewhere and is encouraged by the shifts in phase boundaries with pressure anorthosites are probably the remnants of a crust floating on and crystallizing at the surface of a magma ocean originally of bulk moon composition mineralogical and trace element data suggest that the parental liquid for the most primitive anorthosites had previously crystallized no plagioclase and some but perhaps very little pyroxene hence the bulk moon appears to be similar to that proposed by ringwood 1976 but to have even lower alkalis a subchondritic caal ratio and ree abundances and patterns close to chondritic the mare basalt sources are not directly complementary to the feldspathic crust because experimental and trace element data indicate that they are too magnesian and contain too much high ca pyroxene other crustal rocks such as the mg suite samples are not closely related to anorthosites in addition to their chemical differences they have a different crystallization sequence ol plag px in contrast with the ol px plag inferred for anorthosite parental liquid evolution

the comparative behavior of apatite zircon u pb systems in apollo 14 breccias implications for the thermal history of the fra mauro formation we report secondary ion mass spectrometry sims u pb analyses of zircon and apatite from four breccia samples from the apollo 14 landing site the zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two sims u pb analyses show that the ages of zircon grains range from 4023 24 ma to 4342 5 ma whereas all apatite grains define an isochron corresponding to an age of 3926 3 ma the disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite u pb systems have been completely reset at 3926 3 ma whereas the u pb system of zircon has not been noticeably disturbed at this time the apatite u pb age is slightly older than the ages determined by other methods on apollo 14 materials highlighting need to reconcile decay constants used for the u pb ar ar and rb sr systems we interpret the apatite age as a time of formation of the fra mauro formation if the interpretation of this formation as an imbrium ejecta is correct apatite also determines the timing of imbrium impact the contrast in the pb loss behavior of apatite and zircon places constraints on the temperature history of the apollo 14 breccias and we estimate from the experimentally determined pb diffusion constants and an approximation of the original depth of the excavated samples in the fra mauro formation that the breccias experienced an initial temperature of about 1300 1100 c but cooled within the first five to ten years

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

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

thermal history recorded by the apollo 17 impact melt breccia 73217 abstract lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion mixed with a silica rich glass interpreted to represent an impact melt a study of accessory minerals in a thin section from this breccia 73217 52 identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample equant anhedral zircon grains that probably formed in the gabbronorite referred to as type 1 have consistent u pb ages of 4332 7 ma a similar age of 4335 5 ma was obtained from acicular zircon type 2 grains interpreted to have formed from impact melt a polycrystalline zircon aggregate type 3 occurs as a rim around a baddeleyite grain and has a much younger age of 3929 10 ma similar to the 3936 17 ma age of apatite grains found in the thin section a combined apatite type 3 zircon age of 3934 12 ma is proposed as the age of the serenitatis impact event and associated thermal pulse x ray mapping and electron probe analyses showed that ti is inhomogeneous in the zircon grains on the sub micrometer scale however model temperatures estimated from shrimp analyses of ti concentration in the 10 m diameter spots on the polished surfaces of type 1 and type 2 zircons range between about 1300 and 900 c respectively whereas ti concentrations determined for the type 3 zircon are higher at about 1400 1500 c a combination of u pb ages ti concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 5 ma impact which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon type 2 the 3934 12 ma serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon type 3 this event also reset the u pb system of apatite formed merrillite coronas around some apatite grains and probably re equilibrated some pyroxenes in the clasts although there have been arguments for pre 3 9 ga impacts based on other types of samples the age of the acicular zircon at 4335 5 ma provides the first evidence of impact melt significantly predating the lunar cataclysm our data combined with other chronological results demonstrate the occurrence of pre 3 9 ga impacts on the moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux

was there ever a terminal lunar cataclysm with lunar viscosity arguments about 30 years ago there was a suggestion by several able scientists at the california institute of technology that the moon had undergone a terminal lunar cataclysm this meant that most of the early impact cratering had been concentrated strongly at about the time of formation of the imbrium basin this solution was discussed in many papers and the idea of a cataclysm gradually faded away in about 1990 it was again revived by several scientists the idea of a terminal lunar cataclysm at about the time the imbrium basin was formed was advanced albeit in a somewhat different manner the present paper has been written to analyze the various observations and interpretations that have been advanced to permit a cataclysm it is concluded that the three main proposals which if correct would have permitted a cataclysm to have occurred are each faulty and not consistent with such a cataclysm to demonstrate this conclusion it was necessary to determine absolute ages of various lunar features this meant in part determinations of the existence and nature of lunar crustal viscosity consistent with times of formation of six lunar basins the results of such studies yielded an internally consistent model which requires a long period from the original formation of the moon at about 4 5 byr to a time slightly earlier than that of the formation of the imbrium basin at about 3 84 byr on this model there is no indication of a clustering of events and it is concluded that a terminal lunar cataclysm never occurred