asymmetric crustal growth on the moon indicated by primitive farside highland materials the origin of the dichotomy between the lunar nearside and farside is unclear analysis of spectral reflectance data from the kaguya lunar orbiter indicates a systematic difference in the degree of differentiation in the oldest lunar crustal terrains linking the lunar dichotomy to crystallization of the magma ocean

on the petrogenesis of lunar troctolites new insights into cumulate mantle overturn mantle exposures in impact basins abstract we investigate lunar troctolite petrogenesis with a series of forward models we simulate the cumulate mantle overturn hypothesis by modeling the adiabatic ascent and decompression melting of primary mantle cumulates produced during differentiation of a lunar magma ocean lmo combined equilibrium and fractional crystallization of candidate liquids generated by the melting model can reproduce the predominant constituents of the lunar magnesian suite mg suite troctolites and norites contrary to previous hypotheses model results are consistent with previous studies challenging the proposed and long standing genetic relationship between mg suite and gabbronorites our mg suite petrogenetic model validates a direct temporal and chemical link between mg suite melt production and pressure release melting of primary lmo cumulates if so mg suite crystallization ages 4345 15 ma can be used to constrain the onset and duration of melting associated with mantle overturn based on our model results we propose an alternative mantle overturn hypothesis whereby upwelling olivine dominated cumulates experience decompression melting to produce the mg suite primary melt 1 9 melt at 2 1 gpa but that this melt was extracted from depth akin to lunar picritic glass magmas low degree partial melts at depths corresponding to 1 3 2 5 gpa thus our revised mantle overturn hypothesis reconciles mg suite petrogenesis without the expanse of an olivine dominated upper mantle as suggested by the current paradigms but contradicted by orbital data this hypothesis supports the presence of a low ca pyroxene dominated upper mantle consistent with mantle stratigraphy constrained by experimental and numerical simulations of lmo differentiation and proposed mantle exposures within impact basins

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

disentangling xenon components in nakhla martian atmosphere spallation and martian interior abstract a powdered sample of nakhla was separated into 3 subsamples one was left otherwise untreated one was washed in water and one etched with hno3 removing 6 of the original mass we report results of isotopic analysis of xenon released by laser step heating on aliquots of each of these subsamples some aliquots were neutron irradiated before isotopic analysis to allow determination of i ba and u as daughter xenon isotopes and some were not there is evidence that water soluble phases contain both martian atmospheric xenon and a component with low 129xe 132xe either martian interior xenon or terrestrial atmosphere higher temperature data from unirradiated aliquots of the water and acid treated samples reveal two component mixing one is a trapped xenon component with 129xe 132xe 2 350 0 026 isotopically identical to the martian atmosphere as measured in shock glass from shergottites it is associated with leachable iodine suggesting it is trapped close to grain boundaries it may be a result of shock incorporation of adsorbed atmospheric gas the second component is best explained as an intimate mixture of martian interior xenon and spallation xenon the martian interior component is present at a concentration of 10 12 cm3 stp g 1 132xe around 40 times lower than that observed in chassigny its association with spallation xenon produced from ba and light rare earth elements suggests it is in the feldspathic mesostasis we propose that it was trapped during crystallisation and reflects the mantle source of the parental magma

decadal characterization of indo pacific ocean subsurface temperature modes in soda reanalysis abstractstudies of decadal to multidecadal ocean subsurface temperature variability are fundamental to improving the understanding of low frequency climate signals the present study uses the simple ocean data assimilation soda version 2 2 4 product for the period 1950 2007 to identify decadal modes of variability that characterize the upper indo pacific ocean temperature structure 5 466 m depth an empirical orthogonal function eof analysis of the 10 yr low pass filtered temperature field applied across four depths shows that the dominant mode is characterized by a long term temperature trend with warming at the surface and cooling at the thermocline depth connecting the tropical western pacific with the southern indian ocean via the indonesian seas eof analysis of the detrended 10 yr filtered temperature data and correlation analyses of the eof time series with established large scale climate indices identified the interdecadal pacific oscillation as eof1 the north pacific gyre oscillation as e

martian gases in an antarctic meteorite significant abundances of trapped argon krypton and xenon have been measured in shock altered phases of the achondritic meteorite elephant moraine 79001 from antarctica the relative elemental abundances the high ratios of argon 40 to argon 36 2000 and the high ratios of xenon 129 to xenon 132 2 0 of the trapped gas more closely resemble viking data for the martian atmosphere than data for noble gas components typically found in meteorites these findings support earlier suggestions made on the basis of geochemical evidence that shergottites and related rare meteorites may have originated from the planet mars

a new approach to cosmogenic corrections in 40 ar 39 ar chronometry implications for the ages of martian meteorites abstract anomalously old 40ar 39ar ages are commonly obtained from shergottites and are generally attributed to uncertainties regarding the isotopic composition of the trapped component and or the presence of excess 40ar old ages can also be obtained if inaccurate corrections for cosmogenic 36ar are applied current methods for making the cosmogenic correction require simplifying assumptions regarding the spatial homogeneity of target elements for cosmogenic production and the distribution of cosmogenic nuclides relative to trapped and reactor derived ar isotopes to mitigate uncertainties arising from these assumptions a new cosmogenic correction approach utilizing the exposure age determined on an un irradiated aliquot and step wise production rate estimates that account for spatial variations in ca and k is described data obtained from nwa 4468 and an unofficial pairing of nwa 2975 which yield anomalously old ages when corrected for cosmogenic 36ar using conventional techniques are used to illustrate the efficacy of this new approach for these samples anomalous age determinations are rectified solely by the improved cosmogenic correction technique described herein ages of 188 17 and 184 17 ma are obtained for nwa 4468 and nwa 2975 respectively both of which are indistinguishable from ages obtained by other radioisotopic systems for other shergottites that have multiple trapped components have experienced diffusive loss of ar or contain excess ar more accurate cosmogenic corrections may aid in the interpretation of anomalous ages the trapped 40ar 36ar ratios inferred from inverse isochron diagrams obtained from nwa 4468 and nwa 2975 are significantly lower than the martian atmospheric value and may represent upper mantle or crustal components

a young solidification age for the lunar magma ocean abstract the time at which the lunar magma ocean solidified can be determined from the lu hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean the final magma ocean crystallization product termed urkreep is enriched in incompatible elements including k ree and p we have determined the initial hf isotopic compositions of four samples two kreep basalts and two mg suite norites the incompatible trace element compositions of these samples are controlled by an urkreep component and therefore the initial hf isotopic compositions of these samples represent the hf isotopic evolution of urkreep in order to correct the effects of neutron irradiation on the hf isotopic compositions of these samples we have developed a model that uses the stable hf and sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the hf isotopic composition of the sample we use our corrected results to calculate a 176 lu 176 hf urkreep model age of 4353 37 ma and the 176 lu 177 hf of urkreep to be 0 0153 0 0033 the lu hf model age is concordant with the re calculated sm nd urkreep model age of 4389 45 ma and we take the average of these ages 4368 29 ma to represent the time at which urkreep formed this age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as 142 nd model ages for the formation or re equilibration of mare basalt sources taken together these ages indicate that the moon experienced a widespread large scale magmatic event around 4370 ma most plausibly attributed to solidification of the lunar magma ocean

a new view of lunar ferroan anorthosites postmagma ocean petrogenesis 1 new melting experiments in the range of 0 1 mpa to 4 7 gpa provide the basis for calculations of polybaric crystallization which show that both a 300 km deep magma ocean mo enriched in refractory elements and a 600 km deep magma ocean with refractory element concentrations similar to the earth s upper mantle produce sufficient amounts of differentiated residual liquid capable of crystallizing the troctolitic noritic mineral assemblages that are prevalent in lunar ferroan anorthosites lfa however the both oceans become saturated with augite after 20 30 crystallization of the plagioclase saturated residuum and thereafter are no longer capable of producing noritic assemblages a second set of melting experiments reveals the existence of a previously unrecognized equilibrium augite liquid pigeonite plagioclase and a thermal minimum on the pigeonite plagioclase liquidus surface at 0 6 gpa these equilibria will produce melts of mafic cumulates with lower wo content than those from which the cumulates originally formed in a postmagma ocean environment of overturning mafic cumulates melt thus formed and entrained in a buoyant segregation of plagioclase would then form diapiric masses that would crystallize troctolitic noritic assemblages after intruding upward diapiric intrusions could also provide the local deformational environment needed to produce the apparently high 95 plagioclase modes of the lfa an unstable density profile in the cumulate pile following crystallization of most of the mo should provide the impetus for overturn and the heat for partially remelting mafic cumulates

origin of the lunar highlands mg suite an integrated petrology geochemistry chronology and remote sensing perspective the mg suite represents an enigmatic episode of lunar highlands magmatism that presumably represents the first stage of crustal building following primordial differentiation this review examines the mineralogy geochemistry petrology chronology and the planetary scale distribution of this suite of highlands plutonic rocks presents models for their origin examines petrogenetic relationships to other highlands rocks and explores the link between this style of magmatism and early stages of lunar differentiation of the models considered for the origin of the parent magmas for the mg suite the data best fit a process in which hot solidus temperature at 2 gpa 1600 to 1800 c and less dense 3100 kg m3 early lunar magma ocean cumulates rise to the base of the crust during cumulate pile overturn some decompressional melting would occur but placing a hot cumulate horizon adjacent to the plagioclase rich primordial crust and kreep rich lithologies at temperatures of 1300 c would result in the hybridization of these divergent primordial lithologies producing mg suite parent magmas as urkreep primeval kreep is not the petrologic driver of this style of magmatism outside of the procellarum kreep terrane pkt mg suite magmas are not required to have a kreep signature evaluation of the chronology of this episode of highlands evolution indicates that mg suite magmatism was initiated soon after primordial differentiation 10 m y alternatively the thermal event associated with the mantle overturn may have disrupted the chronometers utilized to date the primordial crust petrogenetic relationships between the mg suite and other highlands suites e g alkali suite and magnesian anorthositic granulites are consistent with both fractional crystallization processes and melting of distinctly different hybrid sources

a combined sm nd rb sr and u pb isotopic study of mg suite norite 78238 further evidence for early differentiation of the moon abstract lunar mg suite norite 78238 was dated using the sm nd rb sr and u pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of mg suite magmatism as well as to provide a direct comparison between the three isotopic systems the sm nd isotopic system yields a crystallization age for 78238 of 4334 37 ma and an initial e nd 143 value of 0 27 0 74 the age initial e nd 143 t i systematics of a variety of kreep rich samples including 78238 and other mg suite rocks kreep basalts and olivine cumulate nwa 773 suggest that lunar differentiation was completed by 4492 61 ma assuming a chondritic uniform reservoir bulk composition for the moon the rb sr isotopic systematics of 78238 were disturbed by post crystallization processes nevertheless selected data points yield two rb sr isochrons one is concordant with the sm nd crystallization age 4366 53 ma the other is 4003 95 ma and is concordant with an ar ar age for 78236 the 207pb 206pb age of 4333 59 ma is concordant with the sm nd age the u pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the pb pb system and may reflect fractionation of u and pb during sample handling despite the disturbed nature of the u pb systems a time averaged 238u 204pb value of the source can be estimated at 27 30 from the pb pb isotopic systematics because kreep rich samples are likely to be derived from source regions with the highest u pb ratios the relatively low value calculated for the 78238 source suggests the bulk moon does not have an exceedingly high value

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

the case for a martian origin of the shergottites nitrogen and noble gases in eeta 79001 abstract nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite eeta 79001 a nitrogen component trapped in the glass but not present in the matrix has a 15n value at least as high as 190 ratios of40ar 14n and15n 14n in the glass are consistent with dilution of a martian atmospheric component 15n 620 160 40ar 14n 0 33 0 03 by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock trapped noble gases in the glass reproduce within error the elemental and isotopic compositions measured in mars atmosphere by viking and are in general agreement with previous measurements except for much lower abundances of neutron generated krypton and xenon isotopes the most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the eeta 79001 glass and that this meteorite and thus the shergottites and probably the nakhlites and chassignites as well originated on mars nitrogen in the non glassy matrix of eeta 79001 amounts to less than 0 5 ppm and has a spallation corrected 15n value in the range 0 to 20 it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen spallogenic noble gases yield single stage exposure ages between 400 000 and 900 000 years depending on irradiation geometry trapped argon may have an unusually low36ar 38ar ratio trapped krypton except for a small excess at80kr is smoothly mass fractionated with respect to either terrestrial or chondritic kr the trapped xenon composition is consistent with addition of neutron capture radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric xe the elemental84kr 132xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio

formation of a martian pyroxenite a comparative study of the nakhlite meteorites and theo s flow the unusual composition of the nakhlites a group of pyroxenitic martian meteorites with young ages presents an opportunity to learn about nonbasaltic magmatic activity on another planet however the limited number of these meteorites makes unraveling their history difficult a promising terrestrial analog for the formation of the nakhlites is theo s flow in ontario canada this atypical 120 m thick flow differentiated in place forming distinct layered lithologies of peridotite pyroxenite and gabbro theo s pyroxenite and the nakhlites share strikingly similar petrographies with concentrated euhedral to subhedral augite grains set in a plagioclase rich matrix these two suites of rocks also share specific petrologic features mineral and whole rock compositional features and size and spatial distributions of cumulus grains the numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion their formation could have involved settling of crystals in a phenocryst laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow the latter case is more likely and requires steady state nucleation and growth of clusters of pyroxene grains and olivine in the nakhlites circulating in a strongly convecting melt pool followed by settling and continued growth in a thickening cumulate pile trapped pockets of intercumulus liquid in the pile gradually evolved finally growing fe enriched rims on cumulus grains with sufficient evolution the melt reached plagioclase supersaturation causing rapid growth of plagioclase sprays and late stage mesostasis growth

meteorite constraints on martian atmospheric loss and paleoclimate abstract the evolution of mars atmosphere to its currently thin state incapable of supporting liquid water remains poorly understood and has important implications for martian climate history martian meteorites contain trapped atmospheric gases that can be used to constrain both the timing and effectiveness of atmospheric escape processes in this paper measurements of xenon isotopes in two ancient martian meteorites alh 84001 and nwa 7034 are reported the data indicate an early episode of atmospheric escape that mass fractionated xenon isotopes culminated within a few hundred million years of planetary formation and little change to the atmospheric xenon isotopic composition has occurred since this time in contrast on earth atmospheric xenon fractionation continued for at least two billion years pujol et al 2011 such differences in atmospheric xe fractionation between the two planets suggest that climate conditions on mars may have differed significantly from those on archean earth for example the hydrogen escape flux may not have exceeded the threshold required for xenon escape on mars after 4 2 4 3 ga which indicates that mars may have been significantly drier than earth after this time

martian atmospheric xenon contents of nakhla mineral separates implications for the origin of elemental mass fractionation abstract analyses of mineral separates from nakhla reveal that martian atmosphere derived xenon is present in olivine and pyroxene in concentrations close to that of the bulk meteorite and at elevated concentrations in mesostasis we argue that neither aqueous alteration nor adsorption followed by shock incorporation are plausible mechanisms for incorporating this gas component into the meteorite and suggest trapping occurred on formation direct dissolution of martian atmosphere in the parent melt is unlikely since it would require a partial pressure of xenon in the martian atmosphere greater than consideration of the i xe system realistically allows the melt appears to have been insufficiently hydrated for water transport to have been solely responsible addition of as little as 500 ppm of martian soil to the melt could account for the observed composition and elemental mass fractionation

a review of lunar chronology revealing a preponderance of 4 34 4 37 ga ages in this study data obtained from sm nd and rb sr isotopic measurements of lunar highlands samples are renormalized to common standard values and then used to define ages with a common isochron regression algorithm the reliability of these ages is evaluated using five criteria that include whether 1 the ages are defined by multiple isotopic systems 2 the data demonstrate limited scatter outside uncertainty 3 initial isotopic compositions are consistent with the petrogenesis of the samples 4 the ages are defined by an isotopic system that is resistant to disturbance by impact metamorphism and 5 the rare earth element abundances determined by isotope dilution of bulk of mineral fractions match those measured by in situ analyses from this analysis it is apparent that the oldest highlands rock ages are some of the least reliable and that there is little support for crustal ages older than 4 40 ga a model age for ur kreep formation calculated using the most reliable mg suite sm nd isotopic systematics in conjunction with sm nd analyses of kreep basalts is 4389 45 ma this age is a good match to the lu hf model age of 4353 37 ma determined using a subset of this sample suite the average modelmore age of 4353 25 ma determined on mare basalts with the 146sm 142nd isotopic system with a peak in pb pb ages observed in lunar zircons of 4340 20 ma and the oldest terrestrial zircon age of 4374 6 ma the preponderance of ages between 4 34 and 4 37 ga reflect either primordial solidification of a lunar magma ocean or a widespread secondary magmatic event on the lunar nearside the first scenario is not consistent with the oldest ages reported for lunar zircons whereas the second scenario does not account for concordance between ages of crustal rocks and mantle reservoirs less

40ar 39ar and cosmic ray exposure ages of nakhlites nakhla lafayette governador valadares and chassigny we present 40ar 39ar dating results of handpicked mineral separates and wholerock samples of nakhla lafayette and chassigny our data on nakhla and lafayette and recently reported ages for some nakhlites and chassigny misawa et al 2006 park et al 2009 point to formation ages of approximately 1 4 ga rather than 1 3 ga that is consistent with previous suggestions of close in time formation of nakhlites and chassigny in lafayette mesostasis we detected a secondary degassing event at approximately 1 1 ga which is not related to iddingsite formation it may have been caused by a medium grade thermal event resetting the mesostasis age but not influencing the k ar system of magmatic inclusions and the original igneous texture of this rock cosmic ray exposure ages for these meteorites and for governador valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3he 21ne and 38ar individual results are similar to literature data the considerable scatter of t3 t21 and t38 ages is due to systematic uncertainties related to bulk rock and target element chemistry production rates and shielding effects this hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and chassigny from a confined martian surface terrain eugster 2003 garrison and bogard 2005 cosmic ray exposure ages from stepwise release age spectra using 38ar and neutron induced 37ar from ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates although not from shielding conditions

isotopic composition of trapped and cosmogenic noble gases in several martian meteorites isotopic abundances of the noble gases were measured in the following martian meteorites two shock glass inclusions from elephant moraine eet 79001 shock vein glass from shergotty and yamato y 793605 and whole rock samples of allan hills alh 84001 and queen alexandra range que 94201 these glass samples when combined with literature data on a separate single glass inclusion from eet 79001 and a glass vein from zagami permit examination in greater detail of the isotopic composition of ne ar kr and xe trapped from the martian atmosphere the isotopic composition of martian ne if actually present in these glasses remains poorly defined the 40ar 36ar ratio of trapped martian atmospheric ar is probably considerably lower than the nominal ratio of 3000 measured by viking and data on impact glasses suggest a value of 1900 the atmospheric 36ar 38ar ratio is 4 0 martian atmospheric kr may be enriched in lighter isotopes by 0 5 amu compared to both solar wind kr and to the martian composition previously reported the isotopic composition of xe in these glasses agrees with that previously reported in the literature the martian atmospheric 36ar 132xe and 84kr 132xe elemental ratios are higher than those reported by viking by factors of 2 5 1 6 depending on the 40ar 36ar ratio adopted and 1 8 respectively and are discussed in a separate paper cosmogenic gases indicate space exposure ages of 2 7 0 6 ma for que 94201 and shergotty and 14 1 ma for alh 84001 small amounts of 21ne produced by energetic solar protons may be present in que 94201 but are not present in alh 84001 or y 793605 the space exposure age for y 793605 is 4 9 0 6 ma and appears to be distinctly older than the ages for basaltic shergottites however uncertainties in cosmogenic production rates still makes somewhat uncertain the number of martian impact events required to produce the exposure ages of martian meteorites

146sm 142nd formation interval for the lunar mantle now extinct 146sm t1 2 103ma was present in the early solar system and lunar basalts might be expected to have inherited small anomalies in 142nd abundances from mantle sources with variable sm nd ratios established during the early global differentiation of the moon 142nd 144nd ratios were measured for several lunar basalts and values of nd142 deviations from terrestrial 142nd 144nd in parts in 104 calculated for them the nd142 values show very small variations attributable to 146sm decay neutron capture near the lunar surface also modified both the samarium and neodymium isotopic compositions of the basalt samples the calculated neutron fluences range from 0 to 7 8 1016 n cm2 both thermal and epithermal neutron fluences were calculated from the sm isotopic compositions of the basalts and used to correct the nd142 values for neutron capture by neodymium well resolved radiogenic enrichments nd142 and 0 17 0 08 2 respectively were measured for lunar meteorite asuka 881757 and apollo 17 basalt 74255 respectively for which the neutron fluences were nil basalts 70135 and 75075 which were exposed to small measureable neutron fluences of 2 5 1015 and 7 7 1015 n cm2 respectively have neutron corrected nd142 values of 0 25 0 15 and 0 29 0 11 respectively the average value of radiogenic nd142 for the three high ti basalts from apollo 17 is 0 22 0 06 neutron capture effects were greater for the other basalts but can be corrected using the measured neutron fluences derived from the samarium isotopic data the neutron corrected nd142 for apollo 12 ilmenite basalt 12056 consistent with its derivation from a highly depleted mantle source also three low ti basalts from apollo 12 and apollo 15 12038 15076 15555 have neutron corrected nd142 which are not resolved from zero and average 0 04 0 06 a fourth low ti basalt 12039 has neutron corrected nd142 but is subject to the largest neutron correction of 0 32 e units kreep basalt 14078 was exposed to only a small neutron fluence of 6 4 1015 n cm2 and has a small deficit of 142nd corresponding to nd142 the average neutron corrected nd142 of three kreep basalts is 0 05 0 04 the nd142 values of the basalts correlate with 147sm 144nd values for their source regions as calculated from their ages and initial nd142 values 147sm 143nd and 146sm 142nd systematics were combined in a three stage model yielding an isochron equation for the lunar mantle formation interval which was calculated to be 238 40 56 ma 2 sm nd ratios for the source regions also are derived from the model and compare favorably with values from conventional geochemical models of mare basalt petrogenesis if the moon were formed by impact of a large mars sized planetesimal with the earth the impact was early enough that the lunar mantle cooled to neodymium isotopic closure by 4 32 ga ago the bulk lunar moon142 evaluated from the mantle isochron at 147sm 144nd chur 0 1967 is 0 01 0 03 2 consistent with derivation of lunar and terrestrial neodymium from a common reservoir

terrestrial and martian weathering signatures of xenon components in shergottite mineral separates abstract xenon isotopic ratios step heating release patterns and gas concentrations of mineral separates from martian shergottites roberts massif rbt 04262 dar al gani dag 489 shergotty and elephant moraine eet 79001 lithology b are reported concentrations of martian atmospheric xenon are similar in mineral separates from all meteorites but more weathered samples contain more terrestrial atmospheric xenon the distributions of xenon from the martian and terrestrial atmospheres among minerals in any one sample are similar suggesting similarities in the processes by which they were acquired however in opaque and maskelynite fractions martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon it is suggested that both martian and terrestrial atmospheric xenon were initially introduced by weathering low temperature alteration processes however the martian component was redistributed by shock accounting for its current residence in more retentive sites the presence or absence of detectable 129xe from the martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock s parent melt variable contents of excess 129xe contrast with previously reported consistent concentrations of excess 40ar suggesting distinct sources contributed these gases to the parent magma

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

noble gases in mineral separates from three shergottites shergotty zagami and eeta79001 this study provides a complete data set of all five noble gases for bulk samples and mineral separates from three martian shergottites shergotty bulk pyroxene maskelynite zagami bulk pyroxene maskelynite and elephant moraine eet a79001 lithology a bulk pyroxene we also give a compilation of all noble gas and nitrogen studies performed on these meteorites our mean values for cosmic ray exposure ages from 3he 21ne and 38ar are 2 48 myr for shergotty 2 73 myr for zagami and 0 65 myr for eeta79001 lith a serious loss of radiogenic 4he due to shock is observed cosmogenic neon results for bulk samples from 13 martian meteorites new data and literature data are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic ray effects while a contribution of this low energy irradiation is strongly indicated for all of the shergottites spallation ne in chassigny allan hills alh 84001 and the nakhlites is fully explained by galactic cosmic ray spallation implanted martian atmospheric gases are present in all mineral separates and the thermal release indicates a near surface siting we derive an estimate for the 40ar 36ar ratio of the martian interior component by subtracting from measured ar in the k poor pyroxenes the small radiogenic component as well as the implanted atmospheric component as indicated from 129xe excesses unless compromised by the presence of additional components a high ratio of 2000 is indicated for martian interior argon similar to that in the martian atmosphere since much lower ratios have been inferred for chassigny and alh 84001 the result may indicate spatial and or temporal variations of 40ar 36ar in the martian mantle

noble gases and chemical composition of shergotty mineral fractions chassigny and yamato 793605 the trapped argon 40 argon 36 ratio and ejection times of martian meteorites we report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene maskelynite mesostasis glass and bulk material of shergotty and of bulk samples from chassigny and yamato 793605 the 40k 40ar isochron for the shergotty minerals yields a gas retention age of 196 ma which is within errors in agreement with previously determined rb sr internal isochron ages argon that was trapped at this time has a 40ar 36ar ratio of 1100 for chassigny and y 793605 we obtain trapped 40ar 36ar ratios of 1380 and 950 respectively using these results and literature data we show that the three shergottites shergotty zagami and que 94001 the lherzolites alh 77005 lew 88516 and y 793605 as well as chassigny and alh 84001 contain a mixture of martian mantle and atmospheric ar whereas the trapped 40ar 36ar ratio of the nakhlites nakhla lafayette and governador valadares cannot be determined with the present data we show that martian atmospheric trapped ar in martian meteorites is correlated with the shock pressure that they experienced hence we conclude that the martian atmospheric gases were introduced by shock into the meteoritic material for the shergotty minerals we obtain 3he 21ne and 38ar based cosmic ray exposure ages of 3 0 ma and for the lherzolite y 793605 4 0 ma which confirms our earlier conclusion that the lherzolites were ejected from mars 1 ma before the shergottites chassigny yields the previously known ejection age of 11 6 ma

the iodine xenon system in clasts and chondrules from ordinary chondrites implications for early solar system chronology abstract we have studied the i xe system in chondrules and clasts from ordinary chondrites cristobalite bearing clasts from parnallee ll3 6 closed to xe loss 1 4 ma after bjurbole feline a feldspar and nepheline rich clast also from parnallee closed at 7 04 0 15 ma two out of three chondrules from parnallee that yielded well defined initial i ratios gave ages identical to bjurbole s within error a clast from barwell l6 has a well defined initial i ratio corresponding to closure 3 62 0 60 ma before bjurbole partial disturbance and complete obliteration of the i xe system by shock are revealed in clasts from julesburg l3 6 and quenggouk h4 respectively partial disturbance by shock is capable of generating anomalously high initial i ratios in some cases these could be misinterpreted yielding erroneous ages a macrochondrule from isoulane n amahar contains concentrations of i similar to ordinary chondrules but unlike most ordinary chondrules contains no radiogenic 129xe this requires resetting 50 ma or more later than most chondrules the earliest chondrule ages in the i xe mn cr and al mg systems are in reasonable agreement this and the frequent lack of evidence for metamorphism capable of resetting the i xe chronometer leads us to conclude that at least the earliest chondrule i xe ages represent formation if so chondrule formation took place at a time when sizeable parent bodies were present in the solar system

chronologic implications for slow cooling of troctolite 76535 and temporal relationships between the mg suite and the ferroan anorthosite suite abstract ages have been obtained using the 87 rb 87 sr 147 sm 143 nd and 146 sm 142 nd isotopic systems for one of the most slowly cooled lunar rocks apollo 17 mg suite troctolite 76535 the 147 sm 143 nd 146 sm 142 nd and rb sr ages derived from plagioclase olivine and pyroxene mineral isochrons yield concordant ages of 4307 11 ma 4299 29 35 ma and 4279 52 ma respectively these ages are slightly younger than the age determined on ferroan anorthosite suite fas rock 60025 and are therefore consistent with the traditional magma ocean model of lunar differentiation in which the mg suite is intruded into the anorthositic crust however the sm nd ages record when the rock passed below the closing temperature of the sm nd system in this rock at 825 c whereas the rb sr age likely records the closure temperature of 650 c a cooling rate of 3 9 c ma is determined using the ages reported here and in the literature and calculated closure temperatures for the ar ar pb pb rb sr and sm nd systems this cooling rate is in good agreement with cooling rates estimated from petrographic observations slow cooling can lower apparent sm nd crystallization ages by up to 80 ma in the slowest cooled rocks like 76535 and likely accounts for some of the variation of ages reported for lunar crustal rocks nevertheless slow cooling cannot account for the overlap in fas and mg suite rock ages instead this overlap appears to reflect the concordance of mg suite and fas magmatism in the lunar crust as indicated by ages calculated for the solidus temperature of 76535 and 60025 of 4384 24 ma and 4383 17 respectively not only are the solidus ages of 76535 and 60025 nearly concordant but the sm nd isotopic systematics suggest they are derived from reservoirs that were minimally differentiated prior to 4 38 ga although the sr isotopic composition of 60025 indicates its source was minimally differentiated the sr isotopic composition of 76535 indicates it underwent fractionation just prior to solidification of the 76535 these observations are consistent with both a magma ocean or a serial magmatism model of lunar differentiation in either model differentiation of lunar source regions must occur near the solidification age of thee samples perhaps the best estimate for the formation age of lunar source regions is the rb sr model age of the 76535 source region age of 4401 32 ma this is in good agreement with sm nd model ages for the formation of ur kreep and suggests that differentiation of a least part of the moon could not have occurred prior to 4 43 ga

constraints on the formation age and evolution of the moon from 142 nd 143 nd systematics of apollo 12 basalts the moon likely formed as a result of a giant impact between proto earth and another large body the timing of this event and the subsequent lunar differentiation timescales are actively debated new high precision nd isotope data of apollo mare basalts are used to evaluate the low ti high ti and kreep mantle source reservoirs within the context of lunar formation and evolution the resulting models are assessed using both reported 146sm half lives 68 and 103 myr the linear relationship defined by 142nd 143nd systematics does not represent multi component mixing and is interpreted as an isochron recording a mantle closure age for the sm nd system in the moon using a chondritic source model with present day 142nd of 7 3 the mare basalt mantle source reservoirs closed at 4 45 09 10 ga t1 2sm146 68 myr or 4 39 14 16 ga t1 2sm146 103 myr in a superchondritic 2 stage evolution model with present day nd142 of 0 mantle source closure ages are constrained to 4 41 08 10 t1 2sm146 68 myr or 4 34 14 15 ga t1 2sm146 103 myr the lunar mantle source reservoir closure ages 4 5 ga may be reconciled by 3 potential scenarios first the moon formed later than currently favored models indicate such that the lunar mantle closure age is near or at the time of lunar formation second the moon formed ca 4 55 to 4 47 ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean lmo for up to ca 200 myr from tidal heating or asymmetric lmo evolution third the lmo crystallized rapidly after early moon formation thus the sm nd mantle closure age represents a later resetting of isotope systematics this may have resulted from a global wide remelting event while current earth moon formation constraints cannot exclusively advocate or dismiss any of these models the fact that u pb ages and hf isotopes for jack hills zircons from australia are best explained by an earth that re equilibrated at 4 4 ga or earlier following the moon forming impact does not favor a later forming moon if magma oceans crystallize in a few million years as currently advocated then a global resetting possibly by a large impact at 4 40 to 4 34 ga such as that which formed the south pole aitken basin best explains the late mantle closure age for the coupled sm nd isotope systematics presented here

preliminary uvlamp determinations of argon partition coefficients for olivine and clinopyroxene grown from silicate melts an ultra violet laser ablation microprobe uvlamp has been applied for the first time to investigate argon partition coefficients for olivines and clinopyroxenes grown from silicate melts at 1 bar argon pressure these preliminary measurements yield crystal melt partition coefficients ranging from 0 138 0 01 to 0 013 0 003 for olivine and 0 589 0 003 to 0 0016 0 0005 for clinopyroxene the higher values may indicate sub microscopic melt inclusions or some other heterogeneous distributions of non equilibrium argon in the crystals the lower values are probably more representative of true partition coefficients and fall at least an order of magnitude below the previously reported experimental data the possibility of anomalous high argon contents for crystals in previous studies is discussed in terms of surface adsorption trapped argon and early partial melting

the parental magma of the nakhla achondrite ultrabasic volcanism on the shergottite parent body the nakhla meteorite is an augite rich igneous cumulate rock magma trapped among the cumulus grains is now represented by fine grained mesostasis and post cumulus overgrowths on cumulus grains the composition of the intercumulus magma calculated by mass balance is ultrabasic sio2 45 and enriched in incompatible elements lalu 6 ci the cumulus augite and intercumulus magma were in chemical equilibrium implying that the intercumulus magma is a sample of nakhla s parental magma complex processes like infiltration metasomatism were therefore unimportant nakhla contains olivine crystals which are as large or larger than the cumulus augite crystals the largest olivine crystals probably xenocrysts preserve core compositions too fe rich to have been in equilibrium with the intercumulus magma other smaller olivine grains may have been phenocrysts the closest terrestrial analogs to nakhla and the other nakhlites are pyroxene rich cumulate layers in differentiated picrite and komatiite flows and shallow sills spinel or garnet was fractionated from the parental magmas of the nakhlites or their parental mantle based on the superchondritic caoal2o3 ratio of nakhla s intercumulus magma and its lack of an eu anomaly the three nakhlites and chassigny are half of the known samples of the shergottite parent body spb one may tentatively infer that ultrabasic volcanism was much more important on the spb than on the moon eucrite parent body or present day earth

an ancient sm nd age for a ferroan noritic anorthosite clast from lunar breccia 67016 strontium and neodymium systematics have been examined in a clast of ferroan noritic anorthosite from apollo 16 breccia 67016 two splits 328 and 326 of the same clast give different sm nd results split 328 gives a well defined internal isochron age of 4 562 0 068 ga and an initial 143nd 144nd ratio of 0 50673 10 corresponding to nd 0 1 0 2 2 optimized error relative to the murchinson carbonaceous chondrite the pyroxene separate from split 326 lies on the same isochron in contrast the plagioclase and whole rock from split 326 fall below this line indicating a small scale disturbance of the sm nd system this may reflect either an isotopic exchange between the plagioclase and a low sm nd mineral or a loss of radiogenic 143nd from the plagioclase possibly during the period of major impacts at 3 9 ga the preservation of an extremely old age for the noritic ferroan anorthosite 67016 328 suggests a rapid cooling of this rock at an early stage in the evolution of the lunar magma ocean this old age is also consistent with giant impact models for the formation of the moon but implies a relatively early event pre 4 50 ga and therefore rapid accretion and differentiation of the terrestrial planets

a valid margules formulation for an asymmetric ternary solution revision of the olivine ilmenite thermometer with applications abstract the olivine ilmenite thermometer of andersen and lindsley 1979 was based on an incorrect formulation for the excess free energy of an asymmetric ternary solution a valid formulation is derived and used to revise the parameters of the olivine ilmenite thermometer for olivine and ilmenite that have equilibrated above 700 c temperature can be calculated from t c 273 12549 p 0 03x fa 0 01099 x gk x il 0 062 10496 x fa 5767 x gk x il x hem 38602 141550x il 47183x gk 5 67 r ln k d 6 52x fa 3 09 x gk x il x hem 16 49 109 46 x il 36 49x gk with k d x il x fo x gk x fa the revised model gives wil gkg 5767 3 09t 0 011p and gexch 7301 8 9t 0 047p t in k p in bars applications include apollo 17 breccias and kimberlites

lunar magma transport phenomena abstract an understanding of the most significant events during the first ga or so of lunar history global differentiation associated with a lunar magma ocean lmo and the later generation of about 107 km3 of mare basalts depends profoundly on magma transport dynamics lmo evolution is dominated by the characteristics of hard turbulent convection of a fluid undergoing phase change quench protocrust formed at the ocean surface in early times is repeatedly disrupted by bolide impact ocean body tides and associated crustal foundering during this epoch the ocean temperature is set by the balance between convective heat brought to the surface and brown body radiation from the surface at later times the convective flow is organized into a number of regimes from top to bottom these include conductive crustal lid a viscous sublayer an inertial core a lower viscous sublayer and a mushy cumulate region crystal settling and flotation is restricted to the viscous sublayer regions dilatancy pumping may prevent the efficient expulsion of intercumulus melt during cumulate formation typical velocities within the inertial region are of order 10 m s because late stage ti rich cumulates are dense relative to the earlier mg rich cumulates hyper to sub solidus convection tends to mix lmo crystallization products numerical simulations that account simultaneously for compositional and thermal buoyancy effects indicate a characteristic mixing time 200 ma statistical measures of mixing variance and spatial correlation length quantify the details of mixing the darcy percolative and vein drain models of melt extraction are briefly discussed because compaction lengths are small 102 m the rate of melt extraction is governed by the balance between melt buoyancy and darcy friction differential melt matrix velocities are of order 10 km ma for nominal values appropriate for mare basalt melt extraction from a heterogeneous mixed lmo source transport of magma through the lunar lithosphere via a connected crack network seems highly probable for the range of inferred discharge rates applicable for lunar mare volcanism lmv 103 to 107 m3 s typical fracture widths and ascent speeds are 10 1 to 50 m and 10 1 to 10 m s respectively a simple model is proposed to evaluate the extent of fractionation as magma traverses cold lunar lithosphere if apollo green glasses are primitive and have not undergone significant fractionation en route to the surface then mean ascent rates of 10 m s and cracks of widths 40 m are indicated lunar tephra and vesiculated basalts suggest that a volatile component plays a role in eruption dynamics the predominant vapor species appear to be co co2 and cos near the lunar surface the vapor fraction expands enormously and vapor internal energy is converted to mixture kinetic energy with the concomitant high speed ejection of vapor and pyroclasts to form lunar fire fountain deposits such as the apollo 17 orange and black glasses and apollo 15 green glass

timing of mantle overturn during magma ocean solidification abstract solidification of magma oceans mos formed early in the evolution of planetary bodies sets the initial condition for their evolution on much longer time scales ideal fractional crystallization would generate an unstable chemical stratification that subsequently overturns to form a stably stratified mantle the simplest model of overturn assumes that cumulates remain immobile until the end of mo solidification however overturning of cumulates and thermal convection during solidification may act to reduce this stratification and introduce chemical heterogeneity on scales smaller than the mo thickness we explore overturning of cumulates before the end of mo crystallization and the possible consequences for mantle structure and composition in this model increasingly dense iron rich layers crystallized from the overlying residual liquid mo are deposited on a thickening cumulate layer overturn during solidification occurs if the dimensionless parameter r c measuring the ratio of the mo time of crystallization m o to the timescale associated with compositional overturn o v g h exceeds a threshold value if overturn did not occur until after solidification this implies that the viscosity of the solidified mantle must have been sufficiently high possibly requiring efficient melt extraction from the cumulate for a given rate of solidification for the lunar mo possible implications for the generation of the mg suites and mare basalt are suggested

the formation and evolution of the moon s crust inferred from the sm nd isotopic systematics of highlands rocks abstract ages determined for magnesian and ferroan anorthosite crustal rock suites overlap suggesting they formed contemporaneously about 4 3 4 5 ga a notable exception is the sm nd age previously determined on mg suite gabbronorite 67667 which is at least 100 ma younger than the youngest ferroan anorthosite new chronologic measurements of 67667 presented here yield concordant sm nd and rb sr mineral isochron ages of 4349 31 ma and 4368 67 ma suggesting the sample is older than previous estimates furthermore a whole rock sm nd isochron of mg suite rocks from the apollo 14 15 16 and 17 landing sites yields an age of 4348 25 ma indicating that mg suite magmatism was widespread and roughly contemporaneous on the lunar nearside analysis of sm nd internal isochron ages confirms that mg suite magmatism was restricted to a period between about 4 33 and 4 35 ga at the apollo 14 15 16 and 17 landing sites and was synchronous with magmatism at the apollo 16 site associated with the ferroan anorthosite suite between 4 35 and 4 37 ga magnesian and ferroan anorthosite suite rocks with ages younger than 4 33 ga appear to have experienced slow cooling in the deep lunar interior so that the ages record when the samples cooled below the closure temperature of the sm nd isotopic system and not the time they crystallized the ages determined for mg suite and ferroan anorthosite suite rocks are concordant with the age determined for the formation of urkreep of 4350 34 ma using the sm nd isotopic systematics of 67667 and measurements completed on norite 78238 troctolite 76535 kreep basalt 15386 and gabbronorite nwa 773 crystallization ages of mg suite and fas are also concordant with the average of 146sm 142nd ages previously determined for the formation of the mare basalt source region of 4333 30 ma the similarity of ages for mg suite magmatism ferroan anorthosite suite magmatism urkreep formation and formation of the mare basalt source regions implies the processes that produced these rocks were petrogenetically linked it also implies that both early stage and late stage lunar magma ocean cumulates formed over a relatively short duration of

martian fluid and martian weathering signatures identified in nakhla nwa 998 and mil 03346 by halogen and noble gas analysis we report argon ar noble gas ar ar ages and halogen abundances cl br i of martian nakhlites nakhla nwa 998 and mil 03346 to determine the presence of martian hydrous fluids and weathering products neutron irradiated samples were either crushed and step heated nakhla only or simply step heated using a laser or furnace and analysed for noble gases using an extension of the 40 ar 39 ar technique to determine halogen abundances the data obtained provide the first isotopic evidence for a trapped fluid that is cl rich has a strong correlation with 40 ar xs 40 ar xs 40 ar measured 40 ar radiogenic and displays 40 ar xs 36 ar of 1000 consistent with the martian atmosphere this component was released predominantly in the low temperature and crush experiments which may suggest a fluid inclusion host for the halogens we observe similar br cl and i cl ratios between the nakhlites and terrestrial reservoirs which is surprising given the absence of crustal recycling organic matter and frequent fluid activity on mars in particular br cl ratios in our nakhla samples especially olivine are consistent with previously analysed martian weathering products and both low temperature and crush analyses show a similar trend to the evaporation of seawater this may indicate that surface brines play an important role on mars and on halogen assemblages within martian meteorites and rocks elevated i cl ratios in the low temperature nwa 998 and mil 03346 releases may relate to in situ terrestrial contamination though we are unable to distinguish between low temperature terrestrial or martian components whilst estimates of the amount of water present based on the 36 ar concentrations are too high to be explained by a fluid component alone they are consistent with a mixed phase inclusion gas and fluid or with shock implanted martian atmospheric argon the observed fluid is dilute low salinity but high br cl and i cl ratios contains a martian atmospheric component and may represent a mixture of a shallow fluid with one derived from fluid circulation within the nakhlite cumulate pile with heat for fluid circulation supplied by either the igneous intrusion or by an impact event

isotopic evidence for a young lunar magma ocean abstract mare basalt sources and ferroan anorthosite suite cumulates define a linear array on a 146sm 144nd versus 142nd 144nd isochron plot demonstrating these materials were derived from a common reservoir at 4336 31 32 ma the minimum proportion of the moon that was in isotopic equilibrium at this time is estimated to be 1 3 of its entire volume based on the geographic extent from which the analyzed samples were collected and the calculated depths from which the samples were derived scenarios in which large portions of the moon were molten to depths of many hundreds of kilometers are required to produce the observed sm nd isotopic equilibrium between the mantle and crustal rocks at 4 34 ga this is a consequence of the fact that limited heating of a solid moon above the blocking temperature of the sm nd isotopic system is insufficient to diffusively homogenize radiogenic nd throughout the mantle and crust there are three scenarios that might account for global scale isotopic equilibrium on the moon relatively late in solar system history including 1 sm nd re equilibration of a solid moon resulting from widespread melting in response to mantle overturn or a very large impact 2 early accretion of the moon followed by delayed cooling due to the presence of an additional heat source that kept a large portion of the moon molten until 4 34 ga or 3 late accretion of the moon followed by rapid cooling of the magma ocean late in solar system history neither density driven overturn of the mantle nor a large impact are likely to homogenize the mantle and crust to the extent required by the sm nd isochron likewise secondary heating mechanisms such as tidal heating or radioactive decay are not efficient enough to keep the moon molten to the depth of the mare basalt source regions for many tens to hundreds of millions of years instead the age of equilibrium between such a compositionally diverse set of rocks produced on a global scale likely records the time of primordial solidification of the moon from a magma ocean this scenario accounts for both the petrogenetic characteristics of lunar rock suites as well as their sm nd isotopic systematics it is supported by the preponderance of 4 35 ga ages obtained for other hypothetical magma ocean crystallization products such as ferroan anorthosite suite rocks and k ree and p enriched cumulates that are thought to represent flotation cumulates of the magma ocean and the last vestiges of magma ocean solidification respectively

the age of ferroan anorthosite 60025 oldest crust on a young moon abstract sm nd isotopic data for mineral separates from the ferroan anorthosite 60025 define a precise isochron of 4 44 0 02ga age this age is roughly 110 m y younger than the formation of the first large solid objects in the solar nebula as recorded by the radiometric ages of the differentiated meteorites in the magma ocean model for early lunar differentiation ferroan anorthosites are the first crustal rocks to form on the moon if the moon is as old as the oldest meteorites the relatively young age determined for 60025 implies either that the magma ocean did not form synchronously with lunar formation or that the magma ocean required over 100 m y before reaching the stage of ferroan anorthosite crystallization alternatively we propose that the accumulated body of radiogenic isotope data for lunar rocks permit the moon to be as young as 4 44 4 51 ga if so isotopic evidence for chemical differentiation on the earth at about this same time suggests that the formation of the moon is reflected in the chemical evolution of the earth this in turn is consistent with the idea that the materials that now make up the moon were derived from the earth perhaps ejected by collision between the earth and another very large planetesimal during the final stages of accumulation of the terrestrial planets terrestrial origin models for the moon lessen the requirement that the earth and moon each have near chondritic relative abundances of the refractory elements and could require that certain chemical and isotopic characteristics of both bodies be considered in the framework of the chemical mass balance of the combined earth moon system

geochronology of an apollo 16 clast provides evidence for a basin forming impact 4 3 billion years ago we examined lithic breccias from the apollo sample collection in order to identify ferroan anorthosite samples suitable for geochronology and better define the age relationships between rocks of the lunar highlands clast 3a is a previously unstudied noritic anorthosite from apollo 16 lithic breccia 60016 with textural evidence of slow subsolidus recrystallization we estimate a cooling rate of 10 c myr and calculate a pyroxene solvus temperature of 1 100 1 000 c pyroxene exsolution lamellae 1 3 m indicate that the last stage of cooling was rapid at 0 2 c year typical of rates observed in thick ejecta blankets we calculate concordant ages from the 147sm 143nd 146sm 142nd rb sr and ar ar isotopic systems of 4 302 28 4 296 39 53 4 275 38 and 4 311 31 ma respectively with a weighted average of 4 304 12 ma the closure temperature of the sm nd system is 855 14 c whereas the closure temperature of the ar ar system is 275 25 c cooling from 855 to 275 c at 10 c myr should result in an age difference between the two isotopic systems of 60 myr the concordant sm nd rb sr and ar ar ages imply that they record the time the rock was excavated by a large impact from the midcrust the ages clearly predate various late accretion scenarios in which an uptick in impacts at 3 8 ga is preceded by a period of relative quiescence between 4 4 and 4 1 ga and instead are consistent with decreasing accretion rates following the formation of the moon

the timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237 abstract ferroan anorthosite suite fas rocks are widely interpreted to represent primordial lunar crust despite their importance in pinpointing the timing of lunar crust formation robust chronological investigations for this rock type are scarce here we report the ar ar rb sr and sm nd isotopic systematics for the fas troctolitic anorthosite 62237 the ar ar isotopic system has been reset by a thermal event at 3710 48 ma and the rb sr isotopic systematics has been disturbed such that a rb sr isochron age cannot be determined however an internal isochron for the sm nd isotopic system has yielded an age of 4350 73 ma mswd 2 0 with an initial 143 ndchur of 0 53 0 26 the mineral and whole rock fractions of 62237 plot on the same internal isochron as fas sample 60025 the combined datasets define an age of 4372 35 ma mswd 4 0 with an initial 143 ndchur of 0 17 0 22 literature sm nd data for fas and mg suite whole rocks also plot on the 60025 62237 isochron the coherence of data from both fas and mg suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical or nearly identical sources in addition the concordance of fas and mg suite ages suggests that primordial crust solidification either involved both magmatic suites or that mg suite magmatism was contemporaneous with fas magmatism within resolution of the sm nd chronometer the ages for fas and mg suite also coincide with the formation ages of the mare basalt source regions and urkreep ferroan anorthosite suite rocks and urkreep are thought to represent primordial lmo solidification products whereas mg suite and the mare basalt source regions are argued to represent mixtures of various lmo crystallization products that were formed during density driven overturn of the lmo the concordance of ages implies that the 4372 35 ma sm nd isochron records the age of mantle overturn and that overturn occurred during or shortly after solidification of the lmo

martian xenon components in shergotty mineral separates locations sources and trapping mechanisms isotopic signatures and concentrations of xenon have been measured in shergotty mineral separates by laser step heating martian atmosphere and martian interior xenon are present as is a spallation component martian atmospheric xenon is 510 times more concentrated in opaque minerals magnetite ilmenite and pyrrhotite and maskelynite than in pyroxenes perhaps reflecting grain size variation this is shown to be consistent with shock incorporation a component consisting of solar xenon with a fission contribution similar to components previously identified in martian meteorites and associated with the martian interior is best defined in the pyroxene dominated separates this component exhibits a consistent 129xe 129xe 132xe 1 2 excess over solar planetary 129xe 132xe 1 04 we suggest that gas present in the melt perhaps a mixture of interior xenon and martian atmosphere was incorporated into the pyroxenes in shergotty as the minerals crystallized

the lunar magma ocean reconciling the solidification process with lunar petrology and geochronology abstract the moon is thought to have originated with a magma ocean that produced a plagioclase flotation crust as solidification proceeded ages of anorthositic crust range over at least 200 million years the model for solidification presented here integrates chemical and physical constraints of lunar magma ocean solidification to determine 1 the final thickness of flotation crust generated by a fractionally solidifying magma ocean 2 the timescale of crystallization before plagioclase is a stable phase 3 the timescale of solidification after the formation of the plagioclase flotation crust and 4 the post overturn lunar mantle composition and structure we find that magma oceans of as much as 1000 km depth are consistent with creating an anorthositic crust 40 to 50 km in thickness solidification of the magma ocean prior to formation of the flotation crust may occur on the order of 1000 years and complete solidification would require additional ten to tens of millions of years reconciling these short model timescales with radiometric dates of crustal samples requires either a very late forming moon combined with finding older crustal ages to be incorrect or calling on tidal heating of the crust by the early earth to prolong solidification gravitationally driven overturn of cumulates during tidal heating provides a mechanism for creating the compositions and ages of the lunar mg suite of crustal rocks further we find that upon crystallization the moon likely began with an azimuthally heterogeneous gravitationally stable mantle after magma ocean cumulate overturn this result may help explain the experimentally determined origin of picritic glasses at similar depths but from different source materials

chronology geochemistry and petrology of a ferroan noritic anorthosite clast from descartes breccia 67215 clues to the age origin structure and impact history of the lunar crust the petrology major and trace element geochemistry and nd ar sr isotopic compositions of a ferroan noritic anorthosite clast from lunar breccia 67215 have been studied in order to improve our understanding of the composition age structure and impact history of the lunar crust the clast designated 67215c has an unusually well preserved igneous texture mineral compositions are consistent with classification of 67215c as a member of the ferroan anorthositic suite of lunar highlands rocks but the texture and mineralogy show that it cooled more rapidly and at shallower depths than did more typical ferroan anorthosites fans incompatible trace element concentrations are enriched in 67215c relative to typical fans but diagnostic signatures such as ti sm sc sm plagiophile element ratios and the lack of zr hf and nb ta fractionation show that this cannot be due to the addition of kreep alternatively 67215c may contain a greater fraction of trapped liquid than is commonly present in lunar fans 147sm 143nd isotopic compositions of mineral separates from 67215c define an isochron age of 4 40 0 11 gyr with a near chondritic initial 143nd of 0 85 0 53 the 40ar 39ar composition of plagioclase from this clast records a post crystallization thermal event at 3 93 0 08 gyr with the greatest contribution to the uncertainty in this age deriving from a poorly constrained correction for lunar atmosphere 40ar rb sr isotopic compositions are disturbed probably by the same event recorded by the ar isotopic compositions trace element compositions of fans are consistent with crystallization from a moderately evolved magma ocean and do not support a highly depleted source composition such as that implied by the positive initial 143nd of the ferroan noritic anorthosite 62236 alternatively the nd isotopic systematics of lunar fans may have been subject to variable degrees of modification by impact metamorphism with the plagioclase fraction being more strongly affected than the mafic phases 147sm 143nd isotopic compositions of mafic fractions from the 4 ferroan noritic anorthosites for which isotopic data exist 60025 62236 67016c 67215c define an age of 4 46 0 04 gyr which may provide a robust estimate for the crystallization age of lunar ferroan anorthosites

lunar feldspathic meteorites constraints on the geology of the lunar highlands and the origin of the lunar crust abstract the composition of the lunar crust provides clues about the processes that formed it and hence contains information on the origin and evolution of the moon current understanding of lunar evolution is built on the lunar magma ocean hypothesis that early in its history the moon was wholly or mostly molten this hypothesis is based on analyses of apollo samples of ferroan anorthosites 90 plagioclase molar mg mg fe mg 75 and the assumption that they are globally distributed however new results from lunar meteorites which are random samples of the moon s surface and remote sensing data show that ferroan anorthosites are not globally distributed and that the apollo highland samples used as a basis for the model are influenced by ejecta from the imbrium basin in this study we evaluate anorthosites from all currently available adequately described lunar highland meteorites representing a more widespread sampling of the lunar highlands than apollo samples alone and find that 80 of them are significantly more magnesian than apollo ferroan anorthosites interestingly luna mission anorthosites collected outside the continuous imbrium ejecta are also highly magnesian if the lunar highland crust consists dominantly of magnesian anorthosites as suggested by their abundance in samples sourced outside imbrium ejecta a reevaluation of the lunar magma ocean model is a sensible step forward in the endeavor to understand lunar evolution our results demonstrate that lunar anorthosites are more similar in their chemical trends and mineral abundance to terrestrial massif anorthosites than to anorthosites predicted in a lunar magma ocean this analysis does not invalidate the idea of a lunar magma ocean which seems a necessity under the giant impact hypothesis for the origin of the moon however it does indicate that most rocks now seen at the moon s surface are not primary products of a magma ocean alone but are products of more complex crustal processes

a chemical model for generating the sources of mare basalts combined equilibrium and fractional crystallization of the lunar magmasphere abstract it is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early moon wide magma ocean or magmasphere however the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal crystal and crystal liquid separation in a convecting magma chamber crystal liquid and crystal crystal separations were similarly inefficient during evolution of the lunar magma ocean lmo allowing for the trapping of interstitial liquid and entrainment of a small proportion of less dense plagioclase into the denser mafic cumulate mush indeed petrography of lunar highlands samples demonstrates this for anorthosites with 1 10 olivine the residual liquid after 80 90 crystallization was very evolved in fact kreepy and even in small proportions 1 5 would have a noticeable effect on the trace element chemistry of melts generated from these cumulates this trapped residual liquid would elevate total ree abundances in the cumulate pile while synchronously deepening the already negative eu anomaly essentially this trapped liquid will make the cumulate more fertile for melting to generate both kreep basalt and mare basalt magmas plagioclase entrained in the mafic cumulate pile adds an essential al component to the high ti basalt source and will moderate the requisite negative eu anomaly in the cumulate early in the evolution of the lunar mantle when the lmo still was largely liquid it is likely that vigorous convection was an important factor in crystallization such convection would allow crystals to remain suspended and in equilibrium with the lmo liquid for relatively long periods of time this extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust previous authors have proposed a three component model for the evolution of high ti mare basalt source regions this model includes kreep early olivine rich high mg cumulates and late ilmeniterich low mg cumulates in various proportions however we propose a model for high ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions this model for the high ti source includes trapped instantaneous residual liquid tirl 1 3 and entrainment of a small 2 5 proportion of plagioclase into the late stage cumulate pile in order to account for both the observed al compositions and trace element characteristics of high ti mare basalts melting of this relatively shallow ilmenite and clinopyroxene bearing late stage cumulate can generate high ti mare basalt magmas furthermore we are in agreement with other workers that only through a process of nonmodal melting will the high ti values for the parent magmas be realized large scale convective overturn of the cumulate pile and mixing of kreep with early and late stage cumulates is not required however localized overturn of the upper tenth of the cumulate pile is likely and in fact required to achieve an appropriate major element balance for the high ti mare basalt source region

ejection times of martian meteorites noble gas isotopic analyses were made for four meteorites whose origin is mars basaltic shergottites queen alexandra range 94201 zagami the iherzolite lew88516 and orthopyroxenite allan hills 84001 the latter one was separated into orthopyroxene chromite and maskelynite fractions ejection times were calculated for all martian meteorites using cosmic ray exposure ages based on 3he 21ne and 38ar concentrations and literature data on the terrestrial ages we discuss the arguments for a scenario in which they were ejected from mars by asteroidal or cometary impact as small meteoroids and were delivered to earth within and up to 15 million years the basaltic shergottites que94201 shergotty and zagami show an ejection age cluster at 2 76 ma the fourth basaltic shergottite eet79001 being younger 0 82 ma lherzolites lew88516 and alh77005 were ejected by a later event 3 84 ma ago all nakhlites nakhla governador valadares and lafayette originate from an event 11 0 ma ago although chassigny is chemically distinct from the nakhlites its ejection age is 11 6 ma whether this is a separate event cannot be decided from the available data finally orthopyroxenite alh84001 was launched from mars 14 4 ma ago the distribution of the delivery times for martian meteorites to earth reflect individual impact events onto five or six different source terrains this result is significant in the context of a comprehensive knowledge of the martian crust composition finally we derived the k 40ar gas retention age of alh84001 mineral fractions adopting two different models assuming a component of martian atmospheric ar with 40ar36ar 1790 trapped by this meteorite we obtained an age of 3390 ma on the other hand assuming no contribution of martian atmospheric ar the resulting k 40ar age is about 4100 ma that is roughly 500 ma younger than the reported sm nd crystallization age

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

stratigraphy of the lunar highland crust depths of burial of lunar samples from cooling rate studies current models of lunar crustal stratigraphy are based primarily on models of impact cratering coupled with what is known about the distribution of rock types in the ejecta blankets associated with the large multiring basins these models postulate an upper crust rich in the ferroan anorthosite component underlain by a more mafic noritic lower crust spudis 1993 we attempted to test these models by determining the depth at which various lunar plutonic rocks formed despite intense brecciation and shock many lunar highland samples retain vestiges of a prior magmatic history for example pyroxene samples preserve a record of their thermal history in such phenomena as cation ordering and exsolution textures by computer simulation of the growth of exsolution lamellae in augite and pigeonite we deduced cooling rates and from these estimated a depth of burial on the basis of a simple model for the thermal evolution of the lunar crust despite the fairly large uncertainties in the calculations it is clear that the ferroan anorthosite samples we studied formed at a depth between 10 and 20 km whereas some members of highland magnesian and alkali suites formed as cumulates in magma chambers intruded into the uppermost layers of the crust as yet there is no evidence to indicate that crystalline rocks from the lower half of the lunar crust are present in the samples returned by the apollo missions most samples derived from the lower crust appear to be impact melts oflow k fra mauro noritic composition

a halite siderite anhydrite chlorapatite assemblage in nakhla mineralogical evidence for evaporites on mars we report the results of a study of a halite siderite anhydrite chlorapatite assemblage in the shergotty nakhla chassigny snc martian meteorite nakhla these minerals are found associated with each other in interstitial areas with halite often being adjacent to or enclosing siderite we suggest the halite and other minerals are martian in origin because 1 the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place 2 textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the nakhla parent and 3 the association with siderite which previous studies have shown has c and o isotopic compositions incompatible with an origin on the earth siderite has the range of compositions caco3 0 1 5 7 mgco3 2 0 40 9 feco3 23 2 87 0 mnco3 1 0 39 9 mol there are two compositional groupings high mnco3 30 mol and low mnco3 high feco3 with a gap identified between the two this may be a miscibility gap or alternatively the two compositional groupings may mark separate generations of carbonate we have not found any textural evidence for the latter explanation but acceptance of the presence of a miscibility gap would require independent work on fe mn carbonates to verify its existence trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains siderite has light rare earth elements 2 2 7 3 c1 greater than heavy rare earth elements 0 32 0 79 c1 without ce or eu anomalies and the anhydrite has a similar pattern these abundances reflect the source composition rather than partitioning or complexing controls they are not typical of hydrothermal signatures which generally do not have such smooth ree abundance patterns the nature of the mineral assemblage suggests that its source rocks on mars were evaporites these may be common in the craters and flood plains of the martian southern highlands two models are suggested in this paper to explain the incorporation of evaporitic material into the nakhla igneous parent it may have happened as a low temperature process 200 c by crystallisation from an aqueous fluid an origin at low temperature is compatible with the available experimental data on siderite stability alternatively we suggest evaporitic material may have been incorporated into the nakhla parent while melt was still present and crystallized 800 c the latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth with residual interstitial melt both high and low temperature models are consistent with the presence of evaporite sediments on mars

timescale of overturn in a magma ocean cumulate abstract the formation and differentiation of planetary bodies are thought to involve magma oceans stages we study the case of a planetary mantle crystallizing upwards from a global magma ocean in this scenario it is often considered that the magma ocean crystallizes more rapidly than the time required for convection to develop in the solid cumulate this assumption is appealing since the temperature and composition profiles resulting from the crystallization of the magma ocean can be used as an initial condition for convection in the solid part we test here this assumption with a linear stability analysis of the density profile in the solid cumulate as crystallization proceeds the interface between the magma ocean and the solid is a phase change interface convecting matter arriving near the interface can therefore cross this boundary via melting or freezing we use a semi permeable condition at the boundary between the magma ocean and the solid to account for that phenomenon the timescale with which convection develops in the solid is found to be several orders of magnitude smaller than the time needed to crystallize the magma ocean as soon as a few hundreds kilometers of cumulate are formed on a mars to earth size planet the phase change boundary condition is found to decrease this timescale by several orders of magnitude for a moon size object the possibility of melting and freezing at the top of the cumulate allows the overturn to happen before complete crystallization the convective patterns are also affected by melting and freezing at the boundary the linearly most unstable mode is a degree 1 translation mode instead of the approximately aspect ratio one convection rolls found with classical non penetrative boundary conditions the first overturn of the crystallizing cumulate on mars and the moon could therefore be at the origin of their observed degree 1 features