antarctic lunar meteorites yamato 793169 asuka 881757 mil 05035 and met 01210 yamm launch pairing and possible cryptomare origin abstract the antarctic lunar meteorite meteorite hills met 01210 is a polymict regolith breccia dominantly composed of mare basalt components one relatively large 2 7 4 7 mm basalt clast in met 01210 met basalt shows remarkable mineralogical similarities to the lunar meteorite crystalline mare basalts yamato y 793169 asuka a 881757 and miller range mil 05035 all four basalts have similar rock texture mineral assemblage mineral composition pyroxene crystallization trend and pyroxene exsolution lamellae the estimated tio 2 contents 2 0 wt of the met basalt and mil 05035 are close to the bulk rock tio 2 contents of y 793169 and a 881757 these similarities suggest that y 793169 a 881757 mil 05035 and the met basalt came from the same basalt flow which we designate the yamm basalt the source basalt pairing of the yamm is also supported by their similar ree abundances crystallization ages approx 3 8 3 9 ga and isotopic compositions low u pb low rb sr and high sm nd the pyroxene exsolution lamellae which are unusually coarse up to a few microns by mare standards imply a relatively slow cooling in an unusually thick lava and or subsequent annealing within a cryptomare reported noble gas and cre data with close launch ages 1 ma and ejection depths deeper than several meters among the four meteorites further indicate their simultaneous ejection from the moon despite the marginally close terrestrial ages pairing in the conventional earth entry sense seems unlikely because of the remote recovery sites among the yamm meteorites the high abundance 68 of mare components in met 01210 estimated from a two component mixing model calculation could have resulted from either lateral mixing at a mare highland boundary or vertical mixing in a cryptomare the proportion of mare materials in met 01210 is greater than in apollo core samples at the mare highland boundary the burial depth several meters deep inferred from the lack of surface irradiation of met 01210 exceeds the typical mare regolith thickness a few meters thus the source of the yamm meteorites is likely a terrain of locally high mare highland mixing within a cryptomare we searched for a possible source crater of the yamm meteorites within the well defined cryptomare based on the multiple constraints obtained from this study and published data an unnamed 1 4 km diameter crater 53 w 44 5 s on the floor of the schickard crater is the most suitable source for the yamm meteorites the 238 u 204 pb value of the yamm basalts is extremely low relative to those of the apollo mare basalts but comparable to those of the luna 24 very low ti basalts the low source indicates a derivation from a less differentiated mantle with a lack of kreep components although the chemical sources of materials and heat source of melting might be independent the heat source that generated the source magma of the yamm and luna 24 basalts may not be related to kreep unlike the case of the apollo basalts the distinct chemical and isotopic compositions of mantle sources between the apollo basalts and the yamm lunar 24 basalts imply differences in mantle composition and thermal evolution between the procellarum kreep terrane pkt and non pkt regions of the nearside

in situ sims u pb dating of phanerozoic apatite with low u and high common pb abstract apatite is a common accessory mineral in various igneous metamorphic and sedimentary rocks it has strong potential to provide important information for geochronology and petrogenesis however precise u th pb dating of apatite especially young samples is difficult due to low abundances of u and less radiogenic pb posing significant challenges for accurate common pb correction an additional issue is the lack of suitable apatite to serve as a standard for in situ analyses here we present both analytical and data reduction protocols for sims in situ u pb analyses on apatite with low u and high common lead with nw 1 apatite as the u pb age standard which was separated from 1160 ma carbonatite in the prairie lake complex in canada apatites from the kovdor carbonatite in the kola peninsula which contains low u mostly 2 5 ppm and high common pb 20 80 yielded weighted average 207pb corrected u pb ages of 375 13 ma kv 8 and kv 18 and 377 11 ma kv a respectively apatites from the quruqtagh ultramafic intrusion in nw china containing low u 2 3 ppm and high common pb 70 yield 207pb corrected u pb age of 805 21 ma the cenozoic durango apatite was dated at 31 2 ma all these new apatite u pb ages are indistinguishable from independently known age constraints to within 2 4 our results demonstrate that a u pb age can be accurately and precisely measured for apatite with low u 50 by sims with a suitable standard and a careful choice of common pb composition

on the elusive isotopic composition of lunar pb highly radiogenic pb isotope compositions determined for volcanic glass beads from the apollo 14 soil sample 14163 are similar to those commonly determined for mare basalts and are correlated with chemical variations observed in the beads this indicates that pb unsupported by in situ u decay has a similar origin in both glass beads and mare basalt samples and is likely to reflect variations of 238u 204pb in the lunar mantle an alternative explanation that this pb is a result of late equilibration with the radiogenic pb present in soil is less likely as it would imply that all other characteristics of glass beads such as their chemistry must also be a consequence of equilibration near the lunar surface regardless of the origin of unsupported pb observed variations of pb isotope compositions in the glass beads and mare basalts appear to be a result of two component mixing between a primitive reservoir with a value similar to the earth s mantle and kreep with a value in excess of several thousand this range cannot be explained by the fractionation of major rock forming minerals from the crystallising lunar magma ocean and instead requires substantial extraction of sulphide late in the crystallisation sequence the proportion of sulphide required to produce the inferred range places limits on the starting of the moon prior to differentiation demanding a relatively high value of about 100 200 low indicated by several basalt samples and previously analysed volcanic glass beads can be explained by the preservation of an early but post ferroan anorthosite sulphide rich reservoir in the lunar mantle while a complete range of pb isotope compositions observed in the glass beads and mare basalts can be interpreted as mixing between this sulphide rich reservoir and kreep

compositional and temporal investigation of exposed lunar basalts in the mare imbrium region abstract this paper presents an updated stratigraphical and compositional study of the exposed maria within the imbrium basin on the moon clementine multispectral data were employed to derive tio2 and feo wt abundance estimates of potentially distinct basaltic flows additionally nasa lunar orbiter images were used to estimate flow ages using crater count statistics mare imbrium shows evidence of a complex suite of low to high ti basaltic lava units infilling the basin over an 800 million year timescale more than a third 37 of identified mare basalts were found to contain 1 3 wt tio2 two other major mare lithological units representing about 25 of the surface each show tio2 values between 3 5 and 7 9 wt the dominant fraction 55 of the sampled maria contain feo between 16 and 18 wt followed by 27 of maria having 18 20 wt and the remaining 18 14 16 wt feo a crater frequency count for diameters 500 m shows that in three quarters of the sampled mare crater counts range between 3 5 and 5 5 10 2 per km2 which translates according to a lunar cratering model chronology into estimated emplacement ages between 3 3 and 2 5 ga a compositional convergence trend between the variations of iron and titanium oxides was identified in particular for materials with tio2 and feo content broadly above 5 and 17 wt respectively suggesting a related petrogenesis and evolution according to these findings three major periods of mare infill are exposed in the imbrium basin despite each period showing a range of basaltic compositions classified according to their tio2 content it is apparent that at least within these local geological settings the igneous petrogenesis generally evolved through time towards more tio2 and feo rich melts

ion microprobe u pb dating of apatite abstract to illustrate in situ sensitive high resolution ion microprobe shrimp analysis of apatites pb isotopes and pb u ratios of five apatite samples with different ages have been measured using shrimp ii ion microprobe at hiroshima university the prap apatite from a carbonatite complex at prairie lake ontario with a 207 pb 206 pb age of 1156 45 ma 2 was used as the standard for 206 pb 238 u 238 u 16 o 238 u calibration apatites from the poohbah lake alkaline complex give 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages of 2726 145 ma 2 and 2701 86 ma 2 in agreement with a conventional tims 207 pb 206 pb age of 2667 10 ma 2 and a k ar isochron age of 2706 46 ma 2 for the remaining three apatite samples the 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages obtained by shrimp are consistent with radiometric and geological ages in the literature

a rb sr and sm nd isotope geochronology and trace element study of lunar meteorite lapaz icefield 02205 abstract rubidium strontium and samarium neodymium isotopes of lunar meteorite lapaz icefield lap 02205 are consistent with derivation of the parent magma from a source region similar to that which produced the apollo 12 low ti olivine basalts followed by mixing of the magma with small amounts 1 2 wt of trace element enriched material similar to lunar kreep rich sample sau 169 the crystallization age of lap 02205 is most precisely dated by an internal rb sr isochron of 2991 14 ma with an initial 87sr 88sr at the time of crystallization of 0 699836 0 000010 leachable ree rich phosphate phases of lap 02205 do not plot on a sm nd mineral isochron indicating contamination or open system behavior of the phosphates excluding anomalous phases from the calculation of a sm nd isochron yields a crystallization age of 2992 85 initial e143nd 2 9 0 8 that is within error of the rb sr age and in agreement with other independent age determinations for lap 02205 from ar ar and u pb methods the calculated 147sm 144nd source ratios for lap 02205 various apollo 12 and 15 basalts and samples with strong affinities to kreep sau 169 nwa 773 15386 are uncorrelated with their crystallization ages this finding does not support the involvement of a common kreep component as a heat source for lunar melting events that occurred after crystallization of the lunar magma ocean

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

determination of planetary basalt parentage a simple technique using the electron microprobe many basaltic meteorites are being discovered in old and new meteorite suites including those from cold deserts e g antarctica and hot desert environments it is important to establish the specific planetary body source proven techniques for establishing planetary parentage include stable isotopic signatures especially oxygen certain elemental ratios in bulk samples and certain elemental ratios in specific minerals some of these techniques are expensive require considerable sample preparation and are adversely affected by weathering processes on the parent body or on earth we have been seeking key major and minor elemental ratios in pyroxene olivine and feldspar that can be measured by the electron microprobe on standard thin sections these ratios may be preserved in unweathered portions of mineral grains and thus see through weathering processes in addition if the sample is too small to provide a representative bulk composition it may still have key information recorded in individual minerals we have found that some of the most useful chemical parameters are fe mn atomic in olivine or pyroxene and the percent anorthite an in plagioclase solid solutions a plot of fe mn in pyroxene and or olivine verses an defines compositional fields that are significantly different for earth mars moon 4 vesta and the angrite parent body this method may be especially powerful when combined with oxygen isotope data

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

mineral chemistry and in situ upb geochronology of the mare basalt northwest africa 10597 implications for low ti mare volcanism around 3 0 ga abstract unbrecciated mare basalts are rare in the lunar meteorite collection found in 2015 northwest africa nwa 10597 is a medium grained low ti mare basalt with a subophitic texture the meteorite consists mostly of mm sized pyroxene and plagioclase with minor olivine spinel ilmenite phosphates silica and trace zr rich minerals such as baddeleyite zirconolite and tranquillityite a portion of plagioclase and silica has been transformed to their high pressure polymorphs due to shock metamorphism nwa 10597 has a low tio2 content 2 9 wt but is relatively enriched in rare earth elements ree laaverage 65 ci with an overall unfractionated pattern except for a negative eu anomaly calculated ree concentrations of parent melts in equilibrium with mg rich pyroxene and ca rich plagioclase suggest no significant assimilation of ree rich melts after the onset of pyroxene crystallization in situ u pb analyses of baddeleyite and apatite reveal a mutually consistent age of 3 0 ga which is also in excellent agreement with that of low ti mare basalts nwa 4734 and lapaz icefield lap 02205 dated with other independent techniques the concordance suggests no significant thermal disturbance in the u pb isotopic system of nwa 10597 although it was heavily shocked nwa 10597 closely resembles nwa 4734 in terms of petrographic texture mineral chemistry and geochronology indicating a pairing relationship

mechanisms for incompatible element enrichment on the moon deduced from the lunar basaltic meteorite northwest africa 032 abstract the lunar meteorite northwest africa nwa 032 is a low ti basalt that has incompatible element abundances and th sm ratios characteristic of the involvement of late stage magma ocean crystallization products urkreep in its petrogenesis this sample is very fine grained and contains terrestrial weather products a progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain rb sr and sm nd ages although many of the leachates as well as the unleached mineral and whole rock fractions contain terrestrial alteration products selected residue fractions yield concordant rb sr and sm nd ages rubidium sr isotopic analyses yield an age of 2947 16 ma with an initial 87 sr 86 sr of 0 700057 17 these characteristics indicate nwa 032 is derived from a source region with an 87 rb 86 sr ratio of 0 044 0 001 this value is higher than all but those determined for kreep basalts and suggests that nwa 032 is derived from a source region that has higher incompatible element abundances than other low ti basalts samarium neodymium isotopic analysis yield a concordant age of 2931 92 ma and an initial e nd of 9 71 0 74 corresponding to a source region with 147 sm 144 nd ratio of 0 246 0 004 the initial nd isotopic composition stands in contrast to the initial sr isotopic composition by requiring nwa 032 to be derived from a source with lower incompatible element abundances than most low ti basalts the source of nwa 032 is therefore unlike those of other lunar basalts modeling of magma ocean cumulate formation demonstrates that unlike other low ti basalt source regions the nwa 032 source is a mixture of olivine pigeonite and clinopyroxene bearing cumulates and only a small amount of urkreep furthermore unlike other mare basalt sources the nwa 032 source does not contain appreciable quantities of plagioclase partial melting models demonstrate that the incompatible element characteristics of the nwa 032 result from formation by smaller degrees of partial melting than other mare basalts thus the incompatible element geochemical signature that is observed in nwa 032 appears to reflect the combined effects of generation from an unusual plagioclase free incompatible element depleted source region by very small degrees of partial melting this study demonstrates that both the presence of urkreep in the source region and small degrees of partial melting generate magmas with similar but not identical incompatible element characteristics in addition it underscores the fact that there is significantly more geochemical diversity on the moon than is represented by samples collected by the american and soviet lunar missions

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

los angeles the most differentiated basaltic martian meteorite los angeles is a new martian meteorite that expands the compositional range of basaltic shergottites compared to shergotty zagami que94201 and eet79001 b los angeles is more differentiated with higher concentrations of incompatible elements e g la and a higher abundance of late stage phases such as phosphates and k rich feldspathic glass the pyroxene crystallization trend starts at compositions more ferroan than in other martian basalts trace elements indicate a greater similarity to shergotty and zagami than to que94201 or eet79001 b but the mg fe ratio is low even compared to postulated parent melts of shergotty and zagami pyroxene in los angeles has 0 7 4 m thick exsolution lamellae 10 times thicker than those in shergotty and zagami opaque oxide compositions suggest a low equilibration temperature at an oxygen fugacity near the fayalite magnetite quartz buffer los angeles cooled more slowly than shergotty and zagami slow cooling coupled with the ferroan bulk composition produced abundant fine grained intergrowths of fayalite hedenbergite and silica by the breakdown of pyroxferroite shock effects in los angeles include maskelynitized plagioclase pyroxene with mosaic extinction and rare fault zones one such fault ruptured a previously decomposed zone of pyroxferroite although highly differentiated the bulk composition of los angeles is not close to the low ca si composition of the globally wind stirred soil of mars

comparative planetary mineralogy v cr al systematics in chromite as an indicator of relative oxygen fugacity we have been developing oxygen barometers based largely on the behavior of v which can occur in four valence states v 2 v 3 v 4 and v 5 and record at least 8 orders of magnitude of f o 2 our first efforts in measuring these valence proportions were by xanes techniques in basaltic glasses from earth moon and mars we now address the behavior of v valence states in chromite in basalts from these bodies with a technique that uses the electron microprobe our first insights into this new technique resulted from running electron probe traverses across spinel grains from core to rim on grains that show zoning from chromite to ulvospinel the zoning profiles showed the normal trends of core to rim decreases of cr al and mg and increases of fe ti and mn however the behavior of v was very different for moon and earth with mars in between in terrestrial basalts v 4 v 3 in lunar basalts v 3 v 4 and in martian basalts v 3 and v 4 are both significant the trends core to rim for the moon show a strong positive correlation of v and cr and negative correlation of v and ti for the earth the trends are just the opposite with a strong negative correlation for v and cr and a strong positive correlation of v and ti chromite in martian basalts showed trends somewhere in between we found that a convenient way to display these data for chromite is a plot showing the relative v cr al ratios these ratios nicely reflect the oxygen fugacity ranges for moon mars and earth

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

exploring the lunar mantle with secondary ion mass spectrometry a comparison of lunar picritic glass beads from the apollo 14 and apollo 17 sites abstract the major element characteristics of lunar picritic beads indicate that their composition approaches that of primary basaltic liquids and therefore may be our best geochemical mineralogical probes of the lunar interior the picritic magmas appear to represent partial melting of a variety of mantle mineral assemblages secondary ion mass spectrometry sims analysis of individual glass beads collected from the apollo 14 a 14 and apollo 17 a 17 landing sites shows that although there is substantial major minor tio 2 al 2 o 3 and trace element variation at each site the glasses from each site have distinguishable trace element signatures ba sr lree hree the incompatible element characteristics of glasses of nearly identical major element chemistry are strikingly different between sites the contrast between the a 14 and a 17 glasses appears to be the result of a higher kreep component in all the a 14 glasses the incorporation of evolved kreep component into high mg magmas is not a result of either assimilation or magma mixing but incorporation during the partial melting episode that produced the picritic magmas this is a more thermally efficient mechanism for incorporation of the kreep component into a picritic magma calculated mantle chemistries based upon glass compositions low degrees of partial melting and mineralogical components used in previous models suggest that the difference between a 14 and a 17 mantle sources is a 0 1 1 4 kreep intercumulate melt this difference will increase with an increase in the degree of partial melting in the model calculations this intercumulate melt may interstitially reside in an evolved high ti cumulate component the juxtaposition of evolved high ti cumulates kreep and primitive low ti cumulates mantle components has been attributed to original mantle inhomogeneities density contrasts during magma ocean crystallization and sinking of high density evolved cumulates into less dense primitive cumulates within the context of either magma ocean or serial magmatism type models regardless of the model the trace element signatures of glasses and the basalts indicate that the character of the a 14 mantle source is intrinsically different from that of the a 17 mantle source this indicates large scale lateral and vertical inhomogeneities in the lunar mantle

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

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

a petrological mineralogical and chemical analysis of the lunar mare basalt meteorite lapaz icefield 02205 02224 and 02226 lapaz icefield lap 02205 02226 and 02224 are paired stones of a crystalline basaltic lunar meteorite with a low ti 3 21 3 43 tio2 low al 9 93 10 45 al2o3 and low k 0 11 0 12 k2o composition they consist mainly of zoned pyroxene and plagioclase grains with minor ilmenite spinel and mesostasis regions large possibly xenocrystic forsteritic olivine grains 3 by mode contain small trapped multiphase melt inclusions accessory mineral and mesostasis composition shows that the samples have with silica oversaturation and late stage liquid immiscibility our section of lap 02224 has a vesicular fusion crust implying that it was at one time located sufficiently close to the lunar surface environment to have accumulated solar wind implanted gases the stones have a comparable major element composition and petrography to low ti low al basalts collected at the apollos 12 and 15 landing sites however the lap stones also have an enriched ree bulk composition and are more ferroan mg numbers in the range of 31 to 35 than similar apollo samples suggesting that they represent members of a previouslyunsampled fractionated mare basalt suite that crystallized from a relatively evolved lunar melt

cryptomare magmatism 4 35 gyr ago recorded in lunar meteorite kalahari 009 the origin and evolution of the moon remain controversial one notable point of controversy being the timing of the beginning and end of basaltic volcanism in the lunar mares or plains lunar research over the past three decades has suggested that mare volcanism occurred mainly after the late heavy bombardment that ended 3 9 billion years ago because most of known mare basalts lunar meteorites and returned apollo and luna samples and remote sensing data indicate crystallization ages younger than 3 9 billion years but a new u pb dating of minerals in the lunar meteorite kalahari 009 gives an age of roughly 4 35 billion years for a lunar basalt this suggests that the volcanism started as early as 4 35 billion years ago relatively soon after lunar formation and differentiation and that kalahari 009 is our first sample of a cryptomare from the moon the result of u pb dating of minerals in a lunar meteorite kalahari 009 is reported analyses of five grains associated with basaltic clasts give an age of 4 35 0 15 billion years these are thought to represent the crystallization ages of parental basalt magma making kalahari 009 one of the oldest known mare basalt this suggests that mare basalt volcanism on the moon started as early as 4 35 billion years ago the origin and evolution of the moon remain controversial1 2 with one of the most important questions for lunar evolution being the timing and duration of basaltic mare magmatism1 3 4 5 6 7 8 here we report the result of ion microprobe u pb dating of phosphates in a lunar meteorite kalahari 009 which is classified as a very low ti mare basalt breccia in situ analyses of five phosphate grains associated with basaltic clasts give an age of 4 35 0 15 billion years these ancient phosphate ages are thought to represent the crystallization ages of parental basalt magma making kalahari 009 one of the oldest known mare basalts we suggest that mare basalt volcanism on the moon started as early as 4 35 gyr ago relatively soon after its formation and differentiation and preceding the bulk of lunar volcanism which ensued after the late heavy bombardment around 3 8 3 9 gyr refs 7 and 8 considering the extremely low abundances of incompatible elements such as thorium and the rare earth elements in kalahari 009 ref 9 and recent remote sensing observations illustrating that the cryptomaria tend to be of very low ti basalt type10 11 12 we conclude that kalahari 009 is our first sample of a very low ti cryptomare from the moon

geochronology of the martian meteorite zagami revealed by u pb ion probe dating of accessory minerals abstract the precise chronology of geological events on mars is hampered by the lack of absolute ages for the martian timescale and the significant uncertainties that result from the extrapolation of the lunar timescale to mars hartmann and neukum 2001 martian meteorites represent the only samples of mars currently available attempts to identify source craters for the meteorites have thus far proven inconclusive hamilton et al 2003 lang et al 2009 mouginis mark and boyce 2012 precluding their use in constraining the absolute martian timescale the majority of the known martian meteorites are basalts shergottites all dated shergottites have mineral separate rb sr or sm nd ages of borg et al 2005 here we report a 238u 206pb age of 182 7 6 9 ma by ion microprobe analysis of baddeleyite zro2 grains in the zagami shergottite there is no correlation between discordancy and baddeleyite grain location relative to shock metamorphism mineral petrography demonstrates that baddeleyite is the result of late stage igneous crystallization and raman spectroscopy shows that baddeleyite has not been transformed by shock into preservable high pressure polymorphs supported by an age of 153 81 ma for phosphate grains obtained using the same method we conclude that zagami crystallized at 180 ma in agreement with previous results from mineral separate geochronology therefore the shergottites represent igneous rocks preferentially ejected from young terrains on mars in a small number of ejection events

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

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

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

clinopyroxene ree distribution coefficients for shergottites the ree content of the shergotty melt abstract ree distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred shergotty intercumulus liquid by in situ analysis with the electron microprobe using samples doped to per cent concentration levels these coefficients exhibit a strong positive correlation with pyroxene wollastonite content recommended values for augites similar in composition to the shergotty cumulus augite cores wo32en48 are d la 0 023 d ce 0 039 d nd 0 10 d sm 0 17 d eu 0 16 d gd 0 20 d yb 0 29 and d lu 0 30 recommended values for pigeonites similar in composition to the shergotty cumulus pigeonite cores wo12en58 are d la 0 002 d ce 0 004 d nd 0 019 d sm 0 031 d yb 0 13 and d lu 0 13 these values resemble distribution coefficients measured for terrestrial basalts of higher mg fe than the shergotty melt rather than those of more evolved siliceous rocks of similar mg fe to the shergotty melt thus it appears that mg fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes ree abundances were computed for the shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions these computed abundances display lree depletion such lree depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the lree enriched patterns required for a two stage sm nd evolution history this result implies that either 1 the melting process was complex e g continuous melting 2 the source region had a more complex history than that of the simple 2 stage model or 3 shergotty does not satisfy the closed system assumption of the calculation and has suffered post magmatic lree enrichment