geochemistry of diogenites still more diversity in their parental melts we report on the major and trace element abundances of 18 diogenites and o isotopes for 3 of them our analyses extend significantly the diogenite compositional range both in respect of mg rich e g meteorite hills met 00425 mgo 31 5 wt and mg poor varieties e g dhofar 700 mgo 23 wt the wide ranges of siderophile and chalcophile element abundances are well explained by the presence of inhomogeneously distributed sulfide or metal grains within the analyzed chips the behavior of incompatible elements in diogenites is more complex as exemplified by the diversity of their ree patterns apart from a few diogenite samples that contain minute amounts of phosphate and whose incompatible element abundances are unlike the orthopyroxene ones the range of incompatible element abundances and particularly the range of dy yb ratios in diogenites is best explained by the diversity of their parental melts we estimate that the feo mgo ratios of the diogenite parental melts range from about 1 4 to 3 5 and therefore largely overlap the values obtained for non cumulate eucrites our results rule out the often accepted view that all the diogenites formed from parental melts more primitive than eucrites during the crystallization of a magma ocean instead they point to a more complex history and suggest that diogenites were derived from liquids produced by the remelting of cumulates formed from the magma ocean

magnetism on the angrite parent body and the early differentiation of planetesimals angrites are among the oldest known pristine basaltic meteorites and record the earliest stages of planet formation and differentiation our paleomagnetic analysis of three angrites found that they record a past magnetic field of 10 microteslas on the angrite parent body extending from 4564 to at least 4558 million years ago because the angrite paleomagnetic record extends beyond the expected lifetime of the early circumstellar disk these paleofields were probably generated internally on the angrite parent body possibly by an early dynamo in a rapidly formed metallic core

evidence for early asteroidal collisions prior to 4 15 ga from basaltic eucrite phosphate u pb chronology abstract the late heavy bombardment lhb hypothesis wherein the terrestrial planets are thought to have suffered intense collisions ca 3 9 billion years ago is under debate coupled with new dynamical calculations re examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts as opposed to a cataclysm to better understand this collisional history records from the asteroidal meteorites are required here we report a uranium lead u pb chronological dataset for eucrite meteorites thought to originate from the asteroid 4 vesta this dataset indicates to a continuous history of collisions prior to 4 15 ga our 207pb 206pb model ages of apatite ca5 po4 3 f cl oh and merrillite ca9namg po4 7 from three brecciated basaltic eucrites juvinas 4150 3 11 6 million years ago ma merrillite only camel donga disturbed around 4570 4370 ma and stannern 4143 0 12 5 ma record multiple thermal metamorphic events during the period of 4 4 4 15 ga we interpret this to mean that vesta or the vestoid cluster underwent multiple impacts and moderate high temperature reheating during this time the ages of 4 4 4 15 ga are distinctly younger than the initial magmatic process on vesta 4 5 ga but are significantly older than a later impact peak based on some interpretations of 40ar 39ar chronologies 3 9 3 5 ga the intense collisions prior to 4 15 ga on vesta are at odds with the conventional lhb hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process conversely the model 207pb 206pb ages of apatite in the unbrecciated basaltic eucrite agoult returned an age of 4524 8 9 6 ma this may represent slow cooling from an earlier global reheating of the crust on vesta at 4 55 ga as documented by other radiometric chronologies the apatite in juvinas recorded a coincident timing of 4516 9 10 4 ma which could be due to either slow crustal cooling or impact

mn cr isotope systematics of the d orbigny angrite we have conducted a detailed study of the mn cr systematics of the angrite d orbigny here we report cr isotopic abundances and mn cr ratios in olivine pyroxene glass chromite and bulk rock samples from d orbigny 53cr excesses in these samples correlate well with their respective mn cr ratios and define an isochron with a slope that corresponds to an initial 53mn 55mn ratio 3 24 0 04 x 10 6 and initial 53cr 52cr ratio of 53 0 30 0 03 at the time of isotopic closure the 53mn 55mn ratio of the d orbigny bulk rock is more than two fold the 53mn 55mn ratio of the angrites lewis cliff 86010 lew and angra dos reis ador and implies an older mn cr age of 4562 9 0 6 ma for d orbigny relative to a pb pb age of 4557 8 0 5 ma for lew and ador one of the most unusual aspects of d orbigny is the presence of glass a phase that has not been identified in any of the other angrites the mn cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite since the mn cr age of d orbigny is 5 ma years older than the angrites lew and ador d orbigny likely represents an earlier stage in the evolution of the angrite parent body

microdistributions and petrogenetic implications of rare earth elements in polymict ureilites polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites including plagioclase enstatite feldspathic melt clasts and dark inclusions this paper investigates the microdistributions and petrogenetic implications of rare earth elements rees in three polymict ureilites elephant moraine eet 83309 eet 87720 and north haig focusing particularly on the mineral and lithic clasts not found in monomict ureilites as in monomict ureilites olivine and pyroxene are the major heavy h ree carriers in polymict ureilites they have light l ree depleted patterns with little variation in ree abundances despite large differences in major element compositions the textural and ree characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body simple ree modeling shows that the most common melt clasts in polymict ureilites can be produced by 20 30 partial melting of chondritic material leaving behind a ureilitic residue the plagioclase clasts as well as some of the high ca pyroxene grains probably represent plagioclase pyroxene rock types on the ureilite parent body however the variety of ree patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event multiple processes probably including shock melting and different sources are required to account for all the ree characteristics observed in lithic and mineral clasts the c rich matrix in polymict ureilites is lree enriched like that in monomict ureilites the occurrence of ce anomalies in c rich matrix dark inclusions and the presence of the hydration product iddingsite imply significant terrestrial weathering a search for 26mg excesses from the radioactive decay of 26al in the polymict ureilite eet 83309 was negative

early stages of core segregation recorded by fe isotopes in an asteroidal mantle ureilites displays 56 fe values higher than average chondrite 29 segregation of fe sulfide melts explains the high 56 fe values in ureilites 30 formation of a core can begin at very low degrees of melting through the circulation of a fe s melt 31 through a silicate mantle 32 33 earth and planetary science letters in press 11 3 15 34 2 35 abstract 36 37 ureilite meteorites are achondrites that are debris of the mantle of a now disrupted 38 differentiated asteroid rich in carbon they provide a unique opportunity to study the differentiation 39 processes of such a body we analyzed the iron isotopic compositions of 30 samples from the ureilite 40 parent body upb including 29 unbrecciated ureilites and one ureilitic trachyandesite alm a 41 which is at present the sole large crustal sample of the upb the 56 fe of the whole rocks fall within a 42 restricted range from 0 01 to 0 11 with an average of 0 056 0 008 which is significantly 43 higher than that of chondrites we show that this difference can be ascribed to the segregation of s 44 rich metallic melts at low degrees of melting at a temperature close to the fe fes eutectic and 45 certainly before the onset of the melting of the silicates 1100 c in agreement with the marked s 46 depletions and the siderophile element abundances of the ureilites these results point to an efficient 47 segregation of s rich metallic melts during the differentiation of small terrestrial bodies 48 49

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

the miller range 090340 and 090206 meteorites identification of new brachinite like achondrites with implications for the diversity and petrogenesis of the brachinite clan miller range mil 090340 and mil 090206 are olivine rich achondrites originally classified as ureilites we investigate their petrography mineral compositions olivine cr valences equilibration temperatures and for mil 090340 oxygen isotope compositions and compare them with ureilites and other olivine rich achondrites we conclude that they are brachinite like achondrites that provide new insights into the petrogenesis of brachinite clan meteorites mil 090340 6 has a granoblastic texture and consists of 97 modal by area olivine fo molar mg mg fe 71 3 0 6 it also contains minor to trace augite chromite chlorapatite orthopyroxene metal troilite and terrestrial fe oxides approximately 80 by area of mil 090206 5 has a granoblastic texture of olivine fo 72 3 0 1 plus minor augite and chromite similar to mil 090340 but also containing minor plagioclase the rest of the section consists of a single crystal of orthopyroxene 11 3 mm poikilitically enclosing rounded grains of olivine fo 76 1 0 6 augite chromite metal and sulfide equilibration temperatures for mil 090340 and mil 090206 calculated from olivine spinel olivine augite and two pyroxene thermometry range from 800 to 930 c in both samples symplectic intergrowths of ca poor orthopyroxene opaque phases fe oxides sulfide metal occur as rims on and veins patches within olivine before terrestrial weathering the opaques were probably mostly sulfide with minor metal all petrologic properties of mil 090340 and mil 090206 are consistent with those of brachinite clan meteorites and largely distinct from those of ureilites oxygen isotope compositions of olivine in mil 090340 18o 5 08 0 30 17o 2 44 0 21 and 17o 0 20 0 12 are also within the range of brachinite clan meteorites and well distinguished from ureilites olivine cr valences in mil 090340 and the granoblastic area of mil 090206 are 2 57 0 06 and 2 59 0 07 respectively similar to those of three brachinites also analyzed here brachina hughes 026 nova 003 they are higher than those of olivine in ureilites even those containing chromite the valence systematics of mil 090340 mil 090206 and the three analyzed brachinites lower fo more oxidized cr are consistent with previous evidence that brachinite like parent bodies were inherently more oxidized than the ureilite parent body the symplectic orthopyroxene sulfide metal assemblages in mil 090340 mil 090206 and many brachinite clan meteorites have superficial similarities to characteristic reduction rims in ureilites however they differ significantly in detail they likely formed by reaction of olivine with s rich fluids with only minor reduction mil 090340 and the granoblastic area of mil 090206 are similar in modal mineralogy and texture to most brachinites but have higher fo values typical of brachinite like achondrites the poikilitic pyroxene area of mil 090206 is more typical of brachinite like achondrites the majority of their properties suggest that mil 090340 and mil 090206 are residues of low degree partial melting the poikilitic area of mil 090206 could be a result of limited melt migration with trapping and recrystallization of a small volume of melt in the residual matrix these two samples are so similar in mineral compositions cr valence and cosmic ray exposure ages that they could be derived from the same lithologic unit on a common parent body

oxygen isotopes in asteroidal materials measurement of the variation of the 17o 16o and 18o 16o ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the solar system this chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material as sampled by meteorites in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios ultimately offering a window back to the very origin of solar system and its primordialoxygen isotopic heterogeneity oxygen is generally a major element in the gas liquid and solid phases that have interacted throughout the evolution of the solar system its chemical properties ensuring that there is abundant opportunity for reactions and exchange the details of which can be recorded in the isotopic ratios of the products such processes can be crudely subdivided into the following six categories early solar nebular isotopic heterogeneity high temperature modifi cation of nebular components metamorphism and melting in asteroids aqueous alteration in asteroids mixing of components brecciation terrestrial weathering it is clear from the isotopic variation in the earliest formed nebular components such as refractory inclusions and chondrules that there was considerable oxygen isotopic heterogeneity in the early solar nebula spanning over 50 and perhaps much greater in both 17o and 18o the origin of this variation is discussed elsewhere in this volume however isotopic exchange between the 16o rich solids and 17o and 18o rich gas phase s resulted in a wide range of isotopic variation in these early formed components offering insight into the conditions of these processes and the evolution of the reservoirs with time and or space once accreted onto the early planetisemals much of the primitive nebular components experienced considerable modification the metamorphism and melting experienced by some of these bodies most probably the result of heating from the decay of short lived radionuclides e g 26al resulted in progressive homogenization within individual bodies of any original isotopic heterogeneity complete homogenization appears to have been achieved in the hed parent body indicating very high levels of melting and the development of a large magma ocean the carbonaceous and ordinary chondrite parent bodies experienced more modest heating sufficient to mobilize water ice that accreted along with the more refractory materials the subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration such as water rock ratio temperature and isotopic signature and thus potentially the origin of the water ice the evolution of the asteroidal bodies has been far from simple and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites the result of breakup reassembly and mixing of different bodies during impact processing the final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering the isotopic compositions of the weathering products depend on location primarily latitude while the magnitude of the weathering is dependent on many factors with some meteorites e g highly reduced enstatite chondrites and the matrix rich co3s particularly susceptible to such effects

two new eucrite breccias from northwest africa this work describes two newly discovered eucrite breccias three presumably paired meteorites all named northwest africa nwa 6105 and nwa 6106 for each meteorite major and minor element compositions of minerals were determined using the electron microprobe pyroxene fe mn co variations and bulk rock oxygen isotope compositions confirm their classification as eucrites variations in mineral compositions and textures are attributed to differences in clast types present i e basaltic or cumulate eucrite the pyroxene compositions support the hypothesis that samples nwa 6105 1 6105 2 and 6105 3 are paired polymict eucritic breccias whereas sample nwa 6106 is a monomict basaltic eucritic breccia two pyroxene geothermometry yields temperatures too low for igneous crystallization the variation in temperatures among samples suggests that metamorphism occurred prior to brecciation

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

highly siderophile elements in ureilites abstract the abundances of the highly siderophile elements hse ru pd re os ir and pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples including monomict augite bearing and polymict lithologies this report adds significantly to the quantity of available pt and pd abundances in ureilites as these elements were rarely determined in previous neutron activation studies the ci normalized hse abundance patterns of all ureilites analyzed here except alha 81101 show marked depletions in the more volatile pd with ci chondrite normalized pd os ratios excluding alha 81101 averaging 0 19 0 23 2 this value is too low to be directly derived from any known chondrite group instead the hse bulk rock abundances and hse interelement ratios in ureilites can be understood as physical mixtures of two end member compositions one component best represented by sample alha 78019 is characterized by superchondritic abundances of refractory hse rhse ru re os ir and pt but subchondritic pd rhse and is consistent with residual metal after extraction of a s bearing metallic partial melt from carbonaceous chondrite like precursor materials the other component best represented by sample alha 81101 is rhse poor and has hse abundances in chondritic proportions the genesis of the second component is unclear it could represent regions within the ureilite parent body upb in which metallic phases were completely molten and partially drained or it might represent chondritic contamination that was added during disruption and brecciation of the upb removal of carbon rich melts does not seem to play an important role in ureilite petrogenesis removal of such melts would quickly deplete the ureilite precursors in re os and as au which is inconsistent with measured osmium isotope abundances and also with literature as au data for the ureilites removal of 26 al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions

phosphate fractionation and spatial patterning in ancient ruins a case study from yucatan abstract as time passes phosphorus p in soils tends to become more tightly bound with minerals phosphate fractionation enables the measurement of loosely versus tightly bound p archaeologists have used p fractionation as a chronometric technique older soils should have greater proportions of p tightly bound with minerals research at chunchucmil a large maya ruin in yucatan mexico has used extractable p concentrations and other lines of evidence to investigate ancient uses of space because contemporary maya villagers use the land among the chunchucmil ruins in a number of ways and because the ancient land surfaces are neither sealed nor deeply buried we used p fractionation to determine whether geochemical signatures of activities such as gardening resulted from ancient or modern inhabitants the fractionation results at chunchucmil are unusual in comparison to studies from other sites furthermore there are strong correlations between the sum of p fractions and carbonate we discuss potential explanations for these patterns in p fractionation data these explanations consider local pedology differential length of ancient occupation and localized dumping of the byproducts of maize processing to examine the chemical signatures of maize processing specifically soaking maize in lime water contemporary samples of sediments and residues from this activity were analyzed our results suggest that dumping water from maize soaking provides a potential explanation for the ancient p patterns

oxygen isotope evidence for rapid mixing of the hed meteorite parent body abstract the 16 o 17 o and 18 o abundances of howardites eucrites and diogenites have been used to assign them to a single hed parent body thought to be asteroid 4 vesta we report the first evidence of oxygen isotopic heterogeneity among hed meteorites indicating incompletely mixed sources new high precision oxygen isotope measurements of 34 hed meteorites reveal that most have the same 17 o consistent with a very rapid early history of large scale mixing on vesta however howardites are on average very slightly enriched in 16 o whereas ibitira caldera pasamonte and alha78132 are 16 o depleted compared to other investigated eucrites the 17 o of ibitira is completely different from all other heds measured some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of vesta others require a separate parent body or indicate that parts of the outer layer of vesta retained some primary isotopic heterogeneity the oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the hed parent body regolith

alkali magmatism on a carbonaceous chondrite planetesimal recent isotopic and paleomagnetic data point to a possible connection between carbonaceous chondrites and differentiated planetary materials suggesting the existence perhaps ephemeral of transitional objects with a layered structure whereby a metal rich core is enclosed by a silicate mantle which is itself overlain by a crust containing an outermost layer of primitive solar nebula materials this idea has not received broad support mostly because of a lack of samples in the meteoritic record that document incipient melting at the onset of planetary differentiation here we report the discovery and the petrologic isotopic characterization of uh154 11 a ferroan trachybasalt fragment enclosed in a renazzo type carbonaceous chondrite cr its chemical and oxygen isotopic compositions are consistent with very low degree partial melting of a vigarano type carbonaceous chondrite cv from the oxidized subgroup at a depth where fluid assisted metamorphism enhanced the na content its microdoleritic texture indicates crystallization at an increasing cooling rate such as would occur during magma ascent through a chondritic crust this represents direct evidence of magmatic activity in a carbonaceous asteroid on the verge of differentiating and demonstrates that some primitive outer solar system objects related to icy asteroids and comets underwent a phase of magmatic activity early in the solar system with its peculiar petrology uh154 11 can be considered the long sought first melt produced during partial differentiation of a carbonaceous chondritic planetary body bridging a previously persistent gap in differentiation processes from icy cometary bodies to fully melted iron meteorites with isotopic affinities to carbonaceous chondrites

petrology and geochemistry of the fine grained unbrecciated diogenite northwest africa 4215 we report on the petrology and geochemistry of northwest africa nwa 4215 an unbrecciated diogenite recovered in the sahara this single stone weighing 46 4 g displays a wellpreserved cumulative texture it consists of zoned xenomorphic orthopyroxene grains on the order of 500 m in size along with a few large chromite crystals 5 vol up to 3 mm accessory olivine and scarce diopside grains occur within the groundmass usually around the chromite crystals minor phases are cristobalite troilite and metal unlike other diogenites orthopyroxenes en76 2wo1 1fs22 7 to en68 6wo5 5fs25 9 olivines fo76 to fo71 and chromites mg 14 3 44 0 cr 42 2 86 5 are chemically zoned the minor element behavior in orthopyroxenes and the intricate chemical profiles obtained in chromites indicate that the zonings do not mirror the evolution of the parental melt we suggest that they resulted from reaction of the crystals with intercumulus melt in order to preserve the observed zoning profiles nwa 4215 clearly cooled significantly faster than other diogenites indeed the cooling rate determined from the diffusion of cr in olivine abutting chromite is in the order of 10 50 c a suggesting that nwa 4215 formed within a small shallow intrusion the bulk composition of nwa 4215 has been determined for major and trace elements this meteorite is weathered and its fractures are filled with calcite limonite and gypsum typical of hot desert alteration in particular the feo cao abundances and most of the trace element concentrations sr ba pb and ree among others are high and indicate a significant contribution from the secondary minerals to remove the terrestrial contribution we have leached with hcl a subsample of the meteorite the residue made essentially of orthopyroxene and chromite has similar major and trace element abundances to diogenites as shown by the shape of its ree pattern or by its high al ga ratio the connection of nwa 4215 with diogenites is confirmed by its o isotopic composition 17o 1 431 0 102 18o 3 203 0 205 17o 0 248 0 005

a model composition of the basaltic achondrite planetoid the basaltic achondrites eucrites diogenites and howardites have compositions on a common oxygen isotope mass fractionation line and probably formed from a chondritic precursor also lying on that same line no chondritic meteorite group has the same isotopic signature as the basaltic achondrites so the oxygen isotope ratios of several known chondritic groups were used to construct a two component mixing model for the composition of the precursor this model does not provide a unique solution as several mixtures of ordinary and carbonaceous precursors will satisfy the isotopic constraints the fe mn mg abundances of the precursors and of the eucrites were used to provide an additional constraint the precursor composition selected for study is a mixture of 70 wt h chondrite with 30 wt cm chondrtte this mixture generates a slightly feo rich silicate precursor that after reduction and separation of an iron sulfide core is compatible with the mantle of the basaltic achondrite planetoid bap having a similar composition to that modeled by dreibus and wanke 1980 partial melting experiments of this h cm precursor composition suggest that eucritic magmas could be formed in such a mantle these experiments also suggest that the mantle must have experienced metal loss to constrain the fe mn ratios and probably significant olivine fractionation as well diogenite precursors may also be generated in this mantle composition as feo reduction and olivine fractionation lead to the formation of sio2 enriched compositions from which diogenite source magmas may be extracted if mixing of material from two very distinct chondritic reservoirs h and cm chondrites is realistic then an asteroid scale mixing process is needed to generate the achondrite precursor large impact events would provide a plausible method for mixing material from reservoirs with quite different oxygen isotope characteristics to assemble the basaltic achondrite planetoid

core formation in asteroids chemical arguments based on the s concentrations in the metallic magmas that formed magmatic iron meteorites and on the lack of pyroxene in pallasites suggest that core formation in asteroids was accompanied by 40 melting of the associated silicates the existence of plagioclase free residues from partial melting such as lodranite meteorites indicates that metal did not segregate from silicates when the amount of partial melting was only 15 thus core formation in asteroids requires substantial melting of an asteroid high interfacial energies between metallic melts and silicates require that metallic spherules sink through partially molten silicates to form cores in asteroids and planetesimals the globule sinking mechanism is not very efficient because silicate crystal liquid mixtures have high yield strengths making it difficult for the globules to sink calculations suggest that the melt fraction must reach 50 before globules can sink alternatively porous flow of molten metal is possible when the amount of metal is high enough to form an interconnected network about 50 this can be achieved by high initial abundances of metal or by gradual removal of silicate melt thereby enriching the residue in the metal sulfide assemblage this also requires removal of 50 silicate partial melt though not necessarily all at once such high amounts of melting were not always attained in asteroids so many asteroids probably consist of partially differentiated silicates and metallic masses that did not segregate to a core s asteroids might represent such partially differentiated bodies

dynamics of metal silicate separation in a terrestrial magma ocean in a terrestrial magma ocean the metal silicate separation involved small metal droplets our goal is to better understand the dynamics of the metal droplet scenario the mechanism of sedimentation in a vigorously convecting and strongly rotating magma ocean may differ significantly from settling droplets in a still fluid in order to systematically study the parameter dependence on the style of motion we utilize two dimensional and three dimensional numerical convection models combined with a tracer based sedimentation method we investigate the characteristic flow patterns resulting from the competing effects of convection and droplet settling and find three styles of motion a temperature dominated style where most droplets remain suspended a droplet dominated style where the droplets separate from the fluid and a style of repetitive motion we find that the droplet dominated style is relevant to the magma ocean in this scenario the droplets settle and form a dense bottom layer one of the key findings of this work is that the formation of the dense bottom layer and therefore the separation of metal droplets from the liquid silicate occurs on a characteristic timescale which is identical with the stokes settling time finally we study the chemical interaction of settling metal droplets and liquid silicate and discuss how accurately a simple parameterized model can describe the chemical equilibration in the metal rain scenario of a terrestrial magma ocean we find agreement between the parameterized model and our two dimensional model our simulations which include thermal convection into the metal rain model confirm the fundamental observation that metal droplets are not permanently suspended in the convecting silicate melt but that they sink and rapidly form a dense layer at the base of the magma ocean

chronology of the angrite parent body and implications for core formation in protoplanets abstract angrites formed by some of the earliest igneous activity in the solar system and provide insights into the early stages of planetary melting and differentiation moreover they are pivotal reference points for early solar system chronology in order to study the processes and timescales of metal segregation in early protoplanets and to assess the distribution of short lived radionuclides in the early solar system the 182 hf 182 w system was applied to a comprehensive suite of angrites 182 hf 182 w isochron ages for angrites are in excellent agreement with previously reported 207 pb 206 pb and 53 mn 53 cr results but are 1 myr older than ages obtained from 26 al 26 mg chronometry these inconsistencies probably reflect a disturbance of the al mg system in the angrite feldspars but could alternatively be explained by a heterogeneous distribution of 26 al in the early solar system based on the hf w results four texturally and temporally resolved groups of angrites can be identified that were derived from at least two distinct mantle sources these mantle sources are the result of separate events of core formation both of which took place within 2 myr of cai formation thus core formation in the angrite parent body did not occur as a single event of metal segregation from a global magma ocean but rather took place under varying conditions by several more local events the disparate hf w systematics of the two distinct angrite source regions indicate that convection in the magma ocean was inefficient in homogenizing the composition of the mantle possibly as a result of a continuous bombardment with small planetesimals during ongoing core formation such impacts could have constantly removed primordial and earlier formed crust facilitating rapid cooling of the magma ocean which solidified as early as 3 6 0 7 myr after cai formation

lead and mg isotopic age constraints on the evolution of the hed parent body the large collection of howardite eucrite diogenite hed meteorites allows us to study the initial magmatic differentiation of a planetesimal we report pb pb ages of the unequilibrated north west africa nwa 4215 and dhofar 700 diogenite meteorites and their mass independent 26mg isotope compositions 26mg to better understand the timing of differentiation and crystallization of their source reservoir s nwa 4215 defines a pb pb age of 4484 5 7 9 myr and has a 26mg excess of 2 3 1 6 ppm whereas dhofar 700 has a pb pb age of 4546 4 4 7 myr and a 26mg excess of 25 5 1 9 ppm we interpret the young age of nwa 4215 as a thermal overprint but the age of dhofar 700 is interpreted to represent a primary crystallization age combining our new data with published mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1 2 myr of our solar system consistent with a short lived early formed magma ocean undergoing convective cooling the other half of the diogenites including both nwa 4215 and dhofar 700 are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least 20 myr

spectral properties of angrites angrites are generally believed to be fragments of a basaltic asteroid that differentiated under relatively oxidizing conditions almost all angrites e g d orbigny lewis cliff lew 86010 and sahara 99555 are composed predominately of anorthite al ti diopside hedenbergite and ca rich olivine except for the type specimen angra dos reis which is composed almost entirely of al ti diopside hedenbergite d orbigny lew 86010 and sahara 99555 also have spectral properties very different from angra dos reis these newly measured angrites all have broad absorption features centered near 1 m with very weak to absent absorption bands at 2 m which is characteristic of some clinopyroxenes the spectrum of angra dos reis has the characteristic 1 and 2 m features due to pyroxene one asteroid 3819 robinson has similar spectral properties to the newly measured angrites in the visible wavelength region but does not appear to spectrally match these angrites in the near infrared

zinc isotopes in heds clues to the formation of 4 vesta and the unique composition of pecora escarpment 82502 the 66zn permil deviation of the 66zn 64zn ratio from a terrestrial standard values for a suite of 20 non antarctic hed howardite eucrite diogenite meteorites and one mesosiderite and for 8 antarctic eucrites and diogenites were measured in order to determine the role of volatization in the formation of their presumed parent body the asteroid 4 vesta the 20 non antarctic heds had 66zn values that ranged from 2 0 to 1 67 with a mean value of 0 01 0 39 2se this range likely represents a small scale heterogeneity due to brecciation induced by multiple impacts the non antarctic eucrites 66zn 0 00 0 58 2se n 12 were isotopically the same as the diogenites 66zn 0 31 0 80 2se n 4 and the howardites 66zn 0 26 0 37 2se n 4 on average non antarctic eucrite falls were isotopically heavier 0 50 than non antarctic finds 1 00 the antarctic finds studied were all unbrecciated samples and they were significantly heavier than the non antarctic samples with a 66zn range of 1 63 to 6 22 for 4 eucrites mean 4 32 and 0 94 to 1 60 for 3 diogenites mean 1 23 excluding one anomalous sample while their zn concentration is significantly lower than the brecciated samples these data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid at which time volatization of lighter zinc isotopes led to an isotopically heavy crust early impact events caused the ejection of these unbrecciated meteorites which were subsequently spared from brecciation caused by multiple additional impacts on the much larger vesta the range of 66zn values and zn concentration for the brecciated heds in this study supports a major contribution to the vestan surface by chondritic impactors 1 30 66zn 0 76 for ordinary and carbonaceous chondrites the anomalous eucrite pca 82502 66zn 7 75 is significantly isotopically lighter than the other heds and is the natural sample with the lightest zn isotopic composition reported in the solar system to date this meteorite most likely originated from a distinct parent body

comment on john s stone a possible fragment of the 1908 tunguska meteorite anfinogenov et al 2014 icarus 243 139 147 anfinogenov et al anfinogenov et al 2014 icarus 243 139 147 argue that a highly silicified gravelite sandstone referred to as john s stone could be a fragment of the long sought after impactor from the 1908 tunguska event we have analyzed the oxygen isotopic composition of two distinct samples of john s stone both kindly provided by dr yana anfingenova both samples had very similar isotopic compositions such that the average of the two was as follows 17o 6 67 0 15 18o 12 78 0 28 17o 0 01 0 01 errors 1sd this value is well within the range for samples of terrestrial quartz bearing sedimentary and metamorphic rocks john s stone has an oxygen isotope composition and mineralogy that is unlike any known group of meteorites if john s stone is extraterrestrial this would imply that it represents a previously unknown type of meteorite as suggested by anfinogenov et al however since sandstones can only form on a parent body with liquid water and by inference also an atmosphere we argue that there are only two possible parent bodies in the solar system the earth and mars mars can be ruled out for several reasons including the oxygen isotopic and noble gas composition of john s stone which is clearly distinct from martian meteorites consistent with the o study the noble gases do not provide any evidence for an extraterrestrial origin of the samples the lack of any cosmogenic noble gases particularly striking in 3he 21ne 38ar is consistent with a terrestrial origin or an extraterrestrial origin under large shielding based on the combined evidence obtained in this study we infer that john s stone is a terrestrial rock unrelated to the tunguska impactor

late accretion as a natural consequence of planetary growth the mantles of the terrestrial planets contain elemental abundances that suggest accretion continued at a late stage after core formation geochemical data of meteorites from differentiated asteroids are consistent with such a late accretion event suggesting that the phenomenon occurred throughout the solar system and was related to planet formation

oxygen isotope and petrological study of silicate inclusions in iie iron meteorites and their relationship with h chondrites abstract the origin of silicate bearing irons especially those in groups iab iiicd and iie is poorly understood as silicate should have separated rapidly from molten metal here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group iie meteorites and a petrological study of silicate inclusions in ten iie irons including those in garhi yasin and tarahumara which have not been described in detail before oxygen isotopes have also been analysed in 20 h chondrites to investigate their possible relationship with the iie irons based on petrographic observations and mineral analysis the silicate bearing iie meteorites have been divided into four types according to the nature of their silicate inclusions 1 primitive chondritic 2 evolved chondritic 3 differentiated with 10 vol orthopyroxene and 4 differentiated with 17 o variation a wide range of values is seen in the dataset as a whole group iie irons with differentiated silicates with the exception of colomera have a range of mean 17 o values that is essentially identical to those of the h4 6 chondrites 0 60 0 77 and 0 61 0 76 respectively colomera inclusions which are differentiated with 17 o value and plot 2 away from the next nearest iie iron however in view of the textural similarities to other iie inclusions a separate source for colomera is deemed unlikely three iie irons with primitive chondritic inclusions garhi yasin netschaevo and techado have relatively low mean 17 o values of 0 56 0 57 as well as relatively reduced silicates with fa 15 17 olivine which have been called hh chondrites given the significant overlap in their oxygen isotope compositions a genetic relationship between iie irons and h chondrites is supported by our new data however derivation of both groups from one parent body seems unlikely instead both groups probably sampled similar precursor materials and accreted at a similar nebular location our data suggest that the iie meteorites formed on an internally heated h hh chondrite like body that experienced the initial stages of differentiation in response to radiogenic heating however prior to full differentiation the iie parent body experienced a major hit and run style collision that resulted in silicate metal mixing the initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates reassembly of the iie parent body produced a large regolith blanket that facilitated subsequent slow cooling the iie parent body has probably experienced numerous subsequent less catastrophic collisions the development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events

oxygen isotope variation in stony iron meteorites asteroidal material delivered to earth as meteorites preserves a record of the earliest stages of planetary formation high precision oxygen isotope analyses for the two major groups of stony iron meteorites main group pallasites and mesosiderites demonstrate that each group is from a distinct asteroidal source mesosiderites are isotopically identical to the howardite eucrite diogenite clan and like them are probably derived from the asteroid 4 vesta main group pallasites represent intermixed core mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites the stony iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early solar system

the heating history of vesta and the onset of differentiation in this work we study the link between the evolution of the internal structure of vesta and thermal heating due to 26 al and 60 fe and long lived radionuclides taking into account the chemical differentiation of the body and the affinity of 26 al with silicates we explored several thermal and structural scenarios differing in the available strength of energy due to the radiogenic heating and in the postsintering macroporosity by comparing them with the data supplied by the heds and the dawn nasa mission we use our results to constrain the accretion and differentiation time as well as the physical properties of the core differentiation takes place in all scenarios in which vesta completes its accretion in 1 4 ma after the injection of 26 al into the solar nebula in all those scenarios where vesta completes its formation in 1 ma from the injection of 26 al the degree of silicate melting reaches 100 vol throughout the whole asteroid if vesta completed its formation between 1 and 1 4 ma after 26 al injection the degree of silicate melting exceeds 50 vol over the whole asteroid but reaches 100 vol only in the hottest outermost part of the mantle in all scenarios where the porosity is lower than 5 vol if the formation of vesta occurred later than 1 5 ma after the injection of 26 al the degree of silicate melting is always lower than 50 vol and is limited only to a small region of the asteroid the radiation at the surface dominates the evolution of the crust which ranges in thickness from 8 to about 30 km after 5 ma a layer about 3 20 km thick is composed of primitive unmelted chondritic material while a layer of about 5 10 km is eucritic

partial melting and melt migration in the acapulcoite lodranite parent body we review petrologic evidence that the acapulcoites and lodranites formed by 1 vol to probably 20 vol whole rock partial melting of a chondritic precursor material at low degrees of partial melting only fe ni fes cotectic melting occurred migration distances for partial melts were short resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents but achondritic textures at high degrees of partial melting both fe ni fes and basaltic plagioclase pyroxene partial melts formed and the melts may have migrated out of the source rock the partial melt residues which are more or less depleted in fe ni fes and plagioclase are the lodranites melt migration was complex most acapulcoites which experienced relatively low degrees of partial melting lost little if any of the partial melt one acapulcoite lew 86220 represents a unique case in which fe ni fes and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region where they were trapped in cases of the relatively high degrees of partial melting experienced by lodranites melts may have been partly selectively or totally removed from the rocks and fe ni fes and or basaltic partial melts may have been removed to different degrees and may in fact have been trapped on occasion in greater than chondritic proportions we model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting our calculations are broadly consistent with observations indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting although the size of the acapulcoite lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained the lack of basaltic rocks in the world s meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by wilson and keil 1991 and ejected into space the diversity of rocks from the acapulcoite lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the s type asteroids

widespread magma oceans on asteroidal bodies in the early solar system our solar system formed about 4 6 billion years ago and within 4 million years small planetary bodies had formed some melting to form volcanic and related rocks two families of meteorites the heds and angrites are thought to have originated from asteroids that melted at this time new oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large scale melting in the early solar system these new results show that early global scale melting was a feature of all the differentiated planets earth moon and mars and asteroids so far sampled immediately following the formation of the solar system small planetary bodies accreted1 some of which melted to produce igneous rocks2 3 over a longer timescale 15 33 myr the inner planets grew by incorporation of these smaller objects4 5 through collisions processes operating on such asteroids strongly influenced the final composition of these planets4 including earth5 currently there is little agreement about the nature of asteroidal igneous activity proposals range from small scale melting to near total fusion and the formation of deep magma oceans2 here we report a study of oxygen isotopes in two basaltic meteorite suites the heds howardites eucrites and diogenites which are thought to sample the asteroid 4 vesta6 and the angrites from an unidentified asteroidal source our results demonstrate that these meteorite suites formed during early global scale melting 50 per cent events we show that magma oceans were present on all the differentiated solar system bodies so far sampled magma oceans produced compositionally layered planetesimals the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features such as earth s high mg si ratio

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

hydrological regime and water budget of the red river delta northern vietnam abstract the red river delta rrd in northern vietnam represents a complex hydrological network of tributaries and distributaries that receive a large and seasonally fluctuating flow of water from the upper red river basin and is also subjected to tidal influence in this study we attempted to assemble a database of discharge estimates within the rrd for 1996 2006 to elucidate the water circulation patterns in the system enable quantification of major water fluxes and assess the water resources availability regular discharge measurements in the rrd are available for three upstream stations while the other hydrological stations provide only water level records however the mike 11 model allowed overall calibration curves to be established which enabled the conversion of available daily mean water level data into discharge values four gauging surveys were conducted under flood and dry season in 2007 and 2008 to experimentally validate these calibration curves after the database was generated a water balance was established for two years with contrasting climatic and hydrological characteristics during the wet year 1996 the main branch of the red river represented the largest input of freshwater to the sea approximately 60 conversely during the dry year 2006 the inputs were more evenly distributed among the three main fluvial branches the total volume annually delivered to the sea from the rrd was approximately 140 and 100 km3 for 1996 and 2006 respectively when the five sub basins within the rrd were evaluated it was shown that the water resources were far from evenly distributed within the area in particular the bui sub basin which has the highest population density and the lowest water resources per unit area is experiencing a critical situation in terms of pressure on water resources

complex origins of silicate veinlets in hed meteorites a case study of northwest africa 1109 we report on the petrography and mineralogy of three types of silicate veinlets in the brecciated eucrite northwest africa nwa 1109 these include fe rich olivine mg rich olivine and pyroxene veinlets the fe rich olivine veinlets mainly infill fractures in pyroxene and also occur along grain boundaries between pyroxene and plagioclase crystals in both nonequilibrated and equilibrated lithic clasts the host pyroxene of fe rich olivine veinlets shows large chemical variations between and within grains the fe rich olivine veinlets also contain fine grained fe3 bearing chromite highly calcic plagioclase merrillite apatite and troilite based on texture and mineral chemistry we argue that the formation of fe rich olivine was related to fluid deposition at relatively high temperatures however the source of fe rich olivine in the veinlets remains unclear magnesium rich olivine veinlets were found in three diogenitic lithic clasts in one of these the mg rich olivine veinlets only occur in one of the fine grained interstitial regions and extend into fractures within surrounding coarse grained orthopyroxene based on the texture of the interstitial materials we suggest that the mg rich olivine veinlets formed by shock induced localized melting and recrystallization pyroxene veinlets were only observed in one clast where they infill fractures within large plagioclase grains and are associated with fine grained pyroxene surrounding coarse grained pyroxene the large chemical variations in pyroxene and the fracture filling texture indicate that the pyroxene veinlets might also have formed by shock induced localized melting and rapid crystallization our study demonstrates that silicate veinlets formed by a range of different surface processes on the surface of vesta

thermal history of northwest africa 5073 a coarse grained stannern trend eucrite containing cm sized pyroxenes and large zircon grains we report on the bulk chemical composition petrology oxygen isotopic composition trace element composition of silicates and degree of self irradiation damage on zircon grains of the eucrite northwest africa nwa 5073 to constrain its formation and postcrystallization thermal history and to discuss their implications for the geologic history of its parent body this unequilibrated and unbrecciated meteorite is a new member of the rare stannern trend eucrites it is mainly composed of elongated zoned pyroxene phenocrysts up to 1 2 cm plagioclase laths up to 0 3 cm in length and is rich in mesostasis the latter contains zircon grains up to 30 m in diameter metal sulfide tridymite and ca phosphates textural observations and silicate compositions coupled with the occurrence of extraordinary fe rich olivine veins that are restricted to large pyroxene laths indicate that nwa 5073 underwent a complex thermal history this is also supported by the annealed state of zircon grains inferred from raman spectroscopic measurements along with u and th data obtained by electron probe microanalyses

vesta s mineralogical composition as revealed by the visible and infrared spectrometer on dawn the dawn spacecraft mission has provided extensive new and detailed data on vesta that confirm and strengthen the vesta howardite eucrite diogenite hed meteorite link and the concept that vesta is differentiated as derived from earlier telescopic observations here we present results derived by newly calibrated spectra of vesta the comparison between data from the dawn imaging spectrometer vir and the different class of hed meteorites shows that average spectrum of vesta resembles howardite spectra nevertheless the vesta spectra at high spatial resolution reveal variations in the distribution of hed like mineralogies on the asteroid the data have been used to derive hed distribution on vesta reported in ammannito et al 2013 and to compute the average vestan spectra of the different hed lithologies reported here the spectra indicate that not only are all the different hed lithologies present on vesta but also carbonaceous chondritic material which constitutes the most abundant inclusion type found in howardites is widespread however the hydration feature used to identify carbonaceous chondrite material varies significantly on vesta revealing different band shapes the characteristic of these hydration features cannot be explained solely by infalling of carbonaceous chondrite meteorites and other possible origins must be considered the relative proportion of heds on vesta s surface is computed and results show that most of the vestan surface is compatible with eucrite rich howardites and or cumulate or polymict eucrites a very small percentage of surface is covered by diogenite and basaltic eucrite terrains are relatively few compared with the abundance of basaltic eucrites in the hed suite the largest abundance of diogenitic material is found in the rheasilvia region a deep basin where it clearly occurs below a basaltic upper crust however diogenite is also found elsewhere although the depth to diogenite is consistent with one magma ocean model its lateral extent is not well constrained

origin of mass independent oxygen isotope variation among ureilites clues from chondrites and primitive achondrites ureilite meteorites are abundant carbon rich primitive achondrites made of coarse grained equilibrated olivine and pyroxene usually pigeonite they probably sample the baked heterogeneous melt depleted mantle of a large once chondritic parent body that was broken up catastrophically while still young and hot heterogeneity in the parent body is inferred from a considerable slope 1 variation from one meteorite to another in oxygen isotopes 2 5 17o 0 2 which correlates with both molar feo mgo range 0 03 0 35 and molar feo mno range 3 57 i e 17o correlates with the redox state no consensus has yet emerged on the cause of these correlated trends one view favors their inheritance via silicates from hot nebular preaccretion processes another invokes smelting reduction of feo by c in the hot parent body here guided mainly by similar trends among equilibrated ordinary and r chondrites studies of their unequilibrated counterparts and work on other primitive achondrites we propose a new model for ureilites in which the parent body accreted nebular ice with high 17o mg rich silicates with low 17o and varying amounts of metallic iron water from the thawing ice then oxidized the metal yielding secondary feo bearing minerals with high 17o that with metamorphism became incorporated into the ureilite silicates feo mgo feo mno and 17o correlate because they rose in unison by amounts that varied spatially depending on the local amount of metal that was oxidized we suggest that the parent body was so large radius 100 km that smelting was inhibited and that carbon played a passive role in ureilite evolution although ureilites are regarded as complicated meteorites we believe our analysis explains their mass independent oxygen isotope trend and related feo variation through well understood processes and enlightens our understanding of the evolution of early planetesimals from cold wet bodies to hot dry ones

reassessing the origin and chronology of the unique achondrite asuka 881394 implications for distribution of 26al in the early solar system abstract the achondrite asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically chemically and texturally distinct from them previous u pb chronology shows that it is very old forming within the first 0 8 ma of the formation of calcium aluminum rich inclusions cais however the age difference between asuka 881394 and other very old solar system materials cais quenched angrites measured with the 26al 26mg and 53mn 53cr extinct radionuclide chronometers and 207pb 206pb chronometer is not the same this could be interpreted to reflect heterogeneity in the distribution of 26al and 53mn in the early solar system given the significant implications for the early solar system chronology if 26al and 53mn are indeed heterogeneously distributed in this study we further investigate the origin of asuka 881394 and the apparent age discordance between short lived and absolute chronometers by focusing on measurement of its e54cr composition renewed measurements of the absolute pb pb age and new high precision measurements of its 26al 26mg systematics new cr isotope data places additional constraints on the origin of asuka 881394 its e 54cr value of 0 37 0 10e is resolvable outside of uncertainty from hed meteorites 0 72 0 10e reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body unlikely to be 4 vesta new pb pb analyses combined with using a directly measured 238u 235u ratio for age calculation result in a recalculated pb pb age of 4564 95 0 53 ma 1 5 ma younger than the previously reported age with this age asuka 881394 remains one of the oldest known achondrites in our solar system new high precision 26al 26mg data produce an initial 26al 27al ratio of 1 48 0 12 10 6 within error of previous data this ratio corresponds to an al mg age of 4563 69 0 36 ma or 4564 83 0 21 ma relative to cais and the d orbigny angrite respectively thus depending on which age anchor is used the 26al 26mg age is either 1 26 ma or 0 12 ma younger than the new pb pb age the latter being unresolved within analytical uncertainty though a potential age discrepancy between 26al 26mg and pb pb could be a result of heterogeneous distribution of 26al we demonstrate with our new high precision mg isotope data in conjunction with petrographic evidence that the mg isotope system has been disturbed in asuka 881394 we suggest that the 26al 26mg system closed to diffusion after the u pb system either due to slow cooling on the parent body or low grade metamorphic re equilibration of mg thus we can satisfactorily explain the observed age discrepancy between 26al 26mg and u pb systems in asuka 881394 without invoking heterogeneous distribution of 26al 27al ratio in the early solar system comparison of the asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26al 27al ratio because these achondrites have significant differences in their e54cr 17o systematics which could be indicative of location within the early protoplanetary disk it is consistent with homogeneous distribution of 26al in the early solar system angrites remain an outlier either because they evolved from a source with a lower 26al 27al ratio or because their 26al 26mg or pb pb data are problematic in either case this suggests that basaltic angrites are questionable as the age anchor for the entire solar system as a whole and other very old well preserved achondrites should be considered for that role

an experimental assessment of the potential of sulfide saturation of the source regions of eucrites and angrites implications for asteroidal models of core formation late accretion and volatile element depletions abstract the geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies although sulfides are important reservoirs for a diverse suite of major and trace elements it is currently unclear whether the interiors of asteroid vesta and the angrite parent body were sulfide liquid saturated during petrogenesis of non cumulate eucrites and volcanic angrites to assess the potential of sulfide liquid saturation in the interiors of these bodies high pressure p temperature t experiments were used to quantify the sulfur concentrations at sulfide saturation scss for volcanic angrites and non cumulate eucrites the sulfide silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation it was found that the measured scss values agree well with the scss values predicted from a previous thermodynamic model for high feo melts to assess the possibility of sulfide liquid saturation of the source regions of non cumulate eucrites and angrites their s abundances were compared with the calculated scss values for their source regions results show that if eucritic and angritic source regions were saturated with fes liquid significant degassing 50 80 of s must have occurred during or following their magmatic emplacement such loss is inconsistent with the s cl zn and rb isotopic compositions of non cumulate eucrites sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of cu and hse in non cumulate eucrites and angrites riches et al 2012 additional calculations were performed to further explore the timing and extent of s loss during crystallization of the vestan magma ocean the assumption of chondritic bulk s abundances of bulk vesta would correspond with extremely high s contents of the eucrite source region s even after consideration of depletion of s due to core formation in light of the s cl zn and rb stable isotopic compositions of eucrites the s abundances in eucrites are most consistent with the hypothesis that the vestan mantle was already strongly depleted in s 70 80 by the time of vestan magma ocean crystallization resulting in more realistic s contents of the eucrite source region s the depletion of s could have been established during initial accretion of vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation wu et al 2018 modeling of the new experimentally determined sulfide silicate partition coefficients and previously reported vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early vestan magma ocean crystallization is also unlikely the lack of sulfide liquid saturation in the source regions of non cumulate eucrites and angrites as well as during early vestan magma ocean solidification shows that current geochemical models of core formation and late accretion remain valid for these bodies

differentiation and magmatic activity in vesta evidenced by 26al 26mg dating in eucrites and diogenites abstract the 26 al 26 mg short lived chronometer has been widely used for dating ancient objects in studying the early solar system here we use this chronometer to investigate and refine the geological history of the asteroid 4 vesta ten meteorites widely believed to come from vesta 4 basaltic eucrites 3 cumulate eucrites and 3 diogenites and the unique achondrite asuka 881394 were selected for this study all samples were analyzed for their 26 mg and 27 al 24 mg ratios in order to construct both whole rock and model whole rock isochrons mineral separation was performed on 8 of the hed s in order to obtain internal isochrons while whole rock al mg analyses of hed s plot on a regression that could be interpreted as a vestan planetary isochron internal mineral isochrons indicate a more complex history crystallization ages obtained from internal 26 al 26 mg systematic in basaltic eucrites show that vesta s upper crust was formed during a short period of magmatic activity at 2 66 0 58 1 39 million years ma after calcium aluminum inclusions after cai we also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of vesta while 26 al was still extant cumulate eucrites crystallized progressively from 5 48 0 60 1 56 to 7 25 ma after cai model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2 88 0 12 0 14 ma after cai i e contemporaneously from the onset of the basaltic eucritic crust based on their cumulate texture we suggest cumulate eucrites were likely formed deeper in the crust of vesta diogenites have a more complicated history and their 26 al 26 mg systematics show that they likely formed after the complete decay of 26 al and thus are younger than eucrites this refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4 vesta from which the basaltic eucrites rapidly crystallized in order to explain the younger age and the complex history of diogenites we postulate that a second episode of magmatism was possibly triggered by a mantle overturn we bring a refined chronology of the geological history of vesta that shows that the asteroid has known a more complex differentiation than previously thought

vesta and extensively melted asteroids why hed meteorites are probably not from vesta abstract most researchers hold that the hed clan of differentiated meteorites originated on vesta largely based on the assumption that nearly all v type asteroids with basaltic reflection spectra are fragments spalled off vesta although it is a reasonable working hypothesis that most of the v type asteroids in the vesta family originated on vesta the spectra are not unique enough to confirm this a sizable fraction may have been produced during the destruction of a differentiated asteroid in the same large region of dynamic space observations of asteroids in the inner asteroid belt show that more than half of the v type asteroids do not belong to the vesta dynamic family iron meteorite evidence shows that at least 26 asteroids experienced extensive melting and would have generated basalts and other differentiated stony meteorites most iron meteorites show high degrees of elemental fractionations that lead to the conclusion that they experienced fractional crystallization it is probable that all these bodies generated basalts there are 9 of these magmatic iron meteorite groups and test criteria mainly based on extreme fractionations indicate that an additional 17 disrupted asteroids hosted fractionally crystallized cores and thus that 26 asteroids experienced extensive melting this estimate is much lower than previous estimates that included nonmagmatic irons within expected planetary heterogeneities the o isotopic composition of heds is the same as that in oxides from iiiab irons the largest magmatic group of iron meteorites e cr 54 values are also very similar in iiiabs and heds the o and cr isotopic ties are much stronger than the spectral tie thus the working hypothesis should be that heds are from the iiiab parent asteroid remote elemental analysis could confirm that heds are not from vesta if future remote analysis measures k contents 0 6 mg g this will indicate that heds did not originate on vesta

presidential address presented 2000 august 28 chicago illinois usa the eucrite vesta story many lines of evidence indicate that meteorites are derived from the asteroid belt but in general identifying any meteorite class with a particular asteroid has been problematical one exception is asteroid 4 vesta where a strong case can be made that it is the ultimate source of the howardite eucrite diogenite hed family of basaltic achondrites visible and near infrared reflectance spectra first suggested a connection between vesta and the basaltic achondrites experimental petrology demonstrated that the eucrites the relatively unaltered and unmixed basaltic achondrites were the product of approximately a 10 melt studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal scale magma ocean mass balance considerations point to a parent body that had its surface excavated but remains intact modern telescopic spectroscopy has identified kilometer scale vestoids between vesta and the 3 1 orbit orbit resonance with jupiter dynamical simulations of impact into vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations hubble space telescope images show a 460 km diameter impact basin at the south pole of vesta it seems that nature has provided multiple free sample return missions to a unique asteroid major challenges are to establish the geologic context of the hed meteorites on the surface of vesta and to connect the remaining meteorites to specific asteroids

impacts on asteroids fragmentation regolith transport and disruption abstract we use a numerical hydrocode model to examine the outcomes of various size impacts into targets the sizes of asteroids 951 gaspra and 243 ida which were imaged by the galileo spacecraft a shock wave fractures the asteroid in advance of crater excavation flow thus for impactors larger than 100 m impacting at 5 3 km s 1 tensile strength is unimportant in these bodies whether they are initially intact or are rubble piles because of the shock induced fracture impact results are controlled by gravity therefore these asteroids are much more resistant to catastrophic disruption than predicted by previous estimates which had assumed that strength was controlling these processes for rock targets the rubble and regolith produced by this fracture can be jolted by the impact redistributing surface material and globally erasing craters the crater ejecta can produce tens of meters of regolith per large event likely consisting of 100 m size boulders mixed with smaller particles the response of kilometer size asteroids to impacts is qualitatively different from that of few centimeter targets in terrestrial experiments making prediction based on such experiments difficult the compositional distribution of delivered meteorites depends on the outcomes of such asteroid impacts

percolative core formation in planetesimals abstract the percolation of liquid iron alloy through crystalline silicates potentially played an important role during core formation in small bodies of the early solar system such as asteroids and planetesimals this is because heat production by radioactive decay of 26 al and 60 fe which is believed to be the main heat source in early formed small planetary bodies will initially cause fe s melts to form well before the silicates start to melt in order to test the feasibility of percolation the effect of pressure on the dihedral angle between fe o s liquid and olivine has been investigated from 1 5 to 5 0 gpa a pressure range that is relevant for the interiors of large asteroids texturally equilibrated dihedral angles increase from 54 to 98 over this pressure range the dihedral angle reaches the critical value of 60 at 2 3 gpa depending on the olivine composition fe this change in dihedral angle is related to the oxygen content of fe o s phase which decreases with increasing pressure because oxygen dissolved in the melt reduces the fe s melt olivine interfacial energy these results show that fe o s liquid can form an interconnected network and percolate through silicate aggregates under conditions of high oxygen fugacity and low pressure even when the melt fraction is small therefore percolation is likely to have been the dominant core formation mechanism in small relatively oxidised planetary bodies with a radius less than about 1300 km

petrogenesis of d orbigny like angrite meteorites and the role of spinel in the angrite source this research was supported by an australian national university phd scholarship to smck who is currently a postdoctoral fellow of the research foundation flanders

relative chronology of crust formation on asteroid vesta insights from the geochemistry of diogenites abstract the eucrites and diogenites are meteorites that probably originate from asteroid 4 vesta the upper part of the crust of this body is certainly composed of eucrites which are basaltic or gabbroic rocks diogenites are ultramafic cumulates whose relationships with eucritic lithologies are unknown here we show that the orthopyroxenes of some diogenites display very deep negative eu anomalies eu eu close to 0 1 or lower the contamination of the parental magmas of diogenites by melts derived by partial melting of the eucritic crust can satisfactorily explain the range of the eu anomalies displayed by diogenites thus these anomalies are the first firm indication that parental melts of diogenites have intruded the eucritic crust and consequently are younger than eucrites

the puerto lapice eucrite puerto lapice is a new eucrite fall castilla la mancha spain 10 may 2007 in this paper we report its detailed petrography magnetic characterization mineral and bulk chemistry oxygen and noble gas isotope systematics and radionuclide data study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature and it contains basaltic and granulitic clasts as well as recrystallized impact melt and breccia fragments shock veins are ubiquitous and show evidence of at least three different shock events bulk chemical analyses suggest that puerto lapice belongs to the main group of basaltic eucrites although it has a significantly higher cr content oxygen isotopes also confirm that the meteorite is a normal member of the hed suite noble gas abundances show typical patterns with dominant cosmogenic and radiogenic contributions and indicate an average exposure age of 19 2 ma and a pu fission xe age well within typical eucrite values cosmogenic radionuclides suggest a preatmospheric size of about 20 30 cm in diameter