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

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

mesosiderite formation on asteroid 4 vesta by a hit and run collision collision and disruption processes of protoplanetary bodies in the early solar system are key to understanding the genesis of diverse types of main belt asteroids mesosiderites are stony iron meteorites that formed by the mixing of howardite eucrite diogenite like crust and molten core materials and provide unique insights into the catastrophic break up of differentiated asteroids however the enigmatic formation process and the poorly constrained timing of metal silicate mixing complicate the assignment to potential parent bodies here we report the high precision uranium lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry to reveal an initial crust formation 4 558 5 2 1 million years ago and metal silicate mixing at 4 525 39 0 85 million years ago the two distinct ages coincide with the timing of the crust formation and a large scale reheating event on the eucrite parent body probably the asteroid vesta this chronological coincidence corroborates that vesta is the parent body of mesosiderite silicates mesosiderite formation on vesta can be explained by a hit and run collision 4 525 4 million years ago that caused the thick crust observed by nasa s dawn mission and explains the missing olivine in mesosiderites howardite eucrite diogenite meteorites and vestoids mesosiderite meteorites may originate from a hit and run impact on the parent asteroid of eucrite meteorites probably vesta as mesosiderite zircon u pb ages are found to coincide with those for eucrites

maskelynite in asteroidal lunar and planetary basaltic meteorites an indicator of shock pressure during impact ejection from their parent bodies abstract maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of 20 30 gpa depending on the ca concentration the proportion of maskelynite rich samples in a basaltic meteorite group correlates with the parent body escape velocity and serves as a shock indicator of launching conditions for eucrites basalts widely presumed to be from vesta vesc 0 36 km s 1 5 of the samples are maskelynite rich for the moon vesc 2 38 km s 1 30 of basaltic meteorites are maskelynite rich for mars vesc 5 03 km s 1 93 of basaltic meteorites are maskelynite rich in contrast literature data show that maskelynite is rare 1 among mare basalts and basaltic fragments in apollo 11 12 15 and 17 soils which were never ejected from the moon angrites are unbrecciated basaltic meteorites that are maskelynite free they were ejected at low to moderate shock pressures from an asteroid smaller than vesta because most impacts that eject materials from a large 100 km parent body are barely energetic enough to do that a collision that has little more than the threshold energy required to eject a sample from vesta will not be able to eject identical samples from the moon or mars there must have been relatively few impacts if any that launched eucrites off their parent body that also imparted shock pressures of 20 30 gpa in the ejected rocks more energetic impacts were required to launch basalts off the moon and mars on average vesta ejecta were subjected to lower shock pressures than lunar ejecta and lunar ejecta were subjected to lower shock pressures than martian ejecta h and ll ordinary chondrites have low percentages of shock stage s5 maskelynite bearing samples 1 and 4 respectively probably reflecting shock processes experienced by these rocks on their parent asteroids in contrast l chondrites have a relatively high proportion of samples containing maskelynite 11 most likely a result of catastrophic parent body disruption 470 ma ago

u pb and hf w dating of young zircon in mesosiderite asuka 882023 mesosiderites are unique stony iron meteorites composed of eucrite like silicates and fe ni metals their formation including silicate metal mixing and metamorphisms provide important insights into early planetary processes in the inner solar system this report describes the first in situ u pb and hf w dating of zircon in a mesosiderite asuka 882023 the u pb 4502 75 ma and hf w 4532 8 5 7 10 5 ma ages may represent timing of the zircon formation which is considerably younger than crustal differentiation of the parent body this evidence combined with earlier studies of chronology implies that mesosiderites were reheated at 4530 4520 ma clearly after the silicate metal mixing

geochemistry and origin of metal olivine clasts and matrix in the dong ujimqin qi mesosiderite the dong ujimqin qi mesosiderite is the first recorded fall of a stony iron meteorite in china according to silicate textures and metal composition this meteorite is classified as a member of subgroup ib instrumental neutron activation analyses inaa of metals show that the matrix metal has lower concentrations of os ir re and pt but higher concentrations of ni and au than the 7 5 cm metal nodule present in the meteorite we attribute these compositional differences to fractional crystallization of molten metal studies of olivine clasts show that feo contents are uniform in individual olivine crystals but are variable for different olivine clasts although concentrations of rare earth elements rees change within olivine clasts they all exhibit a vee shaped pattern relative to ci chondrites the relatively high concentrations of rees in olivine and the shape of ree patterns require a liquid high in rees and especially in light rees as such a liquid was absent from the region where basaltic and gabbroic clasts formed mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed

39ar 40ar ages and thermal history of mesosiderites determinations of 39ar 40ar ages for plagioclase separates of three clasts from the vaca muerta and mount padbury mesosiderites give similar results ages increase monotonically by 0 2 0 3 ga when plotted against increasing 39ar release from the samples and give average values of 3 95 ga when combined with revised ages for mesosiderite analyses previously reported nineteen analyses of thirteen mesosiderites give an average ar ar age of 3 94 0 10 ga the average age at 90 39ar release is 4 13 0 14 ga and the ages at 10 39ar release range over 3 32 3 97 ga it seems unlikely that the 39ar 40ar ages were produced by direct impact heating but they probably are independent evidence for slow cooling deep within the mesosiderite parent body we conclude that the sloped age plateaus and the significant range in ar ar closure temperatures calculated for these mesosiderites 75 340 c are due to significant differences in ar diffusion properties among samples from the ar diffusion data we estimate an approximate mesosiderite cooling rate of 0 2 c ma which is similar to the preferred cooling rate of 0 1 0 5 c ma derived by others from ni diffusion and metal textures the 39ar 40ar ages seem incompatible with very slow cooling rates of 0 2 c ma unless the closure temperatures for ar diffusion and formation of metal cooling textures on the parent body substantially overlap any possible role of fragmentation of the mesosiderite parent body in producing the younger ar ar ages and the very slow cooling rates is unclear

thermal and shock history of mesosiderites and their large parent asteroid to elucidate the geological evolution of the mesosiderite stony iron meteorites and their 3 4 3 8 gy 40ar 39ar ages we have investigated their shock and thermal histories we have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 gpa shock stage s1 s2 three mesosiderites contain a tiny fraction 0 1 overall of mineral fragments that were shocked to shock stage s3 s6 levels the uniformity of shock features within all fragments shows that these fragments were not shocked in situ our shock data for mesosiderites the absence of evidence of 3 6 3 9 gy impact melt the shock history of impact heated ordinary chondrites and the difficulty in quantitatively removing ar in an impact event all suggest that the mesosiderite parent body did not suffer a major impact event 3 6 3 9 gy ago metallographic cooling rates of 0 03 c my at 400 c were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and fe ni p phase diagram cooling rates of 0 01 c my at 425 325 c were estimated from published compositional data for kamacite grains in four mesosiderites these two techniques and four other semiquantitative metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite we infer that cooling rates at 400 c were 0 02 0 03 c my and certainly less than 0 5 c my the inferred cooling rate is too slow to allow ar closure before 4 gy all of the shock thermal and age data for mesosiderites are consistent with slow cooling at depth 1 my after metal and silicate were mixed around 4 4 gy ago thermal models indicate that the mesosiderites probably cooled in an asteroid some 200 400 km in radius

geochemical evaluation of aliphatic and aromatic hydrocarbons in palaeogene source rocks from the latrobe group gippsland basin australia abstract twenty seven hydrocarbon source rocks from the proteacidites asperopolus malvacipollis diversus and lygistepollenites balmei biozones palaeogene latrobe group in the gippsland basin were analysed using gas chromatography mass spectrometry based on the molecular geochemistry of the aliphatic and aromatic fractions together with bulk geochemical data including rock eval pyrolysis and total organic carbon contents the thermal maturity palaeoenvironments and organic matter source of these palaeogene samples were systematically evaluated the results provide a useful comparison with those from upper cretaceous source rocks in the gippsland basin jiang and george 2018 2019 similar to the upper cretaceous rocks the aliphatic hydrocarbons in the paleogene samples are mainly comprised of n alkanes isoprenoids terpanes hopanes and steranes the aromatic hydrocarbons are dominated by naphthalene phenanthrene anthracene fluorene dibenzofuran dibenzothiophene and their alkyl homologues other aromatic hydrocarbons detected include land plant derived compounds e g retene methylretenes cadalene simonellite etc and combustion derived compounds e g chrysene pyrene picene benzo b fluoranthene benzo a anthracene etc these 4 to 6 ring polycyclic aromatic hydrocarbons are more abundant in the paleogene samples compared to the upper cretaceous rocks biomarker and aromatic hydrocarbon distributions indicate that the proteacidites asperopolus and malvacipollis diversus samples are immature or are in the early stage of the oil generation window while the lygistepollenites balmei samples correspond to the low maturity to mature stage the samples were all deposited under sub oxic to oxic conditions in fresh water with low salinity possibly in a lacustrine depositional environment the organic matter inputs are from plankton algae and higher land plants but are dominated by organic matter with a terrigenous origin

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

stony iron meteorites history of the metal phase according to tungsten isotopes abstract the global composition of the early solar system is thought to be roughly chondritic in terms of refractory components and this means that metal and silicate should be present together in early planetesimals to fully understand the metal silicate differentiation process within the eucrite parent body epb it is important to try and identify the metal reservoir that is complementary to the silicate part the isotope 182 of tungsten w a siderophile element is partly formed from the decay of 182 hf and w isotopes are useful for examining metal silicate segregation the w isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites however mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities therefore we undertook the analysis of the metal phase of these stony irons here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body bodies and to assess possible relationships with eucrites all stony iron metals are depleted in radiogenic tungsten by 1 3 to 4 2 e units relative to the terrestrial standard while chondrites for comparison are depleted by 1 9 e units in addition to w isotopic heterogeneity from one stony iron to another there is also w isotopic heterogeneity within individual meteorites a formation model is tentatively proposed where we show that mesosiderites pallasites and eucrites could possibly come from the same parent body several hypotheses are discussed to explain the isotopic heterogeneity the production of cosmogenic tungsten the in situ decay of hafnium present in inclusions and tungsten diffusion processes after metal silicate mixing during the cooling of the meteorites the two latter hypotheses provide the best explanation of our data

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

application of automated image analysis to coal petrography abstract the coal petrologist seeks to determine the petrographic characteristics of organic and inorganic coal constituents and their lateral and vertical variations within a single coal bed or different coal beds of a particular coal field definitive descriptions of coal characteristics and coal facies provide the basis for interpretation of depositional environments diagenetic changes and burial history and determination of the degree of coalification or metamorphism numerous coal core or columnar samples must be studied in detail in order to adequately describe and define coal microlithotypes lithotypes and lithologic facies and their variations the large amount of petrographic information required can be obtained rapidly and quantitatively by use of an automated image analysis system aias an aias can be used to generate quantitative megascopic and microscopic modal analyses for the lithologic units of an entire columnar section of a coal bed in our scheme for megascopic analysis distinctive bands 2 mm or more thick are first demarcated by visual inspection these bands consist of either nearly pure microlithotypes or lithotypes such as vitrite vitrain or fusite fusain or assemblages of microlithotypes megascopic analysis with the aid of the aias is next performed to determine volume percentages of vitrite inertite minerals and microlithotype mixtures in bands 0 5 to 2 mm thick the microlithotype mixtures are analyzed microscopically by use of the aias to determine their modal composition in terms of maceral and optically observable mineral components megascopic and microscopic data are combined to describe the coal unit quantitatively in terms of v for vitrite e for liptite i for inertite or fusite m for mineral components other than iron sulfide s for iron sulfide and veim for the composition of the mixed phases xi i 1 2 etc in terms of the maceral groups vitrinite v exinite e inertinite i and optically observable mineral content m the volume percentage of each component present is indicated by a subscript for example a lithologic unit was determined megascopically to have the composition v 13 i 1 s 1 x1 83 x2 2 after microscopic analysis of the mixed phases this composition was expressed as v 13 i 1 s 1 v63e19i14m4 83 v67e11i13m9 2 finally these data were combined in a description of the bulk composition as v67e16i13m3s1 an aias can also analyze textural characteristics and can be used for quick and reliable determination of rank reflectance our aias is completely software based and incorporates a television tv camera that has optimum response characteristics in the range of reflectance less than 5 making it particularly suitable for coal studies analysis of the digitized signal from the tv camera is controlled by a microprocessor having a resolution of 64 gray levels between full illumination and dark current the processed image is reconverted for display on a tv monitor screen on which selection of phases or features to be analyzed is readily controlled and edited by the operator through use of a lightpen we expect that automated image analysis because it can rapidly provide a large amount of pertinent information will play a major role in the advancement of coal petrography

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

chronology and sources of lunar impact bombardment abstract the moon has suffered intense impact bombardment ending at 3 9 gyr ago and this bombardment probably affected all of the inner solar system basin magnetization signatures and lunar crater size distributions indicate that the last episode of bombardment at about 3 85 gyr ago was less extensive than previously thought we explore the contribution of the primordial mars crosser population to early lunar bombardment we find that mars crosser population initially decays with a 80 myr half life with the long tail of survivors clustering on temporarily non mars crossing orbits between 1 8 and 2 au these survivors decay with half life of about 600 myr and are progenitors of the extant hungaria asteroid group in the same region we estimate the primordial mars crosser population contained about 0 01 0 02 earth masses such initial population is consistent with no lunar basins forming after 3 8 gya and the amount of mass in the hungaria group as they survive longer and in greater numbers than other primordial populations mars crossers are the best candidate for forming the majority of lunar craters and basins including most of the nectarian system however this remnant population cannot produce imbrium and orientale basins which formed too late and are too large to be part of a smooth bombardment we propose that the imbrian basins and craters formed in a discrete event consistent with the basin magnetization signatures and crater size distributions this late impactor shower would be triggered by a collisional disruption of a vesta sized body from this primordial mars crossing population wetherill g w 1975 proc lunar sci conf 6 1539 1561 that was still comparable to the present day asteroid belt a 3 9 gya this tidal disruption lead to a short lived spike in bombardment by non chondritic impactors with a non asteroidal size frequency distribution in agreement with available evidence this body wetherill s object also uniquely matches the constraints for the parent body of mesosiderite meteorites we propose that the present day sources of mesosiderites are multi km sized asteroids residing in the hungaria group that have been implanted there soon after the original disruption of their parent 3 9 gyr ago

unmelted meteoritic debris collected from eltanin ejecta in polarstern cores from expedition ant xii 4 abstract a total of 1 7 g of unmelted meteorite particles have been recovered from fs polarstern piston cores collected on expedition ant xii 4 that contain ejecta from the eltanin impact event most of the mass 1 2 g is a large single specimen that is a polymict breccia similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites most of the remaining mass is in several large individual pieces 20 75 mg each that are polymict breccias fragments dominated by pyroxene and an igneous rock fragment the latter has highly fractionated ree similar to those reported in mafic clasts from mesosiderites other types of specimens identified include fragments dominated by maskelynite or olivine these pieces of the projectile probably survived impact by being blown off the back surface of the eltanin asteroid during its impact into the bellingshausen sea

olivine coronas metamorphism and the thermal history of the morristown and emery mesosiderites abstract coronas are present on all millimeter sized mineral clasts of olivine in the emery and morristown mesosiderites and are a manifestation of high temperature t 850 1100 c metamorphism these coronas formed by reaction and diffusion between olivine and a mesosiderite like matrix assemblage the bulk composition of the coronas can be approximated by a mixture of 10 25 wt olivine and as 90 75 wt metal free matrix except for p and cr which are significantly enriched in coronas phosphorus and cr diffused relatively rapidly to coronas and were derived from a large volume of matrix most likely from metal that was originally enriched in these elements prior to metamorphism the coronas in both meteorites show a similar zone sequence but are systematically thicker in emery 800 m wide than in morristown 350 m wide suggesting that emery experienced more grain growth and more intensive metamorphism than morristown textural relationships suggest that corona formation and high temperature metamorphism occurred largely after intensive millimeter scale brecciation and after or during metal silicate mixing a local equilibrium model can explain many features of the coronas but chemical equilibrium was maintained only on a very small scale overgrowths are present on plagioclase in the coronas of both mesosiderites and probably formed during high temperature metamorphism the compositional interface between core and overgrowth plagioclase is extremely sharp suggesting that cooling rates were 0 1 c y at the peak temperature of metamorphism consistent with high temperature metamorphism occurring in a near surface region of the parent body

k th ti systematics and new three component mixing model of hed meteorites prospective study for interpretation of gamma ray and neutron spectra for the dawn mission abstract the dawn spacecraft carries a gamma ray and neutron detector grand which will measure and map the abundances of selected elements on the surface of asteroid 4 vesta we compare the variability of moderately volatile refractory incompatible element ratios k th and k ti in howardite eucrite and diogenite hed meteorites with those in other achondrite suites that represent asteroidal crusts because these ratios may be accurately measured by grand and likely reflect initial chemical compositions of the hed parent body the k th and k ti variations can differentiate hed meteorites from angrites and some unique eucrite like lithologies the results suggest that k th and ti abundances determined from grand data could not only confirm that vesta is the parent body of hed meteorites but might also allow recognition of as yet unsampled compositional terranes on vesta besides the k th ti systematics study we propose a new three component mixing model for interpretation of grand spectra required because the spatial resolution of grand is coarser than the spectral compositional heterogeneity of vesta s surface the mixing model uses abundances of k ti fe and mg that will be analyzed more accurately than other prospective grand analyzed elements we examine propagated errors due to grand analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model the error investigation suggests that the mixing model can adequately estimate not only the diogenite eucrite mixing ratio but also the abundances of most major and minor elements within the grand propagated errors

oxygen isotopic constraints on the origin and parent bodies of eucrites diogenites and howardites abstract a few eucrites have anomalous oxygen isotopic compositions to help understand their origin and identify additional samples we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites except for five eucrites these meteorites have 17o values that lie within 2 of their mean value viz 0 242 0 016 consistent with igneous isotopic homogenization of vesta the five exceptional eucrites nwa 1240 pasamonte both clast and matrix samples pca 91007 a 881394 and ibitira have 17o values that lie respectively 4 5 5 15 and 21 away from this mean value nwa 1240 has a 18o value that is 5 below the mean eucrite value four of the five outliers are unbrecciated and unshocked basaltic eucrites like nwa 011 the first eucrite found to have an anomalous oxygen isotopic composition the fifth outlier pasamonte is composed almost entirely of unequilibrated basaltic clasts published chemical data for the six eucrites with anomalous oxygen isotopic compositions including nwa 011 exclude contamination by chondritic projectiles as a source of the oxygen anomalies only nwa 011 has an anomalous fe mn ratio but several anomalous eucrites have exceptional na ti or cr concentrations we infer that the six anomalous eucrites are probably derived from five distinct vesta like parent bodies pasamonte and pca 91007 could come from one body these anomalous eucrites like the isotopically normal unbrecciated eucrites with 4 48 gyr ar ar ages are probably deficient in brecciation and shock effects because they were sequestered in small asteroids 10 km diameter during the late heavy bombardment following ejection from vesta like bodies the preservation of vesta s crust and the lack of deeply buried samples from the hypothesized vesta like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets rather than by collisional grinding

evolutionary history of the mesosiderite asteroid a chronologic and petrologic synthesis abstract recent petrologic chemical and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body mpb chronologic measurements indicate that initial melting of the mpb occurred shortly after accretion 4 56 ga ago at about the same time that the eucrites and angrites formed significant crustal remelting 4 47 ga ago probably associated with metal silicate mixing is responsible for forming abundant gabbroic clasts with extreme positive eu anomalies localized impact melting 4 5 3 9 ga ago produced quench textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type 4 mesosiderites collisional disruption and gravitational reassembly 3 9 ga ago is indicated by thorough ar degassing at this time metallographic cooling rates of 1 c ma were likely initiated by this event the history of higher temperature events measured by thermoluminescence was annealed out impact excavation and ejection of deeply buried breccias occurred 3 9 ga ago the 10 160 ma cosmic ray exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the mpb or from a 10 m size fragment it is unlikely that mesosiderites and hed howardite eucrite diogenite samples come from the same parent body there is no evidence for metal silicate mixing with attendant redox effects or major crustal remelting on the hed body bulk howardites have flat chondrite normalized rare earth element ree patterns whereas many bulk mesosiderites have ree patterns with positive eu anomalies and relative enrichments in light ree the nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses