nucleosynthetic osmium isotope anomalies in acid leachates of the murchison meteorite abstract we present osmium isotopic results obtained by sequential leaching of the murchison meteorite which reveal the existence of very large internal anomalies of nucleosynthetic origin e 184 os from 108 to 460 e 186 os from 14 1 to 12 6 e 188 os from 2 6 to 1 6 e 190 os from 1 7 to 1 1 despite these large variations the isotopic composition of the total leachable osmium weighted average of the leachates is close to that of bulk chondrites this is consistent with efficient large scale mixing of os isotopic anomalies in the protosolar nebula the os isotopic anomalies are correlated and can be explained by the variable contributions of components derived from the s r and p processes of nucleosynthesis surprisingly much of the s process rich osmium is released by relatively mild leaching suggesting the existence of an easily leachable s process rich presolar phase or alternatively of a chemically resistant r process rich phase taken together with previous evidence for a highly insoluble s process rich carrier such as sic these results argue for the presence of several presolar phases with anomalous nucleosynthetic compositions in the murchison meteorite the s process composition of os released by mild leaching diverges slightly from that released by aggressive digestion techniques perhaps suggesting that the presolar phases attacked by these differing procedures condensed in different stellar environments the correlation between e 190 os and e 188 os can be used to constrain the s process 190 os 188 os ratio to be 1 275 0 043 such a ratio can be reproduced in a nuclear reaction network for a macs value for 190 os of 200 22 mbarn at 30 kev more generally these results can help refine predictions of the s process in the os mass region which can be used in turn to constrain the amount of cosmoradiogenic 187 os in the solar system and hence the age of the galaxy we also present evidence for extensive internal variation of 184 os abundances in the murchison meteorite a steep anti correlation is observed between e 184 os and e 188 os since 184 os is formed uniquely by the p process this anti correlation cannot be explained by variable addition or subtraction of s process os to average solar system material instead this suggests that p process rich presolar grains e g supernova condensates may be present in meteorites in sufficient quantities to influence the os isotopic compositions of the leachates nevertheless 184 os is a low abundance isotope and we cannot exclude the possibility that the measured anomalies for this isotope reflect unappreciated analytical artifacts

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

osmium isotopic homogeneity in the ck carbonaceous chondrites abstract variable proportions of isotopically diverse presolar components are known to account for nucleosynthetic isotopic anomalies for a variety of elements e g ca ti cr ni sr zr mo ru pd ba nd and sm in both bulk chondrites and achondrites however although large os isotopic anomalies have been measured in acid leachates and residues of unequilibrated chondrites bulk chondrites of various groups iron meteorites and pallasites exhibit os isotopic compositions that are indistinguishable from terrestrial or bulk solar isotopic abundances since the magnitude of nucleosynthetic anomalies is typically largest in the carbonaceous chondrites this study reports high precision os isotopic compositions and highly siderophile element hse concentrations for ten ck chondrites the isotope dilution concentration data for hse and high precision os isotope ratios were determined on the same digestion aliquots to precisely correct for radiogenic contributions to 186os and 187os while acid leached bulk unequilibrated carbonaceous chondrites show deficits of s process os components to the same extent as revealed by unequilibrated enstatite ordinary and rumuruti chondrites equilibrated bulk ck chondrites exhibit no resolvable os isotopic anomalies these observations support the idea that acid resistant carbon rich presolar grains such as silicon carbide sic or graphite are major carriers for nucleosynthetic isotopic anomalies of os the destruction of these presolar grains which are omnipresent in unequilibrated meteorites must have occurred during aqueous alteration and thermal metamorphism early in the ck chondrite parent body history the dispersal of ck chondrites along the iiiab iron meteorite isochron on a 187os 188os versus 187re 188os diagram with re os ratios from 0 032 to 0 083 in combination with the observed redistribution of other hse e g pt pd highlights the influence of parent body processes overprinted by effects of recent terrestrial alteration under the oxidizing conditions prevalent on the ck parent body evident from high abundances of magnetite and limited fe ni metal in ck chondrites these parent body processes made all isotopically anomalous os originally hosted in reduced presolar grains accessible the absence of os isotopic anomalies in ordinary enstatite and now also carbonaceous chondrites implies that the carriers of s and r process os must have been effectively homogenized across the region of chondrite formation and possibly even the entire solar protoplanetary nebula as suggested by the os isotopic compositions of iron meteorites and non anomalous ureilites except for a limited number of ureilites the relative proportions of presolar s and r process carriers of os and other elements such as w in chondrites and most other planetary bodies must have remained constant during all subsequent nebular and planetary processes which appears not to have been the case for other siderophile elements including mo ru and pd the existence of mo ru pd and other siderophile element isotopic anomalies thus appears to be in part controlled by the chemical properties of these elements e g volatility their host phase s e g sic graphite metal sulfides and the nature of the nebular or planetary processes experienced in the early solar system

the distribution of trace elements in carbonaceous chondrites 12 carbonaceous chondrites amongst them representatives of nearly all known petrologic types were analyzed for twenty trace elements by spark source mass spectrography combined with the isotope dilution method data on different element groups refractory moderately volatile and volatile show that the distribution of the trace elements in the carbonaceous chondrites with the exception of renazzo can be well explained by anders two component model this is also valid for the highly metamorphosed cv5 chondrite karoonda furthermore it is observed that the zrhf ratios in the carbonaceous chondrites increase with increasing petrologic type which is interpreted as the result of mixing two components with different zrhf ratios

microscale oxygen isotopic exchange and magnetite formation in the ningqiang anomalous carbonaceous chondrite we report in situ measurements of o isotopic compositions of magnetite olivine and pyroxene in chondrules of the ningqiang anomalous carbonaceous chondrite the petrographic setting of ningqiang magnetite is similar to those in oxidized cv chondrites such as allende where magnetite is found together with ni rich metal and sulfide in opaque assemblages in chondrules both magnetite and silicate oxygen data fall close to the carbonaceous chondrite anhydrous mineral line with relatively large ranges in 18o in magnetite 4 9 to 4 2 and in silicates 15 2 to 4 5 magnetite and silicates are not in o isotopic equilibrium the weighted average 17o 17o 0 52 18o values of magnetite are 1 7 to 3 6 higher than those of the silicates in the same chondrules the petrological characteristics and o isotopic disequilibrium between magnetite and silicates suggest the formation of ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration the weighted average 17o of 3 3 0 3 is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between 17o values of magnetite and silicates in each chondrule these observations indicate that during aqueous alteration in the ningqiang parent asteroid the water rock ratio was relatively low and o isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules

identification of lateral discontinuities via multi offset phase analysis of surface wave data surface wave methods are based on the inversion of observed rayleigh wave phase velocity dispersion curves the goal is to estimate mainly the shear wave velocity profile of the investigated site the model used for the interpretation is 1d hence results obtained wherever lateral variations are present cannot be considered reliable in this paper we study four synthetic models all with a lateral heterogeneity when we process the entire corresponding seismograms with traditional f k approach the resulting 1d profiles are representative of the subsurface properties averaged over the whole length of the receivers lines these results show that classical analysis disregards evidences of sharp lateral velocity changes even when they show up in the raw seismograms in our research we implement and test over the same synthetic models a novel robust automated method to check the appropriateness of 1d model assumption and locate the discontinuities this new approach is a development of the recent multi offset phase analysis with the following further advantages it does not need previous noise evaluation and more than one shot only once the discontinuities are clearly identified we confidently perform classical f k dispersion curve extraction and inversion separately on both sides of the discontinuity thus the final results obtained by putting side by side the 1d profiles are correct 2d reconstructions of the discontinuous s wave distributions obtained without any additional ad hoc hypotheses

origin of isotopic heterogeneity in the solar nebula by thermal processing and mixing of nebular dust we have investigated mo and w isotope compositions in acid leachates and an insoluble residue from the murchison carbonaceous chondrite the new data reveal variable enrichments of s and r process isotopes and demonstrate that several isotopically diverse presolar components are present in murchison the insoluble residue is enriched in s process mo and w evidently due to the enrichment of presolar sic grains in contrast mo and w released by leaching with weak acids are depleted in s process isotopes most likely reflecting the isotope composition of the homogenized portion of the protosolar nebula the mo and w isotope compositions of the different leach steps are broadly correlated as expected from s process nucleosynthesis theory indicating that mo and w are presumably hosted in the same carriers however at the bulk meteorite scale no nucleosynthetic w isotope anomalies have been identified except for ivb iron meteorites in spite of large mo isotope heterogeneities among the same samples this decoupling of mo and w isotopes in bulk meteorites may reflect physical mixing of varying proportions of isotopically diverse presolar components with a normal solar nebula component due to the high w mo ratio and near terrestrial w isotope composition of the latter such mixing has no measurable effect on w isotopes but results in large mo isotope variations alternatively thermal processes within the solar nebula imparted mo isotope heterogeneity on an initially homogeneous mixture of presolar dust while w was not affected removal of volatile mo oxides during the thermal destruction of fragile presolar components would have created isotopically heterogeneous reservoirs of nebular dust accretion of meteorite parent bodies from such variably processed dust would thus result in mo isotope heterogeneities at the bulk meteorite scale other elements such as os and w were not or only slightly affected because they were more refractory during thermal processing and therefore remained isotopically homogeneous thermal processing of presolar dust within the solar nebula can thus account for both isotope heterogeneities observed for some elements and the lack of such isotopic heterogeneity for other elements

molybdenum isotope anomalies in meteorites constraints on solar nebula evolution and origin of the earth the early evolution of the solar nebula involved substantial transport of mass resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources the efficiency of this mixing as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets here we present mo isotopic data for a wide range of samples including ca al rich inclusions chondrites and differentiated meteorites as well as martian and terrestrial samples most meteorites are depleted in s process mo relative to the earth and only the iab iiicd irons angrites and martian meteorites have terrestrial mo isotopic compositions in contrast most ca al rich inclusions are enriched in r process mo but one inclusion is characterized by a large s process deficit molybdenum isotopic anomalies in the bulk meteorites correlate with those in ru exactly as predicted from nucleosynthetic theory but no obvious correlation is apparent between mo and ni anomalies therefore s process mo and ru seem to be hosted in the same carrier which must be distinct from the carrier responsible for isotopic anomalies in the fe group elements ni cr ti furthermore the isotopic heterogeneity in mo and other elements contrasts with the isotopic homogeneity for hf and os indicating that different s process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed the mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies providing evidence for a progressive homogenization of the solar nebula however the carbonaceous chondrites exhibit larger mo anomalies than expected for their age indicating that they received a greater portion of material from the outer solar system where homogenization was slow than other meteorite parent bodies and terrestrial planets compared to the meteorites earth is enriched in s process mo and must have accreted from material distinct from the meteorites combined mo and o isotopic data show that the composition of the earth cannot be reconstructed by any known combination of meteorites implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk earth this is exemplified by the covariation of 92mo and 142nd anomalies in chondrites showing that the 142nd deficit of chondrites compared to the accessible earth may not unequivocally be interpreted as a signature of an early differentiation of the earth however further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the earth

presolar diamond sic and graphite in primitive chondrites abundances as a function of meteorite class and petrologic type abstract acid resistant residues were prepared for eighteen meteorites from seven chondrite classes using improved procedures designed to minimize loss of presolar grains noble gases were measured by stepped pyrolysis and the abundances of five exotic noble gas components were used to determine abundances of diamond sic and graphite diamond separates were prepared to check diamond abundances determined from noble gases diamond and sic were found in unmetamorphosed members of all seven chondrite classes and evidence for graphite was found in orgued ci and ll3 0 3 1 chondrites our data and noble gas and stable isotope data in the literature show that presolar grains were incorporated into all chondrite classes the grains are sited in the fine grained matrix and their abundances within the matrix reflect mainly the degree of metamorphism suffered by the host meteorite the pattern of metamorphic destruction differs from class to class in unequilibrated ordinary chondrites uocs graphite is most easily destroyed followed by sic and then diamond no presolar grains remain in uocs of petrologic type 3 8 whose matrices have been completely recrystallized in eh chondrites diamond and sic survive much more severe metamorphism than in uocs with sic apparently more stable than diamond presolar graphite was not detected in cv3 and c03 chondrites diamond appears to be more stable than sic and graphite again is missing inferred premetamorphic diamond abundances matrix normalized vary by a factor of 2 2 between classes with eh cv and co chondrites having higher initial abundances than orgueil similar diamond sic ratios are observed in most classes and differences in the ratio can be tied to secondary processing suggesting that all chondrite classes sampled the same solar system wide reservoir of presolar grains differences in initial abundances between classes appear to correlate with chemical and physical properties of the host meteorites these bulk properties and variations in presolar grain abundances both may be the result of thermal processing of presolar dust in the accretion disk

the cosmic molybdenum ruthenium isotope correlation dauphas et al astrophys j 565 2002 640 644 reported molybdenum isotopic anomalies in differentiated and bulk primitive meteorites it is shown here that these isotopic anomalies correlate with one another in exactly the way expected from nucleosynthesis theory if different regions of the nebula received different contributions of s process matter synthesized in lowmass agb stars furthermore when bulk measurements are grouped by meteorite class the molybdenum isotopic anomalies correlate with the ruthenium isotopic anomalies recently reported by chen et al lunar planet sci xxxiv 2003 1789 again indicating a heterogeneous distribution of s process matter as molybdenum is only moderately siderophile most of the molybdenum presently in the mantle was delivered before the completion of core formation in contrast because ruthenium is highly siderophile nearly all of the mantle ruthenium was delivered by a late veneer after the end of core formation thus the fact the silicate earth lies on the mo ru cosmic correlation supports the idea that the earth accreted homogeneously stated otherwise the feeding zone of the earth did not change drastically with time as both the bulk of the earth and the late veneer accreted from material from the same mo ru isotopic reservoir d 2004 elsevier b v all rights reserved

rhodium gold and other highly siderophile element abundances in chondritic meteorites abstract the abundances of the highly siderophile elements hse re os ir ru pt rh pd and au and 187os 188os isotope ratios have been determined for a set of carbonaceous ordinary enstatite and rumuruti chondrites using an analytical technique that permits the precise and accurate measurement of all hse from the same digestion aliquot concentrations of re os ir ru pt and pd were determined by isotope dilution icp ms and n tims analysis the monoisotopic elements rh and au were quantified relative to the abundance of ir differences in hse abundances and ratios such as re os 187os 188os pd ir and au ir between different chondrite classes are further substantiated with new data and additional rh and au data including new data for ci chondrites systematically different relative abundances of rh between different chondrite classes are reminiscent of the behaviour of re carbonaceous chondrites are characterized by low average rh ir of 0 27 0 03 1s which is about 20 lower than the ratio for ordinary 0 34 0 02 and enstatite chondrites eh 0 33 0 01 el 0 32 0 01 r chondrites show higher and somewhat variable rh ir of 0 37 0 07 well defined linear correlations of hse in particular for bulk samples of ordinary and el chondrites are explained by binary mixing and or dilution by silicates the hse carriers responsible for these correlations have a uniform chemical composition indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components excepting rumuruti chondrites and au in carbonaceous chondrites these correlations also suggest that metamorphism alteration and igneous processes had negligible influence on the hse distribution on the bulk sample scale depletion patterns for rh pd and au in carbonaceous chondrites other than ci are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites fractionated hse abundance patterns of ordinary enstatite and rumuruti chondrites however are more difficult to explain fractional condensation combined with the removal of metal phases at various times and later mixing of early and late formed metal phases may provide a viable explanation planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the hse abundance patterns of chondrite groups hse abundances of some but not all rumuruti chondrites may be consistent with solid sulphide liquid sulphide fractionation processes during impact induced melting

coupled w os pt isotope systematics in ivb iron meteorites in situ neutron dosimetry for w isotope chronology tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within 2 ma of the formation of cais with the exception of ivb irons that plot to systematically less radiogenic compositions yielding erroneously old ages secondary neutron capture due to galactic cosmic ray gcr irradiation is known to lower the e182w of iron meteorites adequate correction of which requires a measure of neutron dosage which has not been available thus far the w os and pt isotope systematics of 12 of the 13 known ivb iron meteorites were determined by mc icp ms w os pt and tims os on the same dissolutions that yield precise e182w stable os and pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic ray induced burn out of 182w in iron meteorites the w isotope data reveal a main cluster with e182w of 3 6 but a much larger range than observed in previous studies including irons weaver mountains and warburton range that show essentially no cosmogenic effect on their e182w the ivb data exhibits resolvable negative anomalies in e189os 0 6e and complementary e190os anomalies 0 4e in tlacotepec due to neutron capture on 189os which has approximately the same neutron capture cross section as 182w and captures neutrons to produce 190os the least irradiated ivb iron warburton range has e189os and e190os identical to terrestrial values similarly pt isotopes which are presented as e192pt e194pt and e196pt range from 4 4e to 53e 1 54e to 0 32e and 0 73e to 0 20e respectively also identify tlacotepec and dumont as the most gcr damaged samples in w os and w pt isotope space the correlated isotope data back project toward a 0 epsilon value of e192pt e189os and e190os from which a pre gcr irradiation e182w of 3 42 0 09 2 is derived this pre gcr irradiation e182w is within uncertainty of the currently accepted cai initial e182w the pt and os isotope correlations in the ivb irons are in good agreement with a nuclear model for spherical irons undergoing gcr spallation although this model over predicts the change of e182w by 2 indicating a need for better w neutron capture cross section determinations a nucleosynthetic effect in e184w in these irons of 0 14 0 08 is confirmed consistent with the presence of mo and ru isotope anomalies in ivb irons the lack of a non gcr os isotope anomaly in these irons requires more complex explanations for the production of w ru and mo anomalies than nebular heterogeneity in the distribution of s process to r process nuclides

rhenium osmium systematics of calcium aluminium rich inclusions in carbonaceous chondrites the re os isotopic systematics of calcium aluminium rich inclusions cais in chondrites were investigated in order to shed light on the behavior of the re os system in bulk chondrites and to constrain the timing of chemical fractionation in primitive chondrites cais with relatively unfractionated rare earth element ree patterns groups i iii v vi define a narrow range of 187 re 188 os 0 3764 0 4443 and 187 os 188 os 0 12599 0 12717 and high but variable re and os abundances 3209 41 820 ppb os in contrast cais that show depletions in highly refractory elements and strongly fractionated ree patterns group ii also show a much larger range in 187 re 188 os 0 409 0 535 and 187 os 188 os 0 12695 0 13770 and greater than an order of magnitude lower re and os abundances than other groups e g 75 7 680 2 ppb os sixteen bulk cais and cai splits plot within analytical uncertainty of a 4558 ga reference isochron as is expected for materials of this antiquity eight samples however plot off the isochron several possible reasons for these deviations are discussed data for multiple splits of one cai indicate that the nonisochronous behavior for at least this cai is the result of re os reequilibration at approximately 1 6 ga thus the most likely explanation for the deviations of most of the nonisochronous cais is late stage open system behavior of re and os in the asteroidal environment the 187 os 188 os os systematics of cais are consistent with previous models that indicate group ii cais are mixtures of components that lost the bulk of their highly refractory elements in a previous condensation event and a minor second component that provided refractory elements at chondritic relative proportions the high re os of group ii cais relative to other cais and chondrite bulk rocks may have been caused by variable mobilization of re and os during medium to low temperature parent body alteration 4 5 ga ago this model is favored over nebular models which pose several difficulties the narrow range of 187 os 188 os in group i iii v and vi bulk cais and the agreement with 187 os 188 os of whole rock carbonaceous chondrites suggest that on a bulk inclusion scale secondary alteration only modestly fractionated re os in these cais the average of 187 os 188 os for group i iii v and vi cais is indistinguishable from average ci chondrites indicating a modern solar system value for 187 os 188 os of 0 12650 corresponding to a 187 re 188 os of 0 3964 copyright 2001 elsevier science ltd

conditions for pore water convection within carbonaceous chondrite parent bodies implications for planetesimal size and heat production abstract the critical rayleigh number for convective flow of pore water through a permeable spherical body is derived the result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production if the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water then the present calculations suggest that the parent bodies were less than 80 km in diameter the maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water rock ratios in altered carbonaceous chondrites pore water would have flowed through parent bodies larger than 120 km in diameter with typical rock like permeabilities convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other

osmium isotope systematics of ureilites abstract the 187 os 188 os for 22 ureilite whole rock samples including monomict augite bearing and polymict lithologies were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body or bodies the re os ratios of most ureilites show evidence for a recent disturbance probably related to re mobility during weathering and no meaningful chronological information can be extracted from the present data set the ureilite 187 os 188 os ratios span a range from 0 11739 to 0 13018 with an average of 0 1258 0 0023 1 similar to typical carbonaceous chondrites and distinct from ordinary or enstatite chondrites the similar mean of 187 os 188 os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites from the narrow range of the 187 os 188 os distribution in ureilite meteorites it is further concluded that re was not significantly fractionated from os during planetary differentiation and was not lost along with the missing ureilitic melt component the lack of large re os fractionations requires that re os partitioning was controlled by a metal phase and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body

highly siderophile elements and 187re 187os isotopic systematics of the allende meteorite evidence for primary nebular processes and late stage alteration abstract the abundances of highly siderophile elements hse re os ir ru pt and pd as well as 187 re 187 os isotopic systematics were determined for calcium aluminum rich inclusions cais chondrules and matrix separated from the cv3 carbonaceous chondrite allende consistent with prior studies cais are characterized by significant depletions in pd relative to the other hse while the other hse are in generally bulk chondritic relative abundances the depletions in pd can be linked with initial formation of cais via condensation or subsequent processing by evaporative processes chondrules generally have relative hse patterns similar to cais although they have lower absolute abundances palladium depletions in chondrules may reflect solid metal liquid metal fractionation at the time of formation or alternatively be the result of processes that acted on precursor materials matrix samples have nearly chondritic absolute abundances of all hse measured consequently matrix is the only major chondritic component examined here that shows no relative depletion in pd mass balance suggests the existence of an unidentified pd rich carrier although it is possible that the dataset presented here is too limited to represent typical hse abundances of some chondritic components e g chondrules the 187 re 187 os isotopic systematics of only six out of twenty four allende chondritic components analyzed plot within uncertainties of a 4568 ma primordial reference isochron the deviations from the expected isochron most likely reflect late stage open system behavior within the last 2 billion years and in some cases could even have resulted from terrestrial alteration the open system behavior is most readily observed in small millimeter size sub samples of allende consistent with re and or os mobility on that scale

evidence for homogeneous distribution of osmium in the protosolar nebula separate s r and possibly p process enriched and depleted components have been shown to host os in low metamorphic grade chondrites although no measureable os isotopic anomalies have yet been discovered for bulk chondrites here iron meteorites from groups iab iiab iiiab iva and ivb as well as the main group pallasites are examined many of these meteorites show well resolved anomalies in e190os e189os and e186osi the anomalies however differ from those observed in chemically extracted components from chondrites and are interpreted to reflect long term exposure of the meteorites to cosmic rays rather than nucleosynthetic effects a neutron capture model is presented that can well account for observed isotopic variations in 190os and 189os the same model predicts greater enrichment in 186osi than is observed for at least one iron suggesting as yet unaccounted for effects or failings of the model despite the variable anomalies resulting from cosmic ray exposure each of the major meteorite groups examined contains at one member with normal os isotopic compositions that are unresolved from chondritic compositions this indicates that some domains within these meteorites were little affected by cosmic rays these domains are excellent candidates for application of the 182hf 182w system for dating metal silicate segregation on their parent bodies the normal os also implies that os was homogeneously distributed throughout the protosolar nebula on the scale of planetesimal accretion within the current level of analytical resolution the homogeneity in os contrasts with isotopic heterogeneity present for other siderophile elements including mo ru and w the contrast in the scale of anomalies may reflect a late stage injection of s and p process rich material into the coalescing nebula alternately nebular thermal processing and destruction of some presolar host phases of mo ru and w may also be responsible

stable isotopes and the noncarbonaceous derivation of ureilites in common with nearly all differentiated planetary materials abstract the abundant diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents average 3 wt and have long been linked to the so called carbonaceous chondrites although this term is potentially misleading because the high petrologic type carbonaceous chondrites are if anything c poor compared to ordinary chondrites ureilite oxygen isotopic compositions i e diversely negative ccam like 17o viewed in isolation have long been viewed as confirming the carbonaceous chondritic derivation hypothesis however a very different picture emerges through analysis of a compilation of recently published high precision isotopic data for chromium titanium and nickel for ureilites and various other planetary materials ureilites have lower e62ni and far lower e50ti and e54cr than any known variety of carbonaceous chondrite on a plot of e50ti vs e54cr and similarly 17o vs e54cr ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites and the carbonaceous chondrites are separated by a wide margin from all other planetary materials i conclude that notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative 17o the ureilite precursors accreted from preponderantly noncarbonaceous sensu stricto materials despite total depletion of basaltic matter the ureilites retain moderate pyroxene olivine ratios which is an expected outcome from simple partial melting of moderate sio2 feo mgo noncarbonaceous chondritic material but would imply an additional process of major reduction of feo if the precursor material were carbonaceous chondritic the striking bimodality of planetary materials on the e50ti vs e54cr and 17o vs e54cr diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula however an alternative admittedly speculative explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system carbonaceous and materials that accreted in the inner solar system noncarbonaceous including the ureilites

siderophile element anomalies in ck carbonaceous chondrites implications for parent body aqueous alteration and terrestrial weathering of sulfides abstract ck chondrites constitute the most oxidized anhydrous carbonaceous chondrite group most of the fe occurs in magnetite and in feo rich mafic silicates the two observed ck falls karoonda and kobe along with thirteen relatively unweathered ck finds have unfractionated siderophile element abundance patterns in contrast a sizable fraction of ck finds 9 of 24 investigated shows fractionated siderophile abundance patterns including low abundances of ni co se and au the most extreme depletions are in ni 0 24 of normal ck and au 0 14 of normal ck this depletion pattern has not been found in other chondrite groups out of the 74 ck chondrites listed in the meteoritical bulletin database 2006 excluded considerably paired specimens see http tin er usgs gov meteor metbull php we analyzed 24 and subclassified the ck chondrites in terms of their chemical composition and sulfide mineralogy sl siderophiles low six samples for large depletions in ni co se and au 50 of sulfides lost sm siderophiles medium two cks for moderately low ni and co abundances sulfides are highly altered or partly lost sh siderophiles high one specimen for enrichments in ni co se and au normal for unfractionated samples 13 samples the sole sh sample may have obtained additional sulfide from impact redistribution in the parent asteroid we infer that these elements became incorporated into sulfides after asteroidal aqueous processes oxidized nebular metal thermal metamorphism probably also played a role in their mineral siting the siderophile losses in the sl and sm samples are mainly the result of oxidation of pentlandite pyrite and violarite by terrestrial alteration followed by leaching of the resulting phases some antarctic ck chondrites have lost most of their sulfides but retained ni co se and au presumably as insoluble weathering products

pt re os systematics of group iiab and iiiab iron meteorites the pt re os isotopic and elemental systematics of 13 group iiab and 23 group iiiab iron meteorites are examined as has been noted previously for iron meteorite groups and experimental systems solid metal liquid metal bulk distribution coefficients d values for both iiab and iiiab systems show dos dre dpt 1 during the initial stages of core crystallization assuming closed system crystallization the latter stages of crystallization for each core are generally characterized by dpt dre dos the processes governing the concentrations of these elements are much more complex in the iiiab core relative to the iiab core several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the re os pair each model has flaws but in general the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts late additions of primitive metal to either core were likely minor or nonexistent the 187re 187os systematics of the iiab and iiiab groups are consistent with generally closed system behavior for both elements since the first several tens of ma of the formation of the solar system consistent with short lived chronometers the re os isochron ages for the complete suites of iiab and iiiab irons are 4530 50 ma and 4517 32 ma respectively and are similar to previously reported re os ages for the lower ni endmembers of these two groups both isochrons are consistent with but do not require crystallization of the entire groups within 10 30 ma of the initiation of crystallization the first high precision 190pt 186os isochrons for iiab and iiiab irons are presented the pt os isochron ages for the iiab and iiiab irons calculated using the current best estimate of the for 190pt are 4323 80 ma and 4325 26 ma respectively the re os and pt os ages do not overlap within the uncertainties the younger apparent ages recorded by the pt os system likely reflect error in the 190pt decay constant the slope from the pt os isochron is combined with the age from the re os isochron for the iiiab irons to calculate a revised of 1 415 10 12 a 1 for 190pt although additional study of this decay constant is still needed

osmium isotope anomalies in chondrites results for acid residues and related leachates abstract we have investigated os isotope anomalies in acid residues enriched in insoluble organic matter iom extracted from ten primitive chondrites acid leachates and residues of these fractions as well as acid leachates of bulk chondrites osmium isotopic compositions of bulk carbonaceous ordinary and enstatite chondrites are also reported consistent with prior results bulk chondrites have homogeneous os isotope compositions for s r and p process nuclides that are indistinguishable from terrestrial at the current level of resolution in contrast nearly all the iom rich residues are enriched in s process os evidently due to the preferential incorporation of s process enriched presolar grains most likely presolar sic presolar silicate grains that formed in red giant branch rgb or asymptotic giant branch agb stars are also likely hosts of additional s process os in chondrites consistent with one prior study os released by weak acid leaching of bulk chondrites is slightly to strongly enriched in r process nuclides of which the carrier may be fine grained presolar silicates formed in supernovae or unidentified solar phases collectively the different chemically concentrated components in these meteorites are variably enriched in s r and possibly p process os of which the individual carriers must have been produced in multiple stellar environments the lack of evidence for os isotopic heterogeneity among bulk chondrites contrasts with evidence for isotopic heterogeneities for various other elements at approximately the same levels of resolution e g cr mo ru ba sm and nd one possible explanation for this is that the heterogeneities for some elements in bulk materials reflect selective removal of some types of presolar grains as a result of nebular processes and that because of the strong chemical differences between os and the other elements the os was not significantly affected another possible explanation is that late stage injection of isotopically distinct materials into the nebula led to heterogeneous distribution of some elements osmium may not have been affected because of a lack of isotopic contrast with the pre injection well mixed nebula

the relationship between ck and cv chondrites ck chondrites are highly oxidized meteorites containing abundant magnetite and trace amounts of fe ni metal although the group is predominately composed of equilibrated meteorites types 4 6 in recent years a significant number of new samples have been classified as being either ck3 or ck3 anomalous these unequilibrated cks often display a close affinity with members of the cv oxidized subgroup cks and cvs oxidized subgroup may therefore form a continuum and by implication could be derived from a single common parent body to investigate the relationship between these two groups a detailed study of the oxygen isotope composition opaque mineralogy and major and trace element geochemistry of a suite of cv and ck chondrites has been undertaken the results of oxygen isotope analysis confirm the close affinity between cv and ck chondrites while excluding the possibility of a linkage between the co and ck groups magnetites in both cv and ck chondrites show significant compositional similarities but high ti contents are a diagnostic feature of the latter group the results of major and trace element analysis demonstrate that both cv and ck chondrites show overlapping variation supporting evidence for a single common source for both groups comes from their similar cosmic ray exposure age distributions recent reflectance spectral analysis is consistent with both the cvs and cks being derived from eos family asteroids which are believed to have formed by the catastrophic disruption of a single large asteroid thus a range of evidence appears to be consistent with cv and ck chondrites representing samples from a single thermally stratified parent body in view of the close similarity between cv and ck chondrites some modification of the present classification scheme may be warranted possibly involving integration of the two groups one means of achieving this would be to reassigned ck chondrites to a subgroup of the oxidized cvs it is recognized that a full evaluation of this proposal may require further study of the still poorly understood ck3 chondrites

nucleosynthetic strontium isotope anomalies in carbonaceous chondrites abstract precise sr isotopic compositions in samples from sequential acid leaching experiments have been determined for three carbonaceous chondrites allende murchison and tagish lake together with those in the bulk aliquots of these meteorites the chondritic acid leachates and residues were characterized by sr isotope anomalies with variable 84sr values 106 relative deviation from a standard material ranging from 120 to 4700 ppm documenting multiple nucleosynthetic sources within a single meteorite in addition the 84sr patterns across leaching samples for individual chondrites differed from one another the highest 84sr values were observed for leaching step 3 hcl h2o 75 c for allende and murchison likely because of the incorporation of calcium and aluminum rich inclusions cais in contrast extremely low 84sr values were observed in the later fractions steps 6 and 7 for murchison and tagish lake suggesting the existence of s process enriched presolar sic grains derived from agb stars a 84sr 54cr diagram was prepared with the cais and bulk aliquots of carbonaceous chondrites and other meteorites noncarbonaceous that were plotted separately however they still formed a global positive correlation cais presented the highest 84sr and 54cr values whereas carbonaceous chondrites and noncarbonaceous meteorites had intermediate and the lowest 84sr and 54cr values respectively the positive trend was interpreted as resulting from global thermal processing in which sublimation of high 84sr and 54cr carriers generated the excess 84sr and 54cr signatures in cais while noncarbonaceous planetesimals accreted from materials that underwent significant thermal processing and thus had relatively low 84sr and 54cr values apart from the global trend the carbonaceous chondrites and noncarbonaceous meteorites both exhibited intrinsic variations that highlight an isotopic dichotomy similar to that observed in other isotope combinations e g 54cr 50ti 54cr 17o a plausible scenario for creation of the intrinsic variations involves local thermal processing e g flash heating for chondrule formation caused by additional selective destruction of presolar grains differently than that caused by global thermal processing the existence of such a global positive trend and local variations for two meteorite groups suggests a complicated dynamic history for the dust grains with respect to thermal processing material transportation and mixing in the protoplanetary disk prior to planetesimal formation

compositional and petrographic similarities of cv and ck chondrites a single group with variations in textures and volatile concentrations attributable to impact heating crushing and oxidation abstract greenwood et al 2010 gathered data on o isotopic and elemental compositions and reevaluated literature data for cv and ck chondrites they concluded that these two chondrite groups originated on the same parent asteroid with ck chondrites being metamorphosed cv chondrites which are otherwise missing types 4 through 6 to test this interpretation we have gathered new instrumental neutron activation analysis inaa data for cv and ck chondrites and reexamined their petrographic features the new inaa data like the older data show scatter attributable to weathering effects but we conclude that the refractory lithophile abundances are the same in cv and ck in agreement with the greenwood et al interpretation several volatile elements are significantly lower in ck than in cv chondrites among the elements we determine the greatest difference between cv and ck is found for br for which the cv ck ratio is 4 as and sb are about 20 lower in ck than cv and smaller differences are observed for zn ga and se it seems likely that volatiles were lost during impact heating events that also provided the heat responsible for metamorphic recrystallization within statistical uncertainty chondrules in cv and ck chondrites are the same size and have similar textural distributions a significant petrographic difference between ck and cv chondrites cited by kallemeyn et al 1991 was the much higher percentage of igneous rims around cv chondrules however we now recognize that many chondrules in ck3 8 nwa 1559 have igneous rims and in ck4 chondrites igneous rims are recognizable by their associated sulfide rich rings there are no quantifiable cv ck differences in igneous rim abundances we used ca and al maps to show that ck chondrites have cai abundances similar to those of cv chondrites it thus appears that there are no resolvable pre metamorphic petrographic differences between cv and ck chondrites we recommend that the ck designation be abandoned and that the cv group be acknowledged as spanning the range of petrologic types 3 6 we suggest that ck3 chondrites be designated cv3oxk most ck4 chondrites are highly fragmental collisional crushing appears to be much more common than in cv chondrites it seems likely that ck chondrites formed from reduced cv3 materials after the latter was impacted buried aqueously altered and annealed

metamorphosed calcium aluminum rich inclusions in ck carbonaceous chondrites ck chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6 although cks are described as calcium aluminum rich inclusion cai poor objects the abundance of cais in the 18 ck3 6 we analyzed ranges from zero to approximately 16 4 during thermal metamorphism some of the fine grained cais recrystallized as irregular assemblages of plagioclase ca rich pyroxene olivine ca poor pyroxene magnetite coarse grained cais display zoned spinel fassaite destabilization and secondary grossular and spinel secondary anorthite grossular ca rich pyroxene and spinel derive from the destabilization of melilite which is lacking in all cais investigated the al mg isotopic systematics measured in fine and coarsegrained cais from tanezrouft tnz 057 was affected by mg redistribution the partial equilibration of al mg isotopic signatures obtained in the core of a coarse grained cai cg1 cai in tnz 057 may indicate a lower peak temperature for mg diffusion of approximately 540 580 c while grossular present in the core of this cai indicates a higher temperature of around 800 c for the metamorphic event on the parent body of tnz 057 excluding metamorphic features the similarity in nature and abundance of cais in ck and cv chondrites confirms that cvs and cks form a continuous metamorphic series from type 3 to 6

assessment of the 187re decay constant by cross calibration of re os molybdenite and u pb zircon chronometers in magmatic ore systems the past decade has seen renewed interest in 187re 187os geochronology using a variety of matrices including sulfide minerals shales and meteorites the most widely used value of the 187re decay constant 187re is 1 666 0 005 10 11 a 1 0 31 which is based on cross calibration of re os and pb pb chronometers for certain meteorites smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 however other recent studies have yielded alternate values of 187re based upon either direct counting experiments or analysis of meteorites here we provide an independent assessment of 187re using methodology sample materials and preparation of os standard solutions different from those of smoliar et al 1996 combining re os age data for molybdenite formed in magmatic ore deposits with the u pb zircon age of the magmatic rocks a refined 187re value is determined by averaging 11 individual cross calibration experiments spanning ca 2700 ma of earth history using the u decay constants of jaffey jaffey a h flynn k f glendenin l e bentley w c and essling a m 1971 precision measurement of half lives and specific activities of 235u and 238u phys rev 4 1889 1906 a value for 187re of 1 6668 0 0034 10 11 a 1 is determined using the 238u value of jaffey et al 1971 and 235u value of schoene schoene b crowley j l condon d j schmitz m d and bowring s a 2006 reassessing the uranium decay constants for geochronology using id tims u pb data geochim cosmochim acta 70 426 445 a value for 187re of 1 6689 0 0031 10 11 a 1 is determined these values are nominally higher ca 0 1 and ca 0 2 than the value determined by smoliar et al smoliar m i walker r j and morgan j w 1996 re os isotope constraints on the age of group iia iiia iva and ivb iron meteorites science 271 1099 1102 but within calculated uncertainty further refinement of 187re by cross calibrating the molybdenite and u pb zircon chronometers should be possible by utilizing high precision single grain chemical abrasion zircon u pb analyses

chemical compositions of siderophile element rich opaque assemblages in an allende inclusion abstract ten opaque assemblages or fremdlinge weighing 0 17 19 g from an allende type b ca al rich coarse grained inclusion were analyzed by inaa for ca na mn sc ta ti v cr fe ni co w re os ir mo ru pt rh au as sb and se refractory siderophiles are usually enriched by factors of 103 104 relative to cl chondrites but not uniformly from sample to sample c1 chondrite normalized re os ir pt os ru and pt rh ratios vary from 0 81 1 03 0 70 2 25 0 67 3 32 and 0 45 1 43 respectively attempts to model the abundances of refractory siderophiles by condensation from the solar nebula into a single phase assuming ideal solution fail to match the observed combination of subchondritic re os and ir pt ratios even allowing for fractionation of the highest temperature condensates a different model in which all metals whose high temperature crystal structures are hcp condense into one alloy bcc into another and fcc into a third is capable of matching virtually all refractory siderophile fractionations in these fremdlinge in this model all isostructural metals are assumed to form ideal solutions with one another and metals of different structure are assumed to be insoluble in one another after fractionation of high temperature hcp and fcc alloys individual proto fremdlinge formed by mixing of high and low temperature varieties of each of the three alloys either in the nebula before and during cai accretion or during later partial melting of the silicate oxide and metal components of cais

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

ree and hf distribution among mineral phases in the cv ck clan a way to explain present day hf isotopic variations in chondrites chondrites are among the most primitive objects in the solar system and constitute the main building blocks of telluric planets among the radiochronometers currently used for dating geological events sm nd and lu hf are both composed of refractory lithophile element they are thought to behave similarly as the parent elements sm and lu are generally less incompatible than the daughter elements nd and hf during geological processes as such their respective average isotopic compositions for the solar system should be well defined by the average of chondrites called chondritic uniform reservoir chur however while the sm nd isotopic system shows an actual spread of less than 4 in the average chondritic record the lu hf system shows a larger variation range of 28 bouvier a vervoort j d and patchett p j 2008 the lu hf and sm nd isotopic composition of chur constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets earth planet sci lett 273 48 57 to better understand the contrast between sm nd and lu hf systems the ree and hf distribution among mineral phases during metamorphism of karoonda ck and vigarano type cv carbonaceous chondrites has been examined mineral modes were determined from elemental mapping on a set of five ck chondrites from types 3 6 and one cv3 chondrite trace element patterns are obtained for the first time in all the chondrite forming minerals of a given class ck chondrites as well as one cv3 sample this study reveals that ree are distributed among both phosphates and silicates only 30 50 of sm and nd are stored in phosphates at least in chondrites types 3 5 as such they are not mobilized during early stages of metamorphism the remaining fraction of sm and nd is distributed among the same mineral phases these elements are therefore not decoupled during metamorphism of the whole rock total of lu the fraction held in phosphate decreases significantly as the degree of metamorphism increases 30 for types 3 and 4 less than 5 in type 6 in contrast to lu hf is mainly hosted by silicates with little contribution from phosphates throughout the ck metamorphic sequence a significant part of sm and nd are stored in phosphates in types 3 5 and these elements behave similarly during ck chondrite metamorphism that explains the robustness of the sm nd ratios in chondrites through metamorphism and the slight discrepancies observed in the present day isotopic nd values in chondrites on the contrary lu and hf are borne by several different minerals and consequently they are redistributed during metamorphism induced recrystallization the lu hf ratios are therefore significantly disturbed during chondrites metamorphism leading to the high discrepancies observed in present day hf isotopic values in chondrites

the compositional classification of chondrites vii the r chondrite group bulk compositional and petrographic data clearly define the new r rumuruti group of chondrites consisting of rumuruti the only fall alh85151 acfer 217 carlisle lakes dar al gani 013 pca91002 pca91241 y 75302 y 793575 and y 82002 compositional petrographic rare gas and 0 isotopic data strongly suggest that pca91002 and pca91241 are paired the yamato specimens are probably not paired the matrices of the known r chondrites have experienced similar minor degrees of metamorphism petrographic types are 3 8 3 9 with the exception of alh85151 3 6 all except carlisle lakes contain equilibrated r5 r6 clasts petrographically the r chondrites are characterized by a low chondrule matrix modal abundance ratio high states of oxidation reflected by abundant nio bearing olivine with fa37 40 relatively small chondrules mean apparent diameters of 400 abundant up to 11 wt sulfides mainly pyrrhotite and pentlandite and negligible amounts of metallic fe ni refractory lithophile abundances are 0 95 x cl intermediate between those in ordinary chondrites oc and ci chondrites abundances of the volatile elements se and zn are greatly enhanced relative to oc the r chondrites are clearly distinguished from other chondrite groups on the basis of al mn and zn mn abundance ratios the oxygen isotopic data plot roughly along a slope 12 line with whole rock 17o values higher than for any other chondrite group rumuruti acfer 217 alh85151 pca91002 and pca91241 have light dark structures and solar wind implanted rare gases indicating that they are regolith breccias the yamato specimens also have light dark structures and are inferred to be regolith breccias carlisle lakes lacks solar rare gases and is unbrecciated based on similarities in refractory lithophile abundances 00 95 x ci oxygen isotope compositions 17o 0 and refractory inclusion abundances 0 1 vol none have been reported the r chondrites probably belong to a noncarbonaceous superclan of chondrites that also includes ordinary and enstatite chondrites the high oxidation state high matrix chondrule modal abundance ratio relatively low abundance of droplet chondrules and high 17o composition suggest that the r chondrites formed at greater heliocentric distances than the oc

non nucleosynthetic heterogeneity in non radiogenic stable hf isotopes implications for early solar system chronology abstract nucleosynthetic heterogeneity and secondary neutron capture reactions may have important implications for 176 lu 176 hf chronology and modelling of early planetary evolution so far the relevance of these phenomena for the lu hf system has not been explored we therefore have analyzed the non radiogenic stable hf isotope composition 177 hf 178 hf 179 hf and 180 hf of meteorites meteorite components and terrestrial rock samples to identify nucleosynthetic or neutron capture induced variations all analyzed chondrites have uniform 178 hf 177 hf and 180 hf 177 hf values that cannot be resolved from the average terrestrial composition thus there is no evidence for nucleosynthetic heterogeneity in chondrites or earth and these data support the use of a chondritic reference value for the hf isotope composition of the bulk silicate earth this homogeneity contrasts with nucleosynthetic heterogeneities found in lighter elements and provides evidence for a separate synthesis of light and heavy r process nuclei various mesosiderite samples and one lunar meteorite display coupled 178 hf 177 hf and 180 hf 177 hf anomalies that are associated with neutron capture induced deviations in 149 sm 154 sm and 150 sm 154 sm however the analyzed chondrites and an aubrite show only sm isotope anomalies and these are the result of neutron capture the hf isotope anomalies require substantial capture of epithermal neutrons whereas sm anomalies result primarily from thermal neutron capture the non radiogenic stable isotope composition of hf is thus a suitable monitor for epithermal neutron capture reactions the data reveal distinct neutron energy spectra mesosiderites are characterized by high epithermal to thermal neutron fluence ratios whereas the remaining samples show low epithermal to thermal ratios secondary neutron capture may significantly increase the measured 176 hf 177 hf in whole rock meteorite samples without causing a resolvable shift in 176 lu 177 hf thus it could potentially induce scatter in lu hf whole rock isochrons and produce spurious initial 176 hf 177 hf values however the slopes of internal i e mineral isochrons cannot be increased significantly by secondary neutron capture this process therefore cannot account for the unrealistically old 176 lu 176 hf ages e g 4 75 ga of some meteorites

chondrites as samples of differentiated planetesimals abstract chondritic meteorites are unmelted and variably metamorphosed aggregates of the earliest solids of the solar system the variety of metamorphic textures in chondrites motivated the onion shell model in which chondrites originated at varying depths within a parent body heated primarily by the short lived radioisotope 26 al with the highest metamorphic grade originating nearest the center allende and a few other chondrites possess a unidirectional magnetization that can be best explained by a core dynamo on their parent body indicating internal melting and differentiation here we show that a parent body that accreted to 200 km in radius by 1 5 ma after the formation of calcium aluminum rich inclusions cais would have a differentiated interior and ongoing accretion would add a solid undifferentiated crust overlying a differentiated interior consistent with formational and evolutionary constraints inferred for the cv parent body this body could have produced a magnetic field lasting more than 10 ma this hypothesis represents a new model for the origin of some chondrites presenting them as the unprocessed crusts of internally differentiated early planetesimals such bodies may exist in the asteroid belt today the shapes and masses of the two largest asteroids 1 ceres and 2 pallas can be consistent with differentiated interiors conceivably with small iron cores with hydrated silicate or ice silicate mantles covered with undifferentiated crusts

nickel isotope heterogeneity in the early solar system article i nfo ni of bulk iron and chondritic meteorites carbonaceous chondrites have variable positive e 62 ni 0 05 to 0 25 whereas ordinary chondrites have negative e 62 ni 0 04 to 0 09 the ni isotope compositions of iron meteorites overlap with those of chondrites and define an array with negative slope in the e 60 ni versus e 62 ni diagram the ni isotope compositions of the volatile depleted group ivb irons are similar to those of the refractory co cv carbonaceous chondrites whereas the other common magmatic iron groups have ni isotope compositions similar to ordinary chondrites only enstatite chondrites have identical ni isotope compositions to earth and so appear to represent the most appropriate terrestrial building material differences in e 62 ni reflect distinct nucleosynthetic components in precursor solids that have been variably mixed but some of the e 60 ni variability could reflect a radiogenic component from the decay of 60 fe comparison of the e 60 ni of iron and chondritic meteorites with the same e 62 ni allows us to place upper limits on the 60 fe 56 fe of planetesimals during core segregation we estimate that carbonaceous

platinum group element fractionation in a komatiitic basalt lava lake precise pge os ir ru pt pd abundance data for bulk rock samples and mineral separates from a deep differentiated komatiitic basalt lava lake in the vetreny belt baltic shield were obtained using an icp ms id technique the composition of the erupted liquid mgo 5 15 is characterized by a highly fractionated pge pattern pd os n 5 48 with subchondritic os ir n 5 0 43 analysis of separated olivine and chromite revealed that ru os and ir were slightly compatible to moderately incompatible in olivine d 5 1 7 0 8 and were compatible with chromite d 5 100 150 platinum and pd were highly incompatible with olivine d 5 0 08 0 03 and moderately compatible with chromite d 5 1 6 3 3 bulk solid liquid partition coefficients for pges ni cr and cu estimated from the whole rock regressions were os 7 9 ir 6 6 ru 4 5 pt 0 53 pd 0 09 ni 6 2 cr 4 4 and cu 0 01 the highly incompatible behavior of cu and pd indicates that there was no separation of fenicu immiscible sulfide liquid during the differentiation of the lava lake comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included 10 2561 mass fraction of pyrrhotite the data obtained provide new constraints on the effects of fractional crystallization on the pge compositions of mafic ultramafic magmas copyright 2001 elsevier science ltd