187re 187os systematics highly siderophile element s se te abundances in the components of unequilibrated l chondrites abstract the 187re 187os systematics abundances of highly siderophile elements hse re platinum group elements and au te se and s as well as major and minor elements were determined in separated components of two unequilibrated l chondrites que 97008 l3 05 and ceniceros l3 7 the 187re 187os systematics are disturbed in the components of both meteorites most likely due to open system behavior of re during terrestrial weathering of que 97008 and alteration on the l chondrite parent body as indicated by an internal errorchron generated for components of ceniceros the hse abundance patterns suggest that the bulk rock abundances were mainly controlled by two different end members non magnetic fractions display lower re os and hse ir than ci chondrites chondrules metal troilite spherules and fine magnetic fractions are depleted in refractory hse and show higher rh ir pd ir and au ir than in ci chondrites the different hse compositions indicate the presence of unequilibrated alloys and loss of refractory hse rich carrier phases from the precursors of some l chondrite components gold is decoupled from other hse in magnetic fractions and shows chalcophile affinities with a grain size dependent variation similar to s and se presumably inherited from preaccretionary processes tellurium is depleted in all components compared to other analysed siderophile elements and its abundance was most likely controlled by fractional condensation and different geochemical affinities the volatility dependent depletion of te requires different physical and chemical conditions than typical for the canonical condensation sequence as represented by carbonaceous chondrites tellurium also shows variable geochemical behavior siderophile in ceniceros predominantly chalcophile in que 97008 these differences may have been inherited from element partitioning during chondrule formation selenium and s on the other hand are almost unfractionated from each other and only show complementary s se in a few components presumably due to the effects of volatility or metal silicate partitioning during chondrule formation terrestrial weathering had negligible effects on the s se and te systematics

the isotopic composition of uranium and lead in allende inclusions and meteoritic phosphates we measured the isotopic compositions of u and pb in eight ca al rich inclusions from the allende chondrite in whitlockite from the st severin chondrite and in a whitlockite and a whole rock sample from the angra dos reis achondrite ador the total range in 235 u 238 u in the acid soluble phases of the allende inclusions and meteoritic whitlockites is from 1 137 6 to 1 138 3 and to within the error limits 3 20 indistinguishable from normal terrestrial u the 235 u 238 u ratios for whitlockites from st severin and ador are the same as that in the bulk meteorites and are normal the pb isotopic compositions of the acid soluble phases of the allende coarse grained inclusions except c 1 give a mean 207 pb 206 pb model age of4 559 0 015ae which is consistent with the observation of normal u isotopic composition the pb isotopic compositions of coarse grained fun inclusion c 1 and of two fine grained inclusions from allende indicate that the u pb systems of these samples have evolved differently from the rest of the meteorite the 207 pb 206 pb model ages for the whitlockites and for bulk ador are the same as those for st severin whitlockite 4 551 0 004ae the pb results from st severin indirectly constrain the value of the u isotopic abundance to be within 7 4 of normal both the directly measured235u 238u and the pb data on st severin are in complete disagreement with claims of large 235 u excess reported by other workers on the same meteorite the th u ratios of some acid soluble phases of allende inclusions and of meteoritic whitlockite show varying degrees of enrichment of th relative to u by factors of 2 to 25 relative to the bulk meteorites previous studies have shown that 244 pu 238 u ratios in whitlockite are also fractionated relative to the bulk meteorites the observation of no isotopic effects in u reported here does not support the claims of either fractionation of 247 cm 238 u or of a large amount of live 247 cm at the early stage of formation of the solar system using the present data we obtain a limit of 247 cm 235 u 4 10 3 at the time of meteorite formation or differentiation this implies that the last major r process contribution to the material in the protosolar nebula was 10 8 years prior to 26 al formation and injection

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

compositions of large metal nodules in mesosiderites links to iron meteorite group iiiab and the origin of mesosiderite subgroups large mesosiderite metal nodules were studied petrographically and by neutron activation their large sizes minimized the effects of silicate contamination and dilution by fe reduced from feo the remarkable mixture of subequal amounts of metal and basaltic and pyroxenitic silicates in mesosiderites can be explained by the accretion of a core like metal mass from one asteroid onto the surface of another asteroid our data show that the accreted mass had a composition roughly similar to that of the iiiab iron meteorite core consistent with other evidence of close genetic links between these and other related groups the siderophile compositional pattern the very limited compositional range the inferred large projectile target ratio and the electrical interconnectedness of mesosiderite metal indicate that the bulk of the metal was molten when accreted mesosiderites have been divided into plagioclase and tridymiterich subgroup a and orthopyroxene rich subgroup b on the basis of modal silicate mineralogy roughly the same subgroups are defined by metal compositions au as ni and cu are generally higher and w lower in subgroup a than in subgroup b in our sample set a widmanstatten pattern is only observed in nodules from subgroup a a plausible model is that subgroup a best preserves the compositions of the target regolithic silicates and accreted core subgroup b is comprised of these materials together with a minor amount of coremantle materials consisting of olivine and refractory ni poor metal the independent covariation of ga re and ir may in part reflect fractionations during impact heating events the anomalous mesosiderite reckling peak a79015 is strongly depleted in refractory siderophiles and slightly enriched in volatile siderophiles such evolved metal requires a formation distinct from common mesosiderite metal

age of the eucrite caldera from convergence of long lived and short lived chronometers abstract the long lived 147 sm 143nd chronometer has been used in conjunction with the short lived 146 sm 142 nd and 53 mn 53 cr chronometers to constrain the time of formation for the eucrite caldera the combination of these three chronometers has produced results that are consistent with one another and converge on a precise high resolution age of 4537 12 ma within 95 confidence limits so far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite thus this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body most likely between 17 and 41 million years after the formation of the first condensates in the early solar system from these age constraints it is evident that caldera either formed within a magma body on a large 500 km asteroid or in an extensive impact melt on the crust of its parent body

the duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal abstract precise ratios of w and re relative to ir determined by laser ablation icp ms reveal that matrix metal in equilibrated ordinary chondrites eoc exhibits a correlated variation of these ratios in unequilibrated ordinary chondrites uoc the w ir ratio varies by over two orders of magnitude w ir 0 003 0 6 and shows no correlation with any other siderophile element most matrix metal in uoc is depleted in w relative to metal in eoc thus w must enter metal during metamorphism the correlation between w ir and re ir is evident in type 4 chondrites this implies that the abundance and isotopic composition of w is set in ordinary chondrite metal by petrologic type 4 and that the reductant must be exhausted to limit further isotopic exchange as indicated by the 182 hf 182 w ages of ordinary chondrite metal lee and halliday science 274 1996 1876 1879 this offers a means of dating the onset of metamorphic heating of oc parent bodies a discrepancy in ages between 182 hf 182 w lee and halliday science 274 1996 1876 1879 and 207 pb 206 pb gopel et al earth planet sci lett 121 1994 153 171 or 129 i 129 xe brazzle et al geochim cosmochim acta 63 1999 739 760 dating is real and measures the interval of metamorphism the time difference between reduction of w during the onset of heating and isotopic closure of pb or xe by cooling e g t m 2 12 myr for h chondrites the 187 re 187 os and 182 hf 182 w ages are set by the same process metamorphic equilibration establishing an important link between an absolute chronometer t 1 2 187 re 41 6 billion years and a short lived radioisotope system t 1 2 182 hf 9 myr

sm nd chronology and petrogenesis of mesosiderites we have obtained sm nd data from four mesosiderite silicate clasts including three clasts with a variety of textures from the vaca muerta type 1a mesosiderite and one gabbroic clast from the mt padbury mesosiderite the gabbroic vaca muerta pebble 12 and basaltic pebble 16 yield identical 147 sm 143 nd ages of 4 48 0 19 ae and 4 48 0 09 ae respectively while the highly recrystallized pebble 5 gives an age of 4 42 0 02 ae mt padbury yields an age of 4 52 0 04 ae all clasts show a correlation of 142 nd 144 nd with 147 sm 144 nd and provide unequivocal evidence for the presence of live 146 sm at the time of their formation calculated initial 146 sm 144 sm values range from 0 004 pebble 5 to 0 006 pebbles 12 16 and mt padbury and are generally consistent with the 147 sm 143 nd ages however discordance of whole rock leach and residue data and some disagreement between 146 sm 142 nd relative ages and 147 sm 143 nd absolute ages indicate small but significant disturbances to the sm nd systematics the ranges of ages and initial 146 sm 144 sm and 143 nd 144 nd values suggest that each of these silicate clasts underwent a separate protracted evolution on its parent body prior to mixing with metal textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite pebble 12 is extremely lree depleted probably a result of several melt extraction events whereas pebbles 5 and 16 and mt padbury have nearly chondritic sm nd with bulk ree concentrations higher than chondrites by factors of 5 to 15 in general the sm nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation preservation of diverse old ages and the presence of 146 sm imply that metal silicate mixing did not seriously alter the sm nd isotopic memories of these clasts we present a model for metal silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing we show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature depth of burial grain size and diffusion parameters application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing then the clasts must have been buried no deeper than 1 10 m in regolith during the initial high temperature cooling phase in order to prevent the sm nd systems from being extensively reset in order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal silicate mixing and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at 600 700 c the data from this study point to the following history for the mesosiderite parent body 1 differentiation of a silicate parent body within the first 50 m y of solar system history to form diverse parent magmas 2 emplacement of primitive and differentiated mafic magmas near the planetary surface and extensive differentiation in the near surface environment 3 formation and reworking of regolith breccias through impact gardening at the near surface of the body 4 mixing of molten fe ni metal with the regolith followed by rapid cooling 100 150 m y after the origin of the solar system 5 slow cooling from temperatures of 700 c to produce the observed nickel diffusion profiles in the iron phase 6 mild impact metamorphism and brecciation to obtain the much younger k ar ages and 7 recent collisions or perturbations sending mesosiderite fragments into earth crossing trajectories

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

domestic origin of opaque assemblages in refractory inclusions in meteorites experimental studies indicate that opaque assemblages rich in refractory siderophile elements were formed within host calcium and aluminium rich inclusions cais by exsolution oxidation and sulphidization of homogeneous alloys rather than by aggregation of materials in the solar nebula before the formation of cais these opaque assemblages are thus not the oldest known solid materials as was once thought and they do not constrain processes in the early solar nebula before cai formation instead the assemblages record the changing oxygen fugacity experienced by cais during slow cooling in nebular and or planetary environments

the origin and nebular history of the metal phase of ordinary chondrites abstract we present new inaa results for bulk metal from h and ll chondrites both have characteristic elemental patterns similar to l chondrite metal in particular increasing abundances of w and ga from unequilibrated to equilibrated chondrites but a reverse variation of v and cr abundances these characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material the similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor similar to ci or cm material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites co ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups this suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions the chondrites that accreted earliest have preserved their melting characteristics while those that accreted later reequilibrated with the ambient gas at different temperatures the differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies the initial metal composition established during the melting stage 13 wt ni co ni 0 031 is inferred from the metal in highly unequilibrated chondrites the accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction h chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups ll chondrites were accreted at the lowest temperature and are most oxidized compositionally the metal component that would account for the fe si fractionation among h l and ll chondrites is different from the metal formed by melting indicating that the different fe s ratios of h l and ll chondrites were established before their accretion

evidence for successive episodes of condensation at high temperature in a part of the solar nebula abstract a centimeter sized ca al rich inclusion cai 3643 in the allende meteorite has preserved a record of its multiple stages of growth by condensation at high temperature it consists of three distinct concentric layers core mantle and crust the open textured core is composed of hibonite laths interstitial melilite and voids grains of alumina and osir metal nuggets in the inner core that are complementary in composition to rupt rich nuggets in the outer core the core formed out of rapidly aggregated crystals from two episodes of condensation in the same gaseous reservoir mo and w depletions in the nuggets indicate condensation from a relatively oxidizing gas with 60 times higher h 2 oh 2 than solar gas the compact mantle of coarse melilite contains perovskite inclusions and rare nife metal and sulfide grains the compact fine grained crust consists of melilite hibonite spinel perovskite and opaques similar to those in the mantle neither mantle nor crust has been molten each grew by condensation of solids directly onto the surface of the cai the formed cai experienced some metamorphic recrystallization of melilite and volatile alteration producing grossular feldspathoids and hercynite trace element analyses of perovskites and a bulk sample show the mantle and crust to be depleted in refractory elements i e group ii and complementary to the ultra refractory core despite differences in texture and composition the core mantle and crust display a continuity in their modal mineralogical compositions their extreme al 2 o 3 enrichments 53 69 cf 25 45 for other cai s and their trace element complementarity this implies that all the layers of 3643 formed from the same cooling gaseous reservoir within a limited interval of time and space consistent with an origin in a local transient heating event in the cv chondrite formation region of the nebula

age and structure of the southern rockall trough new evidence abstract the rockall trough separates the rockall plateau microcontinent from the shelf and slope west of the british isles the structure and age of the trough has been the source of considerable discussion although widely considered to be of oceanic origin postulated ages for the spreading range from permian to cretaceous new seismic profiles linked to the ipod sites in the bay of biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern rockall trough it is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the albian maastrichtian interval

osmium isotope constraints on contrasting sources and prolonged melting in the proterozoic upper mantle evidence from ophiolitic ru os sulfides and ru os ir alloys this study presents the first extensive data set of os isotopic compositions of primary ru os ir alloys and ru os sulfides derived from two neoproterozoic ophiolite type ultramafic massifs kunar in northern taimyr russia and kraubath in eastern alps austria the study employed a multi technique approach and utilized a number of analytical techniques including hydroseparation electron microprobe analysis negative thermal ionization mass spectrometry n tims and laser ablation attached to multiple collector inductively coupled plasma mass spectrometry la mc icp ms the results identify two different os isotope sources subchondritic and suprachondritic for bedrock ru os sulfides at kraubath whereas detrital ru os ir alloys at kunar are characterized by unradiogenic 187os 188os values indicative of a chondritic or subchondritic mantle source of platinum group elements pge the unradiogenic 187os 188os values yield a very wide range of 187os 188os 0 1094 0 0003 to 0 1241 0 0004 n 19 n tims data which is almost identical to the os isotope composition of bedrock laurite ru os s2 in podiform chromitite within an ophiolitic mantle section at kraubath 0 11249 0 00062 to 0 12437 0 00051 n 17 n tims and la mc icp ms data these results are consistent with a model in which a prolonged history of melting events of parent ultramafic source rocks took place in the oceanic upper mantle in contrast erlichmanite and laurite from banded chromitite within the transition zone at kraubath show high 187os 188os 0 13080 to 0 13212 values indicative of a suprachondritic source of pge this signature might be interpreted as either evidence of an enriched mantle source or an indication of a radiogenic crustal component which was introduced during a subduction related event the os isotope results obtained support the conclusion that the re os system represented by pgm has remained unchanged from the time of formation of the pgm until now despite later thermal events which occurred in the vicinity of ophiolite type complexes

distribution of platinum group elements and rhenium between metallic phases of iron meteorites abstract in situ measurement of the platinum group elements pge and re by laser ablation icpms allows fractionation patterns to be established between host kamacite and ni rich exsolved phases taenite and plessite measurements on two iab iron meteorites canyon diablo and odessa show that for the ni rich phases a there is no significant fractionation for ru and rh b pd is enriched and c ir os and pt are strongly depleted we suggest that in the case of kamacite taenite and plessite the controlling mechanism is ionic size with the order of ions small to large being ir os pt re rh ru pd this progression plots as a smooth curve on a diagram of relative abundance distribution vs ionic size a comparison of pge and re data on iron meteorites with published data from ci chondrites indicate that there is little or no relative fractionation between the elements the exception is re that is clearly enriched in kamacite relative to chondrites this confirms earlier observations on the fractionation of re os between bulk iab irons and ci values 1 metallic veins cutting graphite inclusions within the canyon diablo iab meteorite are interpreted as a melt fraction from the iron meteorite abundance ratios for pge and re between kamacite and the metallic vein are similar to abundance ratios between kamacite and ni rich phases the fact that those elements with large ionic radii rh ru and pd are concentrated in the metallic veins adds weight to the view that they represent a melt fraction our in situ measurements of the pge and re demonstrate that elemental fractionation takes place both during melt segregation and solid state diffusion exsolution the data have important implications for the re os geochronometer and indicate that measurements of187os 186os ratios on individual phases within iron meteorites may provide a potential mineral isochron

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

rhenium osmium isotope systematics in meteorites i magmatic iron meteorite groups iiab and iiiab abstract using resonance ionization mass spectrometry rims re and os abundances were determined by isotope dilution id and 187 os 186 os ratios measured in nineteen iron meteorites eight from group iiab ten from group iiiab and treysa iiib anomalous abundances range from 1 4 to 4800 ppb re and from 13 to 65000 ppb os and generally agree well with previous id and neutron activation naa results the re and os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization addition of late formed metal to produce re os variation in the b subgroups is not essential but cannot be excluded whole rock isochrons for the iiab and iiiab groups are statistically indistinguishable pooled data yield an initial 187 os 186 os of 0 794 0 010 with a slope of 7 92 0 20 10 2 corresponding to a magmatic iron meteorite age of 4 65 0 11 ga using a decay constant of 1 64 10 11 a 1 given the errors in the slope and half life this age does not differ significantly from the canonical chondrite age of 4 56 ga but could be as young as 4 46 ga

182w and 187re 187os systematics of iron meteorites chronology for melting differentiation and crystallization in asteroids w isotopic compositions measured by n tims in five iron meteorite groups are used in conjunction with 187re 187os isotopic data to constrain the timing of metal segregation and crystallization respectively in the iron meteorite parent bodies fractionation of the parent isotope 182hf half life 9 2 ma from the daughter 182w likely occurred during segregation of metal from silicate in the iron meteorite parent bodies if w isotopic equilibrium was achieved between silicate and metal during metal segregation then the w isotopic compositions in the irons should record the timing of this event samples were taken from the five largest iron meteorite groups including the four magmatic groups iiab iiiab iva and ivb and the nonmagmatic iab iiicd group together these groups comprise approximately 90 by weight of iron meteorites classified so far with the exception of the iva group the groups have well defined 187re 187os isochron ages which are interpreted as the time of crystallization of the metal of their parent bodies within each magmatic group samples that crystallized both early and late in the sequence of each core were analyzed to evaluate possible evolution of the w isotopic composition during crystallization all the samples examined display very similar w values within a narrow range of compositions between 5 1 and 3 1 witzation began less than a few percent of additional metal was segregated to the cores moreover the average w values for the five groups have no resolvable difference ranging from 4 08 0 69 iva to 3 30 0 15 iab iiicd these data indicate that all of the iron meteorite parent bodies underwent metal segregation within 5 ma of each other apparently independent of their composition size and time of crystallization of the metal in all groups except the iiiab group there is a resolvable time interval between the timing of metal segregation as constrained by the w isotopic data and the initiation of core crystallization as given by the re os isotopic systematics

chondrule textures and precursor grain size an experimental study chondrule formation appears to have been a major event in the early solar system but chondrule properties do not allow us to distinguish between several possible formation mechanisms the physical nature of the precursors especially grain size must affect the textures of the chondrules they yield when heated we melted precursors of different grain sizes including extremely fine grained crystalline aggregates analogous to nebular condensates to see whether objects resembling most natural chondrules can be crystallized with one minute heating and moderate cooling rates the grain size of the charges depends directly on the grain size of the starting material for temperatures up to very close to the liquidus temperature a single rapid heating of condensate like material thus produces very fine grained chondrules like dark zoned chondrules for a very wide range of peak temperatures it is incapable of generating the observed textural distribution of chondrules which are predominantly porphyritic the simplest model for chondrules a single heating of unmodified condensate material therefore appears unrealistic coarse grained chondrules might be formed from fine grained precursors by extended heating with evaporation leading to coarsening or by multiple reheating events with higher temperatures in subsequent events otherwise an origin from annealed condensates planetary rocks or by condensation of liquid and crystals is required

beringian climate during the late pleistocene and holocene abstract the difference between the climates of the bering land bridge and asiatic western beringia 18 20 ka ago is discussed the climatic reconstructions of the bering land bridge and western beringia are based upon two methods first we discuss the ecological and thermal requirements of the thermophilic weevils coleoptera curculionidae in the genus stephanocleonus second we discuss the effect of climatic continentality on soil temperatures we suggest that for western beringia the climate was suitable for the warm steppe environment mean july air temperatures were at least 10 11 c and an annual sum of daily temperatures above 0 c sdd at the soil surface was up to 2500 c in contrast to these conditions the climate of the bering land bridge was very similar to that found in the modern arctic tundra zone

contrasting re os magmatic fractionation in planetary basalts abstract this work reports the first re os isotopic results for lunar basalts and eucrites both re and os concentrations are found to be very low and re os ratios are close to chondritic this contrasts with the much higher than chondritic re os ratio of terrestrial basalts osmium displays a rather uniform solid liquid fractionation in different planetary bodies moon earth eucrite parent body mars whereas rhenium is about three orders of magnitude more incompatible in terrestrial basalt genesis we argue that this is a function of oxygen fugacity in the source regions of basalts

composition and formation of metal nodules and veins in ordinary chondrites abstract five large metal nodules a composite sample of five small metal nodules one troilite nodule and two metal veins from five ordinary chondrites were analyzed by electron microprobe and neutron activation analysis metal nodules and veins in h chondrites generally consist of large single crystals of kamacite whereas l nodules contain significant taenite most nodules and veins are depleted by large factors ranging up to 240 in refractory siderophiles re os ir pt tungsten normally a refractory siderophile and au as and ga volatile siderophiles have abundance ratios similar to those of the common siderophiles fe co and ni we propose that the metal with extremely low refractory element contents was produced by shock induced vaporization of chondritic material the refractory elements condensed near the point of vaporization and were not transported with the vapor because the shock generated gas was mildly oxidizing w formed volatile oxides the low ni fe ratio and the resulting high kamacite contents may reflect dilution by fe from dissociated fes subsequent condensation of the refractory depleted vapor into fractures and voids formed the metal veins and nodules

the origin and history of the metal and sulfide components of chondrules abstract fourteen siderophile and other non lithophile elements determined in 31 semarkona ll3 0 chondrules by neutron activation analysis are severely fractionated relative to lithophile elements their chondrule whole rock abundance ratios vary by factors of up to 1000 the mean ratio is 0 2 non refractory siderophile abundance patterns in ni rich chondrules are smooth functions of volatility and in ni poor chondrules patterns are more irregular refractory siderophile elements are often fractionated from ni they covary confirming the presence of a refractory metal component the chalcophile element se correlates with br and siderophile elements zinc is uniformly low and uncorrelated with other elements most metal and sulfide in chondrules was probably present in the solar nebula before chondrule formation most siderophile and chalcophile elements were in these materials some fe was also in silicates as were minor amounts of ni co au ge and possibly se the amount of metal formed by reduction during chondrule melting was minor the common metal component in chondrules is similar to and may be the same as the common component involved in the metal silicate fractionation of the ordinary chondrite groups chondrules are depleted in metal chiefly because they sampled metal poor precursor assemblages metal segregation during the molten period and subsequent loss was a minor process that may be responsible for most surface craters on chondrules

drainage reorganization during breakup of pangea revealed by in situ pb isotopic analysis of detrital k feldspar pb isotopes in detrital k feldspar grains provide a powerful provenance tracer for feldspathic sandstones common pb isotopic compositions show broad hundred kilometer scale regional variation and this signature can survive weathering transport and diagenesis the feldspar pb signature can be measured rapidly using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms and careful targeting can avoid inclusions and altered regions within grains here we combine a new pb domain map for the circum north atlantic with detrital k feldspar pb isotopic data from triassic and jurassic sandstones from basins on the irish atlantic margin the pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern triassic sandstones originated from distant archean and paleoproterozoic rocks probably in green land labrador and the rockall bank to the nw implying long 500 km transport across a nascent rift system later jurassic sandstones had a composite paleo and mesoproterozoic source in more proximal sources to the north

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

fine nickel poor fe ni grains in the olivine of unequilibrated ordinary chondrites abstract fine 2 m ni poor 10 mg g fe ni grains are common inclusions in the olivine in porphyritic chondrules in unequilibrated ordinary chondrites the olivine grains appear to be relicts that survived chondrule formation without melting the most common occurrence of this dusty metal is in the core of olivine grains having clear fe poor rims and surrounded either by small euhedral clear olivine grains zoned with feo increasing toward the border of the grains or by large elongated fe poor orthopyroxenes oriented parallel to the chondrule surface and enclosing small round olivine grains various amounts of ca al rich glass are always present the dusty metal is occasionally found in the rims of olivine grains either isolated in the matrix or included in chondrules a rare occurrence is as bands in highly deformed olivines this dusty metal appears to be the product of in situ reduction of feo from the host olivine among the possible reductants h2 or carbonaceous matter ch2 n seem the most likely hydrogen may have been implanted by solar wind or solar flare irradiation but this requires that dissipation of nebular gas occurred before the end of the chondrule formation process carbonaceous matter may have been implanted by shock less likely reductants are nebular co or c dissolved in the olivine lattice the large relict olivine grains may be nebular condensates or more likely fragments broken off earlier generations of chondrules

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

an analytical electron microscopy aem investigation of opaque inclusions in some type 6 ordinary chondrites abstract a large number of ordinary chondrites contains micron sized particles of metal and or troilite dispersed in their silicate grains such metallic phases are responsible for the so called darkening of the silicate grains and might be either precipitates which formed during reduction of the silicates or inclusions injected as a melt during a shock event we have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered metamorphosed ordinary chondrites saint severin ll6 tsarev l6 and kernouve h6 we also looked for remnant shock indices our tem observations suggest the following sequence of events in the three meteorites first a number of relatively strong shock events occurred on the parent body bodies producing an fe fes melt that was injected into silicate grains along a dense network of open fractures most of these shock defects were subsequently erased by high temperature 700 900 c thermal metamorphism some remnants of the shock events are the observed trails of tiny metal and or sulfide inclusions that formed as a result of fracture healing chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high temperature annealing event resulting in ni rich inclusions this effect was especially pronounced in the l and ll chondrites studied during subsequent cooling of the body bodies inclusions of chromite and phosphate precipitated nucleating preferentially on lattice defects dislocations subgrain boundaries and on the metal and sulfide inclusions a later shock event of moderate intensity probably corresponding to the separation of the meteorite from its parent body produced new shock features in the silicate grains of the saint severin meteorite including mechanical twins in diopside and straight free screw dislocations in olivine