26al in chondrules from unequilibrated l chondrites onset and duration of chondrule formation in the early solar system abstract al mg isotope records in sixteen ferromagnesian chondrules from five unequilibrated ordinary chondrites uocs four l chondrites with petrologic grades varying from 3 0 to 3 2 and one l ll chondrite with petrologic grade 3 1 were studied using an ion microprobe to look for presence of the now extinct nuclide 26 al at the time of their formation uocs of low petrologic grades were selected to avoid possible thermal perturbation of al mg isotope records in the analyzed chondrules the presence of resolved 26 mg excess due to decay of 26 al in al rich phases such as glassy mesostasis and plagioclase was found in all the chondrules barring a sole exception the inferred initial 26 al 27 al ratios in the chondrules range from 0 6 1 9 10 5 and most of them have values lying between 0 6 1 1 10 5 our data supports a late formation of chondrules relative to cais suggested in previous studies and indicates a time lag of at least 1 5 ma between cai formation and onset of an intense episode of chondrule formation in the early solar system that lasted less than a million years the paucity of chondrules 26 al 27 al 10 5 rules out active chondrule forming events at earlier epochs the presence of rare chondrules with initial 26 al 27 al 5 in uocs of low petrologic grades 3 0 to 3 2 analyzed by us and also reported previously in uocs of petrologic grade ranging up to 3 4 most probably represent parent body processes leading to disturbance of al mg isotope systematics in these cases an extended duration of chondrule formation lasting more than a couple of million years suggested in some earlier studies needs reassessment a long duration of chondrule formation is also difficult to reconcile with the problem of storage isolation mixing and assimilation of chondrules into their meteorite parent bodies

evolution of protoplanetary disk inferred from 26 al chronology of individual chondrules abstract we review the 26al ages of chondrules in various type 3 0 chondrites the 26al ages of chondrules are 1 3 myr after calcium aluminum rich inclusion cai for ll3 0 co3 0 and acfer 094 ungrouped c 3 0 available data for chondrules in cr chondrites indicate that many chondrules are relatively younger 3 myr although data from chondrules in cr3 0 are not yet available to confirm their younger ages the total ranges for the 26al ages of chondrules in a single chondrite group are more than 0 5 1 myr however most chondrules show relatively narrow range of ages in a single chondrite group 0 2 0 4 myr 1 sd which might be short enough to preserve the group specific chemical and isotope signatures against radial diffusion of solid in the disk distinct oxygen isotope reservoirs might exist in the protoplanetary disk simultaneously which could be spatially separated

the age of the solar system redefined by the oldest pb pb age of a meteoritic inclusion the age of the solar system is defined by the formation of the first solid grains in the solar nebula pb pb age dating of these solids which were later trapped in a meteorite indicates that the solar system is 0 34 1 91 million years older than previously thought the age of the solar system can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto sun this age is estimated by dating calcium aluminium rich inclusions in meteorites these inclusions are considered as the earliest formed solids in the solar nebula their formation marks the beginning for several long and short lived radiogenic clocks that are used to precisely define the timescales of solar system events such as the formation and evolution of planetary bodies1 2 3 here we present the 207pb 206pb isotope systematics in a calcium aluminium rich inclusion from the northwest africa 2364 cv3 group chondritic meteorite which indicate that the inclusion formed 4 568 2 million years ago this age is between 0 3 refs 4 5 and 1 9 refs 1 6 million years older than previous estimates and is the oldest age obtained for any solar system object so far we also determined the 26al 26mg model age of this inclusion and find that it is identical to its absolute pb pb age implying that the short lived radionuclide 26al was homogeneously distributed in the nebular disc surrounding the proto sun from the consistently old ages in the studied inclusion we conclude that the proto sun and the nebular disc formed earlier than previously thought

oxygen isotope compositions of chondrule phenocrysts and matrix grains in kakangari k grouplet chondrite implication to a chondrule matrix genetic relationship abstract to investigate a possible relationship between chondrules and matrix we studied mineralogy mineral chemistry and in situ o isotope compositions of chondrules clastic matrix grains and amoeboid olivine aggregates aoas in the kakangari k grouplet chondrite most olivines and low ca pyroxenes in the kakangari chondrules matrix and aoas have similar magnesium rich compositions fo 95 97 0 3 0 5 wt mno and en 90 96 respectively these rather uniform chemical compositions of the different chondritic components are likely due to partial fe mg mn equilibration during thermal metamorphism experienced by the host meteorite oxygen isotope compositions of olivine and low ca pyroxene grains in chondrules and matrix plot along a slope 1 line on a three o isotope diagram and show a range from 16 o enriched composition similar to that of the sun to 16 o depleted composition similar to the terrestrial o isotope composition most olivines and low ca pyroxenes in chondrules are 16 o poor and plot on or close to the terrestrial mass fractionation line mean 17 o values 2 standard deviations 0 0 0 8 and 0 2 0 9 for olivine and pyroxene respectively consistent with the previously reported compositions of bulk chondrules 17 o 0 16 0 70 in addition to these 16 o poor grains a coarse grained igneous rim surrounding a porphyritic chondrule contains abundant 16 o rich relict olivines 17 o 24 oxygen isotope compositions of olivines and low ca pyroxenes in matrix show a bimodal distribution 12 out of 13 olivine and 4 out of 17 pyroxene grains measured are similarly 16 o rich 17 o 23 5 2 9 others are similarly 16 o poor 17 o 0 1 1 7 due to slow oxygen self diffusion olivines and low ca pyroxenes largely retained their original oxygen isotope compositions the nearly identical o isotope compositions between the chondrule phenocrysts and the 16 o poor matrix grains suggest both chondrules and matrix of kakangari sampled isotopically the same reservoirs in addition the presence of abundant 16 o rich grains in matrix and the chondrule igneous rim suggests both components acquired similar precursor inventories these observations imply that chondrules and matrix in kakangari are genetically related in the sense that material that formed matrix was one of the precursors of chondrules and chondrules and some fraction of matrix experienced the same thermal processing event the 16 o enriched bulk matrix value compared to the bulk chondrules reported previously is likely due to presence of abundant 16 o rich grains in the kakangari matrix

hf w chronometry of core formation in planetesimals inferred from weakly irradiated iron meteorites abstract the application of hf w chronometry to determine the timescales of core formation in the parent bodies of magmatic iron meteorites is severely hampered by 182w burnout during cosmic ray exposure of the parent meteoroids currently no direct method exists to correct for the effects of 182w burnout making the hf w ages for iron meteorites uncertain here we present noble gas and hf w isotope systematics of iron meteorite samples whose w isotopic compositions remained essentially unaffected by cosmic ray interactions most selected samples have concentrations of cosmogenic noble gases at or near the lowermost level observed in iron meteorites and for iron meteorite standards have very low noble gas and radionuclide based cosmic ray exposure ages

microchondrules in three unequilibrated ordinary chondrites we report on a suite of microchondrules from three unequilibrated ordinary chondrites uocs microchondrules a subset of chondrules that are ubiquitous components of uocs commonly occur in fine grained chondrule rims although may also occur within matrix microchondrules have a variety of textures cryptocrystalline microporphyritic radial glassy in some cases their textures and in many cases their compositions are similar to their larger host chondrules bulk compositions for both chondrule populations frequently overlap the primary material that composes many of the microchondrules has compositions that are pyroxene normative and is similar to low ca pyroxene phenocrysts from host chondrules primary material rarely resembles olivine or plagioclase some microchondrules are composed of feo rich material that has compositions similar to the bulk submicron fine grained rim material these microchondrules however are not a common compositional type and probably represent secondary feo enrichment microchondrules may also be porous suggestive of degasing to form vesicles our work shows that the occurrence of microchondrules in chondrule rims is an important constraint that needs to be considered when evaluating chondrule forming mechanisms we propose that microchondrules represent melted portions of the chondrule surfaces and or the melt products of coagulated dust in the immediate vicinity of the larger chondrules we suggest that through recycling events the outer surfaces of chondrules were heated enough to allow microchondrules to bud off as protuberances and become entrained in the surrounding dusty environment as chondrules were accreting fine grained rims microchondrules are thus byproducts of cyclic processing of chondrules in localized environments their occurrence in fine grained rims represents a snapshot of the chondrule forming environment we evaluate mechanisms for microchondrule formation and hypothesize a potential link between the emergence of type ii chondrules in the early solar system and the microchondrule bearing fine grained rims surrounding type i chondrules

26al in plagioclase rich chondrules in carbonaceous chondrites evidence for an extended duration of chondrule formation abstract the 26al 26mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase rich chondrules prcs from 13 carbonaceous chondrites ccs one ungrouped acfer 094 six cr five cv and one co reveal large variations in the initial 26al 27al ratio 26al 27al 0 well resolved 26mg excesses 26mg from the in situ decay of the short lived nuclide 26al t1 2 0 72 ma were found in nine chondrules two from acfer 094 five from the cv chondrites allende and efremovka and one each from the paired cr chondrites eet 92147 and eet 92042 with 26al 27al 0 values ranging from 3 10 6 to 1 5 10 5 data for seven additional chondrules from three cv and two cr chondrites show evidence suggestive of the presence of 26al but do not yield well defined values for 26al 27al 0 while the remaining chondrules do not contain excess radiogenic 26mg and yield corresponding upper limits of 11 2 10 6 for 26al 27al 0 the observed range of 26al 27al 0 in prcs from ccs is similar to the range seen in chondrules from unequilibrated ordinary chondrites uocs of low metamorphic grade 3 0 3 4 however unlike the uoc chondrules there is no clear trend between the 26al 27al 0 values in prcs from ccs and the degree of thermal metamorphism experienced by the host meteorites high and low values of 26al 27al 0 are found equally in prcs from both ccs lacking evidence for thermal metamorphism e g crs and ccs where such evidence is abundant e g cvs the lower 26al 27al 0 values in prcs from ccs relative to most cais are consistent with a model in which 26al was distributed uniformly in the nebula when chondrule formation began approximately a million years after the formation of the majority of cais the observed range of 26al 27al 0 values in prcs from ccs is most plausibly explained in terms of an extended duration of 2 3 ma for the formation of cc chondrules this interval is in sharp contrast to most cais from ccs whose formation appears to be restricted to a narrow time interval of less than 105 years the active solar nebula appears to have persisted for a period approaching 4 ma encompassing the formation of both cais and chondrules present in ccs and raising important issues related to the storage assimilation and mixing of chondrules and cais in the early solar system

high precision oxygen three isotope analyses of anhydrous chondritic interplanetary dust particles abstract oxygen three isotope ratios of three anhydrous chondritic interplanetary dust particles idps were analyzed using an ion microprobe with a 2 m small beam the three anhydrous idps show 17o values ranging from 5 to 1 which overlap with those of ferromagnesian silicate particles from comet wild 2 and anhydrous porous idps for the first time internal oxygen isotope heterogeneity was resolved in two idps at the level of a few per mil in 17o values anhydrous idps are loose aggregates of fine grained silicates 3 m in this study with only a few coarse grained silicates 2 20 m in this study on the other hand wild 2 particles analyzed so far show relatively coarse grained few m igneous textures if anhydrous idps represent fine grained particles from comets the similar 17o values between anhydrous idps and wild 2 particles may imply that oxygen isotope ratios in cometary crystalline silicates are similar independent of crystal sizes and their textures the range of 17o values of the three anhydrous idps overlaps also with that of chondrules in carbonaceous chondrites suggesting a genetic link between cometary dust particles wild 2 particles and most anhydrous idps and carbonaceous chondrite chondrules

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

magnesium diffusion in plagioclase dependence on composition and implications for thermal resetting of the 26al 26mg early solar system chronometer article history experimental data are reported on mg diffusion in plagioclase crystals with a range of anorthite content xan at temperatures between 800 and 1150 c oriented and polished single crystals of anorthite xan 0 93 labradorite xan 0 67 andesine xan 0 43 and oligoclase xan 0 23 were each embedded in powered source material enriched in natural mgo or 25mgo and suspended in a furnace at constant temperature diffusion profiles in quenched samples were measured from the polished surface using sims depth profiling the diffusion coefficient does not depend significantly on the mg concentration gradient and little anisotropy is observed between the b and c directions in labradorite diffusion coefficients increase systematically with decreasing xan and the entire data set is described by ln d 6 06 1 10 7 96 0 42 xan

26al records in chondrules from unequilibrated ordinary chondrites ii duration of chondrule formation and parent body thermal metamorphism abstract fossil records of the now extinct short lived nuclide 26al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites uocs semarkona 3 0 bishunpur and y 791324 both 3 1 and alha76004 3 3 belonging to the ll group to infer their time of formation the present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of uoc chondrule formation nebular processes responsible for chondrule formation started 1 ma after formation of cais and effectively stopped within another one and a half million years in the inner region of the asteroidal belt modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of uoc chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in uoc parent bodies the duration of formation of uoc chondrules is significantly shorter than that for carbonaceous chondrite chondrules

geochemistry and petrogenesis of the early palaeozoic appinite granite complex in the western kunlun orogenic belt nw china implications for palaeozoic tectonic evolution the datong pluton the largest early palaeozoic granitoid in the western kunlun orogenic belt wkob in nw china is a typical appinite granite complex it consists of diorites quartz diorites monzodiorites quartz monzodiorites monzonites quartz monzonites syenites granodiorites and monzogranites laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating yielded crystallization ages of 459 3 ma for the quartz monzonites and 452 5 ma for the monzogranites late ordovician the rocks possess a wide range of sio2 56 0 73 4 wt mgo 0 17 4 55 wt and mg no 25 60 with high k2o 2 83 5 29 wt contents exhibiting high k calc alkaline to shoshonitic traits they are characterized by enrichments in large ion lithophile elements liles and light rare earth elements lrees as well as depletions in high field strength elements hfses the rocks have initial 87sr 86sr ratios of 0 7086 0 7185 negative end t values of 3 72 to 1 79 and ehf t values vary from 1 6 to 4 7 these features are modelled to show that they were most likely derived from an enriched lithospheric mantle source and that fractional crystallization with minor crustal contamination was involved in their petrogenetic process considering the distribution and chronology of the palaeozoic intrusions such as kegang bulong qiukesu yierba north kudi dongbake buya ayilixi and warengzilafu granitoid plutons with ages of c 420 530 ma in conjunction with the palaeozoic metamorphic overprinting in the wkob we propose a divergent double subduction model to explain the destruction of the proto tethys ocean and suggest that the datong pluton was likely emplaced in a post collisional setting following the termination of subduction in response to slab break off

bleached chondrules evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites we present the first detailed study of a population of texturally distinct chondrules previously described by kurat 1969 christophe michel levy 1976 and skinner et al 1989 that are sharply depleted in alkalis and al in their outer portions these bleached chondrules which are exclusively radial pyroxene and cryptocrystalline in texture have porous outer zones where mesostasis has been lost bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4 6 they are most abundant in the l and ll groups apparently less common in h chondrites and absent in enstatite chondrites we used x ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites we studied bleached chondrules from semarkona by ion microprobe for trace elements and h isotopes and by transmission electron microscopy chondrule bleaching was the result of low temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism during aqueous alteration interstitial glass dissolved and was partially replaced by phyllosilicates troilite was altered to pentlandite but pyroxene was completely unaffected calcium rich zones formed at the inner margins of the bleached zones either as the result of the early stages of metamorphism or because of fluid chondrule reaction the mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types the ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories but there is no relationship between the degree of alteration and metamorphic grade a correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter ordinary chondrites were probably open systems after accretion and aqueous fluids may have carried volatile elements with them during dehydration individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching

hf w chronology of ordinary chondrites abstract fifteen h l and ll ordinary chondrites of petrologic types 4 6 have been analyzed for hf w isotope systematics to constrain the chronology internal structure and thermal history of their parent bodies for most samples coarse grained metals plot below the isochrons defined by silicate dominated fractions which consist of variable mixtures of silicate minerals with tiny metal inclusions this offset results from an earlier hf w closure in the large metal grains and provides a new means for simultaneously determining cooling rates and hf w closure ages for individual samples for most type 5 and 6 samples cooling rates and hf w ages are inversely correlated indicating that these samples derive from concentrically zoned bodies in which more strongly metamorphosed samples derive from greater depth these data therefore provide strong evidence for a common onion shell structure for the h l and ll chondrite parent bodies the cooling rates and hf w ages of some type 5 and 6 chondrites overlap indicating that the hf w systematics provide a more robust measure of the thermal history and burial depth of a given sample than the simple petrographic distinction between types 5 and 6 two type 6 samples deviate from the correlation between cooling rates and hf w ages and cooled much faster than expected for their hf w age these samples likely were excavated by impacts that occurred during high temperature metamorphism and prior to complete closure of the hf w system at 10 ma after cai formation as these impacts would have disturbed the asteroid s cooling history these samples likely derive from different bodies than samples with undisturbed cooling histories implying that ordinary chondrites derive from more than just three parent bodies the hf w data reveal that metal silicate fractionation among the h l and ll groups occurred between 2 and 2 7 ma after cai formation and hence was about coeval to chondrule formation as both metal silicate fractionation and chondrule formation occurred prior to chondrite parent body accretion there should be no ordinary chondrite chondrules that are younger than 2 7 ma finally ordinary chondrite precursors had lower hf w ratios than carbonaceous chondrites suggesting that inner and outer solar system materials respectively were chemically distinct even for refractory elements

the paris meteorite the least altered cm chondrite so far the paris chondrite provides an excellent opportunity to study cm chondrules and refractory inclusions in a more pristine state than currently possible from other cms and to investigate the earliest stages of aqueous alteration captured within a single cm bulk composition it was found in the effects of a former colonial mining engineer and may have been an observed fall the texture mineralogy petrography magnetic properties and chemical and isotopic compositions are consistent with classification as a cm2 chondrite there are 45 vol high temperature components mainly type i chondrules with olivine mostly fa0 2 mean fa0 9 with granular textures because of low mesostasis abundances type ii chondrules contain olivine fa7 to fa76 these are dominantly of type iia but there are iiab and iib chondrules ii a b chondrules with minor highly ferroan olivine and iia c with augite as the only pyroxene the refractory inclusions in paris are amoeboid olivine aggregates aoas and fine grained spinel rich ca al rich inclusions cais the cai phases formed in the sequence hibonite perovskite grossite spinel gehlenite anorthite diopside fassaite and forsterite the most refractory phases are embedded in spinel which also occurs as massive nodules refractory metal nuggets are found in many cai and refractory platinum group element abundances pge decrease following the observed condensation sequences of their host phases mn cr isotope measurements of mineral separates from paris define a regression line with a slope of 53mn 55mn 5 76 0 76 106 if we interpret cr isotopic systematics as dating paris components particularly the chondrules the age is 4566 44 0 66 myr which is close to the age of cai and puts new constraints on the early evolution of the solar system eleven individual paris samples define an o isotope mixing line that passes through cm2 and co3 falls and indicates that paris is a very fresh sample with variation explained by local differences in the extent of alteration the anhydrous precursor to the cm2s was co3 like but the two groups differed in that the cms accreted a higher proportion of water paris has little matrix 47 plus 8 fine grained rims and is less altered than other cm chondrites chondrule silicates except mesostasis cai phases submicron forsterite and amorphous silicate in the matrix are all well preserved in the freshest domains and there is abundant metal preserved metal alteration stage 1 of palmer and lauretta 2011 metal and sulfide compositions and textures correspond to the least heated or equilibrated cm chondrites category a of kimura et al 2011 the composition of tochilinite cronstedtite intergrowths gives a pcp index of 2 9 cronstedtite is more abundant in the more altered zones whereas in normal highly altered cm chondrites with petrologic subtype 2 6 2 0 based on the s sio2 and feo sio2 ratios in pcp or tochilinite cronstedtite intergrowths rubin et al 2007 cronstedtite is destroyed by alteration the matrix in fresh zones has ci chondritic volatile element abundances but interactions between matrix and chondrules occurred during alteration modifying the volatile element abundances in the altered zones paris has higher trapped ne contents more primitive organic compounds and more primitive organic material than other cms there are gradational contacts between domains of different degree of alteration on the scale of 1 cm but also highly altered clasts suggesting mainly a water limited style of alteration with no significant metamorphic reheating

late formation of a comet wild 2 crystalline silicate particle pyxie inferred from al mg chronology of plagioclase we examined the al mg isotope systematics of plagioclase in a feo poor ferromagnesian wild 2 particle c2092 7 81 1 0 named pyxie using a 2 m spot three analyses show average 27al 24mg ratio of 65 and excess 26mg value of 0 1 4 5 0 1 4 5 2 indicating no resolvable 26mg excess in the particle the inferred initial 26al 27al 0 ratio of plagioclase in pyxie is estimated as 0 6 4 5 10 6 0 6 4 5 10 6 with an upper limit of 4 10 64 10 6 the result is very similar to that of the feo rich ferromagnesian particle iris ogliore et al 2012 assuming homogeneous distribution of 26al in the early solar system pyxie formed at least 2 6 ma after the oldest ca al rich inclusions this minimum formation age is marginally younger than formation ages of most chondrules in type 3 0 chondrites but comparable with those of mg 98 chondrules in cr3 chondrites considered in conjunction with similar oxygen isotope ratios between pyxie and iris and mg 98 chondrules in cr3 chondrites it is inferred that the ferromagnesian wild 2 particles and mg 98 chondrules in cr3 chondrites formed late in local disk environments that had similar oxygen isotope ratios and redox states

oxygen isotope compositions of chondrules in cr chondrites abstract we report in situ ion microprobe analyses of oxygen isotopic compositions of olivine low ca pyroxene high ca pyroxene anorthitic plagioclase glassy mesostasis and spinel in five aluminum rich chondrules and nine ferromagnesian chondrules from the cr carbonaceous chondrites eet92042 gra95229 and mac87320 ferromagnesian chondrules are isotopically homogeneous within 2 in 17 o the interchondrule variations in 17 o range from 0 to 5 small oxygen isotopic heterogeneities found in two ferromagnesian chondrules are due to the presence of relict olivine grains in contrast two out of five aluminum rich chondrules are isotopically heterogeneous with 17 o values ranging from 6 to 15 and from 2 to 11 respectively this isotopic heterogeneity is due to the presence of 16 o enriched spinel and anorthite 17 o 10 to 15 which are relict phases of ca al rich inclusions cais incorporated into chondrule precursors and incompletely melted during chondrule formation these observations and the high abundance of relict cais in the aluminum rich chondrules suggest a close genetic relationship between these objects aluminum rich chondrules formed by melting of spinel anorthite pyroxene cais mixed with ferromagnesian precursors compositionally similar to magnesium rich type i chondrules the aluminum rich chondrules without relict cais have oxygen isotopic compositions 17 o 2 to 8 similar to those of ferromagnesian chondrules in contrast to the aluminum rich chondrules from ordinary chondrites those from crs plot on a three oxygen isotope diagram along the carbonaceous chondrite anhydrous mineral line and form a continuum with amoeboid olivine aggregates and cais from crs we conclude that oxygen isotope compositions of chondrules resulted from two processes homogenization of isotopically heterogeneous materials during chondrule melting and oxygen isotopic exchange between chondrule melt and 16 o poor nebular gas

neutron capture on pt isotopes in iron meteorites and the hf w chronology of core formation in planetesimals the short lived 182hf 182w isotope system can provide powerful constraints on the timescales of planetary core formation but its application to iron meteorites is hampered by neutron capture reactions on w isotopes resulting from exposure to galactic cosmic rays here we show that pt isotopes in magmatic iron meteorites are also affected by capture of epi thermal neutrons and that the pt isotope variations are correlated with variations in 182w 184w this makes pt isotopes a sensitive neutron dosimeter for correcting cosmic ray induced w isotope shifts the pre exposure 182w 184w derived from the pt w isotope correlations of the iid iva and ivb iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182w 184w of ca al rich inclusions cai the hf w model ages for core formation range from 1 6 1 0 million years ma for the iva irons to 2 7 1 3 ma after cai formation for the iid irons indicating that there was a time gap of at least 1 ma between cai formation and metal segregation in the parent bodies of some iron meteorites from the hf w ages a time limit of 1 5 2 ma after cai formation can be inferred for the accretion of the iid iva and ivb iron meteorite parent bodies consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies

the piancaldoli meteorite a forgotten primitive ll3 10 ordinary chondrite the piancaldoli ordinary chondrite fell in northern italy on august 10 1968 preliminary studies led to its classification as an ll3 4 unequilibrated ordinary chondrite however recent developments in classification procedures have prompted us to re examine its mineralogical petrographic spectroscopic chemical and isotopic features in a multi technique study raman spectra and magnetic properties indicate that piancaldoli experienced minimal thermal metamorphism consistent with its high bulk hydrogen content and the cr contents of ferroan olivines in its type ii chondrules in combination with findings of previous studies our data thus confirm the variability of cr contents in ferroan olivines in type ii chondrules as a proxy of thermal metamorphism furthermore our results reveal that piancaldoli is less altered than previously reported and should be reclassified as an ll3 10 unequilibrated ordinary chondrite our results also imply that the bulk deuterium enrichment as observed in piancaldoli ll3 10 bishunpur ll3 15 and semarkona ll3 00 is a specific signature of the most primitive unequilibrated ordinary chondrites based on our results we propose that to date piancaldoli is the second least altered unequilibrated ordinary chondrite fall after semarkona this work reiterates the importance of meteorite collections worldwide as fundamental resources for studying the formation conditions and evolution of our solar system

26 al 26 mg systematics in chondrules from kaba and yamato 980145 cv3 carbonaceous chondrites we report the mineralogy petrography and in situ measured 26al 26mg systematics in chondrules from the least metamorphosed cv3 vigarano type chondrites kaba and yamato y 980145 two y 980145 chondrules measured show no resolvable excesses in 26mg 26mg a decay product of a short lived t1 2 0 7 ma radionuclide 26al plagioclase in one of the chondrules is replaced by nepheline indicative of thermal metamorphism the lack of 26mg in the y 980145 chondrules is most likely due to disturbance of their 26al 26mg systematics during the metamorphism although kaba experienced extensive metasomatic alteration 300 c it largely avoided subsequent thermal metamorphism and the 26al 26mg systematics of its chondrules appear to be undisturbed all eight kaba chondrules measured show 26mg corresponding to the initial 26al 27al ratios 26al 27al 0 ranging from 2 9 1 7 10 6 to 6 3 2 7 10 6 if cv parent asteroid accreted rapidly after chondrule formation the inferred 26al 27al 0 ratios in kaba chondrules provide an upper limit on 26al available in this asteroid at the time of its accretion the estimated initial abundance of 26al in the cv asteroid is too low to melt it and contradicts the existence of a molten core in this body suggested from the paleomagnetic records of allende carporzen et al 2011 magnetic evidence for a partially differentiated carbonaceous chondrite parent body proc natl acad sci usa 108 6386 6389 and kaba gattacceca et al 2013 more evidence for a partially differentiated cv parent body from the meteorite kaba lunar planet sci 44 abstract 1721

a radiative heating model for chondrule and chondrite formation abstract we propose that chondrules and chondrites formed together during a brief radiative heating event caused by the close encounter of a small m to km scale primitive planetesimal spp with incandescent lava on the surface of a large 100 km scale differentiated planetesimal ldp in our scenario chondrite lithification occurs by hot isostatic pressing hip simultaneously with chondrule formation in accordance with the constraints of complementarity and cluster chondrites thermal models of ldps formed near t 0 predict that there will be a very narrow window of time coincident with the chondrule formation epoch during which crusts are thin enough to frequently rupture by impact volcanism and or crustal foundering releasing hot magma to their surfaces the heating curves we calculate are more gradual and symmetric than the flash heating characteristic of nebular models but in agreement with the constraints of experimental petrology the spp itself is a plausible source of the excess o na and si vapor pressure compared to a solar nebula environment that is required by chondrule observations laboratory experiments demonstrate that feo poor porphyritic olivine chondrules the most voluminous type of chondrule can be made using heating and cooling curves predicted by the flyby model if chondrules are a by product of chondrite lithification then their high volume abundance within well lithified chondritic material is not evidence that they were once widespread within the solar system relatively rare events such as the flybys modeled here could account for their abundance in the meteorite record

short time interval for condensation of high temperature silicates in the solar accretion disk chondritic meteorites are made of primitive components that record the first steps of formation of solids in our solar system chondrules are the major component of chondrites yet little is known about their formation mechanisms and history within the solar protoplanetary disk spd we use the reconstructed concentrations of short lived 26al in chondrules to constrain the timing of formation of their precursors in the spd high precision bulk magnesium isotopic measurements of 14 chondrules from the allende chondrite define a 26al isochron with 26al 27al 1 2 0 2 10 5 for this subset of allende chondrules this can be considered to be the minimum bulk chondrule 26al isochron because all chondrules analyzed so far with high precision 50 chondrules from cv and ordinary chondrites have an inferred minimum bulk initial 26al 27al 1 2 10 5 in addition mineral 26al isochrons determined on the same chondrules show that their formation i e fusion of their precursors by energetic events took place from 0 myr to 2 myr after the formation of their precursors thus showing in some cases a clear decoupling in time between the two events the finding of a minimum bulk chondrule 26al isochron is used to constrain the astrophysical settings for chondrule formation either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at 1 5 my after the start of the solar system or the transport of precursors from the condensation zone to potential storage sites stopped after 1 5 my possibly due to a drop in the disk accretion rate

granoblastic olivine aggregates in magnesian chondrules planetesimal fragments or thermally annealed solar nebula condensates abstract granoblastic olivine aggregates goa have been discovered in some type i magnesian chondrules within carbonaceous chondrites by libourel and krot libourel g krot a n 2007 evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin earth planet sci lett 254 1 8 who proposed an origin from pre existing planetesimals amoeboid olivine aggregates aoa generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites display similar equilibrium texture though on a finer scale for these reasons we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating pressed 88 4 was isothermally heated at temperatures ranging from 1350 1550 c for 1 100 h the 100 h runs yield olivine aggregates with well developed granoblastic texture at all temperatures manifest as a network of randomly oriented and sutured olivine grains with 120 triple junctions individual olivine grains are 4 6 sided and polygonal by 1450 c and equigranular texture is developed at high temperature 1500 1550 c melting of olivine commences at 1450 1500 c and aids in ripening and suturing grain coarsening and grain boundary migration textural equilibrium is clearly met at 1550 c a planetesimal origin cannot be ruled out however the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects goa may have experienced higher degrees of thermal processing than the finer grained aoa if the precursors were the same grain coarsening would have to be accompanied by modification to bulk and isotopic compositions however the precursors could have been olivine condensates formed later than aoa annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of goa prior to their incorporation into chondrules

high precision al mg systematics of forsterite bearing type b cais from cv3 chondrites abstract in order to further elucidate possible temporal relationships between different varieties of calcium aluminum rich inclusions cais we measured the aluminum magnesium isotopic systematics of seven examples of the rare type known as forsterite bearing type b fob inclusions from four different cv3 carbonaceous chondrites allende efremovka nwa 3118 and vigarano the primary phases forsterite al ti rich diopside spinel melilite and anorthite in each inclusion were analyzed in situ using high precision secondary ion mass spectrometry sims in all cases minerals with low al mg ratios all except anorthite yield well defined internal al mg isochrons with a range of initial 26al 27al ratios 26al 27al 0 ranging from 5 30 0 22 10 5 down to 4 17 0 43 10 5 anorthite in all cases is significantly disturbed relative to the isochrons defined by the other phases in the same cais and in several cases contains no resolved excesses of radiogenic 26mg 26mg even at 27al 24mg ratios greater than 1000 the fact that some fobs preserve 26al 27al 0 of 5 2 10 5 close to the canonical value of 5 23 0 13 10 5 inferred from bulk magnesium isotope measurements of cv cais b jacobsen et al 2008 demonstrates that fobs began forming very early contemporaneous with other more refractory cais the range of 26al 27al 0 values further shows that fobs continued to be reprocessed over 200 000 years of nebular history consistent with results obtained for other types of igneous cais in cv chondrites the absence of any correlation between of cai fob formation or reprocessing times with bulk composition or cai type means that there is no temporal evolutionary sequence between the diverse cai types the initial 26mg value in the most primitive fob sj101 is significantly lower than the canonical solar system value of 0 040 0 029

the formation of chondrules petrologic tests of the shock wave model chondrules are millimeter sized rounded igneous rocks within chondritic meteorites their textures and fractionated mineral chemistries suggest that they formed by repeated localized brief minutes to hours melting of cold aggregates of mineral dust in the protoplanetary nebula astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites however a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed it also explains the volatile rich nature of chondrule rims and the chondrite matrix

mn cr relative sensitivity factor in ferromagnesian olivines defined for sims measurements with a cameca ims 1280 ion microprobe implications for dating secondary fayalite abstract the short lived radionuclide 53mn which decays to 53cr with a half life of 3 7 myr is useful for sequencing objects that formed within the first 20 myr of solar system evolution 53mn 53cr relative chronology enables aqueously formed secondary minerals such as fayalite and various carbonates in ordinary and carbonaceous chondrites to be dated thereby providing chronological constraints on aqueous alteration processes in situ measurements of mn cr isotope systematics in fayalite by secondary ion mass spectrometry sims require consideration of the relative sensitivities of the 55mn and 52cr ions for which a relative sensitivity factor rsf 55mn 52cr sims 55mn 52cr true is defined using appropriate standards in the past san carlos olivine fa 10 was commonly used for this purpose but a growing body of evidence suggests that it is an unsuitable standard for meteoritic fayalite fa 90 natural fayalite also cannot be used as a standard because it contains only trace amounts of chromium which makes determining a true 55mn 52cr ratio and its degree of heterogeneity very difficult to investigate the dependence of the mn cr rsf on ferromagnesian olivine compositions we synthesized a suite of compositionally homogeneous mn cr bearing liquidus phase ferromagnesian olivines fa31 99 manganese chromium isotopic measurements of san carlos olivine and synthesized ferromagnesian olivines using the university of hawai i cameca ims 1280 sims show that the rsf for fa10 is 0 9 it increases rapidly between fa10 and fa31 and reaches a plateau value of 1 5 0 1 for fa 34 the rsf is time dependent it increases during the measurements of olivines with fayalite content 50 the rsf measured on ferroan olivine fa 90 is influenced by pit shape whereas the rsf measured on magnesian olivine fa10 is less sensitive to changes in pit shape for these reasons 53mn 53cr systematics of chondritic fayalite fa 90 should be determined using standards of similar composition that are measured under the same analytical conditions as the unknown the 53mn 53cr ages of secondary fayalites fa90 100 in the elephant moraine eet 90161 l3 05 vicencia ll3 2 asuka 881317 cv3 and macalpine hills mac 88107 c3 chondrites 2 4 1 3 1 8 4 0 1 1 1 4 4 2 0 7 0 8 and 5 1 0 4 0 5 myrs after cv cais respectively are 3 myr older when using an rsf measured on a matrix matched fa99 standard rather than on a san carlos olivine the inferred 53mn 53cr ages of fayalite formation are consistent with the ages reported for calcites in cm chondrites measured with similarly matrix matched standards suggesting an early onset of aqueous alteration on the ordinary and carbonaceous chondrite parent bodies heated by decay of 26al

primordial oxygen isotope reservoirs of the solar nebula recorded in chondrules in acfer 094 carbonaceous chondrite abstract highly precise and accurate ion microprobe analyses of oxygen three isotope ratios in chondrules from the acfer 094 one of the most primitive carbonaceous chondrites show that chondrules preserve evidence for oxygen isotope heterogeneity in chondrule forming regions of the solar nebula identical 17 o values in most minerals and glass within each chondrule indicate that the oxygen isotope ratio in chondrule melt did not change during or after crystallization nearly half of the chondrules studied contain small amounts of olivine grains that have an oxygen isotope anomaly relative to other minerals and glass in the same chondrule most chondrules in acfer 094 can be classified into two oxygen isotope groups 17 o 2 and 17 o 5 indicating that the final melting of chondrules occurred within two distinct oxygen isotope reservoirs probably representing the local protoplanetary disk immediately before planetesimal formation one of these reservoirs 17 o 2 is observed from chondrules in other carbonaceous chondrites and from crystalline silicates in comet wild 2 suggesting that crystalline silicates formed in an oxygen isotope reservoir of 17 o 2 were widely distributed in the outer asteroid belt and throughout the outer solar nebula oxygen three isotope ratios of minerals in chondrules from acfer 094 are distributed along a newly defined primitive chondrule minerals pcm line which has slope 1 17 o 0 987 0 013 18 o 2 70 0 11 and intersects the terrestrial fractionation line at 18 o 5 8 0 4 these data are distinct from and plot between the ccam and young and russell lines the pcm line is interpreted to represent the mixing trend of extreme oxygen isotope reservoirs in the early solar system that were the primary oxygen isotope reservoir of solids that accreted to form planets including the earth

early solar system processes recorded in the matrices of two highly pristine cr3 carbonaceous chondrites met 00426 and que 99177 the mineralogy and bulk compositions of the matrices of the cr chondrites met 00426 and que 99177 have been studied using a combination of sem epma and tem techniques the matrices of these two chondrites are texturally chemically and mineralogically similar and are characterized by significant feo enrichments with respect to other cr chondrite matrices nearly flat refractory lithophile patterns variable volatile element patterns and a simple mineral assemblage dominated by amorphous silicate material and fe ni sulfides fine grained crystalline silicate phases such as olivine and pyroxene appear to be extremely rare in the matrices of both meteorites instead the mineralogy of matrices and fine grained rims of both meteorites consists of abundant amorphous feo rich silicate material containing nanoparticles of fe ni sulfides troilite pyrrhotite and pentlandite secondary alteration minerals that are characteristic of other cr chondrites e g renazzo and al rais such as phyllosilicates magnetite and calcite are also rare the texture and mineralogy of the matrices of met 00426 and que 99177 share many features with matrices in the primitive carbonaceous chondrites alh a77307 co3 0 and acfer 094 unique these observations show that met 00426 and que 99177 are very low petrologic type 3 chondrites that have escaped the effects of aqueous alteration unlike other cr chondrites which are typically classified as petrologic type 2 we suggest that these meteorites represent additional samples of highly primitive but extremely rare carbonaceous chondrites of petrologic type 3 00 according to the classification scheme of grossman and brearley 2005 the highly pristine nature of met 00426 and que 99177 provides important additional insights into the origins of fine grained materials in carbonaceous chondrites based on our new observations we infer that the amorphous silicate material and nanosulfide particles that dominate the matrices of these meteorites formed in the solar nebula by rapid condensation of material following high temperature events such as those that formed chondrules

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

outward migration of chondrule fragments in the early solar system o isotopic evidence for rocky material crossing the jupiter gap abstract determining the origins of chondrule precursors is key to constraining how material migrated in the early solar system chondrules that were only partially melted during their formation retain portions of their solid precursors termed relict grains by measuring the chemical and o isotopic compositions of relict grains in chondrules from an unequilibrated ordinary chondrite uoc and renazzo like carbonaceous cr and mighei like carbonaceous cm chondrites we constrain their origins and discuss implications for disk transport within the first 4 million years of the solar system for all three chondrite groups the chemical and o isotopic compositions of dusty olivine grains are sometimes consistent with the reduction of type i feo poor and or type ii feo rich chondrules from the same meteorite group however other dusty olivine grains from the cm chondrites and the uoc are found to be xenocrysts that require an origin from a source distinct from the host meteorite this material plausibly originated as fragments of earlier formed chondrules from another chondrite group or of partially or fully differentiated planetesimals that migrated into an active chondrule forming region multiple cm chondrite dusty olivine chondrules have o isotope compositions that match those of uoc chondrule olivine 17o 0 suggesting an origin from an uoc source this implies that uoc chondrules and or chondrule fragments migrated from the inner solar system outwards to cm chondrite chondrule forming region likely beyond the orbit of jupiter these uoc chondrules or chondrule fragments could have migrated outwards in the protoplanetary disk before the formation of the jupiter gap or

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

oxygen isotope characteristics of chondrules from the yamato 82094 ungrouped carbonaceous chondrite further evidence for common o isotope environments sampled among carbonaceous chondrites high precision secondary ion mass spectrometry sims was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the yamato y 82094 ungrouped 3 2 carbonaceous chondrite twenty one of 21 chondrules have multiple homogeneous pyroxene data 17o 3sd analytical uncertainty 0 7 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data twenty one of 25 chondrules have one or more olivine data matching coexisting pyroxene data such homogeneous phenocrysts 1 are interpreted to have crystallized from the final chondrule melt defining host o isotope ratios and 2 suggest efficient o isotope exchange between ambient gas and chondrule melt during formation host values plot within 0 7 of the primitive chondrule mineral pcm line seventeen chondrules have relict olivine and or spinel with some 17o and 18o values approaching 40 similar to cai or aoa like precursors regarding host chondrule data 22 of 34 have mg s of 98 8 99 5 and 17o of 3 9 to 6 1 consistent with most acfer 094 co cr and cv chondrite chondrules and suggesting a common reduced o isotope reservoir devoid of 16o poor h2o six y 82094 chondrules have 17o near 2 5 with mg s of 64 97 consistent with lower mg chondrules from acfer 094 co cr and cv chondrites their signatures suggest precursors consisting of those forming mg 99 17o 5 1 chondrules plus 16o poor h2o at high dust enrichments three type ii chondrules plot slightly above the pcm line near the terrestrial fractionation line 17o 0 1 their o isotopes and olivine chemistry are like ll3 type ii chondrules suggesting they sampled ordinary chondrite like chondrule precursors finally three mg 99 chondrules have 17o of 6 7 to 8 1 potentially due to 16o rich refractory precursor components the predominance of mg 99 17o 5 1 chondrules and a high chondrule to matrix ratio suggests bulk y 82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules

exploring the efficiency of stepwise dissolution in removal of stubborn non radiogenic pb in chondrule u pb dating abstract chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large scale planetary accretion together with refractory calcium aluminium rich inclusions cais they are the sources of our knowledge of the initial evolution of the early solar system we have investigated a single very large 10 mm in longer dimension chondrule hereafter the mega chondrule a25 2 extracted from the allende cv3 chondrite we characterised texture mineralogy and mineral chemistry of this chondrule and studied its al mg u pb and u isotope systematics we also studied the distribution of u th and pb and measured pb isotopic composition in individual minerals of a25 2 by secondary ion mass spectrometry sims the main difficulty in absolute age determination was the presence of pervasive and resilient non radiogenic pb in the search for the best way to separate radiogenic pb from non radiogenic pb components of terrestrial and asteroidal origins we used various protocols of multi step leaching and assessed their efficiency in generating data suitable for the construction of an isochron testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non radiogenic pb the model age patterns observed in the final hf partial dissolution steps were probably induced by isotopic fractionation although step leaching did not yield fractions with highly radiogenic pb a pb pb isochron age corrected for measured 238u 235u was obtained by 1 data filtering process based on strict analytical and geochemical criteria to include in the pb pb isochron only leaching steps free from terrestrial contamination and 2 arithmetically recombined analyses to cancel the effects of leaching induced isotopic fractionation this extensive data processing yielded the age of 4568 5 3 0 ma which we consider reliable within its uncertainty limits although it is not as precise as and more model dependent than the age that could have been obtained if pb isotopic compositions were more radiogenic the 238u 235u ratio of the mega chondrule is 137 764 0 016 which is similar to the ratios obtained from single chondrules yet slightly different from small pooled allende chondrules the initial 27al 26al ratio inferred from internal isochron obtained from sims al mg isotope measurements is 5 4 6 5 10 6 which corresponds to 4565 0 0 8 ma assuming homogeneous distribution of 26al throughout the protoplanetary disk at the canonical level 5 2 10 5 this age is 3 5 3 1 ma younger than the pb isotopic age calculation of 26al 26mg age assuming initial 27al 26al 0 of 1 36 0 72 10 5 in the cv chondrule forming region yields the age of 4566 4 0 8 which is consistent with the pb isotopic age

the origin of chondrules and chondrites debris from low velocity impacts between molten planetesimals we investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin shelled largely molten planetesimals collided at low speed during accretion this scenario here dubbed splashing stems from evidence that such planetesimals intensely heated by 26al were abundant in the protoplanetary disk when chondrules were being formed approximately 2 myr after calcium aluminum rich inclusions cais and that chondrites far from sampling the earliest planetesimals are made from material that accreted later when 26al could no longer induce melting we show how splashing is reconcilable with many features of chondrules including their ages chemistry peak temperatures abundances sizes cooling rates indented shapes relict grains igneous rims and metal blebs and is also reconcilable with features that challenge the conventional view that chondrules are flash melted dust clumps particularly the high concentrations of na and feo in chondrules but also including chondrule diversity large phenocrysts macrochondrules scarcity of dust clumps and heating we speculate that type i feo poor chondrules come from planetesimals that accreted early in the reduced partially condensed hot inner nebula and that type ii feo rich chondrules come from planetesimals that accreted in a later or more distal cool nebular setting where incorporation of water ice with high 17o aided oxidation during heating we propose that multiple collisions and repeated re accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties and led to so called complementarity and metal depletion in chondrites we suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation

high precision sims oxygen three isotope study of chondrules in ll3 chondrites role of ambient gas during chondrule formation we report high precision sims oxygen three isotope analyses of 36 chondrules from some of the least equilibrated ll3 chondrites and find systematic variations in oxygen isotope ratios with chondrule types feo poor type i chondrules generally plot along a mass dependent fractionation line d 17 o 0 7 with d 18 o values lower in olivine rich ia than pyroxenerich ib chondrules data from feo rich type ii chondrules show a limited range of d 18 o and d 17 o values at d 18 o 4 5 d 17 o 2 9 and d 17 o 0 5 which is slightly 16 o enriched relative to bulk ll chondrites d 17 o 1 3 data from four chondrules show 16 o rich oxygen isotope ratios that plot near the ccam carbonaceous chondrite anhydrous mineral line glass analyses in selected chondrules are systematically higher than co existing minerals in both d 18 o and d 17 o values whereas high ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts our results suggest that the ll chondrite chondrule forming region contained two kinds of solid precursors 1 16 o poor precursors with d 17 o 1 6 and 2 16 o rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites oxygen isotopes exhibited open system behavior during chondrule formation and the interaction between the solid and ambient gas might occur as described in the following model significant evaporation and recondensation of solid precursors caused a large mass dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type ia chondrules with higher bulk mg si ratios type ii chondrules formed under elevated dust gas ratios and with water ice in the precursors in which the ambient h2o gas homogenized chondrule melts by isotope exchange low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high d 17 o 5 in ll the parent body according to our model oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto planetary disk and the ambient gas during chondrule melting events 2010 elsevier ltd all rights reserved

oxygen isotope systematics of chondrule phenocrysts from the co3 0 chondrite yamato 81020 evidence for two distinct oxygen isotope reservoirs high precision oxygen three isotope measurements of olivine and pyroxene were performed on 33 chondrules in the yamato 81020 co3 0 chondrite by secondary ion mass spectrometry in chondrules where oxygen isotopes were measured in both olivine and pyroxene the majority of grains have similar values indicating co magmatic crystallization however many chondrules contain relict grains with unique oxygen isotope ratios a striking feature of yamato 81020 chondrules is a bimodal distribution of oxygen isotope ratios as those with mg 97 phenocrysts range in 17o from 4 8 to 6 5 5 5 group and those with mg 96 36 phenocrysts have 17o values of 2 1 to 3 0 2 5 group a single mg 99 6 barred olivine chondrule has a 17o of 3 3 we discuss that 17o 5 5 chondrules are derivative of a reservoir with limited dust enrichment 100 solar system which yielded a relatively reduced chondrule forming environment in contrast the 17o 2 5 chondrules may have been influenced by 16o poor h2o ice that sublimed and then homogenized with precursor material the addition of h2o when combined with high dust enrichment 1000 solar system and greater bulk fe content could have induced an oxidized environment at high temperatures forming mg 96 36 chondrules among the 33 chondrules studied the al mg relative ages of 20 had been obtained previously comparing the oxygen isotope ratios and the 26al ages of these chondrules it is likely that the 5 5 and 2 5 oxygen isotope reservoirs existed contemporaneously this implies that the snow line was spatially fixed during chondrule formation and separated the co chondrite accretion region into two distinct volumes of precursors

initial 26al 27al in carbonaceous chondrite chondrules too little 26al to melt asteroids we report 26mg excesses correlated with al mg ratios in five chondrules from the primitive co3 0 chondrite yamato 81020 that yield a mean initial 26al 27al ratio of only 3 8 0 7 10 6 about half that of ordinary chondrite oc chondrules even if asteroids formed immediately after chondrule formation this ratio and the mean al content of co chondrites is only capable of raising the temperature of a well insulated co asteroid to 940 k which is more than 560 k too low to produce differentiation the same ratio combined with the higher al content of cv chondrites results in a cv asteroid temperature of 1100 k we calculate that the mean initial 26al 27al ratio of about 7 4 10 6 found in ll chondrules is only able to produce small amounts of melting too little to produce differentiation these results cast serious doubt on the viability of 26al as the heat source responsible for asteroid differentiation inclusion of 60fe raises temperatures about 160 k but this increment is not enough to cause differentiation even of an ll chondrite asteroid

oxygen isotopes in crystalline silicates of comet wild 2 a comparison of oxygen isotope systematics between wild 2 particles and chondritic materials abstract oxygen three isotope ratios of nine crystalline silicate particles from comet wild 2 were measured to investigate oxygen isotope systematics of cometary materials we are able to analyze particles as small as 4 m using an ion microprobe with a 1 2 m beam by locating the analysis spots with an accuracy of 0 4 m three particles of mn rich forsterite known as low iron manganese enriched lime olivine showed extremely 16 o rich signatures 18 o 17 o 50 similar to refractory inclusions in chondrites the three mn rich forsterite particles may have formed by condensation from an 16 o rich solar nebula gas other particles consist of olivine and or pyroxene with a wide range of mg molar mgo feo mgo from 60 to 96 their oxygen isotope ratios plot nearly along the carbonaceous chondrite anhydrous mineral ccam and young and russell lines with 17 o 17 o 0 52 18 o values of 3 0 to 2 5 these data are similar to the range observed from previous analyses of wild 2 crystalline silicates and those of chondrules in carbonaceous chondrites six particles extracted from stardust track 77 show diverse chemical compositions and isotope ratios two mn rich forsterites feo poor pigeonite and three feo rich olivines with a wide range of 17 o values from 24 to 1 6 these results confirmed that the original projectile that formed track 77 was an aggregate 6 m of silicate particles that formed in various environments the 17 o values of ferromagnesian wild 2 particles including data from previous studies increase from 23 to 2 5 with decreasing mg 17 o values of mn rich forsterite particles mg 98 99 8 cluster at 23 those of feo poor particles mg 95 97 cluster at 2 and those of feo rich particles mg 90 scatter mainly from 1 5 to 2 5 compared to chondrules in primitive chondrites the systematic trend between mg and 17 o among the wild 2 particles is most similar to that reported for cr chondrite chondrules we argue that cr chondrites and some cometary materials share multiple common chemical and isotope characteristics we suggest that many of the crystalline silicate particles formed in the outer regions of the asteroid belt or regions that share the common properties and were transported to comet forming regions and accreted into comet wild 2

26al 26mg isotope systematics of the first solids in the early solar system high precision bulk aluminum magnesium isotope measurements of calcium aluminum rich inclusions cais from cv carbonaceous chondrites in several laboratories define a bulk 26al 26mg isochron with an inferred initial 26al 27al ratio of approximately 5 25 10 5 named the canonical ratio nonigneous cv cais yield well defined internal 26al 26mg isochrons consistent with the canonical value these observations indicate that the canonical 26al 27al ratio records initial al mg fractionation by evaporation and condensation in the cv cai forming region the internal isochrons of igneous cv cais show a range of inferred initial 26al 27al ratios 4 2 5 2 10 5 indicating that cai melting continued for at least 0 2 ma after formation of their precursors a similar range of initial 26al 27al ratios is also obtained from the internal isochrons of many cais igneous and nonigneous in other groups of carbonaceous chondrites some cais and refractory grains corundum and hibonite from unmetamorphosed or weakly metamorphosed chondrites including cvs are significantly depleted in 26al at least some of these refractory objects may have formed prior to injection of 26al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk bulk aluminum and magnesium isotope measurements of various types of chondrites plot along the bulk cv cai isochron suggesting homogeneous distribution of 26al and magnesium isotopes in the protoplanetary disk after an epoch of cai formation the inferred initial 26al 27al ratios of chondrules indicate that most chondrules formed 1 3 ma after cais with the canonical 26al 27al ratio

oxygen isotopic constraints on the origin of mg rich olivines from chondritic meteorites chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system the calcium aluminum rich inclusions were condensed from the nebula gas chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk petrographic compositional and isotopic arguments allowed the identification of metal bearing mg rich olivine aggregates among the precursors of magnesian type i chondrules two very different settings can be considered for the formation of these mg rich olivines either a nebular setting corresponding mostly to condensation evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal an ion microprobe survey of mg rich olivines of a set of type i chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the 17o values and the presence of a large range of mass fractionation several within each mode the chemistry and the oxygen isotopic compositions indicate that mg rich olivines are unlikely to be of nebular origin i e solar nebula condensates but are more likely debris of broken differentiated planetesimals each of them being characterized by a given 17o mg rich olivines could have crystallized from magma ocean like environments on partially molten planetesimals undergoing metal silicate differentiation processes considering the very old age of chondrules mg rich olivine grains or aggregates might be considered as millimeter sized fragments from disrupted first generation differentiated planetesimals finally the finding of only a small number of discrete 17o modes for mg rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

thermal and impact history of the h chondrite parent asteroid during metamorphism constraints from metallic fe ni abstract we have studied cloudy taenite metallographic cooling rates and shock effects in 30 h3 6 chondrites to elucidate the thermal and early impact history of the h chondrite parent body we focused on h chondrites with old ar ar ages 4 4 gyr and unshocked and mildly shocked h chondrites as strongly shocked chondrites with such old ages are very rare cooling rates for most h chondrites at 500 c are 10 50 c myr and do not decrease systematically with increasing petrologic type as predicted by the onion shell model in which types 3 5 are arranged in concentric layers around a type 6 core some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types contrary to the onion shell model cloudy taenite particle sizes which range from 40 to 120 nm are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating the three h4 chondrites that were used to develop the onion shell model ste marguerite beaver creek and forest vale cooled through 500 c at 5000 c myr our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by 26al and cooled without disturbance by impact published ar ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating impacts during metamorphism created portales valley and two other h6 chondrites with large metallic veins excavated the fast cooled h4 chondrites around 3 4 myr after accretion and mixed petrologic types metallographic data do not require catastrophic disruption by impact during cooling

contemporaneous formation of chondrules in distinct oxygen isotope reservoirs we investigated the 26al 26mg systematics of type i feo poor chondrules from one of the most primitive carbonaceous chondrites acfer 094 the inferred initial 26al 27al ratios of chondrules from acfer 094 ranged from 4 2 2 0 10 6 to 9 0 1 5 10 6 these chondrules have distinct oxygen isotope ratios 17o 5 2 and 0 though no correlation between inferred initial 26al 27al ratios and oxygen isotope ratios were observed these results indicate the regional oxygen isotope heterogeneity in the solar nebula over the duration of chondrule formation a few low ca pyroxene grains in chondrules have small but resolvable 26mg excesses at the level of 0 1 which may be the result of a partial melting of chondrules by reheating events if this is the case the true inferred initial 26al 27al ratios of these particular chondrule could be slightly lower 10 than estimates using mg isotope ratios of coexisting plagioclase and olivine stable isotope ratios of mg 25mg 24mg of many olivine grains in chondrules are positively fractionated by 1 relative to those in other phases within the same chondrules this suggests that olivine preserved a positively fractionated mg isotope ratios of the chondrule forming melt prior to significant condensation during the cooling stage of chondrule formation

oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early solar system abstract we report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases olivine low ca pyroxene silica and mesostasis of 37 magnesian porphyritic type i chondrules from cv vigarano usnm 477 2 vigarano uh5 mokoia and efremovka and cr eet 92042 eet 92147 eet 87770 el djouf 001 mac 87320 and gra 95229 carbonaceous chondrites in spite of significant variations of the modal proportions of major mineral phases in cr and cv chondrules the same isotopic characteristics are observed i olivines are isotopically homogeneous at the level within a chondrule although they may vary significantly from one chondrule to another ii low ca pyroxenes are also isotopically homogeneous but systematically 16o depleted relative to olivines of the same chondrule and iii all chondrule minerals analyzed show 16o enrichments relative to the terrestrial mass fractionation line enrichments that decrease from olivine spinel to low ca pyroxene and to silica and mesostasis the observation that in most of the type i chondrules studied the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co magmatic and that the glassy mesostasis is not the parent liquid of the olivine the 18o and 17o values of pyroxene and olivine appear to be strongly correlated for all the studied cr and cv chondrules according to 18 o pyroxene 0 72 0 16 0 13 18 o olivine 1 21 0 54 0 79 17 o pyroxene 0 81 0 15 0 13 17 o olivine 0 60 1 00 0 95 these correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas melt mineral reactions mg 2 sio 4 olivine sio 2 melt mg 2 si 2 o 6 pyroxene sio 2 melt sio gas 1 2 o 2 gas these equations imply that 2 3 of the oxygen incorporated by low ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1 3 from the addition of chemical components such as sio from the nebular gas the composition of the nebular gaseous reservoir in which type i cr and cv chondrules formed is determined from the present data set to be 18o gas 3 6 1 and 17o gas 1 8 1 this isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups i e cv cr unequilibrated ordinary and eh thus the oxygen isotopic systematic strongly supports the idea that most of the olivines in type i chondrules are relict grains and that gas melt interaction played a major role in the formation of type i chondrules

magnesium and 54cr isotope compositions of carbonaceous chondrite chondrules insights into early disk processes we report on the petrology magnesium isotopes and mass independent 54cr 52cr compositions 54cr of 42 chondrules from cv vigarano and nwa 3118 and cr nwa 6043 nwa 801 and lap 02342 chondrites all sampled chondrules are classified as type ia or type iab have low 27al 24mg ratios 0 04 0 27 and display little or no evidence for secondary alteration processes the cv and cr chondrules show variable 25mg 24mg and 26mg 24mg values corresponding to a range of mass dependent fractionation of 500 ppm parts per million per atomic mass unit this mass dependent mg isotope fractionation is interpreted as reflecting mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility controlled processes during chondrule formation the cv and cr chondrule populations studied here are characterized by systematic deficits in the mass independent component of 26mg 26mg relative to the solar value defined by ci chondrites which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26al compared to the canonical 26al 27al of 5 10 5 model initial 26al 27al values of cv and cr chondrules vary from 1 5 4 0 10 6 to 2 2 0 4 10 5 the cv chondrules display significant 54cr variability defining a range of compositions that is comparable to that observed for inner solar system primitive and differentiated meteorites in contrast cr chondrites are characterized by a narrower range of 54cr values restricted to compositions typically observed for bulk carbonaceous chondrites collectively these observations suggest that the cv chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the cv parent asteroid whereas cr chondrule predominantly formed from precursor with carbonaceous chondrite like 54cr signatures the observed 54cr variability in chondrules from cv and cr chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir the coupled 26mg and 54cr systematics of cr chondrules establishes that these objects formed from a thermally unprocessed and 26al poor source reservoir distinct from most inner solar system asteroids and planetary bodies possibly located beyond the orbits of the gas giants in contrast a large fraction of the cv chondrules plot on the inner solar system correlation line indicating that these objects predominantly formed from thermally processed 26al bearing precursor material akin to that of inner solar system solids asteroids and planets

short duration thermal metamorphism in cr chondrites cr chondrites are considered as one of the most primitive classes of meteorites most of them experienced a mild aqueous alteration and show no evidence of significant effect of thermal metamorphism we present here a search for low degree metamorphic effects in cr chondrites we studied 15 cr chondrites using different metamorphic indicators 1 structure and ni content of metal grains 2 hydration state of matrix 3 structure and composition of organic matter the different metamorphic indicators show that two of the analyzed cr chondrites gra 06100 and gro 03116 experienced thermal metamorphism indeed all of the metal grains in gra 06100 and half of the metal grains in gro 03116 show ni rich phases the matrix of gra 06100 is almost completely dehydrated and the matrix of gro 03116 is partially dehydrated raman spectra of organic matter in these two meteorites are clearly different from those obtained for organic matter in the other cr chondrites which resemble raman spectra of organic matter in unmetamorphosed cm2 meteorites ir spectra of insoluble organic matter extracted from gra 06100 and gro 03116 show lower carbonyl abundance and higher ch2 ch3 ratio with respect to organic matter of unmetamorphosed chondrites the other cr chondrites analyzed here lack these characteristics and only show a few metal grains with ni rich inclusions our results also show that the metamorphic effects observed in gra 06100 and gro 03116 are different from those observed in type 3 chondrites which experienced long duration metamorphism of radiogenic origin we infer that thermal processing in these two crs extended over a short duration and was triggered by impacts

formation of non magmatic iron meteorite group iie abstract instrumental neutron activation inaa data for metal in 22 nonmagmatic iie meteorites show narrow ranges in ir and other refractory siderophiles the ir range is a factor of 2 6 a factor of 2 smaller than in nonmagmatic iab mg and orders of magnitude smaller than in the large magmatic groups siderophile data show no evidence of fractional crystallization iie irons can be split into two sets a larger main set and a set of 6 cu or fes rich irons elemental concentrations in metal from veins in h5 chondrite portales valley fall within the iie range with the exceptions of co high and ga low h group chondrite and au normalized iie abundances for siderophiles show that iie irons are 30 higher than h in refractory siderophiles re ir and w and are about 30 lower than h chondrites in the volatiles ga and sb inconsistent with proposals that iie irons formed from h chondrites the iie fractionations contrast with those in l chondrites which are about 15 lower than h in the three refractory elements and 40 higher than h in volatiles indicating that iie irons did not form from h chondrites but from a more reduced and siderophile rich kind of ordinary chondrite hh chondrites most o isotope data support a close relationship between iie irons and h or hh chondrites lower 17o in primitive chondritic silicates support an hh classification literature isotopic data for ru and mo also show that iie metal formed from an ordinary chondrite parent it appears that the silicates and metal were formed by melting of a single asteroid there is no evidence for radiogenic 26al heating this the rapid cooling recorded in the sizes of parental gamma crystal in the metal and the absence of fractional crystallization strongly support the hypothesis that iie melting was the result of impacts to summarize the weight of the evidence favors the conclusion that iie meteorites were formed by one or more impacts on an hh asteroid the target probably had a composition like the chondritic materials in netschaevo but was unequilibrated and had much higher porosity

distribution of 26 al in the cr chondrite chondrule forming region of the protoplanetary disk we report on the mineralogy petrography and in situ measured oxygen and magnesium isotope compositions of eight porphyritic chondrules seven feo poor and one feo rich from the renazzo like carbonaceous cr chondrites graves nunataks 95229 grosvenor mountains 03116 pecora escarpment 91082 and queen alexandra range 99177 which experienced minor aqueous alteration and very mild thermal metamorphism we find no evidence that these processes modified the oxygen or al mg isotope systematics of chondrules in these meteorites olivine low ca pyroxene and plagioclase within an individual chondrule have similar o isotope compositions suggesting crystallization from isotopically uniform melts the only exceptions are relict grains in two of the chondrules these grains are 16o enriched relative to phenocrysts of the host chondrules only the feo rich chondrule shows a resolvable excesses of 26mg corresponding to an inferred initial 26al 27al ratio 26al 27al 0 of 2 5 1 6 10 6 2se combining these results with the previously reported al mg isotope systematics of cr chondrules nagashima et al 2014 geochem j 48 561 7 of 22 chondrules 32 measured show resolvable excesses of 26mg the presence of excess 26mg does not correlate with the feo content of chondrule silicates in contrast virtually all chondrules in weakly metamorphosed petrologic type 3 0 3 1 unequilibrated ordinary chondrites uocs ornans like carbonaceous co chondrites and the ungrouped carbonaceous chondrite acfer 094 show resolvable excesses of 26mg the inferred 26al 27al 0 in cr chondrules with resolvable excesses of 26mg range from 1 0 0 4 10 6 to 6 3 0 9 10 6 which is typically lower than 26al 27al 0 in the majority of chondrules from uocs cos and acfer 094 based on the inferred 26al 27al 0 three populations of cr chondrules are recognized the population characterized by low 26al 27al 0 4 0 0 3 0 5 ma after the formation of cais with the canonical 26al 27al ratio although rapid accretion after formation of the major population of cr chondrules is not required by our data

aluminum 26 in calcium aluminum rich inclusions and chondrules from unequilibrated ordinary chondrites in order to investigate the distribution of 26 a1 in chondrites we measured aluminum magnesium systematics in four calcium aluminum rich inclusions cais and eleven aluminum rich chondrules from unequilibrated ordinary chondrites uocs all four cais were found to contain radiogenic 26 mg 26 mg from the decay of 26 a1 the inferred initial 26 al 27 al ratios for these objects 26 al 27 al 0 5 10 5 are indistinguishable from the 26 al 27 al 0 ratios found in most cais from carbonaceous chondrites these observations together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of cais imply that cais in uocs and carbonaceous chondrites formed by similar processes from similar or the same isotopic reservoirs or perhaps in a single location in the solar system we also found 26 mg in two of eleven aluminum rich chondrules the 26 al 27 al 0 ratio inferred for both of these chondrules is 1 10 5 clearly distinct from most cais but consistent with the values found in chondrules from type 3 0 3 1 uocs and for aluminum rich chondrules from lightly metamorphosed carbonaceous chondrites 0 5 10 5 to 2 10 5 the consistency of the 26 al 27 al 0 ratios for cais and chondrules in primitive chondrites independent of meteorite class implies broad scale nebular homogeneity with respect to 26 al and indicates that the differences in initial ratios can be interpreted in terms of formation time a timeline based on 26 al indicates that chondrules began to form 1 to 2 ma after most cais formed that accretion of meteorite parent bodies was essentially complete by 4 ma after cais and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 ma after cais formed type 6 chondrites apparently did not cool until more than 7 ma after cais formed this timeline is consistent with 26 al as a principal heat source for melting and metamorphism

calcium aluminum rich inclusions recycled during formation of porphyritic chondrules from ch carbonaceous chondrites abstract we report on the mineralogy petrography and o isotope compositions of 60 ca al rich inclusions cais incompletely melted during formation of porphyritic chondrules from the ch metal rich carbonaceous chondrites and isheyevo ch cb these include i relict polymineralic cais in porphyritic chondrules ii cais surrounded by chondrule like igneous rims iii igneous pyroxene rich and type c like cais and iv plagioclase rich chondrules with clusters of relict spinel grains 26al 26mg systematics were measured in 10 relict cais and 11 cai bearing plagioclase rich chondrules based on the mineralogy the ch cais incompletely melted during chondrule formation can be divided into grossite rich n 13 hibonite rich n 11 spinel melilite rich n 33 these include plagioclase rich chondrules with clusters of relict spinel grains types mineralogical observations indicate that these cais were mixed with different proportions of ferromagnesian silicates and experienced incomplete melting and gas melt interaction during chondrule formation these processes resulted in partial or complete destruction of the cai wark lovering rims replacement of melilite by na bearing plagioclase and dissolution and overgrowth of nearly end member spinel by chromium and iron bearing spinel only two relict cais and two cai bearing chondrules show resolvable excess of radiogenic 26mg the inferred initial 26al 27al ratios are 1 7 1 3 10 6 3 7 3 1 10 7 1 9 0 9 10 6 and 4 9 2 6 10 6 there is a large range of 17o among the ch cais incompletely melted during chondrule formation from 37 to 5 the unmelted minerals in individual cais however are isotopically uniform and systematically 16o enriched relative to the host chondrules and chondrule like igneous rims which have 17o ranging from 7 to 4 most of the ch cais incompletely melted during chondrule formation are mineralogically and isotopically similar to the ch cais surrounded by wark lovering rims and apparently unaffected by chondrule melting the mineralogy and o isotope compositions of the ch cai bearing chondrules are similar to those of the ch porphyritic chondrules without relict cais we conclude that ch porphyritic chondrules formed by incomplete melting of isotopically diverse solid precursors including mineralogically and isotopically unique cais commonly observed only in ch chondrites therefore the ch porphyritic chondrules must have formed in a distinct disk region where the ch cais were present at the time of chondrule formation because most ch cais avoided chondrule melting we infer that chondrule formation was highly localized these observations preclude formation of ch porphyritic chondrules by splashing of molten planetesimals by impact processing of differentiated planetesimals and by large scale nebular shocks e g shocks driven by disk gravitational instabilities or by x ray flares instead they are consistent with small scale chondrule forming mechanisms proposed in the literature such as nebular processing of dust balls by bow shocks and by current sheets

the microstructure of semarkona and bishunpur abstract the ordinary chondrites semarkona and bishunpur appear to have experienced in situ hydrous alteration here we report the results of the first detailed tem examination of them which supports this conclusion in semarkona all but the most magnesian mafic minerals have been destroyed and primary sulphides most likely troilite have been partially altered to phyllosilicates and ni rich pyrrhotite in bishunpur alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix during hydrous alteration of both meteorites the temperature probably did not exceed 260 c the equilibrium composition of the gas present during the alteration of semarkona was not solar but was dominated by h2o the formation of smectite does not require the presence of liquid water however evidence for the redistribution of elements such as ca zn and si over tens of microns or more implies that there was at least some transient grain boundary fluid present the alteration in semarkona and bishunpur is far from complete thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the cm and ci carbonaceous chondrites superficially the mineralogy mineral chemistry and bulk chemistry of semarkona matrix rims resemble those of some chondritic interplanetary dust particles cidps the ordinary chondrite parent bodies may therefore be a source of some cidps

a discontinuous ca 80 ka record of late quaternary environmental change from lake omapere northland new zealand we present an integrated record of environmental change from lake omapere northland new zealand based on palaeolimnological analysis of a 7 m long core and eight adjunct cores spanning part of the last 80 calendar cal ka the chronology was developed using tephrochronology palaeomagnetism and radiocarbon dating together with climato and palyno stratigraphy two of 14 tephra layers in the cores provide markers for correlating the record with other new zealand climato stratigraphic sequences deep sea cores and the global marine isotope record pollen diatom palaeomagnetic sedimentologic and pigment analyses show that lake omapere currently 2 m deep has had a discontinuous history occupying a shallow basin perched partly within old basalt lavas the initial alkaline lake formed towards the end of mis 5a at ca 80 cal ka presumably because of blockage of local drainage of a swampy alluvial floodplain inundating peat deposits and forest trees including kauri agathis australis the lake filled rapidly to a level 1 2 m above that at present such filling was probably unrelated to climate but a subsequent phase of variable but generally falling lake levels and increasing dystrophy may have been climatically controlled commencing during mis 4 and culminating in periods early in mis 3 when the lake became swampy or dry conditions of non deposition or non preservation obtained for most of the period after ca 55 cal ka rotoehu ash until formation of the modern lake approximately 600 700 cal years ago or soon after as indicated by the presence of tephra 1 identified in part as kaharoa tephra ad 1314 near the top of the core the modern lake originated possibly through damming of the western outlet as a consequence of accelerated erosion accompanying earliest polynesian deforestation an interpretation supported by maori oral tradition the pollen record indicates that beech nothofagus presumably n truncata was much more common in northland during the last glacial and that for this region the relative abundance of nothofagus vs agathis pollen serves as a better indicator of cooler versus warmer intervals during the quaternary than the ratio of tree to non tree pollen however it seems likely that moisture balance was a more critical factor than temperature in vegetation composition and distribution particularly during the lgm and the long periods of hiatus may also be linked to a drier climate than present correlation coefficients between the pollen curves confirm that several tree species halocarpus bidwillii h biformis and phyllocladus alpinus previously palynologically concealed within generic taxonomic groups occurred about two degrees latitude further north and at a much lower altitude than their current limits during cooler or drier phases of the last glacial a temperature depression in northland of 4 c at various times during the last glacial is inferred from these range expansions nevertheless the persistence of widespread forest cover suggests that the late pleistocene climate of northland was less severe than for most of the rest of new zealand and strengthens the argument for a heightened temperature gradient across northern new zealand during the last glacial

chondrule formation by repeated evaporative melting and condensation in collisional debris clouds around planetesimals a synthesis of previous work leads to a model of chondrule formation that involves periodic melting of dispersed dust in debris clouds that were generated by collisions between chondritic planetesimals i suggest that chondrules formed by the passage of nebular shock waves through these dust clumps which temporarily surrounded disrupted planetesimals type i chondrules formed by more intense evaporative heating of fewer particles in tenuous clumps or at the edges of dense clumps and type ii chondrules formed by less intense evaporative heating of more particles deeper within dense clumps chondrules reaccreted by self gravity into the planetesimals mixing with less heated dust and rock this process of disruption melting and reaccretion could have repeated many times in this way chondrite components of various origins and thermal histories could remain preserved in planetesimals as a distinctive mix of materials for extended periods of time while still allowing for a repetitive melting process that converted some of the planetesimal debris into chondrules i also suggest that during chondrule formation the inner solar nebula gas was evolving by the gradual incorporation and heating of icy bodies depleted in 16 o causing a general increase in gaseous d 17 o with time in most places especially close to the snow line in this model early formed type i chondrules in c chondrites with lower d 17 o values were produced inside the snow line and later formed type i and type ii chondrules in c and o chondrites with higher d 17 o values were created nearer the snow line after it had moved closer to the young sun

fe ni metal in primitive chondrites indicators of classification and metamorphic conditions for ordinary and co chondrites we report the results of our petrological and mineralogical study of fe ni metal in type 3 ordinary and co chondrites and the ungrouped carbonaceous chondrite acfer 094 fe ni metal in ordinary and co chondrites occurs in chondrule interiors on chondrule surfaces and as isolated grains in the matrix isolated ni rich metal in chondrites of petrologic type lower than type 3 10 is enriched in co relative to the kamacite in chondrules however ni rich metal in type 3 15 3 9 chondrites always contains less co than does kamacite fe ni metal grains in chondrules in semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and ni rich regions metal in other type 3 chondrites is composed of fine to coarse grained aggregates of kamacite and ni rich metal resulting from metamorphism in the parent body we found that the number density of ni rich grains in metal number of ni rich grains per unit area of metal in chondrules systematically decreases with increasing petrologic type thus fe ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites among the known ordinary and co chondrites semarkona is the most primitive the range of metamorphic temperatures were similar for type 3 ordinary and co chondrites despite them having different parent bodies most fe ni metal in acfer 094 is martensite and it preserves primary features the degree of metamorphism is lower in acfer 094 a true type 3 00 chondrite than in semarkona which should be reclassified as type 3 01

evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin abstract chondrules are the major high temperature components of chondritic meteorites which are conventionally viewed as the samples from the very first generation of undifferentiated planetesimals growing evidences from long and short lived radionuclide chronologies indicate however that chondrite parent asteroids accreted after or contemporaneously with igneous activities on differentiated asteroids questioning the pristine nature of chondrites here we report a discovery of metal bearing olivine aggregates with granoblastic textures inside magnesian porphyritic type i chondrules from the cv carbonaceous chondrite vigarano formation of the granoblastic textures requires sintering and prolonged high temperature 1000 c annealing conditions which are not expected in the solar nebula during chondrule formation but could have been achieved on parent bodies of olivine rich differentiated or thermally metamorphosed meteorites the mineralogy and petrography of the metal olivine aggregates thus indicate that they are relict dunite like lithic fragments which resulted from fragmentation of such bodies the very old pb pb absolute ages and al mg relative model ages of bulk cv chondrules suggest that such planetesimals may have formed as early as the currently accepted age of the solar system 4567 2 0 6 ma

mobility of iron and nickel at low temperatures implications for 60fe 60ni systematics of chondrules from unequilibrated ordinary chondrites abstract the fe and ni isotopic composition of ferromagnesian silicates in chondrules from unequilibrated ordinary chondrites uocs have been used to estimate the initial abundance of the short lived radionuclide 60 fe in the early solar system however these estimates vary widely and there are systematic discrepancies in initial 60 fe 56 fe ratios inferred from in situ and bulk analyses of chondrules a possible explanation is that the fe ni isotope system in uoc chondrules has not remained closed a necessary condition for isotopic dating and fe and ni have been redistributed since the chondrules formed in order to evaluate this we collected high spatial resolution x ray fluorescence xrf maps of uoc chondrules to better understand the distribution and mobility of fe and ni at the low metamorphic temperatures of these chondrites we used synchrotron x ray fluorescence microscopy to map the distribution of fe ni and other elements in portions of 71 chondrules from 8 uocs types 3 00 3 2 the synchrotron xrf maps show clear enrichment of fe and or ni in fractures ranging down to micrometer scale in chondrules from all uocs analyzed for this study regardless of petrologic type and regardless of whether fall or find indicating that there was significant exchange of fe and ni between chondrules and matrix and that the fe ni system was not closed sixty percent of chondrules in semarkona ll3 00 have fe and ni enrichment along fractures while 80 100 of chondrules analyzed from the other uocs show these enrichments mobilization was likely a result of fluid transport of fe and ni during aqueous alteration on the parent body and or during terrestrial weathering in situ and bulk fe ni analyses that incorporate extraneous fe and ni from chondrule fractures will result in lowering the inferred initial 60 fe 56 fe ratios

oxygen isotopic compositions of chondrules from the metal rich chondrites isheyevo ch cbb mac 02675 cbb and que 94627 cbb abstract it has been recently suggested that 1 ch chondrites and the cbb ch like chondrite isheyevo contain two populations of chondrules formed by different processes i magnesian non porphyritic cryptocrystalline and barred chondrules which are similar to those in the cb chondrites and formed in an impact generated plume of melt and gas resulted from large scale asteroidal collision and ii porphyritic chondrules formed by melting of solid precursors in the solar nebula 2 porphyritic chondrules in isheyevo and ch chondrites are different from porphyritic chondrules in other carbonaceous chondrites krot et al 2005 krot et al 2008a krot et al 2008b in order to test these hypotheses we measured in situ oxygen isotopic compositions of porphyritic magnesian type i and ferroan type ii and non porphyritic magnesian and ferroan cryptocrystalline chondrules from isheyevo and cbb chondrites mac 02675 and que 94627 paired with que 94611 using a cameca ims 1280 ion microprobe on a three isotope oxygen diagram 17o vs 18o compositions of chondrules measured follow approximately slope 1 line data for 19 magnesian cryptocrystalline chondrules from isheyevo 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically zoned fe ni metal condensates from cbb chondrites have nearly identical compositions 17o 2 2 0 9 2 3 0 6 and 2 2 1 0 2 respectively these observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in cbb chondrites gounelle et al 2007 are consistent with their single stage origin possibly as gas melt condensates in an impact generated plume in contrast 17o values for 11 type i and 9 type ii chondrules from isheyevo range from 5 to 4 and from 17 to 3 respectively in contrast to typical chondrules from carbonaceous chondrites seven out of 11 type i chondrules from isheyevo plot above the terrestrial fractionation line we conclude that i porphyritic chondrules in isheyevo belong to a unique population of objects suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites ii isheyevo cb and ch chondrites are genetically related meteorites they contain non porphyritic chondrules produced during the same highly energetic event probably large scale asteroidal collision iii the differences in mineralogy petrography chemical and whole rock oxygen isotopic compositions between ch and cb chondrites are due to various proportions of the nebular and the impact produced materials

53mn 53cr dating of aqueously formed carbonates in the cm2 lithology of the sutter s mill carbonaceous chondrite radiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies dolomite is a well documented secondary mineral in cm chondrites and is thought to have formed by precipitation from an aqueous fluid on the cm parent body within several million years of accretion the petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ rather than in the nebular setting here we present 53mn 53cr systematics for dolomite grains in sutter s mill section sm51 1 the mn cr isotope data show well resolved excesses of 53cr correlated with 55mn 52cr ratio which we interpret as evidence for the in situ decay of radioactive 53mn after correcting for the relative sensitivities of mn and cr using a synthetic mn and cr bearing calcite standard the data yield an isochron with slope corresponding to an initial 53mn 55mn ratio of 3 42 0 86 10 6 the reported error includes systematic uncertainty from the relative sensitivity factor when calculated relative to the u corrected pb pb absolute age of the d orbigny angrite sutter s mill dolomites give a formation age between 4564 8 and 4562 2 ma 2 4 5 0 myr after the birth of the solar system this age is contemporaneous with previously reported ages for secondary carbonates in cm and ci chondrites consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation probably occurring via heating from internal 26al decay the high precision isochron for sutter s mill dolomite indicates that late stage processing did not reach temperatures that were high enough to further disturb the mn cr isochron

ion microprobe analyses of oxygen three isotope ratios of chondrules from the sayh al uhaymir 290 ch chondrite using a multiple hole disk the ion microprobe is the only technique capable of determining high precision stable isotope ratios in individual tiny extraterrestrial particles 100 lm in diameter but these small samples present special analytical challenges we produced a new sample holder disk with multiple holes three holes and seven holes in which epoxy disks containing a single unknown sample and a standard grain are cast and polished performance tests for oxygen two isotope analyses using san carlos olivine standard grains show that the new multiple hole disks allow accurate analysis of tiny particles if the particles are located within the 500 lm and 1 mm radius of the center of holes for seven hole and three hole disks respectively using the new seven hole disk oxygen three isotope ratios of eight magnesian cryptocrystalline chondrules approximately 100 lm in diameter from the sayh al uhaymir sau 290 ch chondrite were analyzed by ion microprobe at the university of wisconsin five out of eight chondrules have nearly identical oxygen isotope ratios d 17 o 2 2 0 6 2sd which is consistent with those of magnesian cryptocrystalline chondrules in ch cb and cb chondrites suggesting a genetic relationship i e formation by a common possibly impact heating event the other three chondrules have distinct oxygen isotope ratios d 17 o values from 6 4 to 2 2 given that similar variation in d 17 o values was observed in type i porphyritic chondrules in a ch cb chondrite the three chondrules may have formed in the solar nebula similar to the type i porphyritic chondrules

a long duration of the 16 o rich reservoir in the solar nebula as recorded in fine grained refractory inclusions from the least metamorphosed carbonaceous chondrites abstract oxygen isotope ratios and corresponding 26 al 26 mg isotope systematics of refractory inclusions from the least metamorphosed carbonaceous chondrites acfer 094 c ungrouped 3 00 and yamato 81020 co3 05 were measured with an ion microprobe most of the samples are fine grained refractory inclusions which are considered as condensates from high temperature solar nebular gas the refractory inclusions consistently exhibit 16 o enriched signatures among their interior phases spinel melilite and high ca pyroxene as well as phases within their rim structures spinel high ca pyroxene and adjacent anorthite this observation indicates that aggregated refractory condensates and the formation of rim structures occurred in the same 16 o rich environment evidence for mass dependent isotopic fractionation in oxygen and magnesium which would indicate a later flash heating process was not observed in rims all oxygen isotope data from fine grained cais are distributed between the carbonaceous chondrite anhydrous mineral ccam line and the primitive chondrule mineral pcm regression line based on oxygen isotope data from the acfer 094 chondrules the inferred initial 26 al 27 al ratios 26 al 27 al 0 of spinel melilite rich cais are 4 08 0 75 10 5 to 5 05 0 18 10 5 errors are 2 which are slightly lower than the canonical value of 5 25 10 5 as there is no petrologic evidence for re melting after condensation the lower 26 al 27 al 0 values of these cais indicate either they formed up to 0 3 ma after canonical cais or they formed before 26 al was homogeneously distributed in the solar nebula a pyroxene anorthite rich cai g92 has an 16 o rich signature like other cais but also has an order of magnitude less 26 mg excess in anorthite corresponding to a 26 al 27 al 0 of 5 21 0 54 10 6 as there is no evidence for a later mg isotopic disturbance g92 anorthite is interpreted to have formed by interaction with 16 o rich nebular gas at 2 3 ma after cai formation with the observation that 16 o rich refractory inclusions relatively 16 o poor chondrules and extremely 16 o poor cosmic symplectites within acfer 094 all plot on the pcm line it suggests that 16 o rich nebular gas and extremely 16 o poor primordial volatiles represent mass independent fractionated endmembers in the early solar system and that the pcm line represents a mixing line of these two endmembers

the onset of metamorphism in ordinary and carbonaceous chondrites ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by xray mapping techniques a variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe scanning electron microscopy sem transmission electron microscopy tem and cathodoluminescence cl methods the distribution of cr in feo rich olivine systematically changes as metamorphism increases between type 3 0 and type 3 2 igneous zoning patterns are replaced by complex ones and cr rich coatings develop on all grains cr distributions in olivine are controlled by the exsolution of a cr rich phase probably chromite cr in olivine may have been partly present as tetrahedrally coordinated cr3 separation of chromite is nearly complete by petrologic type 3 2 the abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type tem shows this to be the result of crystallization of albite residual glass compositions systematically change during metamorphism becoming increasingly rich in k glass in type i chondrules also gains alkalis during metamorphism both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules the matrix in the least metamorphosed chondrites is rich in s and na the s is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules alkalis initially leave the matrix and enter chondrules during early metamorphism feldspar subsequently nucleates in the matrix and na re enters from chondrules these metamorphic trends can be used to refine classification schemes for chondrites cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3 0 and 3 1 into types 3 00 through 3 15 on this basis the most primitive ordinary chondrite known is semarkona although even this meteorite has experienced a small amount of metamorphism allan hills alh a77307 is the least metamorphosed co chondrite and shares many properties with the ungrouped carbonaceous chondrite acfer 094 analytical problems are significant for glasses in type ii chondrules as na is easily lost during microprobe analysis as a result existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised

cm and co chondrites a common parent body or asteroidal neighbors insights from chondrule silicates abstract by investigating the petrology and chemical composition of type ii feo rich chondrules in the mighei like carbonaceous cm chondrites we constrain their thermal histories and relationship to the ornans like carbonaceous co chondrites we identified feo rich relict grains in type ii chondrules by their fe mn ratios their presence indicates chondrule recycling among type ii chondrules the majority of relict grains in type ii chondrules are feo poor olivine grains consistent with previous studies chemical similarities between cm and co chondrite chondrules indicate that they had similar formation conditions and that their parent bodies probably formed in a common region within the protoplanetary disk however important differences such as mean chondrule size and the lower abundance of feo poor relicts in cm chondrite type ii chondrules than in co chondrites suggest cm and co chondrules did not form together and they likely originate from distinct parent asteroids despite being aqueously altered many cm chondrites contain pre accretionary anhydrous minerals i e olivine that are among the least thermally metamorphosed materials in chondrites according to the cr 2 o 3 content of their ferroan olivine the presence of these minimally altered pre accretionary chondrule silicates suggests that samples to be returned from aqueously altered asteroids by the hayabusa2 and osiris rex asteroid sample return missions even highly hydrated may contain silicates that can provide information about the pre accretionary histories and conditions of asteroids ryugu and bennu respectively

origin and history of ureilitic material in the solar system the view from asteroid 2008 tc3 and the almahata sitta meteorite asteroid 2008 tc3 approximately 4 m diameter was tracked and studied in space for approximately 19 h before it impacted earth s atmosphere shattering at 44 36 km altitude the recovered samples 680 individual rocks comprise the meteorite almahata sitta ahs approximately 50 70 of these are ureilites ultramafic achondrites the rest are chondrites mainly enstatite ordinary and rumuruti types the goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid almahata sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts however we calculate that the mass of fallen material was 0 1 of the pre atmospheric mass of the asteroid based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples we infer that the lost material was mostly ureilitic therefore 2008 tc3 probably contained only a few percent nonureilitic materials similar to other polymict ureilites except less well consolidated from available data for the ahs meteorite fragments we conclude that 2008 tc3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites we therefore suggest that the immediate parent of 2008 tc3 was the immediate parent of all ureilitic material sampled on earth we trace critical stages in the evolution of that material through solar system history based on various types of new modeling and re evaluation of published data we propose the following scenario 1 the ureilite parent body upb accreted 0 5 0 6 ma after formation of calcium aluminum rich inclusions cai beyond the ice line outer asteroid belt differentiation began approximately 1 ma after cai 2 the upb was catastrophically disrupted by a major impact approximately 5 ma after cai with selective subsets of the fragments reassembling into daughter bodies 3 either the upb before breakup or one of its daughters after breakup migrated to the inner belt due to scattering by massive embryos 4 one daughter after forming in or migrating to the inner belt became the parent of 2008 tc3 it developed a regolith mostly 3 8 ga ago clasts of enstatite ordinary and rumuruti type chondrites were implanted by low velocity collisions 5 recently the daughter was disrupted fragments were injected or drifted into earth crossing orbits 2008 tc3 comes from outer layers of regolith other polymict ureilites from deeper regolith and main group ureilites from the interior of this body in contrast to other models that have been proposed this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 tc3 and other polymict ureilites

records of the moon forming impact and the 470 ma disruption of the l chondrite parent body in the asteroid belt from u pb apatite ages of novato l6 novato a newly observed fall in the san francisco bay area is a shocked and brecciated l6 ordinary chondrite containing dark and light lithologies we have investigated the u pb isotope systematics of coarse cl apatite grains of metamorphic origin in novato with a large geometry ion microprobe the u pb systematics of novato apatite reveals an upper intercept age of 4472 31 ma and lower intercept age of 473 38 ma the upper intercept age is within error identical to the u pb apatite age of 4452 21 ma measured in the chelyabinsk ll5 chondrite this age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the moon forming giant impact on earth the lower intercept age is consistent with the most precisely dated ar ar ages of 470 6 ma of shocked l chondrites and the fossil meteorites and extraterrestrial chromite relicts found in ordovician limestones with an age of 467 3 1 6 ma in sweden and china the lower intercept age reflects a major disturbance related to the catastrophic disruption of the l chondrite parent body most likely associated with the gefion asteroid family which produced an initially intense meteorite bombardment of the earth in ordovician period and reset and degassed at least approximately 35 of the l chondrite falls today we predict that the 470 ma impact event is likely to be found on the moon and mars if not mercury

zoned chondrules in semarkona evidence for high and low temperature processing at least 15 of the 10w feo chondrules in semarkona ll3 0 have mesostases that are concentrically zoned in na with enrichments near the outer margins we have studied zoned chondrules using electron microprobe methods x ray mapping plus quantitative analysis ion microprobe analysis for trace elements and hydrogen isotopes cathodoluminescence imaging and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved mesostases in these chondrules are strongly zoned in all moderately volatile elements and h interpreted as water calcium is depleted in areas ofvolatile enrichment titanium and cr generally decrease toward the chondrule surfaces whereas al and si may either increase or decrease generally in opposite directions to one another mn follows na in some chondru1es but not in others fe and mg are unzoned d h ratios increase in the water rich areas of zoned chondrules mesostasis shows cathodoluminescence zoning in most zoned chondrules with the brightest yellow color near the outside mesostasis in zoned chondrules appears to be glassy with no evidence for devitrification systematic variations in zoning patterns among pyroxene and olivine rich chondrules may indicate that fractionation oflow and high ca pyroxene played some role in ti cr mn si ai and some ca zoning but direct condensation ofelements into hot chondrules secondary melting oflate condensates into the outer portions of chondrules and subsolidus diffusion of elements into warm chondrules cannot account for the sub parallel zoning profiles ofmany elements the presence ofh20 or elemental abundance patterns zoning ofmoderately volatile elements and ca may have been produced by hydration ofchondrule glass without devitrification during aqueous alteration on the parent asteroid this could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material calcium was mainly lost during this process and other nonvolatile elements may have been mobile as well some unzoned 10w feo chondrules appear to have fully altered mesostasis

53 mn 53 cr radiometric dating of secondary carbonates in cr chondrites timescales for parent body aqueous alteration abstract we present 53mn 53cr ages of secondary carbonates in renazzo like cr chondrites determined by secondary ion mass spectrometry the timing of aqueous alteration in cr chondrites has been unconstrained in the literature we measured 53mn 53cr isotope systematics in carbonates from three different cr chondrite lithologies calcite in the interchondrule matrix of renazzo calcite in the matrix of gro 95577 and dolomite in a dark inclusion of renazzo all show excesses in 53cr interpreted as the daughter product from the decay of 53mn the renazzo calcite yields an initial ratio of 53mn 55mn 0 3 6 2 7 10 6 and the renazzo dark inclusion dolomite ranges from 53mn 55mn 0 3 1 1 4 10 6 corrected to the rsf of a calcite standard to 3 7 1 7 10 6 corrected to an inferred dolomite rsf when anchored to the d orbigny angrite the renazzo carbonates yield ages between 4563 6 and 4562 6 ma or 4 3 5 3 myr after the formation of cv cais calcite measured in the heavily altered specimen gro 95577 yields a shallower slope of 53mn 55mn 0 7 9 2 8 10 7 corresponding to a much younger age of 4555 4 ma or 12 6 myr after cai formation the two renazzo ages are contemporaneous with mn cr ages of carbonates in tagish lake ci and cm chondrites but the gro 95577 age is uniquely young these findings suggest that early aqueous alteration on chondritic parent bodies was a common occurrence likely driven by internal heating from 26al decay after accretion the young carbonate ages of gro 95577 suggest that either the cr parent body was sufficiently large to sustain heating from 26al for 8 myr or that late stage impact events supplied heat to the region where gro 95577 originated

well resolved variations in the formation ages for ca al rich inclusions in the early solar system recent whole rock magnesium isotopic data for calcium aluminum rich inclusions cais in chondrite meteorites indicate that the primary nebular fractionation of aluminum from magnesium probably by condensation occurred within 20 000 years at 4 567 ga however high precision multicollector ion microprobe data for diverse cais from vigarano cv3 yield internal isochrons with a clearly resolved spread in initial 26al 27al meaning that cais formed and were reprocessed over a much longer time span primitive unmelted cais have a consistent value of 5 2 0 1 10 5 melted cais range from 5 17 10 5 to 4 24 10 5 and one single object with a complex multistage history has an internal range of 26al 27al 4 77 2 77 10 5 the entire range corresponds to an age span of 0 7 ma thus not all cais formed at time zero and only the most primitive cais should be used as benchmarks for earliest solar system chronology

rates and mechanisms of isotopic exchange background the proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies this trend is likely to continue because of the development of micro analytical techniques e g laser ablation ion microprobe femtomole carrier gas methods etc for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record see mckibben et al 1998 equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems indeed determination of the distribution of the stable isotopes of c o h s in gases fluids and minerals has become a standard and extremely powerful approach in elucidating the temperatures material fluxes rock and fluid origins and time scales associated with ancient and active fluid rock interaction processes in the earth s crust see valley et al 1986 although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by urey 1947 the calibration of fractionation factors has proven to be difficult during the last four decades a great deal of progress has been made in determining the equilibrium fractionations of 18o 16o d h 13c 12c and 34s 32s among fluid and mineral components at high temperatures 400 c both from experimental exchange studies theoretical calculations using vibrational spectra semi empirical calculations using bond type considerations and empirical calibrations based on natural assemblages see references in o neil 1986 kyser 1987 criss 1991 1999 chacko et al this volume of these the experimental method is the most direct because it involves the least number of a priori assumptions despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water rock

the relative formation ages of ferromagnesian chondrules inferred from their initial aluminum 26 aluminum 27 ratios we performed a systematic high precision sims 26al 26mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite bishunpur ll3 1 the chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and or glass the chemical compositions of the chondrules vary from feo poor to feo rich eight chondrules show resolvable 26mg excesses with a maximum 26mg of 1 in two chondrules the initial 26al 27al ratios inferred for these chondrules range between 2 28 0 73 x 10 5 to 0 45 0 21 x 10 5 assuming a homogeneous distribution of al isotopes in the early solar system this range corresponds to ages relative to cais between 0 7 0 2 my and 2 4 0 4 0 7 my the inferred total span of the chondrule formation ages is at least 1 my which is too long to form chondrules by the x wind the initial 26al 27al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine rich chondrules formed earlier than pyroxene rich chondrules though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of si from earlier generations of chondrules followed by addition of si to the precursors of later generation chondrules

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

26al 26mg systematics of chondrules in a primitive co chondrite abstract we have conducted systematic investigations of formation age chemical compositions and mineralogical characteristics of ferromagnesian chondrules in yamato 81020 co3 05 one of the most primitive carbonaceous chondrites to get better understanding of the origin of chemical groups of chondrites the 26 al 26 mg isotopic system were measured in fourteen feo poor type i six feo rich type ii and two aluminum rich al rich chondrules using a secondary ion mass spectrometer excesses of 26 mg in plagioclase 1 0 13 5 are resolved with sufficient precision mostly 0 4 6 6 at 2 level in all the chondrules studied except one chemical zoning of mg and na in plagioclase were investigated in detail in order to evaluate the applicability of 26 al 26 mg chronometer we conclude that the al mg isotope system of the chondrules in y 81020 have not been disturbed by parent body metamorphism and can be used as chronometer assuming homogeneous distribution of 26 al assuming an initial 26 al 27 al ratio of 5 10 5 in the early solar system 26 al 26 mg ages were found to be 1 7 2 5 ma after cai formation for type i 2 0 3 0 ma for type ii and 1 9 and 2 6 ma for al rich chondrules the formation ages of ferromagnesian chondrules in y 81020 are in good agreement with those of l and ll type 3 0 3 1 chondrites in the literature which indicates that common chondrules in the co chondrite were formed contemporaneously with those in l and ll chondrites the concurrent formation of chondrules of co and l ll chondrites suggests that the chemical differences between co and l ll chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk