oxygen isotopic heterogeneities their petrological correlations and implications for melt origins of chondrules in unequilibrated ordinary chondrites abstract ten whole chondrules separated from the dhajala h3 4 hallingeberg l3 and semarkona ll3 chondrites were individually analyzed for bulk element composition by instrumental neutron activation with half of each chondrule subsequently sacrificed for oxygen isotopic analysis and half retained for petrographic and electron microprobe analysis on a three isotope plot 17 o vs 18 o the chondrules neither cluster near their respective chondrite hosts nor in the vicinities of previously recognized chondrite group averages instead they define a trend resolvable into mixing and fractionation components but dominated by mixing in a manner similar to that previously observed for clasts from the ll3 chondrite alha76004 covariations of chondrule isotopic mixing and fractionation parameters with petrological parameters were sought by two variable linear least squares regression analyses however the only two isotopic petrological correlations significant at the 95 confidence level were 17 o vs total bulk fe r 0 68 and mixing parameter m 18 vs bulk weight ratio cao al 2 o 3 mgo r 0 67 other correlations of apparent statistical significance were found by treating the chondrules as separate porphyritic 3 porphyritic olivine pyroxene 1 porphyritic olivine 1 barred olivine and non porphyritic 4 radial pyroxene 1 granular pyroxene cryptocrystalline textural subgroups the reliability of the trends based on so few samples is not clear but the results at least indicate that possible existence of distinct isotopic petrological subgroups of chondrules should be further investigated absence of certain isotopic petrological trends expected as condensation effects argues against direct nebular condensation as the dominant process of chondrule formation instead a model involving melting of heterogeneous solids followed by various degrees of liquid gas exchange is favored in any case chondrule oxygen isotopic evolution was dominated by two component mixing fractional vaporization was at most a second order effect in addition to chondrules parent bodies of unequilibrated ordinary chondrites must have also incorporated a 16 o rich component which might have been fine grained matrix

on the formation of fe ni metal in renazzo like carbonaceous chondrites the fe ni metal within chondrites has been postulated to have originated either through condensation or as a byproduct of chondrule formation to test these hypotheses we studied metal in three renazzo like carbonaceous cr2 chondrites from three petrographic settings inside chondrules on chondrule rims and in the matrix abundances of fe ni co cr and p were determined in situ by electron microprobe and those of os ir pt and au were measured by a newly developed ion microprobe technique the refractory platinum group elements os ir and pt behave coherently in cr2 metal they are either all enriched all depleted or unfractionated with respect to fe and cosmic ratios metal with approximately ci os fe ir fe and pt fe occur primarily in chondrule interiors all metal grains have essentially ci values of ni fe and co fe almost all metal grains have lower than ci ratios of the volatile elements au and p we also estimated the bulk compositions via analyses of phases and modal recombination of a subset of the chondrules whose metal we analyzed the bulk compositions of chondrules are generally unfractionated relative to ci chondrites for elements more refractory than cr but are depleted in more volatile elements abundances of siderophile elements correlate strongly with the metal abundance in the chondrules which implies that siderophile depletions are due to expulsion of metal from the chondrule melts the metal most likely originated during melting via reduction of oxides by c that was part of the chondrule precursor during chondrule heating molten metal efficiently extracted siderophile elements from the silicate melt rim metal consists of two types one is like metal in chondrule interiors and was in the process of being expelled when the chondrules were quenched the other shows systematic depletions in os ir and pt relative to fe and higher concentrations of au and p than interior metal this metal is attributed to recondensation from a vapor depleted in refractory siderophiles a vapor most likely derived via evaporation from chondrules large isolated matrix metal grains comprise the same two groups as rim grains and have the same origins bulk chondrule compositions and siderophile abundance patterns in metal indicate that the precursors of cr2 chondrules had ci like abundances of refractory and moderately volatile elements but was likely depleted in the more volatile elements

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

chondrules in h3 chondrites textures compositions and origins major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated type 3 h group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types barred olivine porphyritic olivine porphyritic pyroxene barred pyroxene radiating pyroxene fine grained although analyses of individual chondrules scatter widely the mean composition of each textural type except barred pyroxene is rather distinct as verified by discriminant function analysis al2o3 tio2 and na3o are correlated in chondrules but al2o3 and cao do not correlate compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition the data suggest that composition played a conspicuous role in producing the observed textures of chondrules though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important if compound chondrules formed and joined when they were still molten or plastic then the data suggest that chondrules of each textural type could have formed together in space or time the correlation of al2o3 and tio2 with na2o and not with cao appears to rule out formation of chondrules by direct equilibrium condensation from the nebula we conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high intermediate and low temperature nebular condensates that underwent melting in space a small percentage of chondrules might have formed by impacts in meteorite parent body regoliths

refractory precursor components of semarkona chondrules and the fractionation of refractory elements among chondrites abstract chondrules from the semarkona ll3 0 chondrite show refractory and common lithophile fractionation trends similar to those observed among the chondrite groups it appears that chondrules are mixtures of a small number of pre existing solid components and we infer that chondrule precursor materials were related to the nebular components involved in the lithophile element fractionations recognized in ordinary chondrites compositional trends among the chondrules can be used to deduce the compositions of these components we use instrumental neutron activation analysis to measure many 20 of the lithophile elements in 30 chondrules the amounts of oxidized iron were calculated from other compositional parameters concentrations of si were estimated using mass balance considerations the data were corrected for the diluting effects of non lithophile constituents plots of lithophile elements versus a reference refractory element such as al show that there were two major chondrule silicate precursor components a refractory olivine rich feo free one and a non refractory sio 2 feo rich one the refractory component probably forms from olivine enriched condensates formed above the condensation temperature of enstatite the non refractory component must have formed from fine grained materials that were able to equilibrate down to lower nebular temperatures chondrite matrix may have had an origin similar to that of the non refractory material and constitutes a third lithophile bearing component that took part in chondrite fractionation processes the low abundance of refractories and mg in ordinary and enstatite chondrites was produced by the loss of materials having a higher refractory element mg ratio than that in the refractory component of chondrules

plagioclase rich chondrules in the reduced cv chondrites evidence for complex formation history and genetic links between calcium aluminum rich inclusions and ferromagnesian chondrules plagioclase rich chondrules prcs in the reduced cv chondrites efremovka leoville vigarano and grosvenor mountains gro 94329 consist of magnesian low ca pyroxene al ti cr rich pigeonite and augite forsterite anorthitic plagioclase feni metal sulfide nodules and crystalline mesostasis composed of silica anorthitic plagioclase and al ti cr rich augite the silica grains in the mesostases of the cv prcs are typically replaced by hedenbergitic pyroxenes whereas anorthitic plagioclase is replaced by feldspathoids nepheline and minor sodalite some of the prcs contain regions that are texturally and mineralogically similar to type i chondrules and consist of forsterite low ca pyroxene and abundant feni metal nodules several prcs are surrounded by igneous rims or form independent compound objects twelve prcs contain relic calcium aluminum rich inclusions cais composed of anorthite spinel high ca pyroxene forsterite and al rich low ca pyroxene anorthite of these cais is generally more heavily replaced by feldspathoids than anorthitic plagioclase of the host chondrules this suggests that either the alteration predated formation of the prcs or that anorthite of the relic cais was more susceptible to the alteration than anorthitic plagioclase of the host chondrules these observations and the presence of igneous rims around prcs and independent compound prcs suggest that the cv prcs may have had a complex multistage formation history compared to a more simple formation history of the cr prcs relatively high abundances of moderately volatile elements such as cr mn and si in the prcs suggests that these chondrules could not have been produced by volatilization of ferromagnesian chondrule precursors or by melting of refractory materials only we infer instead that prcs in carbonaceous chondrites formed by melting of the reduced chondrule precursors magnesian olivine and pyroxene feni metal mixed with refractory materials relic cais composed of anorthite spinel high ca pyroxene and forsterite the mineralogical chemical and textural similarities of the prcs in several carbonaceous chondrite groups cv co ch cr and common presence of relic cais in these chondrules suggest that prcs may have formed in the region s intermediate between the regions where cais and ferromagnesian chondrules originated

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

lithification mechanisms for planetary regoliths the glue that binds it is understood how rocks are made on earth however on the moon mercury and to a lesser extent mars and venus there are distinct rock forming processes that we do not fully comprehend the surfaces and crusts of the inner planetary bodies may retain a history of disruption by hypervelocity impact resulting in the generation of disaggregated materials to several kilometers depth the uppermost component of this is called regolith typically 20 m thick on the moon which is part of a more extensive megaregolith that is up to tens of kilometers thick and which in places may pervade the entire crust of a planetary body it is from these pulverized materials that new rocks are reaggregated to form so called breccias this work reviews regolith and megaregolith structure for the inner planetary bodies and investigates how extraterrestrial breccias are produced three principal formation mechanisms are explored thermal sintering shock sintering and the dynamic interaction of impact generated melt w

origin of sio2 rich components in ordinary chondrites abstract silica rich objects are common minor components in ordinary chondrites oc occurring as fragments and as chondrules their typical paragenesis is orthopyroxene sio 2 with bulk sio 2 65 wt and occasionally with additional olivine and or spinel individual silica rich components src have previously been studied in various types of ocs although there is only one comprehensive study of these objects by brigham et al brigham c a murrell m t yabuki h ouyang z el goresy a 1986 silica bearing chondrules and clasts in ordinary chondrites geochim cosmochim acta 50 1655 1666 several different explanations of how srcs formed have been published the main question is how silica enrichment was achieved because ci chondritic atomic mg si ratio is 1 07 and as a consequence only olivine and pyroxene but no free silica should be stable there are two basic possibilities for the sio 2 enrichment 1 a redox mechanism or magmatic fractionation on the parent body and 2 fractional condensation or recycling of chondrule mesostasis in the solar nebula to better constrain the origin of these objects we measured major and rare earth elements in srcs of various types of ordinary chondrites and in addition we studied silica polymorphism in these objects using an in situ micro raman technique bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene the srcs extend these lines to an sio 2 end member in contrast magmatic trends grossly deviate from these mixing lines concentrations of cao al 2 o 3 and ree in the pyroxenes of the srcs are low 0 01 to 1 ci and the ci normalized ree patterns are virtually flat typical of bulk chondrules but untypical of magmatic trends we therefore conclude that sio 2 rich objects are not of magmatic origin they are the result of fractional condensation in the solar nebula the silica in srcs occurs mainly as tridymite and sometimes as cristobalite or in very rare cases as quartz some sio 2 phases yielded a yet unknown micro raman spectrum which we were unable to identify the often chondrule like shape of srcs as well as the presence of high temperature sio 2 polymorphs lead to the following model for the origin of srcs formation of sio 2 rich precursors in the solar nebula by fractional condensation reheating to temperatures between 1140 and 1968 k thereby forming the srcs probably during the chondrule forming process followed by rapid cooling

thermoluminescence and compositional zoning in the mesostasis of a semarkona group a1 chondrule and new insights into the chondrule forming process a large group a1 porphyritic olivine chondrule in the semarkona ll3 0 chondrite with induced thermoluminescence tl and compositional zoning in its mesostasis has been discovered the chondrule has ca rich and fe poor olivine cao 0 36 0 40 wt fa0 3 0 5 and its mesostasis is highly anorthite normative 52 5 wt the chondrule shows an intense induced tl peak at 300 c with a half width of 180 c the induced tl in the 40 440 c range increases monotonically by a factor of 6 from center to rim while sio2 na2o and mno increase by factors of 1 1 3 6 and 6 respectively the spectrum of the induced tl over the 200 350 c range i e a dominant peak at 570 nm with a half width of about 100 nm and the mn tl correlation suggest mn activated plagioclase is an important constituent of the refractory mesostases in group a1 chondrules the zoning may reflect fractional crystallization soret diffusion transport of volatiles into the chondrule by aqueous alteration a zoned precursor reduction of precursor dust aggregate or recondensation of volatiles lost during chondrule formation the first four possibilities seem unlikely explanations for the zoning of the mesostasis but both reduction of precursor dust aggregate and recondensation of volatiles seem to have played significant roles to the formation of the zoning reducing conditions at high temperatures are suggested to have prevailed during the formation of group a1 chondrules

anorthite rich chondrules in cr and ch carbonaceous chondrites genetic link between ca al rich inclusions and ferromagnesian chondrules anorthite rich chondrules in cr and ch carbonaceous chondrites consist of magnesian low ca pyroxene and forsterite phenocrysts feni metal nodules interstitial anorthite al ti cr rich low ca and high ca pyroxenes and crystalline mesostasis composed of silica anorthite and high ca pyroxene three anorthite rich chondrules contain relic ca al rich inclusions composed of anorthite spinel al diopside and forsterite a few chondrules contain regions which are texturally and mineralogically similar to magnesian type i chondrules and consist of forsterite low ca pyroxene and abundant feni metal nodules anorthite rich chondrules in cr and ch chondrites are mineralogically similar to those in cv and co carbonaceous chondrites but contain no secondary nepheline sodalite or ferrosilite relatively high abundances of moderately volatile elements such as cr mn and si in the anorthite rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only we infer instead that anorthite rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors olivine pyroxenes feni metal mixed with the refractory materials including relic cais composed of anorthite spinel high ca pyroxene and forsterite the observed mineralogical and textural similarities of the anorthite rich chondrules in several carbonaceous chondrite groups cv co ch cr may indicate that these chondrules formed in the region s intermediate between the regions where cais and ferromagnesian chondrules originated this may explain the relative enrichment of anorthite rich chondrules in 16o compared to typical ferromagnesian chondrules russell et al 2000

petrology of feo poor porphyritic pyroxene chondrules in the semarkona chondrite abstract the mineralogy and petrology of feo poor porphyritic pyroxene and olivine rich chondrules in the semarkona ll3 0 chondrite are described in detail in an extension of the textural classification scheme these chondrules are designated types iab and ib in type iab chondrules the proportion of olivine phenocrysts is between 20 80 and in type ib chondrules olivine constitutes

are the source models of the m 7 1 1908 messina straits earthquake reliable insights from a novel inversion and a sensitivity analysis of levelling data

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

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 studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

sulfur isotope composition of putative primary troilite in chondrules from bishunpur and semarkona the sulfur isotopic compositions of putative primary troilite grains within 15 ferromagnesian chondrules 10 feo poor and 5 feo rich chondrules in the least metamorphosed ordinary chondrites bishunpur and semarkona have been measured by ion microprobe some troilite grains are located inside metal spherules within chondrules since such an occurrence is unlikely to be formed by secondary sulfidization processes in the solar nebula or on parent bodies those troilites are most likely primary having survived chondrule forming high temperature events if they are primary they may be the residues of evaporation at high temperatures during chondrule formation and may have recorded mass dependent isotopic fractionations however the supposed primary troilites measured in this study do not show any significant sulfur isotopic fractionations 104 106 k h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors this heating rate may provide a new constraint on the chondrule formation processes

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

chondrules and matrix in the ornans co3 meteorite possible precursor components abstract bulk compositions of 17 chondrules and one sample of matrix from co3 omans were determined by instrumental neutron activation analysis chondrule precursor components deduced from chondrule compositions are characterized by 1 1 common and slightly volatile siderophiles cr and chalcophiles 2 2 refractory lithophiles and na 3 3 mn si and modal pyroxene and 4 4 feo and olivine component 4 may be closely related to matrix material precursor components have now been inferred for oc eh cv cm and co chondrules a mn and si rich component occurs exclusively among carbonaceous chondrite chondrules although there are significant intergroup differences common to chondrules in all chondrite groups are refractory lithophile rich common siderophile rich and with the exception of eh common lithophile rich components this indicates that the siderophile lithophile and refractory lithophile common lithophile fractionations recorded in chondrite whole rocks were established prior to chondrule formation at all nebular locations

metal and associated phases in bishunpur a highly unequilibrated ordinary chondrite it appears that the highly unequilibrated bishunpur ordinary chondrite preserves phase relations acquired during solar nebular processes to a relatively high degree metamorphic temperatures may not have exceeded 300 350 c the major categories of metal are 3 kinds of metal in the metal matrix three kinds in chondrule interiors and 2 kinds in chondrule rims the fine grained matrix metal is highly variable in composition the kamacite co content 7 8 2 0 mg g is within the l group range 6 7 8 2 mg g but extends well above and below its ni content 38 5 mg g is considerably lower than in more equilibrated chondrites and taenite is ni rich 450 mg g and unzoned these compositions imply equilibration at very low temperatures of about 300 350 c it seems unlikely that volume diffusion could account for the observed relatively unzoned phases a better model involves mass transport by grain boundary diffusion and grain growth at the indicated temperatures we find no evidence that the matrix was ever at higher temperatures large 50 650 m polycrystalline metal aggregates consisting of individually zoned crystals are also found in the matrix they probably represent clusters formed in the solar nebula a few large 50 250 m round monocrystalline grains are also present in the matrix metal bearing chondrules tend to be highly reduced they contain low ni metal that occasionally contains si and or cr silicates in these chondrules tend to have low feo feo mgo ratios the si rich metal grains are never in contact with silicates and are always surrounded by troilite with a poorly characterized ca cr sulfide at the metal troilite interface they appear to be high temperature nebular condensates that avoided oxidation even during the chondrule forming process silicon contents drop below our detection limit when the sulfide coating is absent much more common in chondrule interiors are si free spheroidal metal grains not associated with sulfides these have ni and co contents very similar to the si bearing grains and appear to be oxidized variants of the same material the third class of chondrule metal is fine 1 m dusty grains inside individual olivine grains these seem to reflect high temperature in situ reduction of feo from the olivine the composition of kamacite is different in sulfide rich and sulfide poor chondrule rims and in both cases it is dissimilar to the compositions in the chondrule interiors and matrix this indicates that chondrule rims could not have resulted from reactions with the matrix but are primary features acquired prior to accretton

manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites whole chondrule mn cr isochrons are presented for chondrules separated from the chainpur ll3 4 and bishunpur ll3 1 meteorites the chondrules were initially surveyed by instrumental neutron activation analysis ll chondrite normalized mn cr mn fe and sc fe served to identify chondrules with unusually high or low mn cr ratios and to correlate the abundances of other elements to sc the most refractory element measured a subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x ray spectrometer prior to high precision cr isotopic analyses 53cr 52cr correlates with 55mn 52cr to give initial 53mn 55mn i 9 4 1 7 10 6 for chainpur chondrules and 53mn 55mn i 9 5 3 1 10 6 for bishunpur chondrules the corresponding chondrule formation intervals are respectively tlew 10 1 ma for chainpur and 10 2 ma for bishunpur relative to the time of igneous crystallization of the lewis cliff lew 86010 angrite because mn sc correlates positively with mn cr for both the chainpur and bishunpur chondrules indicating dependence of the mn cr ratio on the relative volatility of the elements we identify the event dated by the isochrons as volatility driven elemental fractionation for chondrule precursors in the solar nebula thus our data suggest that the precursors to ll chondrules condensed from the nebula 5 8 2 7 ma after the time when initial 53mn 55mn i 2 8 0 3 10 5 for calcium aluminum rich inclusions cais our preferred value determined from data for a mineral separates of type b allende cai br1 b spinels from efremovka cai e38 and c bulk chondrites mn cr formation intervals for meteorites are presented relative to average i mn 53mn 55mn ch 9 46 10 6 for chondrules mn cr ratios for radiogenic growth of 53cr in the solar nebula and later reservoirs are calculated relative to average i mn 53cr i 9 46 0 08 10 6 0 23 0 08 for chondrules inferred values of mn cr lie within expected ranges thus it appears that evolution of the cr isotopic composition can be traced from condensation of cais via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids measured values of 53cr for individual chondrules exhibit the entire range of values that has been observed as initial 53cr values for samples from various planetary objects and which has been attributed to radial heterogeneity in initial 53mn 55mn in the early solar system estimated 55mn 52cr 0 42 0 05 for the bulk earth combined with 53cr 0 for the earth plots very close to the chondrule isochrons so that the earth appears to have the mn cr systematics of a refractory chondrule thus the earth apparently formed from material that had been depleted in mn relative to cr contemporaneously with condensation of chondrule precursors if as seems likely the earth s core formed after complete decay of 53mn there must have been little differential partitioning of mn and cr at that time

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

chondrules as products of dust collisions with totally molten droplets within a dust rich nebular environment an experimental investigation abstract chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting if barred olivine bo chondrules formed by nearly total melting at their respective liquidi i e over a wide temperature range it is surprising that there are so few totally glassy chondrules the occurrence of fine grained accretionary rims on chondrules suggests mineral dust in the chondrule forming environment which might have collided with totally molten droplets to produce chondrule textures we have therefore conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500 c h and cause nucleation our experiments reproduce the majority of common chondrule textures the production of textures in our experiments is a function of the temperature at which dust is encountered the number of dust grains encountered and to a lesser extent the mineralogy of the encountered mineral grains crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters we have produced some less commonly discussed types of chondrule textures e g barred olivine porphyritic olivine pyroxene bo pop by dust seeding that have not been reproduced by the incomplete melting of starting materials the reproduction of bo pop and excentroradial textures by dust collisions and the scarcity of natural totally glassy chondrules suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures e g feo rich chondrules from these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs however we have reproduced the very fine grained microporphyritic textures characteristic of type ia chondrules only by incomplete melting such chondrules could not have reached average internal temperatures above their liquidus temperatures the conclusion that at least some chondrules formed from total melts that collided with mineral dust during cooling indicates that chondrule formation occurred within a region of the nebula rich in mineral dust

a petrographic chemical and isotopic study of calcium aluminum rich inclusions and aluminum rich chondrules from the axtell cv3 chondrite petrographic compositional and isotopic characteristics were studied for three calcium aluminum rich inclusions cais and four plagioclase bearing chondrules three of them al rich from the axtell cv3 chondrite all seven objects have analogues in allende cv3 and other primitive chondrites yet axtell like most other chondrites contains a distinctive suite of cais and chondrules in common with allende cais cais in axtell exhibit initial 26 al 27 al ratios 26 al 27 al 0 ranging from 5 10 5 to 2 ma after the first cais as in other cv3 chondrites some objects in axtell show evidence of isotopic disturbance axtell has experienced only mild thermal metamorphism 600 c probably not enough to disturb the al mg systematics its cais and chondrules have suffered extensive metasomatism probably prior to final accretion these data indicate that cais and chondrules in axtell and other meteorites had an extended history of several million years before their incorporation into the axtell parent body these long time periods appear to require a mechanism in the early solar system to prevent cais and chondrules from falling into the sun via gas drag for several million years before final accretion we also examined the compositional relationships among the four plagioclase bearing chondrules two with large anorthite laths and two barred olivine chondrules and between the chondrules and cais three processes were examined 1 igneous differentiation 2 assimilation of a cai by average nebular material and 3 evaporation of volatile elements from average nebular material we find no evidence that igneous differentiation played a role in producing the chondrule compositions although the barred olivine compositions can be related by addition or subtraction of olivine methods 2 and 3 could have produced the composition of one chondrule axch 1471 but neither process explains the other compositions our study indicates that plagioclase bearing objects originated through a variety of processes

iron isotopes in chondrules implications for the role of evaporation during chondrule formation we have measured the 57fe of olivines in nine chainpur chondrules all are within error of normal typically 2 1 2 0 most of the chondules could not have lost more than 20 of their feo by rayleigh evaporation and none can have lost more than 61 yet the range of fo contents in these chondrules is fo78 99 9 the isotopic compositions of the chondrules clearly demonstrate that for instance type i chondrules cannot form from type ii chondrules by evaporation of feo under rayleigh conditions the isotopic compositions also place constraints on the minimum cooling rates these chondrules could have experienced these cooling rates must also be equal to or slower than those required to produce the chondrule textures assuming flash heating and evaporation rates like those measured in vacuum the minimum cooling rates necessary to prevent detectable fe isotopic fractionation via rayleigh evaporation approach those needed to produce barred and porphyritic textures the presence of hydrogen in the nebula non linear cooling and other effects will all tend to increase the cooling rates required to prevent 57fe 1 2 0 perhaps by as much as 1 2 orders of magnitude the two most likely ways that the cooling rates required to prevent 57fe 1 2 0 can be kept below those needed to produce barred and porphyritic textures are 1 the ph2 in the nebula was low enough to keep evaporation rates close to those in vacuum or 2 back reaction of chondrules with fe in the gas suppressed isotopic fractionation

the lack of potassium isotopic fractionation in bishunpur chondrules in a search for evidence of evaporation during chondrule formation the mesostases of 11 bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and k isotopic compositions except for six points all areas of the chondrules that were analyzed had 41k compositions that were normal within error typically 3 2s the six anomalous points are probably all artifacts experiments have shown that free evaporation of k leads to large 41k enrichments in the evaporation residues consistent with rayleigh fractionation under rayleigh conditions a 3 enrichment in 41k is produced by 12 loss of k the range of l chondrite normalized k al ratios a measure of the k elemental fractionation in the areas analyzed vary by almost three orders of magnitude if all chondrules started out with l chondrite like k abundances and the k loss occurred via rayleigh fractionation the most k depleted chondrules would have had compositions of up to 41k 200 clearly k fractionation did not occur by evaporation under rayleigh conditions yet experiments and modeling indicate that k should have been lost during chondrule formation under currently accepted formation conditions peak temperature cooling rate etc invoking precursors with variable alkali abundances to produce the range of k al fractionation in chondrules does not explain the k isotopic data because any k that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation if k loss and isotopic fractionation was inevitable during chondrule formation the absence of k isotopic fractionation in bishunpur chondrules requires that they exchanged k with an isotopically normal reservoir during or after formation there is evidence for alkali exchange between chondrules and rim matrix in all unequilibrated ordinary chondrites however melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules which suggests that they at least remained closed since formation if it is correct that some or all melt inclusions remained closed since formation the absence of k isotopic fractionation in them requires that the k isotopic exchange took place during chondrule formation which would probably require gas chondrule exchange potassium evaporated from fine grained dust and chondrules during chondrule formation may have produced sufficient k vapor pressure for gas chondrule isotopic exchange to be complete on the timescales of chondrule formation alternatively our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation

retention of sodium during chondrule melting abstract type i chondrules in unequilibrated ordinary and carbonaceous chondrites tend to be enriched in refractory elements and depleted in volatiles relative to bulk ci type ii chondrules show chondritic concentrations of major and minor lithophile elements with na in particular at or slightly above the appropriate bulk rock values element ratio diagrams for chondrule bulk compositions show that type ii chondrules plot on a mixing line between forsterite and a na phase with na al 1 1 whereas type i compositions can be explained by mixing forsterite with melilite or cai or other refractory component with little na calcium in type ii chondrules must have been added as an al free phase such as diopside if their bulk compositions are manipulated subtracting ca as diopside and fe mg as olivine the residue is 90 albite and 10 silica which would be in pyroxene albite was incorporated into the precursors of type ii chondrules which clearly have not been depleted in na although their initial temperatures overlap with those of type i there is no negative correlation between type i liquidus temperatures 1450 1900 c and na al ratios and hence no indication of na loss from the melt if type i precursors contained albite the most aluminous chondrules would have suffered the most extreme na loss but these have the lowest liquidus temperatures their precursors were na depleted whereas those of type ii were na enriched as a function of the abundance of albite the simplest way to obtain the bulk compositions of chondrules is to assemble condensates into precursors at different temperatures and concentration of solids in the nebula or exceptionally rapid heating is required to preserve na in chondrules after melting substantial exchange of na occurred between chondrules and chondrite matrix during parent body metamorphism to petrologic type 3 6

petrology and mineralogy of type ii feo rich chondrules in semarkona ll3 0 origin by closed system fractional crystallization with evidence for supercooling abstract the petrology of type ii porphyritic olivine chondrules in semarkona ll3 0 has been studied in detail olivines in these chondrules are euhedral fe richand are strongly zoned from cores to rims of grains in feo fa10 30 cr2o3 0 2 0 6 wt mno 0 2 0 7 wt and cao 0 1 0 4 wt interstitial mesostasis is rich in si aland ca and is glassy with abundant microcrystallites minor minerals include troilite fe ni metaland chromite some olivine grains contain euhedral fayalite rich cores that are probably produced during initial supercooling of the chondrule melt rare relict grains of forsteritic olivine have compositions very similar to olivines in type ia chondrules in semarkona and may result from disaggregation of such chondrules apart from these relics all properties of type ii chondrules can be described by closed system fractional crystallization of droplets which were essentially entirely molten the chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks cooling rates of the order of 1000 c h are indicated from peak temperatures of around 1600 c the chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization there is no evidence for chondrule nebula exchange either during or after crystallization or any secondary processes such as aqueous alteration or thermal metamorphism bulk composition data suggest that precursor material included a refractory ca al ti rich component similar to one of the precursor components of type ia chondrulesand a non refractory fe and si rich component type ia chondrules may have formed from type ii chondrules by loss of fe and volatiles alternatively the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different fe mg ratios

aluminum 26 in h4 chondrites implications for its production and its usefulness as a fine scale chronometer for early solar system events in order to investigate whether or not 26al can be used as a fine scale chronometer for early solar system events we measured with an ion microprobe mg isotopes and al mg ratios in separated plagioclase olivine and pyroxene crystals from the h4 chondrites ste marguerite forest vale beaver creek and quenggouk and compared the results with the canonical 26al 27al ratio for ca al rich inclusions cais for ste marguerite sm and forest vale fv pb pb and mn cr ages have previously been determined gpel et al 1994 polnau et al 2000 polnau and lugmair 2001 plagioclase grains from these two meteorites show clear excesses of 26mg the 26al 27al ratios inferred from these excesses and from isotopically normal mg in pyroxene and olivine are 2 87 0 64 x 10 7 for sm and 1 52 0 52 x 10 7 for fv the differences between these ratios and the ratio of 5 x 10 5 in cais indicate time differences of 5 4 0 1 ma and 6 1 0 2 ma for sm and fv respectively these differences are in agreement with the absolute pb pb ages for cais and sm and fv phosphates but there are large discrepancies between the u pb and mn cr system for the relative ages for cais sm and fv for example mn cr ages of carbonates from kaidun are older than the pb pb age of cais however even if we require that cais are older than these carbonates the time difference between this adjusted cai age and the mn cr ages of sm and fv require that 26al was widely distributed in the early solar system at the time of cai formation and was not mostly present in cais a feature of the x wind model proposed by shu and collaborators gounelle et al 2001 shu et al 2001 from this we conclude that there was enough 26al to melt small planetary bodies as long as they formed within 2 ma of cais and that 26al can serve as a fine scale chronometer for early solar system events

feo rich porphyritic pyroxene chondrules in unequilibrated ordinary chondrites abstract a suite of feo rich type ii porphyritic olivine pyroxene pop chondrules has been studied in detail data for ten chondrules from semarkona ll3 0 are emphasized and one chondrule from chainpur ll3 4 and two from parnallee ll3 6 are included as further examples of certain properties the chondrules contain phenocrysts of pyroxene and olivine in varying proportions and have fe fe mg 10 mol in the silicate minerals four pyroxene minerals are present clinoenstatite orthorhombic enstatite pigeonite and augite pyroxene phenocrysts may consist of all four minerals with progressive overgrowths in order of increasing wo content enstatite may occur independently of clinoenstatite but ca rich pyroxenes pigeonite and augite always nucleate on low ca pyroxene phenocrysts pyroxene and olivine phenocrysts may be independent of each other or may be intergrown and commonly show hopper morphologies indicative of rapid growth pyroxenes and olivines are commonly strongly zoned as a result of fractional crystallization of the chondrules in addition low ca pyroxene may contain ghost regions of slightly more feo and minor element rich material in the cores of grains these regions are partially resorbed and appear to be derived from previous crystallization of essentially the same chondrule oscillatory zoning is observed in enstatite in two chondrules and is believed to be the result of disquilibrium kinetic effects relict grains including a forsterite relict and a partially reduced feo rich relict in the same chondrule and clinoenstatite cores of pyroxene phenocrysts in another chondrule are evidence for recycling and mixing of chondrule material all of the chondrules are highly disequilibrium assemblages they appear to have cooled from essentially molten droplets with little volatile loss or recondensation occurring during the heating event peak temperatures were probably close to liquidus temperatures 1500 1700 c and cooling rates were probably of the order of hundreds of degrees per hour this description will serve as a basis for future interpretation of the effects of metamorphism on porphyritic pyroxene rich chondrules

mineral equilibrium in fractionated nebular systems we investigated the equilibrium mineral assemblages in chemically fractionated nebular systems using a computer routine that finds the set of minerals and gases which minimizes the gibbs free energy of a system with stipulated elemental abundances diagrams are presented showing the equilibrium mineralogy as a function of temperature 400 2300 k for unfractionated solar material and five fractionated systems the fractionated systems were defined by mixing in various proportions the following four volatility components that solar material can be divided into refractory dust carbonaceous matter ices and h2 gas dust enrichment is seen to increase temperatures of condensation evaporation and the fe2 content of mafic minerals and to permit existence of stable melt phases enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites but with mafic minerals that are far more reduced fe2 poor than those in primitive enstatite chondrites enrichment of dust organics and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above 450 k

oxygen isotope ratios of feo poor chondrules in cr3 chondrites influence of dust enrichment and h2o during chondrule formation abstract we present detailed electron microprobe analyses and oxygen three isotope measurements by high precision secondary ion mass spectrometry on 45 type i feo poor chondrules fragments and 3 type ii feo rich chondrule fragments from meteorite hills 00426 and queen alexandra range 99177 two of the most primitive cr3 chondrites type i chondrules fragments have mg s defined as the mg of constituent olivine and or low ca pyroxene ranging from 94 2 to 99 2 type ii chondrule fragments have mg s of 53 63 oxygen three isotope measurements plot on the slope 1 primitive chondrule mineral pcm line within chondrules 17 o 17 o 0 52 18 o values of coexisting olivine pyroxene and plagioclase are homogeneous with propagated uncertainties of 0 3 this indicates each phase crystallized from the final chondrule melt and that efficient oxygen isotope exchange occurred between ambient gas and chondrule melt among type i chondrules there is a well defined increase in 17 o from 5 9 to 1 as mg s decrease from 99 2 to 96 type ii chondrule fragments are comparatively 16 o poor 17 o 0 2 0 6 the relationship between mg and 17 o among type i chondrules confirms that addition of a 16 o poor oxidizing agent to the highest mg chondrule precursors resulted in forming lower mg cr chondrules using aspects of existing equilibrium condensation models and a mass balance we estimate that type i cr chondrules formed at dust enrichments of 100 200 from dusts with 0 0 8 times the atomic abundance of ice relative to ci dust the type ii chondrule fragments are predicted to have formed at ci dust enrichments near 2500

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

correlation between relative ages inferred from 26al and bulk compositions of ferromagnesian chondrules in least equilibrated ordinary chondrites we have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26al 27al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites semarkona ll3 0 and bishunpur ll3 1 the initial 26al 27al ratios of these chondrules were obtained by kita et al 2000 and mostefaoui et al 2002 corresponding to relative ages from 0 7 0 2 to 2 4 0 4 0 7 myr after calcium aluminum rich inclusions cais by assuming a homogeneous distribution of 26al in the early solar system the measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and thus the chondrules in this study are regarded as representatives of ferromagnesian chondrules the relative ages of the chondrules appear to correlate with bulk abundances of si and the volatile elements na k mn and cr but there seems to exist no correlation of relative ages neither with fe nor with refractory elements younger chondrules tend to be richer in si and volatile elements our result supports the result of mostefaoui et al 2002 who suggested that pyroxene rich chondrules are younger than olivine rich ones the correlation provides an important constraint on chondrule formation in the early solar system it is explained by chondrule formation in an open system where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation