oxygen isotopic evolution of amoeboid olivine aggregates in the reduced cv3 chondrites efremovka vigarano and leoville amoeboid olivine aggregates aoas from the reduced cv chondrites efremovka vigarano and leoville consist of forsteritic olivine feni metal and a refractory component composed of spinel al diopside anorthite secondary ferrous olivine and alkali rich minerals nepheline and sodalite commonly observed in the oxidized cvs are rare mineralogy and chemical compositions of aoas are similar to those predicted by equilibrium thermodynamic condensation models suggesting that aoas formed primarily by gas solid condensation over a narrow temperature range slightly below the temperatures over which most ca al rich inclusions cais formed aoas in the reduced cvs preserve a 1st generation 16o rich signal 17 18o 40 similar to that observed in many cais suggesting that these refractory objects originated from a common source in the solar nebula in fact aoas and many fine grained cais may have formed by the same processes but at slightly different temperatures and can be considered a single class of refractory objects alteration of the aoas is manifested by differing extents of 16o depletion in original aoa minerals feo enrichment in olivine and formation of interstitial very fine grained na bearing phases from the six aoas and one fine grained melilite pyroxene rich cai examined in this study five distinct patterns of alteration were identified 1 one unaltered aoa from vigarano is characterized by 16o rich forsterite without feo rich rims and interstitial na bearing phases 2 weak alteration in the melilite pyroxene rich cai is characterized by incomplete 16o depletion in some melilite and precipitation of na bearing phases near the cai rim 3 oxygen isotopic composition and mineralogy are correlated in two aoas from leoville with 16o rich olivine 16o poor anorthite and a range of intermediate compositions in al diopside this pattern is consistent with model diffusion between original grains and a 16o poor reservoir during a relatively short term 60 yr high temperature 900 1100 c event 4 original forsterite has been enriched in feo but remained 16o rich in one aoa from vigarano this result is consistent with the slower rate of diffusion of o than fe and mg in olivine at least some interstitial phases are 16o rich and na bearing phases are abundant in this aoa 5 in contrast oxygen isotopic composition and fo content are correlated in two aoas from efremovka the olivine in these aoas tends to have forsteritic 16o rich cores and feo rich 16o depleted rims the general correlation between oxygen isotopic composition and fo content is difficult to model by diffusion and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains independent evidence for aqueous alteration of the efremovka aoas is provided by oh rich signals detected during ion beam sputtering of some of the 16o poor olivine elevated 16oh count rates and order of magnitude increases in 16oh detected during single analyses reflect trapping of an aqueous phase in 16o depleted olivine an elevated 16oh signal was also detected in one analysis of relatively 16o poor melilite in the melilite pyroxene cai from vigarano suggesting that this object also was altered by aqueous fluid

oxygen isotopes in asteroidal materials measurement of the variation of the 17o 16o and 18o 16o ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the solar system this chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material as sampled by meteorites in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios ultimately offering a window back to the very origin of solar system and its primordialoxygen isotopic heterogeneity oxygen is generally a major element in the gas liquid and solid phases that have interacted throughout the evolution of the solar system its chemical properties ensuring that there is abundant opportunity for reactions and exchange the details of which can be recorded in the isotopic ratios of the products such processes can be crudely subdivided into the following six categories early solar nebular isotopic heterogeneity high temperature modifi cation of nebular components metamorphism and melting in asteroids aqueous alteration in asteroids mixing of components brecciation terrestrial weathering it is clear from the isotopic variation in the earliest formed nebular components such as refractory inclusions and chondrules that there was considerable oxygen isotopic heterogeneity in the early solar nebula spanning over 50 and perhaps much greater in both 17o and 18o the origin of this variation is discussed elsewhere in this volume however isotopic exchange between the 16o rich solids and 17o and 18o rich gas phase s resulted in a wide range of isotopic variation in these early formed components offering insight into the conditions of these processes and the evolution of the reservoirs with time and or space once accreted onto the early planetisemals much of the primitive nebular components experienced considerable modification the metamorphism and melting experienced by some of these bodies most probably the result of heating from the decay of short lived radionuclides e g 26al resulted in progressive homogenization within individual bodies of any original isotopic heterogeneity complete homogenization appears to have been achieved in the hed parent body indicating very high levels of melting and the development of a large magma ocean the carbonaceous and ordinary chondrite parent bodies experienced more modest heating sufficient to mobilize water ice that accreted along with the more refractory materials the subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration such as water rock ratio temperature and isotopic signature and thus potentially the origin of the water ice the evolution of the asteroidal bodies has been far from simple and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites the result of breakup reassembly and mixing of different bodies during impact processing the final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering the isotopic compositions of the weathering products depend on location primarily latitude while the magnitude of the weathering is dependent on many factors with some meteorites e g highly reduced enstatite chondrites and the matrix rich co3s particularly susceptible to such effects

mineralogical controls on the oxygen isotopic compositions of uocs abstract we report the results of oxygen isotopic analyses made on mineral separates prepared from chondrules and whole meteorites uocs feldspar glass mainly chondrule mesostasis is more 16o poor than the olivine low ca pyroxene from which it was separated this relationship is true both for chondrules and whole meteorites from the three uoc groups feldspar and glass have exchanged to a higher degree with 16o poor gases than ferromagnesian minerals because of the susceptibility of framework silicates and glass to gas solid diffusion olivine and low ca pyroxene have preserved primitive isotopic compositions in this way mineralogy is the dominant control on the oxygen isotopic compositions of uocs the similarity of chondrule and whole meteorite isotope compositions suggests that for many uocs matrix and chondrules have undergone exchange with the same isotope reservoirs this exchange postdates chondrule formation and is believed to have occurred after consolidation with the surrounding chondrite matrix in a parent body regolith after this gas solid exchange some isotopic equilibration took place between feldspar and the other relatively 16o rich solids as a result of metamorphism a progressive decrease in the values of 18ofeldspar glass 18oolivine pyroxene for bulk meteorites occurs with increasing petrologic subtype from 8 0 00 for a type 3 2 to 2 5 for a type 3 9 a chemical origin for the initial 16o poor gas reservoirs in the h l and ll parent bodies by processes of mass independent fractionation is considered more likely than nucleosynthesis feldspar glass separated from adrar 003 l ll3 2 may be in or close to having isotopic equilibrium with the 16o poor gases for l or ll chondrites on this basis and assuming that the gas was dominated by h2o and its isotopic composition lay along a line of slope 1 corresponding to what has been called the equilibrated chondrite line a tentative upper limit for gas solid isotopic exchange in uocs can be calculated which gives 370 c

evidence in co3 0 chondrules for a drift in the o isotopic composition of the solar nebula several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher 17o 17o 0 52 x 18o values than those recorded in unaltered chondrules other recent studies have shown that in sets of chondrules from carbonaceous chondrites 17o tends to increase as the feo contents of the silicates increase we report a comparison of the o isotopic composition of olivine phenocrysts in low feo fa1 type i and high feo fa15 type ii porphyritic chondrules in the highly primitive co3 0 chondrite yamato 81020 in agreement with a similar study of chondrules in co3 0 alh a77307 by jones et al 2000 17o tends to increase with increasing feo we find that 17o values are resolved but only marginally between the two sets of olivine phenocrysts in two of the high feo chondrules the difference between 17o of the late formed high feo phenocryst olivine and those in the low feo cores of relict grains is well resolved although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by yurimoto and wasson 2002 it appears that during much of the chondrule forming period there was a small upward drift in the 17o of nebular solids and that relict cores preserve the record of a different and earlier nebular environment

relationship among o mg isotopes and the petrography of two spinel bearing compound chondrules the petrological properties and o and al mg isotopic compositions of two spinel bearing chondrules from the allende cv chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry a coarse spinel grain in a barred olivine bo chondrule is less enriched in 16o 17o 5 17o 17o 0 52 18o whereas smaller spinel grains in a plagioclase rich chondrule member of a compound chondrule are extremely 16o rich 17o 17 and the spinels have a strongly serrated character the petrological features and 16o enrichments of the spinels in the plagioclase rich chondrule indicate that spinels originating in coarse grained ca al rich inclusions cais were incorporated into chondrule precursors and survived the chondrule forming event the degree of 16o excesses among minerals within each chondrule is correlated to the crystallization sequences this evidence suggests that the o isotopic variation among minerals may have resulted from incomplete exchange of o isotopes between 16o rich chondrule melt and 16o poor nebular gas aqueous alteration also has changed the o isotope compositions in the mesostasis the feldspathic mesostasis in the bo chondrule shows a disturbed mg al isochron indicating that the bo chondrule experienced secondary alteration while plagioclase in the plagioclase rich chondrule member of the compound chondrule shows slight 26mg excesses corresponding to 26al 27al 0 4 6 4 0 2 10 6 nepheline formed by secondary alteration shows no detectable excess the al mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the allende parent body

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

oxygen isotopes of chondritic components we review the oxygen isotopic compositions of chondrite components refractory inclusions chondrules and matrix and their inter and intra crystalline oxygen isotopic distributions primary oxygen isotopic compositions acquired before planetesimal accretion are easily disturbed by parent body processes such as aqueous alternation and thermal metamorphism primary or original oxygen isotopic compositions of refractory inclusions ca al rich inclusions and amoeboid olivine aggregates distribute along a slope 1 line on the three oxygen isotope diagram over the range of 60 17 o 18 o 16 o rich and 17 o 18 o rich during refractory inclusion formation on the other hand primary minerals of most chondrules have small isotopic variations and are enriched in 17 o and 18 o relative to refractory inclusions suggesting that chondrule formation occurred in 17 o 18 o rich nebular gas however rare 16 o rich chondrules have been found suggesting some overlap in the timing of formation of chondrules and refractory inclusions in the solar nebula oxygen isotopic compositions of matrix grains distribute along the slope 1 line over the same range as refractory inclusions and chondrules the similarity in oxygen isotopic distributions suggests that matrix was originally a mechanical mixture of nebular dusts co generated with chondrules and refractory inclusions presolar grains with oxygen isotopic compositions that are clearly distinct from those of solar nebular materials are a trace component of chondrite matrices based on these oxygen isotopic characteristics more than 99 5 of the solid materials in the nebula formed locally in the solar nebula and the remainder formed in interstellar space the astrophysical setting of chondrite component formation in the early solar system is also discussed refractory inclusions and 16 o rich matrix dusts formed around the inner edge of the solar nebula on the other hand most chondrules and 17 o 18 o rich matrix dusts seem to have formed elsewhere in the solar nebula efficient large scale radial mixing of the solar nebular materials may have been an essential process in the formation of chondritic planetesimals

thermal history of the enstatite chondrites from silica polymorphs here we report the results of our petrologic and mineralogical study of enstatite e chondrites in order to explore their thermal histories we studied silica phases in 20 e chondrites by laser micro raman spectroscopy to determine the silica polymorphs they contain silica phases are commonly present in e chondrites and their polymorphs reflect the physical conditions of formation the samples studied here include eh3 5 el3 6 e chondrite melt rocks and an anomalous e chondrite we identified quartz tridymite cristobalite and silica glass in the samples studied eh4 5 and eh melt rocks are divided into high and low temperature classes based on niningeritealabandite solid solutions eh3 el3 and some eh melt rocks of the high temperature class contain tridymite and cristobalite we suggest that tridymite and cristobalite crystallized in chondrules and e chondrite melts followed by rapid cooling leading to the survival of these silica polymorphs eh4 and el4 chondrites also contain tridymite and cristobalite in their chondrules indicating that these silica polymorphs survived low temperature metamorphism as estimated from opaque mineral geothermometers because of the sluggishness of the transition to a more stable polymorph tridymite and cristobalite in el6 chondrites reflect the high temperature processes experienced by these meteorites on the other hand some eh5 chondrites and eh melt rocks of the low temperature class contain quartz which may be a product of the transition from tridymite or cristobalite during a long period of low temperature metamorphism although the thermal history of e chondrites have been previously estimated from opaque minerals such compositions mainly reflect low temperature processes however we can reconstruct the primordial thermal processes and subsequent cooling histories of e chondrites from their silica polymorphs the e chondrites have complicated thermal histories which produced the observed variations among them

a single asteroidal source for extraterrestrial ordovician chromite grains from sweden and china high precision oxygen three isotope sims analysis we examined oxygen three isotope ratios of 48 extraterrestrial chromite ec grains extracted from mid ordovician sediments from two different locations in sweden and one location in south central china the ages of the sediments similar to 470 ma coincide with the breakup event of the l chondrite parent asteroid elemental compositions of the chromite grains are generally consistent with their origin from l or ll chondrite parent bodies the average delta o 17 parts per thousand deviation from the terrestrial mass fractionation line measured in situ from 15 mu m spots by secondary ion mass spectrometry sims of ec grains extracted from fossil meteorites from thorsberg and brunflo are 1 17 0 09 parts per thousand 2 sigma and 1 25 0 16 parts per thousand respectively and those of fossil micrometeorites from thorsberg and puxi river are 1 10 0 09 parts per thousand and 1 11 0 12 parts per thousand respectively within uncertainty these values are all the same and consistent with the l chondrite group average delta o 17 1 07 0 18 parts per thousand but also with the ll chondrite group average delta o 17 1 26 0 24 parts per thousand clayton et al 1991 we conclude that the studied ec grains from correlated sediments from sweden and china are related and most likely originated in the same event the l chondrite parent body breakup we also analyzed chromites of modern h l and ll chondrites and show that their delta o 17 values coincide with averages of delta o 17 of bulk analyses of h l and ll chondrites this study demonstrates that in situ oxygen isotope data measured by sims are accurate and precise if carefully standardized and can be used to classify individual extraterrestrial chromite grains found in sediments c 2009 elsevier ltd all rights reserved less

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

on the formation of the tunnel valleys of the superior lobe central minnesota abstract when considering the formation of tunnel valleys most researchers have appealed to basal melting as the primary source of water erosion of the tunnel valleys can then be accomplished by steady state drainage over a long period or by storage and subsequent catastrophic release however field relationships in a large system of tunnel valleys formed by the superior lobe in central minnesota indicate that another source of water must be considered the minnesota tunnel valleys are composed of individual segments 10 20 km long and the segments commonly terminate in subaerial outwash fans at recessional ice margin positions eskers associated with the tunnel valleys are also composed of short segments frequently beginning at moulin kames and terminating at the head of outwash fans the dominant source of the water responsible for tunnel valley formation was seasonal meltwater from the glacier surface that reached the bed through moulins and crevasses the apparent continuity of the valleys resulted from the headward development of the englacial drainage system during ice retreat

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

role of gas melt interaction during chondrule formation abstract it is generally believed that most chondrules formed in the protoplanetary disk by melting of fine grained solid precursor materials and that during these melting events chondrules behaved as closed systems both chemically and isotopically i e the observed range in chondrule bulk chemistry and modal mineralogy was entirely inherited from solid precursors that experienced melting to varying degrees followed by igneous crystallization accompanied by very limited if any evaporation recondensation our mineralogical and chemical study of magnesium rich porphyritic type i chondrules in primitive ordinary and carbonaceous chondrites revealed that chondrule glasses are not located on the subtraction lines of olivine or low ca pyroxene in appropriate phase diagrams indicating that type i chondrules do not obey closed system crystallization we suggest that gas melt interaction played a major role in evolution of mineralogy bulk chemical and isotopic compositions of type i chondrules chondrules are thus complex objects composed of an inherited solid component and a melt component that equilibrated with nebular gas suggesting that chondrules formed in regions with high ambient gas partial pressures as a result chondrule glasses melts are therefore thermochemical and isotopic sensors of the surrounding nebular gas in the chondrule forming regions of the protoplanetary disk and can be used for evaluating its physico chemical conditions

oxygen isotopic compositions of low feo relicts in high feo host chondrules in acfer 094 a type 3 0 carbonaceous chondrite closely related to cm with one exception the low feo relict olivine grains within high feo porphyritic chondrules in the type 3 0 acfer 094 carbonaceous chondrite have 17o 17o 0 52 18o values that are substantially more negative than those of the high feo olivine host materials these results are similar to observations made earlier on chondrules in co3 0 chondrites and are consistent with two independent models 1 nebular solids evolved from low feo low 17o compositions towards high feo more positive 17o compositions and 2 the range of compositions resulted from the mixing of two independently formed components the two models predict different trajectories on a 17o vs log fe mg olivine diagram but our sample set has too few values at intermediate fe mg ratios to yield a definitive answer published data showing that acfer 094 has higher volatile contents than co chondrites suggest a closer link to cm chondrites this is consistent with the high modal matrix abundance in acfer 094 49 vol acfer 094 may be an unaltered cm chondrite or an exceptionally matrix rich co chondrite chondrules in acfer 094 and in co and cm carbonaceous chondrites appear to sample the same population textural differences between acfer 094 and cm chondrites are largely attributable to the high degree of hydrothermal alteration that the cm chondrites experienced in an asteroidal setting

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

oxygen isotopes in chondrule olivine and isolated olivine grains from the co3 chondrite allan hills a77307 we have measured o isotopic ratios in a variety of olivine grains in the co3 chondrite allan hills alh a77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites oxygen isotopic ratios of olivines in this chondrite are variable from delta 17 o 15 5 to 4 5 parts per thousand and delta 18 o 11 5 to 3 9 parts per thousand with delta 17 o varying from 10 4 to 3 5 parts per thousand forsteritic olivines fa 1 are enriched in o 16 relative to the bulk chondrite whereas more feo rich olivines are more depleted in o 16 most ratios lie close to the carbonaceous chondrite anhydrous minerals ccam line with negative values of delta 17 o although one grain of composition fa 4 has a mean delta 17 o of 1 6 parts per thousand marked o isotopic heterogeneity within one feo rich chondrule is the result of incorporation of relic o 16 rich mg rich grains into a more o 16 depleted host isolated olivine grains including isolated forsterites have similar o isotopic ratios to olivine in chondrules of corresponding chemical composition this is consistent with derivation of isolated olivine from chondrules as well as the possibility that isolated grains are chondrule precursors the high o 16 in forsteritic olivine is similar to that observed in forsterite in cv and ci chondrites and the ordinary chondrite julesburg and suggests nebula wide processes for the origin of forsterite that appears to be a primitive nebular component

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

petrology and oxygen isotope compositions of chondrules in e3 chondrites chondrules in e3 chondrites differ from those in other chondrite groups many contain near pure endmember enstatite fs 1 some contain si bearing feni metal cr bearing troilite and in some cases mg mn and ca sulfides olivine and more feo rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules in some cases the feo rich grains contain dusty inclusions of metal the oxygen three isotope ratios 18o 17o of olivine and pyroxene in chondrules from e3 chondrites which are measured using a multi collection sims show a wide range of values most enstatite data plots on the terrestrial fractionation tf line near whole rock values and some plot near the ordinary chondrite region on the 3 isotope diagram pyroxene with higher feo contents 2 10 wt feo generally plots on the tf line similar to enstatite suggesting it formed locally in the ec enstatite chondrite region and that oxidation reduction conditions varied within the e3 chondrite chondrule forming region olivine shows a wide range of correlated 18o and 17o values and data from two olivine bearing chondrules form a slope 1 mixing line which is approximately parallel to but distinct from the ccam carbonaceous chondrite anhydrous mixing line we refer to this as the ecm enstatite chondrite mixing line but it also may coincide with a line defined by chondrules from acfer 094 referred to as the pcm primitive chondrite mineral line ushikubo et al 2011 the range of o isotope compositions and mixing behavior in e3 chondrules is similar to that in o and c chondrite groups indicating similar chondrule forming processes solid gas mixing and possibly similar 16o rich precursors solids however e3 chondrules formed in a distinct oxygen reservoir internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in e3 chondrites suggesting incomplete melting of the chondrules survival of minerals from previous generations of chondrules and chondrule recycling olivine possibly a relict grain in one chondrule has an r chondrite like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs calcium aluminum rich inclusions cais in e3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups however chondrules from e3 chondrites differ markedly from those in other chondrite groups from this we conclude that chondrule formation was a local event but cais may have all formed in one distinct place and time and were later redistributed to the various chondrule forming and parent body accretion regions this also implies that transport mechanisms were less active at the time of and following chondrule formation

oxygen isotopic compositions of relict and host grains in chondrules in the yamato 81020 co3 0 chondrite we report the oxygen isotope compositions of relict and host olivine grains in six high feo porphyritic olivine chondrules in one of the most primitive carbonaceous chondrites co3 0 yamato 81020 because the relict grains predate the host phenocrysts microscale in situ analyses of o isotope compositions can help assess the degree of heterogeneity among chondrule precursors and constrain the nebular processes that caused these isotopic differences in five of six chondrules studied the 17o 17o 0 52 18o compositions of host phenocrysts are higher than those in low feo relict grains the one exception is for a chondrule with a moderately high feo relict both the fayalite compositions as well as the o isotope data support the view that the low feo relict grains formed in a previous generation of low feo porphyritic chondrules that were subsequently fragmented it appears that most low feo porphyritic chondrules formed earlier than most high feo porphyritic chondrules although there were probably some low feo chondrules that formed during the period when most high feo chondrules were forming

petrographic and oxygen isotopic study of refractory forsterites from r chondrite dar al gani 013 r3 5 6 unequilibrated ordinary and carbonaceous chondrites we have conducted petrographic chemical and in situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated r chondrite refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low feo and high refractory lithophile element rle contents refractory forsterites are typically enriched in 16o relative to normal olivine independent of the bulk o isotope ratios of the parent meteorites analyses of refractory forsterites spread along a 17o mixing line with 17o ranging from 2 to 10 due to similarities in oxygen isotopes and chemical compositions we conclude that refractory forsterites of various types of chondrites come from a single common reservoir implications of this hypothesis for the chemical and o isotope evolution of silicates in the early solar nebular are discussed

extreme crustal oxygen isotope signatures preserved in coesite in diamond the anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor1 recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons2 early crustal growth is thought to have been enhanced by the addition of slab derived magmas leaving an eclogite residuum in the upper mantle beneath the cratons3 but the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean floor basalt and intermediate stage subduction zone eclogites and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean floor basalts we present here the oxygen isotope composition of eclogitic mineral inclusions analysed in situ in diamonds using an ion microprobe secondary ion mass spectrometer the oxygen isotope values of coesite a polymorph of sio2 inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle but are typical of subduction zone meta basalts and accordingly provide strong support for the link between altered ocean floor basalts and mantle eclogite xenoliths

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

condensation of major elements during chondrule formation and its implication to the origin of chondrules abstract two glassy refractory al rich chondrules in semarkona ll3 0 the most primitive unequilibrated ordinary chondrite provide direct evidence for condensation of si and mg on melt droplets during cooling the chondrules are completely rounded rich in ca and al and poor in fe and alkalis they have extraordinarily abundant glass 70 80 vol with a subordinate amount of forsterite as the only crystalline phase that occurs mostly rimming the chondrule edge the groundmass glass is concentrically zoned in terms of si with an outward increase which is overlapped with local heterogeneity of mg and al induced by crystallization of forsterite the outward increase of si mostly compensated by al cannot be formed solely by crystallization of forsterite from a homogeneous melt in a closed system combined with skeletal or dendritic morphology and sector zoning of forsterite it is suggested that si condensed onto totally molten droplets initial melts accompanied by nucleation and rapid growth of forsterite with lowering temperature the initial melts the compositions of which were estimated from the ca contents of the first crystallized forsterite are very similar to type c cai but are notably poorer in mg and si than the bulk chondrules indicating condensation of mg in addition to si with an atomic ratio of mg si 3 2 the condensation after the nucleation of forsterite took place below 1300 c under cooling at 70 c h and amounted to 30 wt of the current chondrule this study suggests a model that a short time and local shock heating event induced melting of type c cai and concomitant evaporation of dusts ferromagnesian chondrules of earlier generation and their fragments to generate mg and si rich gas which condensed onto the melt droplets upon cooling accompanying condensation of type i chondrules

compositions and textures of relic forsterite in carbonaceous and unequilibrated ordinary chondrites several percent of the olivine in the c2 c3 and unequilibrated ordinary chondrites uoc can be distinguished by blue cathodoluminescence cl and an unusual composition for forsterite this olivine has the following textural features 1 1 forms cores in single olivine grains 2 2 shows subhedral to euhedral boundaries against rim olivine 3 3 rarely contains inclusions 4 4 has embayments containing olivine like that of the rim 5 5 occurs within chondrules especially in uoc meteorites the blue olivine is always fe poor 0 25 feo 1 0 and shows the following average and maximum values al2o3 0 25 0 5 tio2 0 05 0 09 cao 0 5 0 8 cr2o3 0 15 0 5 and mno 0 02 0 15 vanadium is present within a single olivine and within all blue olivines al ca and ti are strongly positively correlated as are mn fe and cr in olivine surrounding the blue the blue cores are not zoned but each element shows a marked change at the boundary of the blue with al showing the most rapid change these are interpreted as diffusion profiles between rim and core olivine textures suggest initial free growth probably from a gas and later addition of olivine by liquid crystallization to form single crystals or chondrules the unusual olivine composition indicates high temperature growth from a refractory rich reservoir with al entering olivine in tetrahedral coordination vapor growth is suggested as the process allowing the high minor element levels the occurrence of blue olivine in all primitive meteorites indicates that it is relic material which was widespread prior to chondrule and hence meteorite formation similarities in composition exist between this relic olivine and olivine of cosmic dust and deep sea particles pointing to this olivine being a common component in all primitive extraterrestrial material

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

calcium aluminum rich inclusions and amoeboid olivine aggregates from the cr carbonaceous chondrites calcium aluminum rich refractory inclusions cais in cr chondrites are rare 1 vol fairly small 500 m and irregularly shaped and most of them are fragmented based on the mineralogy and petrography they can be divided into grossite hibonite rich melilite rich and pyroxene anorthite rich cais other types of refractory objects include fine grained spinel melilite pyroxene aggregates and amoeboid olivine aggregates aoas some of the pyroxene anorthite rich cais have igneous textures and most melilite rich cais share similarities to both the fluffy and compact type a cais found in cv chondrites one major difference between these cais and those in cv cm and co chondrites is that secondary mineral phases are rare in situ ion microprobe analyses of oxygen isotopic compositions of 27 cais and aoas from seven cr chondrites demonstrate that most of the cais are 16o rich 17o of hibonite melilite spinel pyroxene and anorthite 22 and isotopically homogeneous within 3 4 likewise forsterite spinel anorthite and pyroxene in aoas have nearly identical 16o rich compositions 24 17o 20 deg in contrast objects which show petrographic evidence for extensive melting are not as 16o rich 17o less than 18 o secondary alteration minerals replacing 16o rich melilite in melilite rich cais plot along the terrestrial fractionation line most cr cais and aoas are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes which is reflected in their relatively homogeneous 16o rich compositions it is likely that these objects or their precursors condensed in an 16o rich gaseous reservoir in the solar nebula in contrast several igneous cais are not very enriched in 16o probably as a result of their having melted in the presence of a relatively 16o poor nebular gas if the precursors of these cais were as 16o rich as other cr cais this implies either temporal excursions in the isotopic composition of the gas in the cai forming regions and or radial transport of some cai precursors into an 16o poor gas the absence of oxygen isotope heterogeneity in the primary minerals of melilite rich cais containing alteration products suggests that mineralogical alteration in cr chondrites did not affect oxygen isotopic compositions of their cais

compositional evolution of the protoplanetary disk oxygen isotopes of type ii chondrules from cr2 chondrites chondrules are the dominant component of chondritic meteorites and attest to high temperature transient heating events within the protoplanetary disk they provide valuable constraint on the disk environments in which they formed and potentially the evolution of primitive planetary materials in the disk the oxygen isotopic composition of cr2 chondrite type ii chondrules was investigated our data show significant variation in the isotopic compositions of the chondrules with no petrographic or geochemical means to identify what chondrule will plot where on a three isotope diagram although we cannot rule out that these chondrules may have come from another chondrite forming region we argue in context of type i chondrules from cr2 chondrites that cr2 type ii chondrules record changes in solid and gas composition during formation due to the vaporization of icy bodies in localized regions of the inner disk

extremely rapid cooling of a carbonaceous chondrite chondrule containing very 16o rich olivine and a 26mg excess we describe a phenocryst in a co chondrite type ii chondrule that we infer to have formed by melting an amoeboid olivine aggregate aoa this magnesian olivine phenocryst has an extremely 16o rich composition 17o 17o 0 52 18o 23 it is present in one of the most pristine carbonaceous chondrites the co3 0 chondrite yamato 81020 the bulk of the chondrule has a very different 17o of 1 thus the 17o range within this single chondrule is 22 the largest range encountered in a chondrule we interpret the o isotopic and fe mg distributions to indicate that a fine grained aoa assemblage was incompletely melted during the flash melting that formed the chondrule some fe mg exchange but negligible o isotopic exchange occurred between its core and the remainder of the chondrule a diffusional model to account for the observed fe mg and o isotopic exchange yields a cooling rate of 105 to 106 k hr 1 this estimate is much higher than the cooling rates of 101 to 103 k hr 1 inferred from furnace simulations of type ii chondrule textures e g lofgren 1996 however our cooling rate applies to higher temperatures near 1900 k than are modeled by the crystal growth based cooling rates we observed a low 26al 27al initial ratio 4 6 3 0 10 6 in the chondrule mesostasis a value similar to those in ordinary chondrites kita et al 2000 if the 26al 27al system is a good chronometer then chondrule i formed about 2 ma after the formation of refractory inclusions

oxygen isotope heterogeneity in chondrules from the mokoia cv3 carbonaceous chondrite we report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the mokoia oxidized cv3 chondrite bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination and oxygen isotope ratios of individual grains mostly olivine were obtained in situ on polished mounts using secondary ion mass spectrometry sims our results can be compared with data obtained previously for the oxidized cv3 chondrite allende bulk oxygen isotope ratios of mokoia chondrules form an array on an oxygen three isotope plot that is subparallel to and slightly displaced from the ccam carbonaceous chondrite anhydrous minerals line the best fit line for all cv3 chondrite chondrules has a slope of 0 99 and is displaced significantly by 17o 2 5 from the young and russell slope one line for unaltered calcium aluminum rich inclusion cai minerals oxygen isotope ratios of many bulk cais also lie on the cv chondrule line which is the most relevant oxygen isotope array for most cv chondrite components bulk oxygen isotope ratios of most chondrules in mokoia have 18o values around 0 and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values in general it appears that chondrule mesostases have higher 18o values than olivines in the same chondrules our bulk chondrule data spread to lower 18o values than any ferromagnesian chondrules that have been measured previously two chondrules with the lowest bulk 18o values 7 5 and 11 7 contain olivine grains that display an extremely wide range of oxygen isotope ratios down to 17o 18o around 50 in one chondrule in these chondrules there are no apparent relict grains and essentially no relationships between olivine compositions which are homogeneous and oxygen isotopic compositions of individual grains heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates followed by solid state chemical diffusion without concomitant oxygen equilibration alternatively oxygen isotope exchange between an 16o rich precursor and an 16o poor gas may have taken place during chondrule formation and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains if this is the case we can infer that the earliest nebular solids from which chondrules formed had 17o and 18o values around 50 similar to those observed in refractory inclusions

ore deposition in a proterozoic incipient rift zone environment a tentative model for the filipstad grythyttan hjulsjo region bergslagen sweden the 1 9 1 8 ga bergslagen supracrustal series comprises an early volcanic stage represented by the lower leptite group an initial rift stage by the middle leptite group a rift stage by the upper leptite halleflinta and slate group metabasites and the granite granophyre suite and a post rift stage by conglomerate beds remobilized granite granophyres and the hyttsjo gabbro tonalite suite the formation and subsequent alteration of iron manganese and sulfide skarn ores in the supracrustal series involve

oxygen and magnesium isotopic compositions of amoeboid olivine aggregates from the semarkona ll3 0 chondrite amoeboid olivine aggregates aoas in the ll3 0 semarkona chondrite have been studied by secondary ion mass spectrometry the aoas mainly consist of aggregates of olivine grains with interstitial al ti rich diopside and anorthite oxygen isotopic compositions of all phases are consistently enriched in 16o with 17 18o 50 the initial 26al 27al ratios are calculated to be 5 6 0 9 2 10 5 these values are equivalent to those of aoas and fine grained calciumaluminum rich inclusions fgis from pristine carbonaceous chondrites this suggests that aoas in ordinary chondrites formed in the same 16o rich calcium aluminum rich inclusion cai forming region of the solar nebula as aoas and fgis in carbonaceous chondrites and subsequently moved to the accretion region of the ordinary chondrite parent body in the solar nebula

16 o enrichments in aluminum rich chondrules from ordinary chondrites the oxygen isotopic compositions of seven al rich chondrules from four unequilibrated ordinary chondrites were measured in situ using an ion microprobe on an oxygen three isotope plot the data are continuous with the ordinary chondrite ferromagnesian chondrule field but extend it to more 16o enriched values along a mixing line of slope 0 83 0 09 with the lightest value recorded at 18o 15 7 1 8 and 17o 13 5 2 6 if al rich chondrules were mixtures of ferromagnesian chondrules and cai material their bulk chemical compositions would require them to exhibit larger 16o enrichments than we observe therefore al rich chondrules are not simple mixtures of these two components three chondrules exhibit significant internal isotopic heterogeneity indicative of partial exchange with a gaseous reservoir porphyritic al rich chondrules are consistently 16o rich relative to nonporphyritic ones suggesting that degree of melting is a key factor and pointing to a nebular setting for the isotopic exchange process because al rich chondrules are closely related to ferromagnesian chondrules their radiogenic mg isotopic abundances can plausibly be applied to help constrain the timing or location of chondrule formation

the oxygen isotope composition of chondrules and isolated forsterite and olivine grains from the tagish lake carbonaceous chondrite abstract the oxygen isotope compositions obtained by laser fluorination of hand picked separates of isolated forsterite isolated olivine and chondrules from the tagish lake carbonaceous chondrite describe a line 17o 0 95 18o 3 24 r2 0 99 similar to the trend known for chondrules from other carbonaceous chondrites the isolated forsterite grains fo99 6 99 8 18o 7 2 to 5 5 17o 9 6 to 8 2 are more 16o rich than the isolated olivine grains fo39 6 86 8 18o 3 1 to 5 1 17o 0 3 to 2 2 and have chemical and isotopic characteristics typical of refractory forsterite chondrules contain olivine fo97 2 99 8 with oxygen isotope compositions 18o 5 2 to 5 9 17o 8 1 to 1 2 that overlap those of isolated forsterite and isolated olivine an inverse relationship exists between the 17o values and fo contents of tagish lake isolated forsterite and chondrules the chondrules likely underwent greater exchange with 16o poor nebular gases than the forsterite the oxygen isotope compositions of the isolated olivine grains describe a trend with a steeper slope 1 1 0 1 r2 0 94 than the carbonaceous chondrite anhydrous mineral line ccamslope 0 95 the isolated olivine may have crystallized from an evolving melt that exchanged with 16o poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite the primordial components of the tagish lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups especially cms its alteration history has its closest affinities to ci carbonaceous chondrites

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 isotopes in chondrules and coarse grained chondrule rims from the allende meteorite abstract the oxygen isotopic compositions of porphyritic chondrules and coarse grained chondrule rims from the allende cv chondrite form a 16 o mixing trend situated near the carbonaceous chondrite anhydrous minerals ccam mixing line lying 0 1 0 7 higher in 17 o coarse grained rims are depleted in 16 o relative to their enclosed chondrules chondrules and rims exhibit significant correlations between their oxygen isotopic and chemical compositions objects rich in 16 o tend to have higher contents of refractory lithophiles and alkalies objects poor in 16 o tend to have higher contents of feo and volatile siderophiles cv chondrite matrix material is chemically and isotopically similar to some coarse grained chondrule rims previous work showed that o isotope data for cv barred olivine bo chondrules plot just below the terrestrial fractionation tf line near 18 o 4 well off the ccam line the allende data suggest that three solid precursor components are required to account for the o isotopic compositions of cv chondrules and rims 1 a high temperature refractory alkali and 16 o rich component perhaps near 18 o 40 17 o 42 relative to smow 2 a low temperature feo rich refractory and 16 o poor component lying near the ccam line perhaps near its intersection with the tf line at 18 o 10 and 3 an additional component required to explain the composition of bo chondrules

the primitive matrix components of the unique carbonaceous chondrite acfer 094 a tem study the mineralogical and chemical characteristics of the fine grained matrix 53 m of the unique primitive carbonaceous chondrite acfer 094 have been investigated in detail by scanning electron microscopy sem and analytical transmission electron microscopy atem generally the fine grained matrix represents a highly unequilibrated assemblage of an amorphous material small forsteritic olivines 200 300 nm low ca pyroxenes 300 400 nm and fe ni sulfides 100 300 nm the matrix is basically unaffected by secondary processes only minor amounts of serpentine and ferrihydrite as products of hydrous alteration are present texturally the amorphous material acts as a groundmass to olivines pyroxenes and sulfides mostly exhibiting rounded or elongated morphologies only very few clastic mineral grains have been found the texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment as such the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites the non clastic matrix olivines fo98 99 and pyroxenes en97 100 are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material the formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons rapid cooling caused the observed intergrowths of clino orthoenstatite in the mg rich matrix pyroxenes although some similarities exist comparing the fine grained matrix of acfer 094 with the matrices of the unequilibrated c03 chondrite alha77307 and the unique type 3 chondrite kakangari acfer 094 remains unique since it contains the highest measured concentrations of circumstellar sic and the second highest of diamond highest is orgueil it seems reasonable to suggested that at least parts of the amorphous material in the fine grained matrix may be of circumstellar origin