origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites optical microscopy electron probe microanalysis scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in bishunpur krymka semarkona chainpur all ll3 tieschitz and sharps both h3 the size distribution of grains in matrix in bishunpur obeys a power law rims around chondrules tend to be finer grained than matrix and may be layered all rims and matrices studied contain clasts of forsteritic olivine and low ca pyroxene in bishunpur the clasts may be cemented by amorphous glue rich in normative albite and partly altered to smectite rims in krymka sharps chainpur and tieschitz have a densely packed olivine groundmass with a grain size of about 0 1 m in krymka groundmass olivine is fo20 30 and forms interlocking dendrites various fe oxides are present in rims and matrices grain size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules from analyses matrices rims in the ll group apparently are enriched in a feldspathic component tieschitz rims are slightly enriched but here the feldspathic component is present as white matrix sharps rims are not enriched in a feldspathic component opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules partly induced by contraction of protopyroxene on inversion to clinopyroxene fragmented silica rich chondrule mesostases reacted with fe oxides and na to form the groundmass of fayalitic olivine and feldspathic glue a low temperature nebular or pre solar component is limited to 3 vol of each meteorite so equilibrium condensate was not the carrier of volatiles such as tl and bi

oxygen isotopes in r chondrite magnetite and olivine links between r chondrites and ordinary chondrites ion microprobe studies yield 17o 17o 0 52 18o values in magnetite from the rumurti chondrite rc pca91241 which is paired with pca91002 of 3 1 to 3 9 slightly higher than that in o from whole rock r samples despite 17o values in whole rock rcs that are much by ca 1 6 higher than in whole rock ll chondrites the 17o in r magnetite is much lower by ca 2 than the values 4 to 7 from ll3 semarkona and ngawi choi et al 1998 the 18o values in pca magnetite 15 to 10 are the lowest known in meteorites well below the range in semarkona 4 to 9 on a 17o 18o diagram both magnetite data sets form linear arrays with slopes of ca 0 7 indicating mixing of o from different isotopic reservoirs the slopes and intercepts of the two arrays are similar enough to permit them to be segments of a single array this suggests that in rcs and ll chondrites magnetite formed from the same raw materials by the same processes probably by aqueous alteration of metal in an asteroidal setting we observed 17o values in olivines and pyroxene from rcs ranging from 1 2 to 2 9 and 18o from 1 4 to 9 1 these compositions scatter in the same general range observed in chondrules from ordinary chondrites the similarity in the o isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which rcs formed from nebular components similar to those in h chondrites but with a matrix chondrule ratio several times higher in the rcs and with more extensive aqueous alteration in the rcs than in known h chondrites we postulate that the matrix in r chondrites has 17o higher than whole rock values we suggest that the original 17o value of h2o in the rc body was similar to that incorporated into the ordinary chondrites previously estimated by choi et al 1998 to be ca 7 in the ll parent body

chondrules from chainpur ll 3 reduced parent rocks and vapor fractionation abstract nineteen chondrules from the chainpur ll 3 chondrite were analyzed for their bulk lithophile element contents and mineral chemistries chondrule compositions are highly fractionated the following compositional groups can be distinguished 1 refractory depleted in na and k 2 k depleted k ci na ci 3 approximately chondritic na k ci na ci and 4 k enriched k ci na ci the most refractory chondrules are depleted in k 0 11 ci abundances normalized to sc na 0 26 mn 0 52 and cr 0 71 therefore we conclude that the refractory group was mainly produced by vapor fractionation during the chondrule forming event the majority of chondrules in enriched in na 1 37 1 40 ci but fractionated in k which ranges from 0 4 ci k depleted to 2 84 ci k enriched for the fractionation of k from na we suggest a separation mechanism via alkali sulfides uncorrelated fractionations of ca mn cr eu yb sm give supporting evidence for such a mechanism these findings imply that the pre chainpur matter went through highly reducing conditions high h2 o2 and s2 o2 ratios before it was sampled by the chondrule forming process most chondrules of our suite have similar fa contents in olivine 11 21 and are enriched in volatile elements from this we conclude that these chondrules were formed under oxidizing conditions in a relatively dense atmosphere with high partial pressures of volatile elements alkalis halogens a proto planetary atmosphere could have provided these conditions the fractionation process proposed should also be applicable to fractionations on a large scale like the na k and mg si fractionations among chondrites

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

mid infrared spectroscopy of refractory inclusions cais in cv and co chondrites we present laboratory mid infrared absorption spectra 2 5 m to 16 0 m of powdered calcium aluminum rich inclusions cais and matrix separated from the carbonaceous chondrites allende cv3 2 vigarano cv3 3 and ornans co3 3 two groups of spectra with different features were found for the cai in the first group spectra are dominated by spinel pyroxene and sodalite nepheline where main features occur at 9 3 m 10 3 m and 11 3 m in the second group characteristic minerals are spinel and melilite with typical band maxima at 11 0 m and 12 3 m and a broad feature between 14 0 m and 15 0 m the position of the broad spinel feature probably depends on its iron content comparison of band positions in spectra from the cai components to observed circumstellar emission spectra indicates the potential occurrence of cai like material pyroxene and spinel rich features could occur in spectra of dust around the herbig ae star hd104237 the t tauri star hen3 600 and the post agb star r sge melilite and spinel rich components possibly appear in the spectrum of hd104237 hen3 600 04187 1927 r sge and the planetary nebula hb 12 there is also indication for a spinel component in dust from the herbig ae be star hd179218 the spectra of the agb stars r cas and aps show no features of cai type spinel

a survey of clasts and large chondrules in ordinary chondrites we report the results of a survey of clasts and large 5 mm chondrules macrochondrules within the 833 ordinary chondrites of the natural history museum collection thirty six macrochondrules and 24 clasts were identified and studied macrochondrules have textures and mineral assemblages like normal chondrules and so share a common origin clasts show evidence for fracturing from larger bodies and can be classified as either 1 chemically fractionated if they have major and trace element compositions differing substantially from most chondrules and clasts 2 impact melt clasts if they have microporphyritic textures and signs that indicate they are derived from shock melted chondritic material 3 microporhyritic clasts if they are similar to the last category but lack evidence for derivation through shock melting or 4 indeterminate clasts forming a diverse class that includes all those clasts that do not fit into the other categories

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

upper mesosphere summer echoes detected with the antarctic syowa hf radar 1 peculiar hf radar echoes observed at syowa station antarctica 69 0 s 39 6 e are reported the echoes occurred in summer at slant ranges between 180 first range gate and 315 km under very quiet geomagnetic conditions they are characterized by a duration of less than 80 min quasi periodic oscillations of velocity and echo power with periods of 5 20 min velocities between 40 and 20 ms 1 and narrow spectral widths less than 40 ms 1 from data of june july december 1997 and january 1998 we identified such echoes on 4 days in december and january summer but not in june and july winter although range resolution of the hf radar 45 km is very poor we believe that our echoes are antarctic polar mesosphere summer echoes pmse relying on that the echo characteristics are very similar to the arctic pmse

trace element contents of chondrule rims and interchondrule matrix in ordinary chondrites abstract the abundances of forty elements have been measured in the rims and matrices of the unequilibrated ordinary chondrites semarkona bishunpur chainpur and krymka the rims appear to be mixtures of matrix like material and material that was volatilized during chondrule formation iron silicon manganese and copper all appear to have behaved as volatiles during chondrule formation and their enrichments are generally confined to the rims suggesting recondensation of these elements was rapid and localized this is supported by petrologic observations that show many chondrules were still partially molten or plastic when the rims accreted some rims also exhibit a decrease in both their feo content and the degree of mineral re equilibration away from the chondrule the alkali metals are enriched in both the rims and matrices the alkalis may have recondensed onto the matrix as well as the rims but their mobility is such that they have probably undergone extensive redistribution after accretion some na may even have been able to diffuse back into chondrules either during chondrule cooling or during metamorphism alteration most refractory lithophiles are depleted with respect to a1 in the rims and matrices in all four uocs these depletions are probably due to postaccretional redistribution of these elements into phosphates and other secondary minerals that form in and around metal sulfide grains the matrix is probably composed of chondrule fragments and processed nebular dust but the presence of carbonaceous presolar grains in the matrix indicates that there must be unprocessed presolar silicates present too on average the presolar silicates presumably have a ci like composition but a ci like signature is not seen in the matrix compositions simple estimates suggest that the abundances of carbonaceous presolar grains are consistent with there being at most 10 wt o presolar silicates present in uoc matrix which would be hard to detect in this study

a unique high mn fe microgabbro in the parnallee ll3 ordinary chondrite nebular mixture or planetary differentiate from a previously unrecognized planetary body the study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies we studied a unique microgabbro fragment from the parnallee ll3 unequilibrated ordinary chondrite the fragment which was originally identified by its ophitic to sub ophitic texture exhibits features characteristic of lunar and terrestrial tholeiitic basalts extreme compositional zoning in clinopyroxene wo 10 en 65 fs 25 to wo 15 en 2fs 83 a multiply saturated major element composition similar to mid ocean ridge basalt with 3 1 wt na 2o and 0 15 wt k 2o and uniformly enriched rare earth elements c 10 c1 a high bulkmno feo ratio 0 064 distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment however the absence of fe metal and the extreme enrichment of feo up to 40 wt in late crystallizing pyroxferroite requires the last crystallization event to have occurred in a relatively oxidizing environment we suggest the microgabbro formed by partial melting in a planetary body after removal of metallic fe examination of possible planetary source materials such as alkali rich eucritic material a volatile depleted c1 carbonaceous chondrite or h group chondrite shows that multiple stage fractionation is required to produce a melt with the feo mgo ratio of the microgabbro from these materials the increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites uoc suggests the picture of uoc as primitive assemblages of unprocessed material is overly simplistic

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

record of low temperature alteration in asteroids most chondritic materials experienced diverse styles of secondary alteration resulting in formation of hydrous and anhydrous oxygen bearing minerals silicates oxides and carbonates phyllosilicates magnetite calcite dolomite breunnerite ferrous olivine hedenbergite wollastonite grossular monticellite forsterite andradite nepheline sodalite brearley 2003 2005 and references therein petrographic mineralogic oxygen and short lived isotope systematics 26al 26mg 53mn 53cr 129i 129xe suggest that alteration occurred in the presence of aqueous solutions under variable physico chemical conditions temperature water rock ratio ph f o2 and fluid compositions in an asteroidal setting it started within 1 2 m y after formation of the cv cais was multistage and lasted up to 15 m y krot et al 2006 and references therein here we review bulk o isotopic compositions of chondritic materials o isotopic compositions of secondary minerals produced during asteroidal alteration and possible effects of fluid assisted thermal metamorphism on o isotopic exchange in primary high temperature minerals melilite and anorthite in the cv and co cais the implications of these data for understanding temperatures of alteration water rock ratios and oxygen isotopic composition of water that probably accreted into chondrite parent bodies in the form of ice are discussed the inferred or measured o isotopic composition of the meteoritic water is close to the terrestrial fractionation line 17o 2 it is very different from the inferred o isotopic compositions of the sun 17o 25 clayton 2002 yurimoto and kuramoto 2004 hashizume and chaussidon 2005 lyons and young 2005 suggesting significant evolution of o isotopic composition of the gaseous reservoir in the inner protoplanetary disk over its short 3 5 m y life time

trace element distributions within ordinary chondrite chondrules implications for chondrule formation conditions and precursors the concentrations of thirty six lithophile elements in the major silicate phases of chondrules have been determined by ion microprobe two types of porphyritic chondrule olivines normal and incompatible rich have been identified the incompatible rich olivines which have forsterite contents that are greater than fo98 appear to have formed under highly reducing conditions the olivine and low ca pyroxene grains in porphyritic chondrules have relatively unfractionated l chondrite normalized ree abundances compared to expectations from equilibrium distribution coefficients this is consistent with porphyritic chondrules having formed at cooling rates of the order of 1000 c h or faster the alkali metal na k rb and cs to al ratios in chondrule glasses vary by two orders of magnitude the lower values possibly being the result of volatile loss from chondrules no correlation was found between alkali metal depletions in chondrule glasses and enrichments in the surrounding rims but any such correlation may have been destroyed by the diffusive redistribution of these elements in rims that is likely to occur even under mild metamorphic conditions the refractory lithophiles with the exception of mg are all highly incompatible in silicate melts and are as a result concentrated in the chondrule glasses the refractory lithophiles in the glass are in most cases unfractionated from one another suggesting that neither vapor phase nor crystal liquid fractionation played a major role in the formation of chondrules or their precursors based largely on the observation that the refractory lithophiles are concentrated in the chondrule glasses it is suggested that previously reported correlations between these elements in bulk chondrule analyses are due to chondrules themselves as opposed to refractory condensates having been their own immediate precursors a simple monte carlo simulation using the compositions of the major chondrule silicate phases reported here supports this hypothesis it is argued that the upper limit of about one observed in the atomic na al ratios of chondule glasses is not due to an albitic precursor but simply reflects that for na to be incorporated into the structure of the melt it must take part in the coupled substitution naalo2 sio2

on the separation of relatively buoyant components from subducted lithosphere subducted oceanic lithosphere consists of a sandwich of component layers morb underlain by depleted parent mantle these layers give rise to intrinsic density contrasts within the slab which have been hypothesized to cause component separation after subduction we explore this process through numerical models of simple layered slabs and through convection models which incorporate large local chemical density contrasts we show that the components of positive and negative buoyancy density contrast component volume must be almost equal in order to cause significant component separation or to selectively block deep penetration of buoyant slab components in the transition zone published petrological constraints on component densities and relative volumes indicate that such buoyancy neutralization is not achieved in subducted slabs therefore component separation should not occur on the timescale associated with the sinking of slabs through the upper mantle and transition zone

earliest planetary melting the view from meteorites abstract the eucrite basaltic meteorites and iron meteorites from the cores of asteroids have ages that testify to planetary formation and differentiation 4 55 4 60 ga ago these ages are indistinguishable from those of the ordinary chondrites mixtures of silicate sulphide and metal that did not melt after their formation rare inclusions in the ordinary chondrites have textures and or fractionated rare earth element patterns indicating that they are the products of basaltic melts thus some basaltic rocks predate the accretion of the ordinary chondrites four inclusions are briefly described one contains evidence for the former presence of live 26al another exhibits an ophitic texture extreme fe enrichment in the pyroxenes and an abnormally high mno feo ratio this inclusion is from a differentiated body of which we have identified no other sample

the iodine xenon system in clasts and chondrules from ordinary chondrites implications for early solar system chronology abstract we have studied the i xe system in chondrules and clasts from ordinary chondrites cristobalite bearing clasts from parnallee ll3 6 closed to xe loss 1 4 ma after bjurbole feline a feldspar and nepheline rich clast also from parnallee closed at 7 04 0 15 ma two out of three chondrules from parnallee that yielded well defined initial i ratios gave ages identical to bjurbole s within error a clast from barwell l6 has a well defined initial i ratio corresponding to closure 3 62 0 60 ma before bjurbole partial disturbance and complete obliteration of the i xe system by shock are revealed in clasts from julesburg l3 6 and quenggouk h4 respectively partial disturbance by shock is capable of generating anomalously high initial i ratios in some cases these could be misinterpreted yielding erroneous ages a macrochondrule from isoulane n amahar contains concentrations of i similar to ordinary chondrules but unlike most ordinary chondrules contains no radiogenic 129xe this requires resetting 50 ma or more later than most chondrules the earliest chondrule ages in the i xe mn cr and al mg systems are in reasonable agreement this and the frequent lack of evidence for metamorphism capable of resetting the i xe chronometer leads us to conclude that at least the earliest chondrule i xe ages represent formation if so chondrule formation took place at a time when sizeable parent bodies were present in the solar system

36cl 26al and o isotopes in an allende type b2 cai implications for multiple secondary alteration events in the early solar system we measured 36cl 36s and 26al 26mg systematics and o isotopes of secondary phases in a moderately altered type b2 cai cai 2 from the allende cv3 chondrite cai 2 has two distinct alteration domains the anorthite grossular an grs domain that mostly consists of anorthite and grossular and the na rich domain that mostly consists of sodalite anorthite and fe bearing phases large 36s excesses up to 400 corresponding to an initial 36cl 35cl ratio of 1 4 0 3 x 10 6 were observed in sodalite of the na rich domain but no resolvable 26mg excesses were observed in anorthite and sodalite of the na rich domain the initial 26al 27al ratio 4 4 x 10 7 if we assume that the 36cl 36s and the 26al 26mg systematics were closed simultaneously the 36cl 35cl ratio would have to be on the order of 10 2 when cais were formed in contrast to sodalite in na rich domain significant 26mg excesses up to 35 corresponding to an initial 26al 27al ratio of 1 2 0 2 x 10 5 were identified in anorthite of the an grs domain the 26al 26mg systematics of secondary phases in cai 2 suggest that cais experienced multiple alteration events some of the alteration processes occurred while 36cl half life is 0 3 myr and 26al half life is 0 72 myr were still alive whereas others took place much later assuming that 26al was homogeneously distributed in the solar nebula our study implies that alteration of cais occurred as early as within 1 5 myr of cai formation and as late as 5 7 myr after

sodium chlorine rich mesostases in chainpur ll3 and parnallee ll3 chondrules forty six chondrules from chainpur ll3 4 and 39 chondrules and clasts from parnallee ll3 6 have been sectioned and searched for na cl rich phases by electron probe microanalysis epma oxygen isotopic compositions i xe ages and ion probe data were also obtained on some of these chondrules sodium cl rich glass and microcrystalline sodalite na4al3si3o12cl nepheline naalsio4 scapolite na4al3si9o24cl have been identified in 7 of the chainpur and 8 of the parnallee samples these phases are present in chondrule mesostases or in one case the plagioclase of a barred olivine chondrule none of the chondrules contain 5 vol na cl rich phases in the chainpur chondrules they originated through partial devitrification of silica undersaturated rare earth element ree na and cl rich mesostases two processes have been identified that led to the formation of these mesostases in two of the chondrules which consist mainly of low ca pyroxene the extended metastable crystallization of low ca pyroxene created silica undersaturated ree rich residua barium and cl enrichments in nepheline and scapolite of one chondrule suggest that there was also an influx of alkalis and cl during crystallization of the low ca pyroxene similarly another one of the chainpur chondrules mainly composed of olivine phenocrysts is markedly enriched in cl 10 oc as there is no evidence of corrosive metasomatism in any of the chondrules cl and alkali enrichment is believed to have occurred when they were still partially molten the chondrules were derived from normal o isotopic reservoirs so the postulated influx of ba na and cl did not occur on an exotic parent body trace amounts of nepheline and sodalite present in two parnallee chondrules crystallized from small na cl ree rich residua following extended crystallization of anorthite an i xe age of 5 0 ma post bjurbole obtained on one of these parnallee chondrules dates the crystallization of feldspathoid and thus formation of the chondrule

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

evidence for primitive nebular components in chondrules from the chainpur chondrite abstract the least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system these chondrules are excellent indicators of the physical and chemical nature of the solar nebula we separated 36 chondrules from the chainpur ll3 4 chondrite and analyzed each for 20 elements and petrographic properties sampling biases were minimized as far as possible chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components the identity of these components can be deduced from chondrule compositions the dominant components appear to be 1 a mixture of metal and sulfide with composition similar to whole rock metal and sulfide 2 refractory ir rich metal 3 refractory olivine rich silicates 4 low temperature pyroxene rich silicates and possibly 5 a component containing the more volatile lithophiles most of the textural types of chondrules formed from the same set of precursor components in some cases chondrules having different textures are almost identical in composition a few unusual chondrule types seem to mainly consist of uncommon nebular components possibly indicating different modes of formation etching experiments confirm that chondrule rims are enriched in metal troilite and moderately volatile elements relative to the bulk chondrules however a large fraction of the volatiles remains in the unetched interior

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

discovery mineral paragenesis and origin of wadalite in a meteorite the mineral wadalite ideal and simplified formula ca 6 al 5 si 2 o 16 cl 3 has been discovered for the first time in a meteorite specifically in coarse grained igneous type b calcium aluminum rich inclusions cais from the cv carbonaceous chondrite allende we report the results of electron microprobe scanning electron microscopy and transmission electron microscopy analyses of wadalite bearing assemblages in the allende cais and propose that wadalite formed by metamorphic reaction between akermanitic melilite and anorthite likely mediated by chlorine bearing fluids petrographic relationships support the likelihood of multistage alterations by fluids of different chemistries interspersed or coinciding with thermal metamorphic episodes on the allende parent asteroid fluid involvement in metamorphism of allende cais implies that these objects experienced open system alteration after accretion into the cv chondrite parent asteroid which may have resulted in disturbances of their oxygen and magnesium isotope systematics

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