the cr renazzo type carbonaceous chondrite group and its implications abstract a petrologic geochemical and oxygen isotopic study of eight meteorites identified as cr renazzo type chondrites reveals that they have a set of similar properties that distinguishes them from other chondrite groups and confirms cr as a carbonaceous chondrite group characteristics of the cr group include 1 1 abundant large multilayered fe ni metal rich type i chondrules 2 2 abundant matrix and dark inclusions di that contain microchondrules microfragments and magnetite framboids and platelets 3 3 unique assemblages of serpentine and chlorite rich phyllosilicates and ca carbonates 4 4 ca carbonate rims on chondrules 5 5 abundant fe ni metal with a positive ni vs co trend and a solar ni co ratio 6 6 low abundance of refractory rich inclusions 7 7 amoeboid olivine aggregates aoa with mn rich and mn poor forsterite and 8 8 oxygen isotopic compositions that form a unique mixing line the cr chondrites are breccias consisting of two major components the large layered chondrules and the matrix dark inclusions al rais is an anomalous member in having a higher abundance of the dark inclusion component 70 vol and this accounts for its having a higher volatile content than other members of the group the conditions that prevailed during aqueous alteration of the cr chondrites differed from those of other hydrated chondrite groups and may be the result of a higher temperature environment than that for the ci and cm chondrites the cr chondrite components chondrules matrix dark inclusions etc experienced aqueous alteration to differing degrees prior to assembly of the host chondrites and dark inclusions represent a lithology that was altered by water to a greater degree than the rest of the meteorite the overall degree of hydration in cr chondrites varies among meteorites with al rais being the most hydrated the oxygen isotopic variation of the whole rocks chondrules and matrices may be related to degree of hydration indicating that cr chondrites interacted with a water rich component having heavy oxygen and that was the same as ci and cm water

oxygen isotope studies of carbonaceous chondrites abstract the carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula these isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions we present a comprehensive set of whole rock analyses of cv co ck cm cr ch and ci chondrites as well as selected components of some of these meteorites a simple model is developed which describes the isotopic behavior during parent body aqueous alteration processes the process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition

the oxygen isotope record in murchison and other carbonaceous chondrites the ranges of 18o and 17o in components of the murchison c2 chondrite exceed those in all other meteorites analyzed previous authors have proposed that c2 chondrites are the products of aqueous alteration of anhydrous silicates a model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes the minimum number two of initial isotopic reservoirs is assumed two major stages of reservoir interaction are required one at high temperature to produce the16o mixing line observed for the anhydrous minerals and another at low temperature to produce the matrix minerals the isotopic compositions severely constrain the conditions of the low temperature process requiring temperatures 44 extension of the model to the data on c1 chondrites requires aqueous alteration in a warmer wetter environment

phyllosilicates in the mokoia cv carbonaceous chondrite evidence for aqueous alteration in an oxidizing environment abstract most cv chondrites contain little if any phyllosilicate mineralization a petrographic and transmission electron microscopy study of the mokoia cv carbonaceous chondrite shows that the matrix chondrules aggregates and inclusions all contain considerable amounts of phyllosilicates those in the matrix are mostly fe bearing saponite while those in chondrules aggregates and inclusions are both febearing saponite and na rich phlogopite serpentine occurs as minor coherent intergrowths within the phlogopite the saponite resulted from aqueous alteration of fe rich olivine and the phlogopite resulted from alteration of ca rich clinopyroxene anorthite spinel and ca al si rich glass iron rich olivines in the mokoia matrix contain extremely thin planar zones parallel to 100 with diffuse streaks along a in their diffraction patterns such planar zones are not known from terrestrial olivines saponite forms preferentially along these zones with its 001 planes roughly parallel to olivine 100 the planar zones probably consist of precipitates of fe oxide or fe hydroxide that were produced during oxidation of olivine while it cooled from a high temperature by formation of the fe rich precipitates olivine became fe deficient and thus was structurally disturbed during subsequent hydration saponite formed by preferentially replacing these regions in olivine simultaneously part of the fe was mobilized and precipitated as magnetite along the olivine peripheries the mineralogy and occurrence of phyllosilicates in mokoia differ from those in the ci and cm chondrites the differences suggest that aqueous alteration of the three meteorite groups probably occurred under a variety of conditions

mineralogy micromorphology and chemical analysis of andosols on the island of sao miguel azores abstract a study concerning soil genesis was conducted on surface and buried volcanic soils of sao miguel island azores of eight soil profiles three were selected for detailed study of pedogenesis a series of techniques which include chemical micromorphological xrd and dxrd analysis micropicking on impregnated soil blocks tem eds and ir spectroscopy analyses were used the soils showed an incipient to moderate degree of weathering as typically occurs in many young volcanic districts four different stages of soil development were clearly identified the first stage of soil development was characterised by dominance of primary minerals and some allophane and ferrihydrite formation with an feo fed ratio of about 0 86 lom soil the second stage of soil development was indicated by halloysite and ferrihydrite formation in the lower horizons along with an feo fed ratio of about 0 62 san surface soil the third stage of soil development was characterized by halloysite and ferrihydrite formation in the lower horizon but also by clay illuviation and stronger pumice weathering and an feo fed ratio of about 0 51 san buried soil the fourth stage of soil development concerned the lix soils buried soils older than 5000 years bp which had a much higher content in hydrated halloysite and considerable amounts of goethite and hematite these soils exhibit the most expressed weathering features and an feo fed ratio of about 0 25 the presence of young surface soils and well developed buried soils can be explained by i the longer pedogenesis of buried soils or possibly by ii the presence of a climate more seasonal than that of the present day

fundamental processes controlling the first stage of alteration of a basalt glass by seawater an experimental study between 200 and 320 c the first stage of alteration of a basalt glass by seawater has been studied experimentally between 200 and 320 c under vapor pressure following both the chemical evolution of the reacting solutions during the experiments and the chemical and mineralogical transformations of the altered glass surface via microprobe scanning transmission electron microscope and resonant nuclear reactions the alteration is controlled by diffusion of dissolved species through an altered layer and not by surface chemical reaction selective removal of cations with respect to sio2 leads to the formation of a thick porous protonated surface layer enriched in sio2 aqueous diffusion of dissolved species from the reacting glass and from solution through the pores of this layer results in the formation of an amorphous silicate gel which is the precursor of normal crystalline clays saponites appearing in a later stage computer simulations of the alteration generated with the eq3 6 computer software package account for the chemical composition of the rim and the sequence of secondary minerals found in the experiments

oxygen isotopes in magnetite and fayalite in cv chondrites kaba and mokoia we report in situ measurements of o isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized cv chondrites kaba and mokoia in both meteorites the magnetite and the secondary olivine fayalite fa90 100 have o isotopic compositions near the terrestrial fractionation tf line the mean 17o 17o 0 52 18o value is about 1 in contrast the compositions of nearby primary chondrule low feo olivines fa1 2 are well below the tf line 17o values range from 3 to 9 krot et al 1998 summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting the compositions of magnetite and fayalite in kaba and mokoia imply that the o isotopic composition of the oxidant was near or somewhat above the tf line in mokoia the fayalite and magnetite differ in 18o by 20 whereas these same materials in kaba have virtually identical compositions the difference between mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water rich environment at low temperatures 300 k in contrast the similar compositions of these phases in kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment with the o coming entirely from the precursor magnetite and silica the 17o of the oxidant incorporated into the cv parent body as phyllosilicates or h2o appears to have been much 7 8 lower than that in that incorporated into the ll parent body choi et al 1998 which suggests that the o isotopic composition of the nebular gas was spatially or temporally variable

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

mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites abstract the degree of compositional variation of fine grained minerals displayed by the members within any carbonaceous chondrite group i e ci cm cv cr is a direct reflection of the range of aqueous alteration assemblages present matrix and fine grained chondrule rims within any particular carbonaceous chondrite are mineralogically nearly identical to one another but not necessarily similar in bulk elemental composition even though they have subsequently experienced post accretional secondary processing aqueous alteration under identical conditions this may indicate preferential alteration of one component brecciation exchange of components between rims and matrix and or accretion of matrix and rims from mineralogically or compositionally distinct materials we propose that co chondrites experienced parent body conditions of low f o2 low water rock ratios and temperatures below 50 c cr chondrites experienced higher water rock ratios potentially higher temperatures 150 c and a wide range of f o2 the alteration mineralogy of cv chondrites indicates water rock ratios at the high end at least of the range for cr chondrites essebi and mac 87300 cm chondrites experienced temperatures below 50 c low f o2 and low water rock ratios except eet 83334 which probably experienced relatively higher f o2 and b 7904 and y 86720 which experienced postalteration temperatures in the range 500 700 c most ci chondrites experienced temperatures between 50 and 150 c relatively high water rock ratios and variable f o2 with a late stage increase in the latter due possibly to brecciation and consequent release of methane y 82162 witnessed post alteration heating possibly as high as 400 c

accretionary dust mantles in cm chondrites evidence for solar nebula processes abstract the texture and modal composition of fourteen carbonaceous chondrites of the cm group were studied in detail to unravel the origin of their various components and the evolution of their parent body ies the analysis and mapping of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse grained primordial chondritic components chondrules chondrule fragments refractory inclusions particles rich in hydrous silicates pcp and various mineral fragments are coated by fine grained serpentine and tochilinite rich dust the dust mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material the dust mantles termed accretionary dust mantles and their host lithology called primary accretionary rock are interpreted as products of accretion processes in the solar nebula an origin by processes on the parent body is excluded for textural and mineralogical reasons most cm chondrites are affected by secondary impact induced brecciation of variable intensity except for yamato 791198 which is composed entirely of a primary accretionary rock obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation the cm chondrites y74662 haripura cold bokkeveld kivesvaara mighei pollen nogoya y793321 alh 83100 murray and murchison consist of clasts of primary accretionary rock embedded in a fine grained clastic matrix derived from primary rock material the volume percentage of the clastic matrix ranges from 5 y74662 to 76 murchison bells and essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved the evaluation of literature data indicates that solar wind implanted noble gases are absent in the primary accretionary rock component of cm chondrites they are only present in brecciated samples it must be concluded from the relation between the content of trapped solar wind and degree of brecciation that 1 the solar noble gases are residing exclusively in the clastic matrix and 2 some cm chondrites are regolith breccias and others are fragmental or monomict breccias both types were formed on the parent body although the latter were never exposed to the solar wind based on the results of textural chemical and mineralogical investigations we conclude that a major portion of the water bearing phases in cm chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent cm parent body this process requires special physicochemical conditions which allow the crystallization of oh bearing silicates e g higher density of the nebula gas or special precursor planetesimals of the cm chondrite parent body

progressive alteration in cv3 chondrites more evidence for asteroidal alteration abstract the oxidized cv3 chondrites can be divided into two major subgroups or lithologies bali like cv3oxb and allende like cv3oxa in which chondrules calcium aluminum rich inclusions cais and matrices show characteristic alteration features weisberg et al 1997 krot et al 1997d kimura and ikeda 1997 the cv3oxb lithology is present in bali kaba parts of the mokoia breccia and possibly in grosnaja and allan hills alh 85006 it is characterized by the presence of the secondary low ca phyllosilicates saponite and sodium phlogopite magnetite ni rich sulfides fayalite fa 90 ca fe rich pyroxenes fs10 50wo45 50 and andradite phyllosilicates replace primary ca rich minerals in chondrules and cais which suggests mobilization of ca during aqueous alteration magnetite nodules are replaced to various degrees by fayalite ca fe rich pyroxenes and minor andradite fayalite veins crosscut fine grained rims around chondrules and extend into the matrix thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures 300 c in the presence of aqueous solutions oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large 18ofayalite magnetite fractionation 20 we infer that phyllosilicates magnetite fayalite ca fe rich pyroxenes and andradite formed at relatively low temperatures 300 c by fluid rock interaction in an asteroidal environment secondary fayalite and phyllosilicates are virtually absent in chondrules and cais in the cv3oxa lithology which is present in allende and its dark inclusions axtell alha81258 alh 84028 lewis cliff lew 86006 and parts of the mokoia and vigarano breccias instead secondary nepheline sodalite and fayalitic olivine are common fayalitic olivine in chondrules replaces low ca pyroxenes and rims and veins forsterite grains it also forms coarse lath shaped grains in matrix secondary ca fe rich pyroxenes are abundant we infer that the cv3oxa lithology experienced alteration at higher temperatures than the cv3oxb lithology the presence of the reduced and cv3oxa lithologies in the vigarano breccia and cv3oxa and cv3oxb lithologies in the mokoia breccia indicates that all cv3 chondrites came from one heterogeneously altered asteroid the metamorphosed clasts in mokoia krot and hutcheon 1997 may be rare samples of the hotter interior of the cv asteroid we conclude that the alteration features observed in the oxidized cv3 chondrites resulted from the fluid rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced cv3 chondrites

aqueous alteration and brecciation in bells an unusual saponite bearing cm chondrite the petrological and mineralogical characteristics of the unusual cm2 chondrite bells have been investigated in detail by scanning electron microscopy sem electron microprobe analysis epma and transmission electron microscopy tem bells is a highly brecciated chondrite which contains few intact chondrules a very low abundance of refractory inclusions and is notable in having an unusually high abundance of magnetite which is disseminated throughout the fine grained matrix fragmental olivines and pyroxenes are common and based on compositional data appear to have been derived from chondrules as a result of extensive brecciation the fine grained mineralogy of matrix in bells differs considerably from other cm chondrites and has closer affinities to matrix in ci chondrites the dominant phases are fine grained saponite interlayered with serpentine and phases such as tochilinite and cronstedtite which are typical of cm chondrite matrices are entirely absent pentlandite pyrrhotite magnetite anhydrite calcite and rare ti oxides also occur as accessory phases based on its oxygen and noble gas isotopic compositions zadnik 1985 rowe et al 1994 bells can be considered to be a cm2 chondrite although its bulk composition shows some departures from the typical range exhibited by this group however these variations in bulk chemistry are entirely consistent with the observed mineralogy of bells the unusual fine grained mineralogy of bells matrix can be reasonably attributed to the combined effects of aqueous alteration and advanced brecciation in a parent body environment extensive brecciation has assisted aqueous alteration by reducing chondrules and mineral grains into progressively smaller grains with high surface areas which are more susceptible to dissolution reactions involving aqueous fluids this has resulted in the preferential dissolution of fe rich chondrule olivines which are now completely absent in bells although present in other cm chondrites the formation of saponite in bells probably resulted from the dissolution of relatively silica rich phases such as pyroxene and olivine that were derived from chondrules the result of such dissolution reactions would be to increase the activity of silica in the fluid phase at least on a localized scale stabilizing saponite in preference to serpentine an increase in asio2 would also have destabilized preexisting cronstedtite which may have reacted to form magnetite and mg fe serpentine under conditions of constant o2

aqueous alteration of chondrules in the cm carbonaceous chondrite allan hills 81002 implications for parent body alteration abstract alh 81002 is a cm carbonaceous chondrite that has petrographic characteristics suggesting that it experienced a high degree of aqueous alteration but has entirely escaped parent body brecciation in this study we report the results of a systematic investigation of the alteration of more than 35 chondrules in alh 81002 that belong to specific well defined chondrule types our results show that the extent of chondrule alteration is almost exclusively controlled by the properties of the chondrule phases although these chondrules taken as a group show highly variable degrees of alteration no evidence for differences in the alteration histories of chondrules has been found instead the widely varying rates of alteration of chondrule phases such as forsteritic and fayalitic olivine clinoenstatite orthopyroxene and augite are sufficient to explain the diversity of chondrule alteration in alh 81002 chondrules with similar primary textural and mineralogical characteristics e g type ia type iia etc show remarkably similar degrees of alteration detailed studies of the alteration products of chondrules show that they retain a detailed record of the alteration history of the chondrules from the earliest to last stages and provide strong evidence for the compositional evolution of the altering fluid contrary to what might be expected the fluid composition appears to be the dominant controlling factor on the composition of serpentines that replace olivines pyroxenes and mesostasis the consistent degree of alteration observed in chondrules of the same type in alh 81002 indicates that they were altered under the same conditions for similar lengths of time such conditions are most consistent with alteration in a parent body environment in addition the compositional and textural uniformity of rim material chondrule alteration phases and vein fillings in chondrule components throughout alh 81002 also argue strongly that alteration occurred after accretion several observations on alh 81002 provide additional constraints and modifications on existing models of in situ parent body alteration for example inferences on the degree of alteration of cm chondrites based on the extent of brecciation as a parameter of the alteration degree appear to be compromised in addition models which suggest that tochilinite serpentine pcp rich objects have resulted from the break up of chondrules during brecciation are also inconsistent with our observations on alh 81002

fayalite silica association in unequilibrated ordinary chondrites evidence for aqueous alteration on a parent body we report ten occurrences of high fayalite fa56 99 mol four with fa 82 mol olivine in association with silica in type 3 ordinary chondrites pyroxene with high fs contents is much less common fs contents do not exceed 66 mol and most maxima are 50 mol in those cases where the amount of fayalite is minor relative to that of silica the fayalite forms a layer on the silica and shows textural evidence of formation by reaction of silica with oxidized fe the latter seems to have resulted from reaction of metallic fe ni with an oxidant most likely h2o vapor the fayalite is generally in contact with pyroxene and in one case olivine having much lower fe fe mg ratios indicating that lattice diffusion has been minimal formation of fayalite from sio2 explains the low mg content of this olivine the mg was sequestered inside the lattices of mafic minerals and was thus inaccessible in contrast the moderately high mn contents of the fayalite indicate that an appreciable fraction of the mn in the precursor assemblage was accessible it was probably sited in the matrix in the form of tiny poorly crystallized oxide grains produced by nebular condensation at temperatures too low to permit diffusion into forsterite or enstatite the reaction of sio2 with feo produced by oxidation during metamorphism of fe ni can also account for fayalitic olivine associated with sio2 microspherules in the fine grained matrices of type 3 ordinary chondrites and because matrix is sio2 normative for other occurrences of fayalite in matrix the presence of mn in the fayalitic rims on the olivine of carbonaceous chondrites does not require a nebular origin

extended chronologies of aqueous alteration in the cm2 carbonaceous chondrites evidence from carbonates in queen alexandra range 93005 abstract the antarctic cm2 carbonaceous chondrite que 93005 contains four compositionally distinct carbonates namely breunnerite calcite dolomite and a ca poor dolomite these carbonates can form monomineralic grains or may be intergrown as bimineralic grains consisting of dolomite plus breunnerite and dolomite plus calcite or polymineralic grains containing an intergrowth of breunnerite ca poor dolomite and calcite carbonates in all grain types have inclusions of fe ni sulphides and or mg fe phyllosilicates in the bimineralic grains dolomite crystallised first to be overgrown by breunnerite or partially replaced by calcite polymineralic grains are concentrically layered with breunnerite crystallising first on pore margins to be later etched then overgrown and partially replaced by ca poor dolomite that was itself partly dissolved prior to being overgrown by calcite calcite and dolomite have also cemented fractures that cross cut the fine grained rims to aqueously altered chondrules and were formed by expansion of the chondrules during their hydration overall the sequence of mineralisation in que 93005 was 1 dolomite 2 breunnerite 3 ca poor dolomite then 4 calcite this secular change in carbonate composition and mineralogy reflects changing solution composition and probably also provenance mg fe phyllosilicates replaced dolomite breunnerite and ca poor dolomite prior to calcite crystallisation and most or all of the sulphides formed after both the phyllosilicates and calcite following sulphide crystallisation the edges of carbonate grains were abraded either by impact gardening or as a consequence of fluidisation of the matrix during rapid loss of gas or vapour determination of the crystallisation age of dolomite via the mn cr system indicates that aqueous alteration of que 93005 began on or before 3 93 0 23 ma after the formation of the oldest solar system solids overall the water rock ratio and fo2 during alteration of que 93005 was similar to that of the cm1s and cr1s but the lower degree of alteration of que 93005 overall suggests that alteration timescales were shorter possibly due to loss of intergranular liquid water during fluidisation

comparison of the trace element composition of tagish lake with other primitive carbonaceous chondrites a meteorite fall on 18 january 2000 was detected by u s defense department satellites which established its pre impact orbit fresh samples were collected from frozen tagish lake in british columbia a week later and some properties of these samples reveal it to be a unique meteorite we characterized tagish lake and 8 other samples using icpms and rnaa data for 47 elements reveal that each of 9 carbonaceous chondrites of different type exhibit the orgueil normalized plateaus expected for members of such types trends evident in tagish lake differ from all other carbonaceous chondrites including ci and cm samples of tagish lake collected later show similar patterns affected by weathering

deuterium enrichments in chondritic macromolecular material implications for the origin and evolution of organics water and asteroids here we report the elemental and isotopic compositions of the insoluble organic material iom isolated from several previously unanalyzed meteorites as well as the reanalyses of h isotopic compositions of some previously measured samples alexander et al 2007 the iom in ordinary chondrites ocs has very large d enrichments that increase with increasing metamorphism and decreasing h c the most extreme d value measured being almost 12 000 we propose that such large isotopic fractionations could be produced in the oc parent bodies through the loss of isotopically very light h2 generated when fe was oxidized by water at low temperatures 200 c we suggest that similar isotopic fractionations were not generated in the iom of cv and co chondrites with similar metamorphic grades and iom h c ratios because proportionately less water was consumed during metamorphism and the remaining water buffered the h isotopic composition of the iom even a h was being lost from it hydrogen would also have been generated during the alteration of ci cm and cr carbonaceous chondrites the iom in these meteorites exhibit a considerable range in isotopic compositions but all are enriched in d as well as 15n relative to terrestrial values we explore whether these enrichments could also have been produced by the loss of h2 but conclude that the most isotopically anomalous iom compositions in meteorites from these groups are probably closest to their primordial values the less isotopically anomalous iom has probably been modified by parent body processes the response of iom to these processes was complex and varied presumably reflecting differences in conditions within and between parent bodies the d enrichments associated with h2 generation along with exchange between d rich iom and water in the parent bodies means that it is unlikely that any chondrites retain the primordial h isotopic composition of the water ice that they accreted the h isotopic compositions of the most water rich chondrites the cms and cis are probably the least modified and their compositions d 25 suggest that their water did not form at large radial distances from the sun where ice is predicted to be very d rich yet models to explain the o isotopic composition of inner solar system bodies require that large amounts of ice were transported from the outer to the inner solar system

high temperature condensation of iron rich olivine in the solar nebula abstract olivine is the most abundant mineral in many chondritic meteorites it is also the first major mg silicate to condense from a solar gas upon cooling the composition of olivine provides important information concerning its origin results of calculations are presented for the fe mn and ni contents of olivine condensing from a gas with solar composition but variable oxygen fugacity a basic computational method is described that takes into account the depletion of the gas in mg and si during condensation the first olivine that condenses from a solar gas has less than 10 2 mol feo independent of the oxygen fugacity of the otherwise solar gas at lower temperatures when most of the mg is condensed more feo can be incorporated in forsterite by condensation if the oxygen fugacity is sufficiently high the feo mno ratio of the first condensing olivine is above 1000 decreasing with decreasing temperature and increasing oxygen fugacity to values of around 100 which are actually observed in fayalitic olivine rims in allende the ni content of condensed olivine is below 10 7 mol even at high oxygen fugacities indicating that condensation of nio in olivine is a very unlikely process any ni in olivine in the ppm range is therefore indicative of metal olivine equilibration under increasingly oxidizing conditions h 2 o h 2 ratios of 0 01 0 1 chromite is an early phase to condense forming before forsterite isolated olivine grains in allende and olivine on the outside of chondrules have feo rich rims texture composition and morphology of these rims indicate that they formed by condensation from a gas our calculations suggest that the gas was enriched by a factor of 100 in oxygen relative to solar composition gas and that mg was almost completely condensed when rim formation occurred the sharp boundary between forsteritic and fayalitic olivine suggests an increase in oxygen fugacity and rapid formation of fayalitic olivine by condensation cr enrichment at the rim interface with the underlying forsterite probably reflects stability of chromite under oxidizing conditions conventional models of feo bearing silicate formation require oxidation of metal silicate assemblages at temperatures as low as 400 k it is shown here that diffusion at this temperature is so slow that the time needed for formation of the observed feo bearing phases would exceed the lifetime of the solar nebula other oxygen fugacity indicators reflect similar conditions for example the hercynite component of spinel in fine grained al rich inclusions indicates equilibration with a gas of similar composition as that responsible for condensation of fayalitic olivine suggesting formation of feo rich spinel by gas solid reactions mechanisms for producing oxidizing conditions in the solar nebula are briefly discussed

siderophile element anomalies in ck carbonaceous chondrites implications for parent body aqueous alteration and terrestrial weathering of sulfides abstract ck chondrites constitute the most oxidized anhydrous carbonaceous chondrite group most of the fe occurs in magnetite and in feo rich mafic silicates the two observed ck falls karoonda and kobe along with thirteen relatively unweathered ck finds have unfractionated siderophile element abundance patterns in contrast a sizable fraction of ck finds 9 of 24 investigated shows fractionated siderophile abundance patterns including low abundances of ni co se and au the most extreme depletions are in ni 0 24 of normal ck and au 0 14 of normal ck this depletion pattern has not been found in other chondrite groups out of the 74 ck chondrites listed in the meteoritical bulletin database 2006 excluded considerably paired specimens see http tin er usgs gov meteor metbull php we analyzed 24 and subclassified the ck chondrites in terms of their chemical composition and sulfide mineralogy sl siderophiles low six samples for large depletions in ni co se and au 50 of sulfides lost sm siderophiles medium two cks for moderately low ni and co abundances sulfides are highly altered or partly lost sh siderophiles high one specimen for enrichments in ni co se and au normal for unfractionated samples 13 samples the sole sh sample may have obtained additional sulfide from impact redistribution in the parent asteroid we infer that these elements became incorporated into sulfides after asteroidal aqueous processes oxidized nebular metal thermal metamorphism probably also played a role in their mineral siting the siderophile losses in the sl and sm samples are mainly the result of oxidation of pentlandite pyrite and violarite by terrestrial alteration followed by leaching of the resulting phases some antarctic ck chondrites have lost most of their sulfides but retained ni co se and au presumably as insoluble weathering products

fayalite in the kaba and mokoia carbonaceous chondrites abstract the olivines in the kaba and mokoia cv3 carbonaceous chondrites range from almost pure forsterite fo99 6 to almost pure fayalite fa99 9 individual grains of fayalite of such high purity have not previously been reported from meteorites they can reach 100 m in diameter and occur in the matrix chondrules forsterite enstatite aggregates and rims around ca al rich inclusions chondrules and aggregates the fayalite is three to nine times richer in mn than in bulk ci chondrites many grains are associated with magnetite troilite and pentlandite an assemblage that suggests relatively oxidizing conditions we propose that the fayalite formed through reaction of sio g released by decomposition of enstatite with magnetite and sulfide the lower thermal limit for fayalite formation is determined by the above reaction whereas the upper thermal limit is controlled by magnetite stability thermodynamic calculations indicate that fayalite coexisting with magnetite and sulfides could have formed in a nebular environment that had a h 2 o h 2 ratio substantially greater than the canonical solar nebula coexisting olivines having endmember compositions indicate that they experienced little or no thermal metamorphism

aqueous alteration in the kaba cv3 carbonaceous chondrite the kaba cv3 carbonaceous chondrite has undergone pervasive aqueous alteration that produced fe bearing saponite in chondrules and matrix saponite in chondrules is coarse grained and shows a crystallographic orientation relationship such that c of saponite parallels a of enstatite matrix saponite exhibits a variety of textures including coarse and fine grained clusters isolated packets and sparse lamellar replacements of fe rich olivine the coarse grained clusters were probably derived from altered chondrules that were fragmented during regolith gardening the fine grained clusters and isolated packets formed from material released during the partial dissolution of matrix silicates much saponite is associated with submicron fe ni sulfides suggesting that conditions were not oxidizing during aqueous alteration we believe that the alteration occurred after accretion and on the kaba parent body because saponite is present throughout the sample by analogy to terrestrial occurrences the saponite in kaba probably formed at low temperatures 100 c the distribution of saponite in kaba is determined by local bulk compositions enstatite in chondrules has been altered more extensively than the fine grained olivine in matrix the alteration products in kaba resemble those in other aqueously altered carbonaceous chondrites such as the mokoia cv3 chondrite and in certain ci chondrites there are however significant differences kaba lacks the abundant high al phyllosilicates in chondrules and in cais reported from mokoia also lacking in kaba is the serpentine and ferrihydrite found in orgueil alteration products in kaba matrix are dissimilar from those in co3 chondrites even though their prealteration mineralogies were similar an observation that probably reflects different temperatures during alteration

evidence for low temperature growth of fayalite and hedenbergite in macalpine hills 88107 an ungrouped carbonaceous chondrite related to the cm co clan the carbonaceous chondrite macalpine hills mac 88107 has bulk composition and mineralogy that are intermediate between those of co and cm chondrites this meteorite experienced minor alteration and a low degree of thermal metamorphism petrologic type 3 1 and escaped post accretional brecciation the alteration resulted in the formation of fayalite fa90 100 al free hedenbergite fs50wo50 phyllosilicates saponite serpentine intergrowths magnetite and ni bearing sulfides pyrrhotite and pentlandite fayalite and hedenbergite typically occur as veins which start at the opaque nodules in the chondrule peripheries crosscut fine grained rims and either terminate at the boundaries with the neighboring fine grained rims or continue as layers between these rims these observations suggest that fayalite and hedenbergite crystallized after accretion and compaction of the fine grained rims fayalite also overgrows isolated forsteritic fa1 5 and fayalitic fa20 40 olivine grains without any evidence for fe mg interdiffusion it also replaces massive magnetite sulfide grains the initial 53mn 55mn ratio of 1 58 0 26 10 6 in the mac 88107 fayalite corresponds to an age difference between the formation of fayalite and refractory inclusions in allende of either 9 or 18 ma depending upon the value of the solar system initial abundance of 53mn used in age calculations formation of secondary fayalite and hedenbergite requires mobilization and transport of ca si and fe either through a high temperature gaseous phase hua and buseck 1995 or low temperature aqueous solution krot et al 1998a b the high temperature nebular model for the origin of fayalite hua and buseck 1995 fails to explain a formation of fayalite hedenbergite assemblages after accretion of fine grained rims that lack any evidence for high temperature processing b extreme fractionation of refractory lithophile elements of similar volatility ca and al in hedenbergite and c absence of fe mg interdiffusion along fayalite forsterite boundaries we conclude that fayalite and hedenbergite in mac 88107 formed during late stage low temperature approximately 150 200 c aqueous alteration the data for mac 88107 extend the evidence for an early onset of aqueous activity on chondrite parent bodies and reinforce the conclusion that liquid water played an important role in the chemical and mineralogical evolution of the first chondritic planetesimals

secondary calcium iron rich minerals in the bali like and allende like oxidized cv3 chondrites and allende dark inclusions we have characterized ca fe rich silicates salite hedenbergite pyroxenes fs10 50wo45 50 andradite ca3fe2si3o12 kirschsteinite cafesio4 and wollastonite ca3si3o9 in the type i chondrules and matrices in the bali like and allende like oxidized cv3 chondrites and allende dark inclusions in type i chondrules in the bali like cv3 chondrites metal is oxidized to magnetite magnetite sulfide nodules are replaced by ca fe rich pyroxenes with minor andradite and pure fayalite we infer that ca fe rich pyroxenes andradite fayalite magnetite and phyllosilicates which occur in mesostases formed at relatively low temperatures 300 c in the presence of aqueous solutions thermodynamic analysis of phase relations in the si fe ca o h system and large o isotopic fractionation of the coexisting magnetite and fayalite 20 krot et al 1998 are consistent with this interpretation in type i chondrules in the allende like cv3 chondrites and dark inclusions magnetite sulfide nodules are replaced by ca fe rich pyroxenes and ferrous olivine low ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine it appear that the ca fe rich pyroxenes predate formation of ferrous olivine the latter postdates formation of talc and biopyriboles brearley 1997 the allende dark inclusions are crosscut by ca fe pyroxene andradite veins and surrounded by ca rich rims that consist of ca fe rich pyroxenes andradite wollastonite and kirschsteinite calcium rich veins and rims formed after aggregation and lithification of the dark inclusions the rimmed dark inclusions show zoned depletion in ca which is due to a lower abundance of ca fe rich pyroxenes close to the rim calcium was probably leached from the inclusions and redeposited along their edges we infer that the allende like chondrites and dark inclusions experienced similar aqueous alteration to the bali like chondrites and were metamorphosed subsequently which resulted in loss of aqueous solutions and dehydration of phyllosilicates we conclude that ca fe rich silicates in the oxidized cv3 chondrites and allende dark inclusions are secondary and resulted from aqueous fluid rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous ices and anhydrous materials the latter were possibly mineralogically similar to the reduced cv3 chondrites

coupled organic synthesis and mineral alteration on meteorite parent bodies the hypothesis that the soluble fraction of the organic compounds present in carbonaceous chondrite meteorites was formed during aqueous alteration of the parent body was tested with masstransfer reaction path calculations in these calculations we start with likely compositions of the original parent body and asteroidal fluids that are far from thermodynamic equilibrium and metastable and stable equilibrium constraints are imposed as the total gibbs free energy of the parent body environment is minimized the results of these calculations suggest that the classes of soluble organic compounds present in carbonaceous chondrite meteorites could have formed during relatively low temperature aqueous alteration of the meteorite parent body or bodies the main controls on the potential for synthesis and transformation of organic compounds were the oxidation state of the rock fluid system the bulk composition of that system and the temperatures that were achieved during the alteration event or events it also appears that the alteration mineral assemblages were influenced by the presence of soluble organic compounds and reaction among them

the compositional classification of chondrites vii the r chondrite group bulk compositional and petrographic data clearly define the new r rumuruti group of chondrites consisting of rumuruti the only fall alh85151 acfer 217 carlisle lakes dar al gani 013 pca91002 pca91241 y 75302 y 793575 and y 82002 compositional petrographic rare gas and 0 isotopic data strongly suggest that pca91002 and pca91241 are paired the yamato specimens are probably not paired the matrices of the known r chondrites have experienced similar minor degrees of metamorphism petrographic types are 3 8 3 9 with the exception of alh85151 3 6 all except carlisle lakes contain equilibrated r5 r6 clasts petrographically the r chondrites are characterized by a low chondrule matrix modal abundance ratio high states of oxidation reflected by abundant nio bearing olivine with fa37 40 relatively small chondrules mean apparent diameters of 400 abundant up to 11 wt sulfides mainly pyrrhotite and pentlandite and negligible amounts of metallic fe ni refractory lithophile abundances are 0 95 x cl intermediate between those in ordinary chondrites oc and ci chondrites abundances of the volatile elements se and zn are greatly enhanced relative to oc the r chondrites are clearly distinguished from other chondrite groups on the basis of al mn and zn mn abundance ratios the oxygen isotopic data plot roughly along a slope 12 line with whole rock 17o values higher than for any other chondrite group rumuruti acfer 217 alh85151 pca91002 and pca91241 have light dark structures and solar wind implanted rare gases indicating that they are regolith breccias the yamato specimens also have light dark structures and are inferred to be regolith breccias carlisle lakes lacks solar rare gases and is unbrecciated based on similarities in refractory lithophile abundances 00 95 x ci oxygen isotope compositions 17o 0 and refractory inclusion abundances 0 1 vol none have been reported the r chondrites probably belong to a noncarbonaceous superclan of chondrites that also includes ordinary and enstatite chondrites the high oxidation state high matrix chondrule modal abundance ratio relatively low abundance of droplet chondrules and high 17o composition suggest that the r chondrites formed at greater heliocentric distances than the oc

53mn 53cr dating of fayalite formation in the cv3 chondrite mokoia evidence for asteroidal alteration fayalite grains in chondrules in the oxidized aqueously altered cv3 chondrite mokoia have large excesses of radiogenic chromium 53 these excesses indicate the in situ decay of short lived manganese 53 half life 3 7 million years and define an initial53mn 55mn ratio of 2 32 0 18 10 6 this ratio is comparable to values for carbonates in ci and cm chondrites and for several classes of differentiated meteorites mokoia fayalites formed 7 to 16 million years after allende calcium aluminum rich inclusions during hydrothermal activity on a geologically active asteroid after chondritic components had ceased forming in the solar nebula

magnetite in comet wild 2 evidence for parent body aqueous alteration the mineralogy of comet 81p wild 2 particles collected in aerogel by the stardust mission has been determined using synchrotron fe k x ray absorption spectroscopy with in situ transmission xrd and x ray fluorescence plus complementary microraman analyses our investigation focuses on the terminal grains of eight stardust tracks c2112 4 170 0 0 c2045 2 176 0 0 c2045 3 177 0 0 c2045 4 178 0 0 c2065 4 187 0 0 c2098 4 188 0 0 c2119 4 189 0 0 and c2119 5 190 0 0 three terminal grains have been identified as near pure magnetite fe3o4 the presence of magnetite shows affinities between the wild 2 mineral assemblage and carbonaceous chondrites and probably resulted from hydrothermal alteration of the coexisting feni and ferromagnesian silicates in the cometary parent body in order to further explore this hypothesis powdered material from a cr2 meteorite nwa 10256 was shot into the aerogel at 6 1 km s 1 using a light gas gun and keystones were then prepared in the same way as the stardust keystones using similar analysis techniques to the eight stardust tracks a cr2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel