origin of fayalitic olivine rims and lath shaped matrix olivine in the cv3 chondrite allende and its dark inclusions we have studied an allende dark inclusion by optical microscopy scanning electron microscopy electron microprobe analysis and transmission electron microscopy the inclusion consists of chondrules isolated olivines and matrix which as in the allende host is mainly composed of 5 20 m long lath shaped fayalitic grains with a narrow compositional range fa42 2 and nepheline olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5 10 m wide fayalitic rims fa39 2 and 100 1000 m wide translucent zones which consist of 5 20 m long lath shaped fayalitic grains fa41 1 intergrown with nepheline these fayalitic olivines like those in the matrix of the dark inclusion contain 10 20 nm sized inclusions of chromite hercynite and fe ni sulfides the fayalitic rims around remnant olivines are texturally and compositionally identical to those in allende host suggesting that they have similar origins chondrules are surrounded by opaque rims consisting of tiny lath shaped fayalitic olivines 1 3 m long intergrown with nepheline as in the allende host fayalitic olivine veins may crosscut altered chondrules fine grained chondrule rims and extend into the matrix indicating that alteration occurred after accretion we infer that fayalitic olivine rims and lath shaped fayalites in allende and its dark inclusions formed from phyllosilicate intermediate phases this explanation accounts for 1 the similarity of the replacement textures observed in the dark inclusion and allende host to aqueous alteration textures in cm chondrites 2 the anomalously high abundances of al and cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in allende and allende dark inclusions 3 abundant voids and defects in lath shaped fayalites in the allende dark inclusion which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed cm ci chondrites we conclude that the matrix and chondrule rims in allende were largely converted to phyllosilicates and then completely dehydrated the allende dark inclusions experienced diverse degrees of aqueous hydrothermal alteration prior to complete dehydration the absence of low ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in allende is consistent with the lower resistance of low ca pyroxene to aqueous alteration relative to forsteritic olivine hydro thermal processing of allende probably also accounts for the low abundance of planetary noble gases and interstellar grains and the formation of nepheline sodalite salite hedenbergite pyroxenes wollastonite kirschsteinite and andradite in chondrules and ca al rich inclusions

fine grained rims in the allan hills 81002 and lewis cliff 90500 cm2 meteorites their origin and modification antarctic cm meteorites allan hills alh 81002 and lewis cliff lew 90500 contain abundant fine grained rims fgrs that surround a variety of coarse grained objects fgrs from both meteorites have similar compositions and petrographic features independent of their enclosed objects the fgrs are chemically homogeneous at the 10 m scale for major and minor elements and at the 25 m scale for trace elements they display accretionary features and contain large amounts of volatiles presumably water they are depleted in ca mn and s but enriched in p all fgrs show a slightly fractionated rare earth element ree pattern with enrichments of gd and yb and depletion of er gd is twice as abundant as er our results indicate that those fgrs are not genetically related to their enclosed cores they were sampled from a reservoir of homogeneously mixed dust prior to accretion to their parent body the rim materials subsequently experienced aqueous alteration under identical conditions based on their mineral textural and especially chemical similarities we conclude that alh 81002 and lew 90500 likely have a similar or identical source

non nebular origin of dark mantles around chondrules and inclusions in cm chondrites abstract our examination of nine cm chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in cm chondrites are not discrete fine grained rims acquired in the solar nebula as modeled by metzler et al accretionary dust mantles in cm chondrites evidence for solar nebula processes geochim cosmochim acta 56 1992 2873 2897 nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles we infer that the mantles were produced from porous nebular materials on the cm parent asteroid by impact compaction a process that produces the lowest porosity adjacent to chondrules and inclusions compaction was followed by aqueous alteration that formed tochilinite serpentine ni bearing sulfide and other secondary products in voids in the interchondrule regions metzler et al reported a correlation between mantle thickness and the radius of the enclosed object in yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation r2 0 44 if we limit consideration to central objects with radii 35 m a moderate correlation is also found in que 97990 we suggest that impact induced shear of a plum pudding like precursor produced the observed mantles these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions some mantles in cm chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity typically a gray uniform inner layer is surrounded by an outer layer consisting of darker silicates with bse bright speckles the cm chondrite objects characterized as primary accretionary rocks by metzler et al did not form in the nebula but rather on the parent body the absence of solar flare particle tracks and solar wind implanted rare gases in these clasts reflect their lithified nature and low surface volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles the clasts are analogous to the light colored metamorphosed clasts in ordinary chondrite regolith breccias which also lack solar flare particle tracks and solar wind gas

a hydrated clast in the mokoia cv3 carbonaceous chondrite evidence for intensive aqueous alteration in the cv parent body abstract we report the discovery of a highly hydrated clast in the mokoia cv3 carbonaceous chondrite that contains eight chondrules and one amoeboid olivine inclusion aoi embedded in a matrix dominated by hydrous phyllosilicates anhydrous silicates in the chondrules and aoi were extensively replaced by phyllosilicates the matrix has a composition intermediate between saponite and serpentine and contains abundant framboidal magnetite being similar to the matrices of the ci chondrites the mineralogy and texture indicate that the clast resulted from intensive aqueous alteration of a precursor equivalent to the host mokoia meteorite and it can be regarded as the cv2 type lithology the results indicate that there was in fact an extremely wet region within the mokoia parent body and that this clast escaped subsequent thermal metamorphism this clast reveals abundant evidence of progressive and dynamic aqueous alteration processes the chondrules were replaced inward from their margins and the alteration products were disaggregated and mixed into the matrix as a result the chondrules increasingly became smaller in size and irregular in shape some of the chondrules were separated into pieces the results indicate that a major proportion of the matrix materials were produced by the alteration of chondrules and inclusions and that there was a stage in which the alteration products and the original matrix material were extensively and uniformly mixed together the presence of the clast supports a previously proposed model which suggests that hydrated chondrules inclusions and the rims surrounding them in the host mokoia chondrite are clasts produced by brecciation within a wet region of the parent body and that they were subsequently transported and incorporated into the dry matrix

mineralogy and petrology of chondrules and inclusions in the mokoia cv3 chondrite all objects 100 m in apparent diameter in five polished thin sections of the mokoia cv3 chondrite were studied and classified number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined three major types of olivine chondrules were observed igneous chondrules recrystallized chondrules and chondrules that appear to be accretional aggregates coarse grained cai s have igneous textures and mineral parageneses while fine grained cai s are aggregates containing varying proportions of al rich concentric objects ca rich chaotic material and inclusion matrix chondrules and refractory inclusions in mokoia and allende are broadly similar in texture and mineral chemistry but mokoia refractory inclusions contain phyllosilicates rather than feldspathoids and melilite rich cai s are more abundant in allende we think that most cai s formed during the metamorphism partial melting and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula in the process ca rich melt appears to have been physically separated from al rich residues producing the observed fractionation of ca from al into distinct constituents of cai s some cai s may be aggregates of devitrified amorphous metastable condensates inclusion matrix may have condensed from silicate rich vapors produced during distillation mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material

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

geochemistry of the pliocene red clay formation in the chinese loess plateau and implications for its origin source provenance and paleoclimate change abstract recent field observations and sedimentological studies suggest the eolian origin of the late tertiary red clay formation underlying the pleistocene loess soil sequence in the loess plateau thereby extending the wind blown record from 2 6 ma down to 7 0 ma in north central china to address the source provenance of the red clay major and trace element concentrations were analyzed on the samples from a north south loess transect of the loess plateau and a red clay section at jiaxian results show that there is a good agreement of both major and trace element compositions between the last glacial interglacial loess soil units and the jiaxian red clay record the ree distribution patterns of the loess soil and red clay samples are remarkably similar in shape with enriched lree and fairly flat hree profiles and clear negative eu anomaly the geochemical features of the red clay are also identical to those of upper continental crust these chemical characteristics provide further evidence for eolian sedimentation of the red clay from the relations both between na2o al2o3 and k2o al2o3 ratios and between u pb and th pb ratios the eolian loess and red clay materials must have been subject to thorough sedimentary differentiation and moderate chemical weathering in the source area the chemical similarity between soil unit s1 and the red clay implies that such an environment as occurred in the last interglacial period may have already formed in northwestern china during the late tertiary comparison of down section variations in elemental ratios with field observations of pedogenic development of the red clay suggests that cao al2o3 sr rb and sr ba are more sensitive to climate changes than magnetic susceptibility and can be used to reconstruct the climate history of the late cenozoic

evolution of chaka salt lake in nw china in response to climatic change during the latest pleistocene holocene abstract the late pleistocene and holocene hydrologic balance of chaka salt lake in the eastern qaidam basin of nw china was studied based on the analysis of lithostratigraphy mineralogy total organic carbon and total nitrogen from a 9 0 m long sediment core an age depth model for the lake sediments is based on eight accelerator mass spectrometry ams 14 c measurements of organic matter and a 1700 year radiocarbon reservoir correction the pitzer model was used to calculate the sequence of minerals precipitated as a function of salinity assuming that the initial lake water was recharged from adjacent rivers and springs lake salinity values were derived from a good match between the calculated and observed mineral sequences our multi proxy based hydrologic reconstruction of chaka lake indicates that it was a clastic dominated freshwater lake between 17 2 and 11 4 cal ka bp which may have resulted from the input of glacial water into the lake at that time during the lateglacial and holocene a warm climatic regime predominated between 13 9 and 12 7 cal ka bp and then a cold climatic regime prevailed between 12 7 and 11 4 cal ka bp these warm and cold periods correlate with the bolling allerod and younger dryas events in the region beginning at 11 4 cal ka bp a saline or hypersaline lake developed which may have resulted from increased summer insolation and temperatures these conditions persisted throughout the holocene modeling indicates that lake salinity fluctuated between 66 and 223 ppt from 11 4 to 7 2 cal ka bp and then increased to 223 322 ppt between 7 2 and 6 0 cal ka bp when most regions of china recorded high moisture availability i e the so called mid holocene climatic optimum lake salinity decreased to 66 223 ppt during a short time period between 6 0 and 5 3 cal ka bp possibly caused by reduced evaporation subsequently salinity values rapidly increased to 223 322 ppt between 5 3 and 5 2 cal ka bp and a hypersaline lake with a salinity higher than 324 ppt similar to modern chaka salt lake became established around 5 2 cal ka bp this establishment may have been related to the weakening of asian monsoon regional comparisons of paleoclimatic records indicate that large temporal and spatial variations exist in the occurrence and intensity of drought over western china since the lateglacial

fine grained rims surrounding chondrules in the tagish lake carbonaceous chondrite verification of their formation through parent body processes abstract a petrographic and electron microscopic study of the carbonate poor lithology of the tagish lake carbonaceous chondrite reveals that most chondrules contain many pseudomorphs of opaque nodules and are surrounded by phyllosilicate rich altered zones that were formed by replacing enstatite and opaque nodules along the chondrule peripheries most chondrules and other coarse grained components are surrounded by fine grained rims which are commonly disaggregated and partly lacking the altered zones and the rims are compositionally and texturally similar although they exhibit some differences in secondary minerals in comparison the rims and the host matrix show more significant differences in bulk chemical composition texture and mineralogy the observations suggest that the chondrules and the rims experienced aqueous alteration simultaneously whereas the rims and the matrix experienced aqueous alteration under distinct conditions we also found a clast that contains multiple coarse grained components embedded in a matrix and numerous smaller matrix clasts the coarse grained components in the clast have no rims and the matrices of the clasts are mineralogically identical to the rims the results suggest that the chondrules other coarse grained components and their rims generically referred to as chondrules rims and the clasts originated from a common precursor region in the meteorite parent body that was different from the location where the host meteorite was finally lithified that is the chondrules rims are actually clasts produced by brecciation and later transported and incorporated into the present host matrix the rims are therefore remnants of matrix material that formerly filled interspaces between the chondrules and other coarse grained components this model is essentially consistent with those previously proposed for the carbonate rich lithology of tagish lake and the hydrated chondrules rims in the vigarano and mokoia cv3 chondrites

accretionary rims on inclusions in the allende meteorite abstract many inclusions in allende particularly those with irregular shapes are surrounded by a sequence of thin layers which differ from one another in texture mineralogy and mineral chemistry the layer underlying all others contains either ia pyroxene needles olivine clumps of hedenbergite and andradite ib olivine doughnuts or ic rectangular olivine crystals the next layer outward ii contains tiny

matrix material in type 3 chondrites occurrence heterogeneity and relationship with chondrules abstract matrix material in type 3 chondrites forms rims on chondrules metal sulfide aggregates ca al rich inclusions and chondritic clasts it also forms lumps up to a millimeter in size which may contain coarser silicates chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized mean concentrations of mg na al and ca in matrix occurrences show up to fivefold variations in a single chondrite variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between h l and ll matrices such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates compositions of chondrules and their matrix rims are normally unrelated although rim compositions are correlated with those of matrix lumps inside chondrules a single chondrule was found with a composition nearly identical to that of its internal matrix lump suggesting that some chondrules may have formed from matrix material matrix lumps are as heterogeneous as chondrules but mean chondrule and matrix compositions differ even allowing for possible loss of metallic fe ni during chondrule formation since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for fe mg exchange our matrix samples cannot represent typical chondrule precursor materials

on the origin of rim textures surrounding anhydrous silicate grains in cm carbonaceous chondrites fine grained optically opaque rims coat individual olivine and pyroxene grains in cm matrices and chondrules bulk chemical analyses and observations of these rims indicate the presence of phyllosilicates and disseminated opaques because phyllosilicates could not have survived the chondrule formation process chondrule silicate rims must have formed entirely by late state aqueous reactions as such these textures provide a useful benchmark for isolating alteration features from more complex cm matrix materials both chondrule silicate and matrix silicate rims exhibit morphological features commonly associated with advancing stages of replacement reactions in terrestrial serpentinites contacts between many matrix silicate rims and the adjacent matrix materials suggest that these rims formed entirely by aqueous reactions in a parent body setting this contrasts with previous assertions that rim textures can only form by the accretion of nebular dust but does not imply an origin for the rims surrounding other types of cm core components such as chondrules

characterization of micron sized fe ni metal grains in fine grained rims in the y 791198 cm2 carbonaceous chondrite implications for asteroidal and preaccretionary models for aqueous alteration the presence of apparently unaltered micron sized fe ni metal grains juxtaposed against hydrated fine grained rim materials in the cm2 chondrite yamato y 791198 has been cited as unequivocal evidence of preaccretionary alteration we have examined the occurrence composition and textural characteristics of 60 fe ni metal grains located in fine grained rims in y 791198 using scanning electron microscopy sem and electron microprobe analysis in addition three metal grains prepared by focused ion beam fib sample preparation techniques were studied by transmission electron microscopy tem the metal grains are heterogeneously distributed within the rims electron microprobe analyses show that all the metal grains are kamacite with minor element contents p cr and co that lie either within or close to the range for other cm2 metal grains x ray maps obtained by electron microprobe show s p and or ca enrichments on the outermost parts of many of the metal grains z contrast stem imaging of fib prepared fe ni metal grains show the presence of a small amount of a lower z secondary phase on the surface of the grains and within indentations on the grain surfaces energy filtered tem eftem compositional mapping shows that these pits are enriched in oxygen and depleted in fe relative to the metal these observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of fe oh 2 a common oxidation product of fe metal the presence of such a layer could have inhibited further alteration of the metal grains these findings are consistent with alteration by an alkaline fluid as suggested by zolensky et al 1989 but the location of this alteration remains unconstrained because y 791198 was recovered from antarctica and therefore may have experienced incipient terrestrial alteration however we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in antarctica and that alteration in an extraterrestrial environment is more probable although the presence of unaltered or incipiently altered metal grains in these fine grained rims could be interpreted as evidence for preaccretionary alteration we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body

olivine rich rims surrounding chondrules in the mokoia cv3 carbonaceous chondrite further evidence for parent body processes abstract fine grained rims surrounding chondrules and inclusions in the mokoia cv3 carbonaceous chondrite can be divided into phyllosilicate rich and olivine rich types we present a petrographic and electron microscopic study of the olivine rich rims and their host objects referred to as chondrules olivine rich rims the olivine rich rims consist mainly of fe rich olivine and very minor phyllosilicate saponite their host chondrules contain minor saponite and phlogopite which resulted from aqueous alteration of anhydrous silicates mineralogical and compositional characteristics of the chondrules olivine rich rims suggest that they experienced mild thermal metamorphic effects the rims commonly contain veins of coarse grained fe rich olivine magnetite and fe ni sulfides the chondrules show abundant evidence of alteration along their peripheries and the alteration textures suggest a mechanism for rim formation by replacement of the chondrules initially enstatite and opaque nodules preferentially reacted to form coarse platy fe rich olivine crystals which were subsequently divided into finer grains forsterite was also replaced by fe rich olivine as the alteration advanced these fe rich olivines were disaggregated mixed with simultaneously produced saponite and formed rims in contrast the surrounding matrix shows no evidence of such alteration and metamorphism these observations indicate that the chondrules olivine rich rims did not experience these secondary processes in their present setting the results suggest that the chondrules olivine rich rims experienced extensive replacement reactions in an environment in which aqueous fluids existed but only in minor amounts they have probably also undergone simultaneous and or subsequent mild thermal metamorphism we suggest that the chondrules olivine rich rims are actually clasts transported from a relatively dry region in the parent body that was different from the region where mokoia was finally lithified

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

aqueous alteration of carbonaceous chondrites evidence for preaccretionary alteration a review in most groups of carbonaceous chondrites minerals occur that are formed due to aqueous alteration in the nebula and or within meteorite parent bodies for determining the evolution of materials in the early solar system it is of significant importance to clearly identify evidence for either nebular or planetary aqueous alteration therefore results from the study of chondrites have fundamental implications for ideas concerning nebular dynamics gas solid interactions in the nebula and accretionary processes considering the topic of this review it is important to define nebular and parent body processes the solar nebula activity should include condensation processes of high and low temperature components as well as processes of chemical fractionation and grain size sorting mixing of solids and gas and interactions between early formed solids with the remaining gas but it should exclude processes that occurred in small uncompacted protoplanetary objects that may have been totally destroyed again before accretion of the final meteorite parent bodies therefore the term preaccretionary instead of nebula is used in this paper to include all these processes that may have occurred in small precursor planetesimals currently there is no doubt that parent body aqueous alteration is a fundamental process in the evolution of several groups of carbonaceous chondrites however due to textural and mineralogical observations and chemical analyses strong arguments have been found also indicating preaccretionary aqueous alteration of distinct components in carbonaceous chondrites in this paper evidence for preaccretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail the strongest evidence for preaccretionary alteration comes from studies of cm chondrites the survival of highly unequilibrated mineral assemblages in accretionary rims and sharp contacts between chondrule glass and surrounding phyllosilicates are only two important arguments for preaccretionary alteration features among many others discussed in the paper similar observations were also made in cr ch and related chondrites due to the small abundance of water bearing phases in co and cv chondrites the origin of phyllosilicates in these groups is less clear a preaccretionary origin of hydrous phases in ca al rich inclusions cais has also been suggested by several scientists in ci chondrites no strong indications for such processes have been found mainly due to heavy brecciation and severe parent body alteration

fine grained rim mineralogy of the cold bokkeveld cm chondrite abstract a chrysotile like phase cronstedtite polygonal serpentine pentlandite and finely intergrown tochilinite comprise the fine grained rim fgr mineralogy of the cold bokkeveld cm chondrite transmission electron microscope images combined with compositional data indicate reaction among cronstedtite the chrysotile like phase and polygonal serpentine the mg mg fe ratios of the cronstedtite are higher than those reported for the less altered murchison cm chondrite cronstedtite grains exhibit layer separations particularly at their boundaries the fgrs surround different chondrule types but have similar bulk compositions and mineralogy ca is depleted in the fgrs relative to the bulk cm chondrite the fgrs display non uniform thicknesses especially where they coat embayed chondrule areas and they exhibit grain size coarsening outward from the chondrules they enclose fgr formation in cold bokkeveld is most plausibly explained by multiple accretionary episodes during which progressively coarser dust was deposited onto chondrules presumably in the solar nebula the compositional and mineralogic data are consistent with aqueous alteration on the parent body

fayalite in the vigarano cv3 carbonaceous chondrite occurrences formation age and conditions abstract we have performed petrographic characterization 53mn 53cr age determination and thermodynamic stability evaluations of fayalite in vigarano meteorite that belongs to the reduced subgroup of cv3 chondrites vigarano is a breccia consisting of clasts which are separate chondrules surrounded by olivine rich fine grained materials four out of twenty three explored clasts contain fayalites that represent materials of the bali like oxidized subgroup of cv3 chondrites the fayalites fa 80 with grain sizes typically

microdistribution of primordial noble gases in cm chondrites determined by in situ laser microprobe analysis decipherment of nebular processes abstract in situ noble gas analyses using a laser microprobe with a beam diameter of 50 100 m were performed on thin slices of murchison and yamato y 791198 cm carbonaceous chondrites in order to see microdistribution of primordial noble gases petrographic observations prior to noble gas analyses showed that the two meteorite slices are entirely composed of chondrules pcps poorly characterized phases and other chondritic components that are rimmed by layers of fine grained dust based on an existing classification scheme for texture of cm chondrites the two samples are classified as primary accretionary rocks pars the noble gas analyses showed that primordial noble gases are rich in the fine grained rims around chondrules in the two meteorite slices this suggests that large amounts of carrier phases of the primordial noble gases in cm chondrites are located in these rims which seem to have accreted on the surfaces of chondrules prior to the formation of the meteorite parent bodies rims on different chondrules within a given meteorite showed similar concentrations of heavy primordial noble gases suggesting that phase q is distributed homogeneously in the rims and possibly in the nebular region where the chondrules had acquired their dust rims 20 ne diamond 20 ne q ratios are relatively constant among rims in a given meteorite indicative of homogeneous mixing of interstellar diamonds and phase q on a 10 g scale one location in a rim around a chondrule in the murchison sample showed an enrichment of 22 ne which suggests presence of carrier phases of ne e in the rim a stepped heating analysis of a chip of y 791198 was carried out to characterize the noble gases of bulk par abundances of primordial noble gases are only 50 of those of rims around chondrules consistent with the relative abundances of gas rich fine grained material and gas poor chondrules and inclusions in par solar gases were not detected which implies that pars the primitive cm bodies had grown to at least centimeter size in a dense solar nebula which shielded these objects from direct exposure to solar wind

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 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

chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in cv chondrites constraints on the evolution of nebular dust abstract a correlation of petrography mineral chemistry and in situ oxygen isotopic compositions of fine grained olivine from the matrix and of fine and coarse grained olivine from accretionary rims around ca al rich inclusions cais and chondrules in cv chondrites is used here to constrain the processes that occurred in the solar nebula and on the cv parent asteroid the accretionary rims around leoville vigarano and allende cais exhibit a layered structure the inner layer consists of coarse grained forsteritic and 16o rich olivine fa1 40 and 17o 24 to 5 the higher values are always found in the outer part of the layer and only in the most porous meteorites whereas the middle and the outer layers contain finer grained olivines that are more fayalitic and 16o depleted fa15 50 and 17o 18 to 1 the cv matrices and accretionary rims around chondrules have olivine grains of textures chemical and isotopic compositions similar to those in the outer layers of accretionary rims around cais there is a correlation between local sample porosity and olivine chemical and isotopic compositions the more compact regions the inner accretionary rim layer have the most mgo and 16o rich compositions whereas the more porous regions outer rim layers around cais accretionary rims around chondrules and matrices have the most mgo and 16o poor compositions in addition there is a negative correlation of olivine grain size with fayalite contents and 17o values however not all fine grained olivines are feo rich and 16o poor some small 20 olivine grains in the outer layers of the cai accretionary rims and in the matrix show significant enrichments in 16o 17o 20 to 10 we infer that the inner layer of the accretionary rims around cais and at least some olivine grains in the finer portions of accretionary rims and cv matrices formed in an 16o rich gaseous reservoir probably in the cai forming region grains in the outer layers of the cai accretionary rims and in the rims around chondrules as well as matrix may have also originated as 16o rich olivine however these olivines must have exchanged o isotopes to variable extents in the presence of an 16o poor reservoir possibly the nebular gas in the chondrule forming region s and or fluids in the parent body the observed trend in isotopic compositions may arise from mixtures of 16o rich forsterites with grain overgrowths or newly formed grains of 16o poor fayalitic olivines formed during parent body metamorphism however the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of 16o rich forsterite and subsequently exchanged fe mg and o isotopes in situ in the presence of aqueous solutions i e fluid assisted thermal metamorphism

aeolian deposits at the southeastern margin of the tengger desert china implications for surface wind strength in the asian dust source area over the past 20 000years abstract independent dust provenance data play an important role in understanding changes in the atmospheric dust loading over geological time major elemental compositions and grain size characteristics of the aeolian deposits xiangshan xs section at the southeastern margin of the tengger desert in northwestern china demonstrate that the deposits are an intermediate product of the transportation of aeolian materials from the dust source area deserts to the dust depositional regions chinese loess plateau that were likely formed by dust storm processes the 40 m and 63 178 m fractions and the ratio of sio2 tio2 of the xs section were used as proxies to reconstruct the surface wind regime over the last 20 kyr the results show that the surface winds were much stronger and fluctuate acutely from 19 1 to 11 4 kyr the winds gradually attenuated after the beginning of the holocene and reached a minimum between 4 0 and 3 5 kyr bp a sharp increase in surface winds occurred after 2 5 kyr bp the surface winds in the asian dust source area are largely consistent with the changes in winter monsoon strength documented by the chinese loess deposits also the stronger surface winds from 19 1 to 11 4 kyr coincide with the high mineral dust concentration determined from the greenland ice core except for the period of bolling allerod b a when the atmospheric dust may have been washed out by enhanced global hydrological cycles although dust was still emitted from source areas this suggests that the stronger surface winds in dust source areas can enhance the atmospheric dust loadings during the late glacial the weakening surface wind regime during the holocene may have greatly contributed to the decrease in dust concentration of the remote dust records the paleo environmental changes in the study area since 42 5 kyr bp are broadly in agreement with other adjacent records however the possible warm and humid spell of 42 5 19 1 kyr bp still requires verification

phyllosilicate rich chondrule rims in the vigarano cv3 chondrite evidence for parent body processes abstract a petrographic and scanning electron microscopic study of the vigarano cv3 carbonaceous chondrite reveals that a small proportion 5 of the fine grained rims surrounding chondrules consist mainly of phyllosilicates saponite and serpentine and minor but variable amounts of ferrihydrite most of the rims consist mainly of fe rich olivine the phyllosilicate rich rims and the chondrules that they enclose show abundant evidence of aqueous alteration low ca pyroxene and olivine at chondrule margins have been partially replaced by phyllosilicate rich materials producing embayments on chondrule surfaces fe ni metal fe sulfide and magnetite in the chondrules have also been partially replaced by fe hydroxide due to preferential replacement of low ca pyroxene and olivine sulfide magnetite and fe hydroxide remain exposed at chondrule margins we discovered a clast 0 7 0 9 mm in size containing several chondrules embedded in a phyllosilicate rich matrix that is almost identical in mineralogy texture and composition to the rims we also found numerous small clasts 20 400 m in size consisting of a similar phyllosilicate rich material in the host vigarano meteorite many of the clasts include fragments of chondrules amoeboid olivine aggregates and ca al rich inclusions these clasts also show abundant evidence of aqueous alteration which is similar to that observed in the rims and their host chondrules from these observations we conclude that the chondrule rim assemblages and the clasts are a part of the same precursor chondrite which was fragmented during brecciation on the meteorite parent body we interpret that a portion or portions of the vigarano parent body was aqueously altered and the interchondrule matrix was largely converted to phyllosilicates and ferrihydrite this hydrated portion was later fragmented and brecciated by impacts as fragmentation occurred preferentially along the phyllosilicate rich porous matrix most of the ejected clasts consisted of a chondrule rimmed by remnant matrix the clasts were rounded by abrasion during brecciation and transportation and finally mixed into an anhydrous portion in the regolith from which the vigarano meteorite was derived some of the phyllosilicate rich rims show evidence for partial dehydration during heating before final incorporation into the regolith these results taken together suggest that the vigarano parent body was spatially heterogeneous in degree of aqueous alteration and thermal metamorphism and the host vigarano meteorite is a breccia consisting of numerous clasts from many different locations

matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 origins and evidence for diverse primitive nebular dust components abstract the mineralogical and chemical characteristics of dark matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 have been investigated in detail by scanning electron microscopy electron microprobe analysis and transmission electron microscopy matrix and fine grained rims on chondrules and other objects are compositionally homogeneous on the scale of 10 m and there are no compositional differences between matrix and rims mineralogically both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of si and fe rich amorphous material olivine pyroxene fe ni metal magnetite pentlandite pyrrhotite anhydrite and mixed layer phyllosilicate phases several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics which appear to represent basic fine grained units of nebular dust the microstructures of these different components show that they have experienced significantly different thermal histories unlike the ordinary chondrites an origin for any significant component of the matrix or rims in alha77307 from chondrules is improbable based on compositional and microstructural evidence the matrix and rim components in alha77307 formed by disequilibrium condensation processes as fine grained amorphous dust that is represented by the abundant amorphous component in the matrix such condensation could have occurred under a variety of conditions at different times and locations within the solar nebula or possibly earlier in a circumstellar environment subsequent thermal processing of this primitive condensate material in a variety of environments in the nebula resulted in partial or complete recrystallization of the fine grained dust the intimate association of fine grained components with disparate compositions and thermal histories shows that mixing of fine grained dust within the nebula must have been extremely thorough

mineralogy of fine grained rims in the alh 81002 cm chondrite abstract the mineralogy of fine grained rims in alh 81002 a relatively primitive cm carbonaceous chondrite has been determined by using transmission electron microscopy the most abundant phase is mg rich serpentine which occurs in small 20 nm crystals with cylindrical or fibrous morphologies cronstedtite an fe rich phyllosilicate occurs as relatively large 100 to 2500 nm platy crystals some cronstedtite is coherently intergrown with tochilinite in many cases cronstedtite has been partially altered to serpentine the compositions and textures of these two phases provide a mineralogical and morphologic alteration sequence that parallels the known compositional trend for cm chondrite matrix accessory minerals embedded within the phyllosilicates include chlorite pentlandite gypsum olivine kamacite taenite and chromite regions containing only anhydrous minerals also occur the hydrated and anhydrous regions are in direct contact with each other suggesting that the rims accreted material from multiple reservoirs

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

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

alteration processes in the cv chondrite parent body based on analysis of nwa 2086 meteorite analysis of the nwa 2086 cv3 chondrite showed a matrix chondrule ratio of 52 similar to bali mokoia and grosanaja nearly twice as many chondrule fragments as intact ones demonstrate that an early fragmentation phase occurred prior to final accretion after this event no substantial mechanical change or redeposition is evident rims with double layered structures were identified around some chondrules which in at least one case is attributed to an accretionary origin the rim s outer parts with a diffuse appearance were formed by in situ chemical alteration during this later process mg content decreased fe content increased and olivine composition was homogenized producing a rim composition close to that of the matrix this alteration occasionally happened along fractures and at confined locations and was probably produced by fluid interactions iron oxides are the best candidate for a small grain sized alteration product however technical limitations in the available equipment did not allow exact phase identification these results suggest that nwa 2086 came from a location possible more deeply buried in the cv parent body than mokoia or bali and suffered less impact effects although there is no evidence of sustained thermal alteration this meteorite may represent a sample of the cv parent asteroid interior and provide a useful basis for comparison with other cv meteorites in the future

isotope geochemistry of quaternary deposits from the arid lands in northern china abstract quaternary deposits of various types lacustrine desert sand river loess and loess like deposits from the southwestern tarim basin north tianshan northeast china and the central loess plateau exhibit an nd 0 range from 5 to 12 8 and a 147 sm 144 nd range from 0 110 to 0 140 their nd depleted mantle model ages are between 1 50 and 1 92 ga with a mean value of 1 7 ga th sc ratios vary over a wide range from 0 1 to 2 comparison of these geochemical signatures with those of the old australian shales and modern pacific ocean dust suggests a continuous input of mantle derived material into the crust over geological time the deposits from the north tianshan show clearly different chemical and isotopic features with significantly higher sm nd high 143 nd 144 nd low 87 sr 86 sr and low la lu and th sc ratios relative to those in the deposits from the other areas this indicates a geological control on the local source which has an overall young age and an enrichment of basic components the ce 0 values of two loess samples from the central loess plateau are 1 8 and 3 9 corresponding to depleted mantle model ages of 2 2 and 3 4 ga like the rb sr system the evolution of the la ce isotopic system in the crust may be to some extent decoupled from the sm nd system due to la ce fractionation by sedimentary processes the loess deposits and the desert sand are not distinguishable in isotopic composition but show somewhat different chemical compositions we conclude that the loess can have originated from desert processes since the difference in the chemistry of the desert sand and loess may result from mineral sorting during desert processes and aeolian transportation the loess deposits on the central loess plateau chemically and isotopically resemble the desert loess deposits in the southwestern tarim basin but do not resemble those of the north tianshan accordingly the loess on the plateau may have a source region in the tarim basin but clearly not in the north tianshan

indicators of parent body processes hydrated chondrules and fine grained rims in the mokoia cv3 carbonaceous chondrite abstract a petrographic and electron microscopic study of the mokoia cv3 carbonaceous chondrite shows that all of the chondrules and inclusions 400 m in diameter and most of their fine grained rims studied referred to as chondrules rims contain various amounts of hydrous phyllosilicates mostly saponite formed by aqueous alteration of anhydrous silicates the rims mainly consist of fine grained olivine and saponite in varying proportions and contain crosscutting veins of fe rich olivine the boundaries between the chondrules and their rims are irregular and show abundant evidence of aqueous alteration interactions between them in contrast the host matrix contains very minor amounts of saponite and shows no evidence of such extensive aqueous alteration the boundaries between the chondrules rims and the matrix are sharp and show no traces of the matrix having been involved in the alteration of the chondrules rims these observations indicate that the aqueous alteration in the chondrules rims did not occur in the present setting we suggest that the chondrules rims are actually clasts transported from a location on the meteorite parent body different from where the mokoia meteorite was from the aqueous alteration of the chondrules rims probably occurred there the veins in the rims were originally fractures produced in an interchondrule matrix by impacts these were later filled by fe rich olivine during aqueous activity this location was then involved in impact brecciation and individual chondrules were ejected as clasts with remnants of the matrix surrounding them during the continuing brecciation those chondrule rim clasts were transported mixed with anhydrous matrix grains and finally lithified to the present meteorite therefore the rims are fragmented remnants of a former matrix textures characterized by fine grained rims surrounding chondrules in chondrites have been widely thought to have formed in the solar nebula before they accreted into their parent bodies however our results suggest that some textures may not be explained by such an accretionary model instead the multi stage parent body process modeled for the mokoia rim formation may be a more plausible explanation

matrices of type 3 ordinary chondrites primitive nebular records petrologic studies were made on the fine grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype the matrix minerals in order of abundance are olivine fo99 to fo9 enstatite or bronzite augite or subcalcic augite albite fe ni metal troilite magnetite spinel mgal2o4 chromite and calcite fe and mg rich fluffy particles and albite like particles are also major constituents the chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium showing that the matrix materials were hardly heated after their formation textural relationships indicate that magnesian olivine was formed after ca pyroxene followed by intermediate to iron rich olivine intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions the observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules they were mostly the products of condensation and reaction among solids and or between solids and the ambient gas mostly at low temperatures and thus they contain records of primitive processes in the nebula in order to explain the presence of olivines more iron rich than fo50 the presence of free sio2 or a high activity of sio2 in the gas is necessary which was not shown in previous thermochemical calculations mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites indicating that they were formed at different oxygen fugacities the rims of chondrules and surrounding matrix materials must have accreted onto chondrules during turbulent movements of the nebula

indicators of aqueous alteration and thermal metamorphism on the cv parent body microtextures of a dark inclusion from allende abstract an unusual dark clast in the allende cv3 chondrite termed allende af which was previously interpreted as a primary aggregate formed in the solar nebula kurat et al 1989 palme et al 1989 was re examined our study reveals abundant evidence suggesting that it probably experienced extensive aqueous alteration and subsequent thermal metamorphism on the meteorite parent body allende af contains numerous rounded to oval shaped inclusions embedded in a dark matrix the inclusions consisting predominantly of fine grains of fe rich olivine have internal textures suggesting that they are pseudomorphs after chondrules several inclusions appear to be replaced cais veins filled with fibrous olivine grains occur abundantly in both inclusions and matrix some veins up to 4 mm in length penetrate several inclusions providing strong evidence that aqueous alteration occurred after accretion the fibrous morphology of olivine in veins and inclusions suggests that the olivine was produced by dehydration and thermal transformation of phyllosilicate that had been formed by aqueous alteration olivine grains in the matrix contain numerous micro inclusions of fe ni sulfide which were probably incorporated during transformation from phyllosilicate allende af is probably related to the fine grained variety of dark inclusions reported from cv3 chondrites that has been described as the type containing abundant porous aggregates of fe rich olivine by johnson et al 1990 many dark inclusions previously described appear to be similar in texture and mineralogy to allende af and probably experienced similar secondary process on the meteorite parent body the wide variation in texture of dark inclusions can be explained by different degrees of aqueous alteration that preceded thermal metamorphism the size distribution of chondrule pseudomorphs and the abundance of cai pseudomorphs suggest that the precursor of allende af is a cv type chondrite probably allende itself oxygen isotopic and chemical compositions are consistent with this interpretation the cv parent body has been commonly thought to have escaped major secondary processing however allende af provides evidence that extensive aqueous alteration and thermal metamorphism have occurred locally on the cv parent body

dark inclusions in the mokoia cv3 chondrite evidence for aqueous alteration and subsequent thermal and shock metamorphism mokoia is a cv3 chondrite that contains abundant phyllosilicate mineralization we present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions dis that we found in mokoia the overall texture and constituent minerals of the dis resemble those in the host meteorite fe bearing saponite and na rich phlogopite the same phyllosilicates as in the host meteorite occur in the dis which strongly suggests that the dis have a similar alteration history to the host meteorite however the dis show several distinct differences from the host meteorite olivine grains in the di matrices are more homogeneous in fe fe mg ratio than those in the host meteorite matrix phyllosilicates in the dis are less abundant than in the host meteorite and they have been dehydrated to various extents these characteristics suggest that the dis have experienced higher degree of thermal metamorphism than the host meteorite in addition the matrices in the dis are more compacted than those in the host meteorite most olivine grains in the dis show undulatory extinction in transmitted crossed polarized light and some show planar fractures while such olivine grains are rare in the host meteorite two of the dis contain si mg fe and o rich melt veins these characteristics indicate that most dis have been shocked to shock stage s3 s4 while the host meteorite is shock stage s1 virtually unshocked thermal metamorphism of the dis was likely caused by shock heating these results are consistent with the contention previously proposed for the dis in cv3 chondrites i e the dis have experienced aqueous alteration and subsequent dehydration on the cv parent body we suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the mokoia parent body

redistribution of chondrules in a carbonaceous chondrite parent body a model abstract carbonaceous chondrites mainly consist of chondrules and inclusions embedded in a fine grained matrix this texture is widely believed to have formed primarily by direct accretion of solar nebular materials although it may have been modified to various extents by subsequent parent body processes recently we studied all chondrules and inclusions larger than 400 m in diameter and their rims referred to as chondrules rims in the mokoia cv3 carbonaceous chondrite using a scanning electron microscope and found that the chondrules rims experienced various degrees of aqueous alteration and that some also exhibit evidence of thermal metamorphism the mineralogical and petrographic characteristics of the chondrules rims suggest that the alteration and metamorphism occurred within the meteorite parent body in contrast however the surrounding matrix does not show evidence of such alteration and metamorphism these findings indicate that the alteration and metamorphism of the chondrules rims did not occur in situ based on these results we proposed a model that the chondrules rims are actually clasts transported from regions in the parent body different from the location where the host meteorite was finally lithified if it can be assumed that the chondrules and inclusions studied are representative of all chondrules and inclusions in mokoia the results and interpretation pose a fundamental challenge regarding the formation of the whole mokoia lithology that is it cannot be explained by either direct accretion of the solar nebula or conventional parent body brecciation we propose a model for the development of the mokoia lithology through formation of chondrules rims and fine matrix grains by fragmentation in different regions in the parent body followed by transportation mixing and accumulation in a fluid state and finally lithification of those objects these processes may have been repeated cyclically within the parent body

experimental shock metamorphism of the murchison cm carbonaceous chondrite abstract a series of shock recovery experiments were carried out on the murchison cm carbonaceous chondrite by using a single stage propellant gun the murchison samples were shocked in nine experiments at peak pressures from 4 to 49 gpa the recovered samples were studied in detail by using an optical microscope a scanning electron microscope and an electron probe microanalyzer chondrules are flattened in the plane of the shock front at 4 to 30 gpa the mean aspect ratio of chondrules increases from 1 17 to 1 57 roughly in proportion to the intensity of shock pressure up to 25 gpa at 25 to 30 gpa the mean aspect ratio does not increase further and chondrules show increasingly more random orientations and degrade their preferred orientations and at 35 gpa they are extensively disrupted most coarse grains of olivine and pyroxene are irregularly fractured fracture density increases with increasing shock pressure and at 30 gpa almost all are thoroughly fractured with subgrains of local shock melting occurs as melt veins and pockets at 20 to 30 gpa fracture filling veins of fine grains of matrix are also produced at 25 to 30 gpa the melts and the fine grains seem to result mainly from frictional heating due to displacement along fractures at 35 gpa melting occurs pervasively throughout the matrix the melts are mainly produced from the matrix however they are consistently more enriched in fe s and ca which indicates that these elements are selectively incorporated into the melts the melts contain tiny spherules of fe ni metal fe sulfide and numerous vesicles at 49 gpa the matrix is totally melted and coarse grains of olivine are partially melted the melts contain much larger vesicles 50 300 m in diameter than those in the samples shocked at lower pressures which indicates that much more intense devolatilization and gas expansion took place for the purpose of comparing shock thermal effects between the experimentally shocked samples and naturally shocked targets surface materials in the murchison parent body we calculated internal energy increase for compression by multiple shock wave reflections experimental case and for compression by a single shock wave natural case the results suggest that postshock thermal effects observed at each experiment may be attained by impact on the natural targets at a considerably lower shock pressure than the peak shock pressure from the results of our experiments and calculations we conclude that if the murchison parent body were shocked on the surface at pressures higher than 25 gpa shocked material would probably undergo drastic increase in the degree of comminution and simultaneous generation of strong expansive forces on pressure release thus the results support the hypothesis of scott et al 1992 that volatile rich carbonaceous chondrites shocked above 20 to 30 gpa escaped from the parent body and formed particles that are too small to survive as meteorites

hydrous and anhydrous alterations of chondrules in kaba and mokoia cv chondrites chondrules in the bali like cv chondrite kaba and the allende like portion of the mokoia breccia have been studied to explore the relationship between hydrous alteration to form phyllosilicates and anhydrous alteration resulting in secondary olivine zonation replacement of enstatite by ferroan olivine and formation of feldspathoids nepheline and sodalite all kaba chondrules experienced extensive hydrous alteration whereas anhydrous alteration was minor and resulted only in the olivine zonation on the other hand all of the mokoia chondrules experienced both extensive anhydrous and hydrous alteration bronzite rims formed between relic enstatite grains and phyllosilicates in both kaba and mokoia during the hydrous alteration petrographic observations indicate that phyllosilicates in mokoia postdate formation of the secondary ferroan olivine and feldspathoids we conclude that anhydrous alteration in kaba and mokoia predated hydrous alteration and took place before accretion of chondrules into the cv parent asteroid

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

mineralogy aqueous alteration and primitive textural characteristics of fine grained rims in the y 791198 cm2 carbonaceous chondrite tem observations and comparison to alha81002 abstract transmission electron microscopy tem was used to study the microtextural and mineralogical characteristics of fine grained rims in the unbrecciated cm2 chondrites y 791198 and alha81002 in an effort to provide constraints on the origins of the rims themselves our tem observations show that the rims in y 791198 are composed of two distinct types of region sulfide poor and sulfide rich that are intermixed in a complex manner at the micron to submicron level the sulfide poor regions are largely composed of amorphous silicate material or nanocrystalline serpentine but rare fibrous and coarse grained serpentine grains have also been identified no fine grained cronstedtite or tochilinite were observed although coarse grained lumps of tochilinite are present in the rims in contrast the sulfide rich regions are characterized by the presence of a myriad nanometer sized fe ni sulfide grains pentlandite with some ni rich pyrrhotite embedded within an amorphous silicate similar in composition to that of the sulfide poor regions the sulfide rich regions also contain rare phases such as olivine and fe ni metal grains with grain sizes that are always 100 nm in size z contrast scanning transmission electron microscopy stem reveals that the fine grained rims consist of a mosaic of irregularly shaped sulfide poor and sulfide rich regions with sizes of about 0 2 0 5 m that have been compacted together during parent body lithification despite aqueous alteration the distinct mineralogical characteristics of these different regions are preserved on a fine scale and probably represent primitive heterogeneity in the dust from which these rims formed serpentine is much better developed and more widespread in the fine grained rims of alha81002 than y 791198 complex mats of serpentine fibers are commonly found and cronstedtite and tochilinite are plentiful anhydrous minerals such as olivine are rare and have usually been replaced by serpentine like y 791198 all the fine grained rims studied in alha81002 show the same mineral assemblages and textural characteristics throughout and between rims the homogeneity of the mineralogy textural relationships and degree of hydration in the rims of these two chondrites is more consistent with parent body alteration than with pre accretionary alteration

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

fine grained dust rims in the tagish lake carbonaceous chondrite evidence for parent body alteration the tagish lake carbonaceous chondrite consists of heavily aqueously altered chondrules cais and larger mineral fragments in a fine grained phyllosilicate dominated matrix the vast majority of the coarse grained components in this meteorite are surrounded by continuous 1 5 to 200 m wide fine grained accretionary rims which are well known from meteorites belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate texturally the fine grained rims in tagish lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object they typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix compared to the matrix the rims are much more finegrained and characterized by a significantly lower porosity the rims consist of an unequilibrated assemblage of phyllosilicates fe ni sulfides magnetites low ca pyroxenes and forsteritic olivines and are except for a much lower abundance of carbonates very similar to the tagish lake matrix electron microprobe and synchrotron x ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in ca x ray elemental mapping and mineralogical observations indicate that ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix the analyses also show that ca is strongly fractionated from al in the rims whereas there is no fractionation of the ti al ratios our data suggest that the fine grained rims in tagish lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body this pervasive alteration removed any potential evidence for preaccretionary alteration but did not change the overall texture of the tagish lake meteorite

nepheline and sodalite in chondrules of the ningqiang carbonaceous chondrite implications for a genetic relationship with those in the matrix abstract ningqiang is an ungrouped carbonaceous chondrite that has a chemical and mineralogical affinity to cv3 chondrites the ningqiang matrix has distinctly higher abundances of na k and al than cv3 matrices a recent study by matsumoto et al 2014 revealed that the major proportions of these elements can be attributed to the presence of nepheline and sodalite scanning electron microscopy revealed that all of the ningqiang chondrules studied show abundant evidence of extensive na fe metasomatism only a small proportion of the chondrules contain primary mesostases in their cores but the mesostases in their mantles were replaced by fine grains of nepheline sodalite fe rich olivine and hedenbergite the mesostases in the majority of the chondrules were completely replaced by fine grains of the same secondary minerals most opaque nodules were also largely replaced by various fine grained secondary minerals nepheline sodalite form veins penetrating the primary mesostases providing evidence that aqueous fluids were involved in the alteration reactions the nepheline sodalite in the mesostases contain various amounts of inclusions of fe rich olivine diopside hedenbergite fe sulfides and magnetite the mineralogical features of the nepheline sodalite in the mesostases are almost identical to those in the meteorite matrix these results suggest that a significant fraction of the nepheline sodalite grains in the ningqiang matrix originated from the nepheline sodalite produced in chondrules and refractory inclusions and that they were disaggregated and mixed into the matrix these processes can be explained consistently by the model of the dynamic formation of chondrite lithology in a parent body proposed by tomeoka and ohnishi 2015 we suggest that after a ningqiang precursor with a cv3 like lithology was metasomatized it was fragmented causing the disaggregation of the fine grained host matrix and the fine grained altered mesostases including nepheline sodalite and opaque nodules in the chondrules the chondrules were thereby separated into multiple fragments subsequently during transportation in a fluidized state all these materials were homogenously mixed together and later underwent accumulation and lithification