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

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

visualisation and quantification of cv chondrite petrography using micro tomography abstract micro computed tomography is a non destructive technique that allows the study of 3d meteorite petrography the technique produces a unique and instructive visualisation of the meteorite for quantifying its components we studied the overall petrography of the two cv chondrites allende and mokoia to constrain their formation histories a set of movies and stereographic images detail the 3d petrography component modal abundances agree with previous reports and modal abundance differences between allende and mokoia support the chondrule matrix complementarity and that chondrules and matrix formed from the same chemical reservoir we identified two types of chondrules a normal type and one where a normal type i or ii chondrule is almost completely encapsulated by an opaque rich layer this layer was probably acquired during a late stage condensation process the appearance of opaques in chondrules and matrix is different not supporting a genetic relationships between these low abundances of compound chondrules 1 75 vol in allende and 2 50 vol in mokoia indicate low chondrule densities and or low relative component velocities in chondrule formation regions porosities on a scale

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

density porosity and magnetic susceptibility of carbonaceous chondrites abstract we report physical properties bulk and grain density magnetic susceptibility and porosity measured using nondestructive and noncontaminating methods for 195 stones from 63 carbonaceous chondrites grain densities over the whole population average 3 44 g cm 3 ranging from 2 42 g cm 3 ci1 orgueil to 5 66 g cm 3 cb bencubbin magnetic susceptibilities in log units of 10 9 m3 kg 1 averaged log 4 22 ranging from 3 23 cv3 axtell to 5 79 cb bencubbin porosities averaged 17 ranging from 0 for a number of meteorites to 41 for one stone of the co ornans notably we found significant differences in porosity between the oxidized and reduced cv subgroups with the porosities of cvo averaging approximately 20 and cvr porosities approximately 4 overall porosities of carbonaceous chondrite falls trend with petrographic type from type 1 ci near 35 type 2 cm cr averaging 23 type 3 cv co 21 to type 4 ck and some co averaging 15 there is also a significant decrease in porosity between meteorites of shock stage s1 and those of s2 indicative of shock compression

fayalitic olivine in cv3 chondrite matrix and dark inclusions a nebular origin fayalitic olivine fa32 is the major component of the matrices and dark inclusions of cv3 and other unequilibrated chondrites it occurs most commonly as rims veins and halos in and around chondrule silicates in the allende type cv3oxa chondrites and to a much lesser extent in the reduced cv3r and bali type cv3oxb chondrites the olivines have distinctive platy tabular and lath or irregular shaped crystals with the ratio of the two types varying widely in cv3oxb chondrites matrix fayalitic olivines range up to fag99 9 whereas in the other cv3 chondrites the range is much smaller the platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor and the nature of the occurrences suggests that cv3 matrices are unequilibrated mixtures of nebular materials we argue that the parent body hydration dehydration model has numerous inconsistencies that make this hypothesis highly unlikely these include 1 there is no direct evidence linking fayalitic olivine to precursor phyllosilicates 2 dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines 3 fayalitic olivine clearly predates the formation of the hydrous phases in cv3 chondrites and is one of the phases that breaks down to form phyllosilicates keller et al 1994 4 the unequilibrated nature of the matrix including fine scale zoning in 10 m sized fayalitic olivine crystals would not survive the parent body metamorphism required in the dehydration model 5 a dark inclusion in the ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material zolensky et al 1997 which probably formed by radiation damage this indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting 6 some allende chondrules contain unaltered primary anhydrous glassy mesostasis in contact with the host matrix e g ikeda and kimura 1995 chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario 7 single platy and barrel shaped crystals of fayalitic olivine are present in accretionary rims in calcium aluminum rich inclusions cais macpherson and davis 1997 which developed in the nebula 8 matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine scott et al 1984 which indicates a nebular origin 9 oxygen isotopic compositions of allende matrix and dark inclusions strongly indicate little or no hydration for allende and its components clayton 1997 we favor a nebular vaporization recondensation model in which vaporization of chondritic dust produced a fayalite rich vapor followed by formation of the fayalitic olivine by direct recondensation from the vapor epitactic growth on surfaces of existing forsterite and enstatite in chondrules and replacement of existing forsterite and enstatite by gas solid exchange

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

olivine in terminal particles of stardust aerogel tracks and analogous grains in chondrite matrix abstract the dearth of both major and minor element analyses of anhydrous silicate phases in chondrite matrix has thus far hindered their comparison to the wild 2 samples we present 68 analyses of olivine fa 0 97 in the coarse grained terminal particles of stardust aerogel tracks and a comprehensive dataset 10 3 analyses of analogous olivine grains 5 30 m isolated in ci cm cr ch co cv3 oxidized cv3 reduced c3 ungrouped acfer 094 and ningqiang l ll 3 0 4 eh3 and kakangari chondrite matrix these compositions reveal that wild 2 likely accreted a diverse assortment of material that was radially transported from various carbonaceous and ordinary chondrite forming regions the wild 2 olivine includes amoeboid olivine aggregates aoas refractory forsterite type i and type ii chondrule fragments and or microchondrules and rare relict grain compositions in addition we have identified one terminal particle that has no known compositional analog in the meteorite record and may be a signature of low temperature aqueous processing in the kuiper belt the generally low cr content of feo rich olivine in the stardust samples indicates that they underwent mild thermal metamorphism akin to a petrologic grade of 3 05 3 15

petrography and oxygen isotopic compositions in refractory inclusions from co chondrites fine grained ca al rich inclusions fgis in yamato 81020 co3 0 and kainsaz co3 1 co3 2 chondrites have been studied by secondary ion mass spectrometry the fgis from yamato 81020 consist of aggregates of hibonite spinel melilite anorthite diopside and olivine grains with no petrographic evidence of alteration in contrast the fgis from kainsaz commonly contain alteration products such as nepheline from replacement textures and chemical compositions of altered and unaltered fgis we conclude that the alteration products formed by decomposition of melilite and anorthite all phases in the yamato 81020 fgis are enriched in 16o with 17 18o 40 except for one fgi that experienced melting oxygen isotopic compositions of melilite anorthite some spinel and diopside in kainsaz fgis changed from 17 18o 40 toward 0 by aqueous alteration alteration products in fgis are depleted in 16o relative to primary phases with 17 18o 0 these results show that fgis in co chondrites commonly had 16o rich compositions in the solar nebula the original 16o rich fgis were modified to 16o poor compositions during aqueous alteration in the parent body

aspects of the crustal structure of the western mojave desert california from seismic reflection and gravity data seismic and gravity data taken along line 1 of the 1982 consortium for continental reflection profiling cocorp mojave desert survey n s profile 30 km long have been used to characterize the upper crust north of the san andreas fault in the western mojave block of southern california consortium for continental reflection profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds two way travel time the resultant common depth point cdp sections provided starting models for generating a refined geologic cross section using a combination of ray tracing forward modeling and gravity interpretation the forward modeling was used to validate the existence of faults and constrain their dips the gravity data were used to refine the overall model particularly in poor data areas on the cdp sections gravity data taken along three nearby profiles parallel to primary line of section were also used to determine the structural trend results from the first two seconds indicate the presence of a series of ene striking reverse faults beneath the late tertiary and quaternary sedimentary cover of the western mojave the faults dip northward and offset the sediment basement interface the largest such feature has an apparent throw of 1 8 km and exhibits a subtle scarp at the earth s surface suggesting holocene displacement the orientation of these faults although not an instantaneous representation of the present day stress field is consistent with nnw compression across the western mojave block and wnw striking san andreas fault as determined from nearby focal mechanisms and in situ stress measurements the faults also appear to be closing small sedimentary basins in the mojave block which may have formed during an earlier extensional phase similar to what is happening on a much larger scale in the los angeles basin to the south of the san andreas fault reflections between 2 and 5 s coupled with the local geology and gravity modeling are consistent with the presence of the pelona rand schist in the subsurface beneath the western mojave the upper surface of the schist i e vincent rand thrust equivalent rises southward toward the san andreas fault where it is displaced vertically up to the south at least 5 km along the e w trending hitchbrook fault such that the schist crops out between the hitchbrook and subparallel san andreas to the south the same structure may exist beneath the tehachapi mountains with the roles of the hitchbrook and san andreas faults played by the north and south branches of the garlock fault respectively the rising or arching of the basement toward the san andreas fault and toward the garlock is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western mojave furthermore the arching is also consistent with a strong component of fault normal compression

the mineral chemistry and origin of inclusion matrix and meteorite matrix in the allende cv3 chondrite the two textural varieties of olivine rich allende inclusions rimmed and unrimmed olivine aggregates consist primarily of a porous fine grained mafic constituent inclusion matrix that differs from the opaque meteorite matrix of cv3 chondrites by being relatively depleted in sulfides metal grains and perhaps carbonaceous material olivine is the most abundant mineral in allende inclusion matrix clinopyroxene nepheline sodalite and ti al pyroxene occur in lesser amounts olivine in unrimmed olivine aggregates type 1a inclusions is ferrous and has a narrow compositional range fo50 65 olivine in rimmed olivine aggregates type 1b inclusions is on average more magnesian with a wider compositional range fo53 96 olivine grains in the granular rims of type 1b inclusions are zoned with magnesian cores fo 80 and ferrous rinds fo 70 ferrous olivines fo 65 in both varieties of inclusions commonly contain significant amounts of al2o3 as much as 0 7 wt cao as much as 0 4 wt and tio2 as much as 0 2 wt refractory elements that probably occur in submicroscopic inclusions of ca al ti rich glass rather than in the olivine crystal structure defocussed beam analyses of allende matrix materials demonstrate that 1 inclusion matrix in type 1a inclusions is more enriched in olivine and feo than inclusion matrix in the cores of type 1b inclusions 2 opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix 3 the bulk compositions of type 1a and type 1b inclusions overlap and 4 excluding sulfides and metal the bulk compositions of allende matrix materials cluster in a complementary pattern around the bulk composition of c1 chondrites inclusion matrix and meteorite matrix in allende and other cv3 chondrites are probably relatively primitive nebular material but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing non equilibrium conditions from supercooled supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula cv3 meteorite matrix contains in addition a proportion of interstellar material that was heated but not vaporized in the nebula granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula the mineral chemistry of matrix olivine in allende seems to have been established by three different processes non equilibrium vapor solid condensation recrystallization and partial melting in the nebula and femg equilibration without textural homogenization in the meteorite parent body

26al 26mg systematics of ca al rich inclusions amoeboid olivine aggregates and chondrules from the ungrouped carbonaceous chondrite acfer 094 we report in situ magnesium isotope measurements of 7 porphyritic magnesium rich type i chondrules 1 aluminum rich chondrule and 16 refractory inclusions 14 ca al rich inclusions cais and 2 amoeboid olivine aggregates aoas from the ungrouped carbonaceous chondrite acfer 094 using a cameca ims 6f ion microprobe both aoas and 9 cais show radiogenic 26mg excesses corresponding to initial 26al 27al ratios between 5 x 10 5 and 7 x 10 5 suggesting that formation of the acfer 094 cais may have lasted for 300 000 years four cais show no evidence for radiogenic 26mg three of these inclusions a corundum rich a grossite rich and a pyroxene hibonite spherule cai are very refractory objects and show deficits in 26mg suggesting that they probably never contained 26al the fourth object without evidence for radiogenic 26mg is an anorthite rich igneous type c cai that could have experienced late stage melting that reset its al mg systematics significant excesses in 26mg were observed in two chondrules the inferred 26al 27al ratios in these two chondrules are 10 3 7 4 10 6 and 6 0 3 8 10 6 errors are 2 suggesting formation and myr after cais with the canonical 26al 27al ratio of 5 10 5 these age differences are consistent with the inferred age differences between cais and chondrules in primitive ordinary ll3 0 ll3 1 and carbonaceous co3 0 chondrites

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

mineralogy and petrology of amoeboid olivine inclusions in co3 chondrites relationship to parent body aqueous alteration petrographic and mineralogic studies of amoeboid olivine inclusions aois in co3 carbonaceous chondrites reveal that they are sensitive indicators of parent body aqueous and thermal alteration as the petrologic subtype increases from 3 0 to 3 8 forsteritic olivine fa0 1 is systematically converted into ferroan olivine fa60 75 we infer that the fe si and o entered the assemblage along grain boundaries forming ferroan olivine that filled fractures and voids as temperatures increased fe 2 from the new olivine exchanged with mg 2 from the original aoi to form diffusive haloes around low feo cores cations of mn 2 ca 2 and cr 3 were also mobilized the systematic changes in aoi textures and olivine compositional distributions can be used to refine the classification of co3 chondrites into subtypes in subtype 3 0 olivine occurs as small forsterite grains fa0 1 free of ferroan olivine in petrologic subtype 3 2 narrow veins of feo rich olivine have formed at forsterite grain boundaries with increasing alteration these veins thicken to form zones of ferroan olivine at the outside aoi margin and within the aoi interior by subtype 3 7 there is a fairly broad olivine compositional distribution in the range fa63 70 and by subtype 3 8 no forsterite remains and the high fa peak has narrowed fa64 67 even at this stage there is incomplete equilibration in the chondrite as a whole e g data for coarse olivine grains in isna co3 8 chondrules and lithic clasts show a peak at fa39 we infer that the mineral changes in aoi identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes 3 3 largely reflect diffusive exchange within and between mineral grains without the aid of fluids

refractory inclusions in the unique carbonaceous chondrite acfer 094 abstract acfer 094 is an unshocked nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions the refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work and to report new or unusual inclusion types a total of 289 ca al rich inclusions in the thin section and 67 among the disaggregated material having a total of 31 different mineral assemblages were found inclusions are largely free of secondary alteration products and are typically 200 m across the most common are gehlenitic melilite spinel perovskite spinel perovskite and spinel with a thin silicate rim typically melilite diopside such rims and thicker mantles are very common among acfer 094 inclusions and they exhibit a variety of zoning patterns with respect to akermanite and feo contents in the thin section about 13 of the inclusions contain hibonite and approximately 5 are grossite bearing in the disaggregated material the percentages are 14 and 9 respectively comparable to previous work among the unusual inclusions are a fine grained porous ti rich hibonite spinel perovskite melilite inclusion with a compact coarse ti poor hibonite spinel melilite clast two inclusions in which hibonite has reacted to form grossite two inclusions with feo rich spinel and a small object consisting of fassaite enclosing euhedral spinel the first fragment of a type b inclusion reported from acfer 094 inclusions similar to those found in cm or cv chondrites are rare acfer 094 contains a distinctive population of inclusions the population dominated by small melilite bearing inclusions is most similar to that of co chondrites a distinguishing feature is that in acfer 094 almost every phase in almost every refractory inclusion contains 0 5 1 5 wt feo a lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to feo bearing fluid prior to accretion into the acfer 094 parent body there are no known nebular conditions under which the refractory phases found in the present samples could acquire feo enrichments to the observed levels the most likely setting is therefore in an earlier feo rich parent body the inclusions were ejected from this parent body mixed with typical cais chondrules amoeboid olivine aggregates and amorphous material and incorporated into the acfer 094 parent body

hydrothermal experiments on alteration of ca al rich inclusions cais in carbonaceous chondrites implication for aqueous alteration in parent asteroids carbonaceous chondrites contain ca al rich inclusions cais which consist mainly of refractory minerals including gehlenite spinel perovskite and hibonite which are predicted to be the first phases formed from early solar nebula on the basis of thermodynamic calculations however many cais contain secondary minerals such as nepheline calcite and phyllosilicates the formation process of secondary minerals in cais is still controversial whether they were produced by the reaction with solar nebula or by aqueous alteration in parent asteroids we performed hydrothermal experiments on gehlenite spinel and diopside which are common in cais in carbonaceous chondrites to study aqueous alteration in parent asteroids mixtures of gehlenite with sio2 and or al2o3 and those of sio2 with al2o3 were also employed to study products of these systems which have a close relationship to crystallization of secondary minerals in cais in fluid in parent asteroids hydrogrossular calcite and amorphous al si were obtained from gehlenite by hydrothermal experiments whereas both spinel and diopside escaped alteration suggesting that gehlenite is the weakest mineral among these refractory minerals against alteration this result is consistent with observation of cais analcime nepheline hydrate and tobermorite were obtained from the runs for mixtures of gehlenite with sio2 whereas the products from mixtures of al2o3 with sio2 were analcime nepheline hydrate and hydrosodalite these results suggest that the alteration process of gehlenite is as follows 1 gehlenite was dissolved by fluid in chondrite parent asteroids to release ca al and si from the surface 2 co32 in fluid reacted with ca to crystallize calcite 3 na al and si in fluid crystallized nepheline hydrate hydrosodalite and or analcime 4 ca al and si in fluid crystallized hydrogrossular without co32 and excess ca and si crystallized ca si hydrate such as tobermorite heating experiments on several minerals obtained through hydrothermal experiments were also performed to study thermal metamorphism in parent asteroids hydrogrossular was converted into hydrogrossular with a lower h2o content and analcime and nepheline hydrate were converted into nepheline tobermorite is known to be converted into wollastonite with increasing temperature the results of experimental studies suggest that most secondary minerals found in cais in carbonaceous chondrites were produced from gehlenite by aqueous alteration and the subsequent thermal metamorphism in parent asteroids

an experimental study of enstatite formation reaction between forsterite and si rich gas experiments on the reaction between forsterite and si rich gas were carried out in a vacuum furnace using a newly designed double capsule technique a thin layer of enstatite was formed on the surface of the forsterite by the reaction the thickness of the enstatite layer changes in proportion to the square root of time parabolic rate law indicating that the rate is controlled by diffusion in the enstatite layer the activation energy and the logarithm of the pre exponential term of the parabolic rate constant were obtained at505 188 kj mol and3 6 5 6 cm2 s respectively it is difficult to definitely determine the mechanism of the reaction from these values but consideration of strain generation due to volume change during the reaction may suggest that the counter diffusion of silicon and magnesium takes place along the grain boundary in the enstatite layer the degree of the reaction of forsterite with si rich residual gas in the primordial solar nebula is discussed based upon the measured reaction rate and it is suggested that very little enstatite was formed by this reaction in the cooling nebula if the grain size of the forsterite crystals was controlled by homogeneous nucleation theory

aqueous alteration of the bali cv3 chondrite evidence from mineralogy mineral chemistry and oxygen isotopic compositions abstract a petrographic geochemical and oxygen isotopic study of the bali cv3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body deformation textures are common and include flattened chondrules a well developed foliation and the presence of distinctive 100 planar defects in olivine the occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration the secondary minerals produced during the alteration event include well crystallized mg rich saponite framboidal magnetite and ca phosphates the alteration products are not homogeneously distributed throughout the meteorite but occur in regions adjacent to relatively unaltered material such as veins of altered material following the foliation the alteration assemblage formed under oxidizing conditions at relatively low temperatures oxygen isotopic compositions for unaltered regions in bali fall within the field for other cv3 wholerocks however the oxygen isotopic compositions of the heavily altered material lie in the region for the cm and cr chondrites the heavy isotope enrichment of the altered regions in bali suggest alteration conditions similar to those for the petrographie type 2 carbonaceous chondrites

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

thermochemical stability of low iron manganese enriched olivine in astrophysical environments low iron manganese enriched lime olivine grains are found in cometary samples returned by the stardust mission from comet 81p wild 2 similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix lime olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar but enrichment of solar composition vapor in a dust of chondritic composition causes the feo mno ratio of olivine to increase the compositions of lime olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition the compositional zoning of lime olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine without re equilibration at lower temperatures a similar history is likely for lime olivines found in comet samples and in interplanetary dust particles lime olivine is not likely to persist in nebular conditions in which silicate liquids are stable

the matrices of unequilibrated ordinary chondrites implications for the origin and history of chondrites abstract the matrices of sixteen unequilibrated ordinary chondrites all witnessed falls were studied microscopically in transmitted and reflected light and analyzed by electron microprobe selected specimens were also studied by scanning electron microscopy these studies indicate that the fine grained opaque silicate matrix of type 3 unequilibrated chondrites is compositionally mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by larimer and anders 1967 1970 examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed with the alteration ranging in intensity from quite mild where the matrix has been only slightly altered to a more severe metamorphism that has completely recrystallized the opaque matrix most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules the metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves the present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed plus the subsequent metamorphic alteration it experienced

ca al rich inclusions amoeboid olivine aggregates and al rich chondrules from the unique carbonaceous chondrite acfer 094 i mineralogy and petrology based on their mineralogy and petrography 200 refractory inclusions studied in the unique carbonaceous chondrite acfer 094 can be divided into corundum rich 0 5 hibonite rich 1 1 grossite rich 8 5 compact and fluffy type a spinel melilite rich 50 3 pyroxene anorthite rich 7 4 and type c pyroxene anorthite rich with igneous textures 1 6 ca al rich inclusions cais pyroxene hibonite spherules 0 5 and amoeboid olivine aggregates aoas 30 2 melilite in some cais is replaced by spinel and al diopside and or by anorthite whereas spinel pyroxene assemblages in cais and aoas appear to be replaced by anorthite forsterite grains in several aoas are replaced by low ca pyroxene none of the cais or aoas show evidence for fe alkali metasomatic or aqueous alteration the mineralogy textures and bulk chemistry of most acfer 094 refractory inclusions are consistent with their origin by gas solid condensation and may reflect continuous interaction with sio and mg of the cooling nebula gas it appears that only a few cais experienced subsequent melting the al rich chondrules arcs 10 wt bulk al2o3 consist of forsteritic olivine and low ca pyroxene phenocrysts pigeonite augite anorthitic plagioclase spinel feni metal and crystalline mesostasis composed of plagioclase augite and a silica phase most arcs are spherical and mineralogically uniform but some are irregular in shape and heterogeneous in mineralogy with distinct ferromagnesian and aluminous domains the ferromagnesian domains tend to form chondrule mantles and are dominated by low ca pyroxene and forsteritic olivine anorthitic mesostasis and fe ni metal nodules the aluminous domains are dominated by anorthite high ca pyroxene and spinel occasionally with inclusions of perovskite have no or little feni metal and tend to form cores of the heterogeneous chondrules the cores are enriched in bulk ca and al and apparently formed from melting of cai like precursor material that did not mix completely with adjacent ferromagnesian melt the inferred presence of cai like material among precursors for al rich chondrules is in apparent conflict with lack of evidence for melting of cais that occur outside chondrules suggesting that these cais were largely absent from chondrule forming region s at the time of chondrule formation this may imply that there are several populations of cais in acfer 094 and that mixing of normal cais that occur outside chondrules and chondrules that accreted into the acfer 094 parent asteroid took place after chondrule formation alternatively there may have been an overlap in the cai and chondrule forming regions where the least refractory cais were mixed with fe mg chondrule precursors this hypothesis is difficult to reconcile with the lack of evidence of melting of aoas which represent aggregates of the least refractory cais and forsterite grains

vapor condensed phase processes in the early solar system abstract equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in cm2 and cv3 chondrites many refractory inclusions in cm2 chondrites contain a relatively sio2 poor assemblage spinel hibonite grossite perovskite corundum that represents a high temperature stage of condensation and some may be pristine condensates that escaped later melting compact type a and type b refractory inclusions consisting of spinel melilite perovskite ca rich clinopyroxene anorthite in cv3 chondrites are more sio2 rich and equilibrated with the solar nebular gas at a slightly lower temperature textures of many of these objects indicate that they underwent melting after condensation crystallizing into the same phase assemblage as their precursors the ti3 ti4 ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity was approximately 8 5 log units below that of the iron wustite buffer making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun relative to ci chondrites these inclusions are uniformly enriched in a group of elements e g ca ree zr ta ir that are chemically diverse except for their high condensation temperatures in a system of solar composition the enrichment factor 17 5 can be interpreted to mean that these objects represent either the first 5 7 wt of the condensable matter to condense during nebular cooling or the residue after vaporization of 94 3 of a ci chondrite precursor the mg and si isotopic compositions of types a and b inclusions are mass fractionated by up to 10 and 4 amu respectively when interpreted in terms of rayleigh fractionation during evaporation of mg and si from the inclusions while they were molten the isotopic compositions imply that up to 60 of the mg and up to 25 of the si were evaporated and that approximately 80 of the enrichment in refractory cao al2o3 relative to more volatile mgo sio2 in the average inclusion is due to initial condensation and approximately 20 due to subsequent evaporation the mineralogical composition including the ti3 ti4 ratio of the pyroxene in inti a particle sampled from comet wild 2 by the stardust spacecraft is nearly identical to that of a type b inclusion indicating that comets contain not only the lowest temperature condensates in the form of ices but the highest temperature condensates as well the feo feo mgo ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition requiring s only 1 or 2 log units below iron wustite more than 105 times higher than that of a solar gas various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant feo in olivine at temperatures above those where fe mg interdiffusion in olivine ceases one is by vertical settling of dust toward the nebular midplane enriching a region in dust relative to gas because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition a higher results from total vaporization of the region but the factor by which theoretical models have so far enriched the dust is 10 times too low another is by transporting icy bodies from the outer part of the nebula into the hot inner part where vaporization of water ice occurs not only does this method fail to make the needed by a factor of 30 1000 but it also ignores simultaneous evaporation of carbon bearing ices that would make the even lower

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

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

calcium aluminum rich inclusions in enstatite chondrites i mineralogy and textures abstract like calcium aluminum rich inclusions cais from carbonaceous and ordinary chondrites enstatite chondrite cais are composed of refractory minerals such as spinel perovskite al ti diopside melilite hibonite and anorthitic plagioclase which may be partially to completely surrounded by halos of na cl rich minerals porous aggregate and compact textures of the refractory cores in enstatite chondrite cais and rare wark lovering rims are also similar to cais from other chondrite groups however the small size 100 m low abundance 1 by mode in thin section occurrence of only spinel or hibonite rich types and presence of primary ti v oxides and secondary geikelite and ti fe sulfides distinguish the assemblage of enstatite chondrite cais from other groups the primary mineral assemblage in enstatite chondrite cais is devoid of indicators e g oldhamite osbornite of low o fugacities thus high temperature processing of the cais did not occur under the reducing conditions characteristic of enstatite chondrites implying that either 1 the cais are foreign to enstatite chondrite forming regions or 2 o fugacities fluctuated within the enstatite chondrite forming region in contrast secondary geikelite and ti fe sulfide which replace perovskite indicate that alteration of perovskite occurred under reducing conditions distinct from cais in the other chondrite groups we have not ascertained whether the reduced alteration of enstatite chondrite cais occurred in a nebular or parent body setting we conclude that each chondrite group is correlated with a unique assemblage of cais indicating spatial or temporal variations in physical conditions during production or dispersal of cais

nwa 1152 and sahara 00182 new primitive carbonaceous chondrites with affinities to the cr and cv groups we have investigated the mineralogy petrography bulk chemistry and light element isotope composition of the ungrouped chondrites north west africa nwa 1152 and sahara 00182 nwa 1152 contains predominantly type 1 porphyritic olivine po and porphyritic olivinepyroxene pop chondrules chondrule silicates are magnesium rich fo98 8 1 2 n 36 fs2 3 2 1 wo1 2 0 3 n 23 matrix comprises 40 vol of the sample and is composed of a micron sized silicate groundmass with larger silicate sulfide magnetite and fe ni metal ni 50 wt grains phyllosilicates were not observed in the matrix refractory inclusions are rare 0 3 vol and are spinel pyroxene aggregates or amoeboid olivine aggregates melilite is absent from the refractory inclusions sahara 00182 contains predominantly type 1 po chondrules pop chondrules are less common most chondrules contain blebs of and are often rimmed with fe ni metal and sulfide chondrule phenocrysts are magnesium rich fo92 2 0 6 n 129 fs4 4 1 8 wo1 3 1 1 n 16 matrix comprises 30 vol of the meteorite and is predominantly sub micron silicates with rare larger silicate gains matrix fe ni metal mean ni 5 8 wt and sulfide grains are up to mm scale no phyllosilicates were observed in the matrix refractory inclusions are rare 1 1 vol and melilite is absent the oxygen isotope composition of nwa 1152 falls within the range of the cv chondrites with 17o 3 43 18o 0 70 and is similar to sahara 00182 17o 3 89 18o 0 19 grossman and zipfel 2001 based on mineralogical and petrographic characteristics we suggest nwa 1152 and sahara 00182 show many similarities with the cr chondrites however oxygen isotopes suggest affinity with the cvs thus neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics both samples demonstrate the complexity of inter and intra group relationships of the carbonaceous chondrites whatever their classification nwa 1152 and sahara 00182 represent a source of relatively pristine solar system material

simultaneous calculation of metamorphic fluid fluxes reaction rates and fluid rock interaction timescales using a novel inverse modeling framework abstract to study metamorphic carbonation at greenschist facies conditions in the sw scottish highlands a novel inverse modeling framework which combines solutions of the transport equation with a global optimization method of differential evolution was developed using this framework we calculated simultaneously time integrated and time averaged metamorphic fluid fluxes of 83 4 35 4 m 3 m 2 and 10 10 1 0 5 m 3 m 2 s 1 respectively a time averaged reaction rate constant of 10 12 7 10 2 m s 1 and comparable timescales for fluid flow and fluid driven reaction of 10 4 3 0 5 yr and 10 2 7 5 2 yr respectively these parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity our estimates are 1 consistent with single pass flow of fluid released by metamorphic devolatilization 2 within the range where heat is transported by conduction and matter is transported by advection 3 in agreement with an emerging consensus that metamorphic events are relatively short lived and 4 supportive of applying laboratory based estimates of kinetic parameters to metamorphic systems based on a sensitivity analysis we show that 1 selecting the diffusion coefficient rather than fluid velocity reaction rate or flow duration as an input parameter yields more robust estimates of metamorphic fluid flow parameters and 2 ignoring reaction dependent porosity and reaction rates can result in an order of magnitude uncertainty in best fit flow parameters evaluated from concentration profiles finally similarity between our calculated time averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the quasi stationary state assumption to quantify metamorphic fluid flow parameters

fluffy type a ca al rich inclusions in the allende meteorite abstract allende fluffy type a s fta s are a distinct sub group of ca al rich inclusions whose primary mineral assemblage consists of al rich melilite ak 0 33 spinel that is commonly very v rich perovskite and frequently hibonite some contain relatively coarse grained melilite up to 1 5 mm that is intensely kink banded and commonly reversely zoned hibonite and v rich spinel others contain much finer grained and strain free melilite 50 m and have not been found to contain hibonite or v rich spinel some fta s contain both coarser and finer grained melilite and textural relationships indicate that the latter is replacing the former fta s are characterized by extremely irregular shapes and 60 75 volume per cent of fine grained secondary alteration products many are aggregates of innumerable nodules each of which is surrounded by a wark lovering type rim sequence these nodules are frequently separated from one another by matrix like clastic rim material other fta s do not have nodular structure structural and mineralogical characteristics of their wark lovering rims suggest that fta s did not achieve their shapes by deformation of a liquid or a hot plastic solid in contrast to those in type b inclusions formation of reverse zoning in the coarser grained melilite crystals in fta s cannot be understood in terms of crystallization from a liquid but are readily explainable by condensation from a solar nebular gas during a period of falling pressure further evidence against a liquid origin is the wide range of spinel compositions within individual coarser grained fta s the fact that the reversely zoned melilite crystals cannot have been produced in any kind of sublimation or distillation process precludes formation of these inclusions as volatilization residues fta s are aggregates in some of which are preserved vapor solid condensate grains that formed at high temperature in the solar nebula

oxygen isotopic distribution in an amoeboid olivine aggregate from the allende cv chondrite primary and secondary processes the oxygen isotopic distribution in an amoeboid olivine aggregate aoa tta1 02 from the allende cv3 chondrite has been determined by secondary ion mass spectrometry the irregular shaped tta1 02 5 3mm consists mostly of olivine grains of ca 5 m in diameter olivine grains of mg rich fo95 and fe rich fo60 composition are in direct contact with each other with a sharp compositional boundary oxygen isotopic compositions of fe rich olivine grains are 16o poor 17o 5 whereas mg rich olivine is 16o rich 17o 25 several al rich inclusions ca 500 m in diameter are enclosed by olivine grains in the aoa oxygen isotopic compositions of spinel and fassaite in al rich inclusions are 16o rich 17o 20 whereas those of anorthite nepheline and phyllosilicate are 16o poor 17o 5 we propose the following sequence of events during the formation of aoas in the allende meteorite 1 formation of al rich inclusions with 16o rich oxygen isotopic composition 2 accretion of mg rich olivine grains with 16o rich oxygen isotopic composition around al rich inclusions 3 accretion into parent body and 4 aqueous alteration in the parent body which led to crystallization of 16o poor minerals fe rich olivine anorthite nepheline and phyllosilicate this is reflecting reactions among primary 16o rich aoa minerals and aqueous fluid having 16o poor oxygen isotopic composition fe rich olivine grains precipitated from aqueous fluids which partially dissolved pre existing mg rich olivine grains sintering and mg fe diffusion occurred during thermal metamorphism anorthite nepheline and phyllosilicate in al rich inclusions replaced primary anorthite or melilite during the aqueous alteration stage

regional bowen ratio controls on afternoon moist convection a large eddy simulation study this study examines the effect of the regional bowen ratio the ratio of the domain averaged surface sensible heat flux shf to latent heat flux lhf on afternoon moist convection with a temporally evolving but spatially uniform surface available energy over a mesoscale domain under a weak capping inversion we run large eddy simulation of the afternoon convective boundary layer cbl we first prescribe a small of 0 56 a wet surface and then the reversed large of 1 80 a dry surface by switching the shf and lhf fields the perturbation fields of the fluxes are prescribed with the fourier spectra of 3 is horizontal wave number strong mesoscale heterogeneity and 0 homogeneity the large cases have strong vertical buoyancy fluxes and produce more vigorous updrafts in the heterogeneous large surface case with the removal of convective inhibition over a mesoscale subdomain of large shf deep convection develops in the heterogeneous small surface case convective clouds develop but do not progress into precipitating convection in the homogeneous surface cases randomly distributed shallow clouds develop with significantly more and thicker clouds in the large case co spectral analyses confirm the more vigorous turbulent thermals in the large cases and reveal that the moisture advection by the surface heterogeneity induced mesoscale flows makes the correlation between mesoscale temperature and moisture perturbations change from negative to positive which facilitates the mesoscale pool of high relative humidity air just above the cbl top a necessary condition for deep convection

determination of the petrologic type of cv3 chondrites by raman spectroscopy of included organic matter abstract this paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine cv3 chondrites allende axtell bali mokoia grosnaja efremovka vigarano leoville and kaba the maturity of the organic matter in matrix determined by raman spectroscopy has been used as a powerful metamorphic tracer independent of the mineralogical context and extent of aqueous alteration this tracer has been used along with other metamorphic tracers such as fe zoning in type i chondrules of olivine phenocrysts presolar grain abundance and noble gas abundance bulk and p3 component the study shows that the petrologic types determined earlier by induced thermoluminescence were underestimated and suggests the following values pt allende axtell 3 6 pt bali mokoia grosnaja 3 6 pt efremovka leoville vigarano 3 1 3 4 pt kaba 3 1 the most commonly studied cv3 allende is also the most metamorphosed bali is a breccia containing clasts of different petrologic types the attribution suggested by this study is that of clasts of the highest petrologic types as pointed out by iom maturity and noble gas bulk abundance cv3 chondrites have complex asteroidal backgrounds with various degrees of aqueous alteration and or thermal metamorphism leading to complex mineralogical and petrologic patterns fe mg chemical zoning in olivine phenocrysts on the borders of type i chondrules of porphyritic olivine and pyroxene rich textural types has been found to correlate with the metamorphism grade this suggests that chemical zoning in some chondrules often interpreted as exchanges between chondrules and nebular gas may well have an asteroidal origin furthermore the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration this does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through fe mg equilibration on the other hand it may provide clues on the degree of aqueous alteration vs thermal metamorphism and on the timing of both processes in particular mokoia experienced significant aqueous alteration after the metamorphism peak whereas grosnaja which has similar metamorphism grade did not

the onset of metamorphism in ordinary and carbonaceous chondrites ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by xray mapping techniques a variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe scanning electron microscopy sem transmission electron microscopy tem and cathodoluminescence cl methods the distribution of cr in feo rich olivine systematically changes as metamorphism increases between type 3 0 and type 3 2 igneous zoning patterns are replaced by complex ones and cr rich coatings develop on all grains cr distributions in olivine are controlled by the exsolution of a cr rich phase probably chromite cr in olivine may have been partly present as tetrahedrally coordinated cr3 separation of chromite is nearly complete by petrologic type 3 2 the abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type tem shows this to be the result of crystallization of albite residual glass compositions systematically change during metamorphism becoming increasingly rich in k glass in type i chondrules also gains alkalis during metamorphism both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules the matrix in the least metamorphosed chondrites is rich in s and na the s is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules alkalis initially leave the matrix and enter chondrules during early metamorphism feldspar subsequently nucleates in the matrix and na re enters from chondrules these metamorphic trends can be used to refine classification schemes for chondrites cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3 0 and 3 1 into types 3 00 through 3 15 on this basis the most primitive ordinary chondrite known is semarkona although even this meteorite has experienced a small amount of metamorphism allan hills alh a77307 is the least metamorphosed co chondrite and shares many properties with the ungrouped carbonaceous chondrite acfer 094 analytical problems are significant for glasses in type ii chondrules as na is easily lost during microprobe analysis as a result existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised

shock metamorphism of carbonaceous chondrites abstract we have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites we find that sixty nine carbonaceous chondrites can be assigned to four shock stages si to s4 largely on the basis of shock effects in olivine the least shocked chondrite groups are cm2 and co3 thirty six out of thirty eight members are classified as shock stage s1 differences between the mean shock levels of certain groups of chondrites e g co3 and cv3 are probably due to stochastic differences in the sampling of their parent bodies however there is an overall tendency for the mean shock level of carbonaceous chondrites and ordinary chondrites to increase with increasing petrologic type that may reflect real differences in the shock level of near surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6 the greater porosity of types 2 3 causes higher post shock temperatures and melting at lower pressures and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure we suggest that type 2 3 material shocked above 20 30 gpa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites in the cv3 group there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism analogous to that discovered in ordinary chondrites by sneyd et al 1988 suggesting that shock rather than static overburden pressure is responsible for chondrule flattening we infer that the correlations are due to collapse of pores under shock pressures of at least 5 10 gpa and that shock is an important process affecting many physical properties of type 2 4 chondrites

alteration of al rich inclusions inside amoeboid olivine aggregates in the allende meteorite lightly altered al rich inclusions in amoeboid olivine aggregates have cores containing primary melilite fassaite spinel perovskite and no secondary alteration products in moderately altered inclusions whose cores now contain only fassaite spinel perovskite melilite was replaced by a fine grained mixture of grossular anorthite feldspathoids and perovskite was partially replaced by ilmenite in heavily altered inclusions fassaite has been replaced by a mixture of phyllosilicates ilmenite and the remaining primary phases are spinel perovskite in very heavily altered inclusions no primary phases remain the spinel having reacted to form either phyllosilicates or a mixture of olivine feldspathoids this sequence of alteration reactions may reflect successively lower solar nebular equilibration temperatures during alteration sio2 na2o k2o feo cr2o3 h2o and cl were introduced into the inclusions and cao was lost mgo may have been lost during the melilite reaction and added during formation of phyllosilicates electron microprobe analyses indicate that the phyllosilicates are a mixture of na rich phlogopite and chlorite or alrich serpentine thermodynamic calculations suggest that at a solar nebular water fugacity of 10 6 na rich phlogopite could have formed from fassaite at 470 k and chlorite from na rich phlogopite at 328 k olivine mantling al rich inclusions is not serpentinized suggesting that these objects stopped equilibrating with the nebular gas above 274 k

fayalitic olivine in matrix of the krymka ll3 1 chondrite vapor solid growth in the solar nebula fayalitic olivine fa54 94 is a ubiquitous component in the matrix of krymka ll3 1 as well as in other highly unequilibrated chondrites ordinary and carbonaceous in krymka the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as 1 isolated platelets 2 clusters of platelets 3 euhedral to subhedral crystals 4 overgrowths of platelets on forsteritic olivine and 5 fluffy porous aggregates from transmission electron microscope tem investigation the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material the fayalitic olivine in the matrix of krymka is compositionally similar to olivine with platy morphology in the matrix of some cv3 chondrites and both have similar fe mn ratios but important morphological differences indicate that their relationship needs to be explored further textural and compositional data indicate that the fayalitic olivine in the matrix of krymka as well as in some other unequilibrated ordinary chondrites formed prior to final lithification of the meteorite and probably prior to parent body accretion we find that formation of the fayalitic olivine by vapor solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of krymka we propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich dust during a period of enhanced dust gas ratio in the nebula

amoeboid olivine aggregates with low ca pyroxenes a genetic link between refractory inclusions and chondrules amoeboid olivine aggregates aoas in primitive unmetamorphosed and unaltered carbona ceous chondrites are uniformly 16 o enriched 17 o 20 and consist of forsterite fa2 feni metal and a refractory component individual cais and fine grained minerals interspersed with forsterite grains composed of al diopside anorthite spinel and exceptionally rare melilite a u k15 some cais in aoas have compact igneous textures melilite in aoas is replaced by a fine grained mixture of spinel al diopside and anorthite spinel is corroded by anorthite or by al diopside in 10 of 500 aoas studied in the cr cv cm co ch cb and ungrouped carbonaceous chondrites acfer 094 adelaide and lew85332 forsterite is replaced to a various degree by low ca pyroxene there are three major textural occurrences of low ca pyroxene in aoas i thin 10 m discontinuous layers around forsterite grains or along forsterite grain boundaries in aoa peripheries ii haloes and subhedral grains around feni metal nodules in aoa peripheries and iii thick up to 70 m continuous layers with abundant tiny inclusions of feni metal grains around aoas aoas with low ca pyroxene appear to have experienced melting of various degrees in the most extensively melted aoa in the cv chondrite leoville only spinel grains are relict forsterite anorthite and al diopside were melted this aoa has an igneous rim of low ca pyroxene with abundant feni metal nodules and is texturally similar to type i chondrules based on these observations and thermodynamic analysis we conclude that aoas are aggregates of relatively low temperature solar nebular condensates originated in 16 o rich gaseous reservoir s probably cai forming region s some of the cais were melted before aggregation into aoas many aoas must have also experienced melting but of a much smaller degree than chondrules before and possibly after aggregation melilite and spinel reacted with the gaseous sio and mg to form ca tschermakite caal2sio6 diopside camgsi2o6 solid solution and anorthite solid or incipiently melted olivine in some aoas reacted with gaseous sio in the cai or chondrule forming regions to form low ca pyroxene mg2sio4 sio g h2o g mg2si2o6 h2 g some low ca pyroxenes in aoas may have formed by oxidation of si bearing feni metal mg2sio4 si in feni 2h2o g mg2si2o6 2h2 g and by direct gas solid condensation mg g sio g h2o g mg2si2o6 s h2 g from fractionated mg si ratio solar nebular gas although bulk compositions of aoas are rather similar to those of type i chondrules on the projection from spinel onto the plane ca2sio4 mg2sio4 al2o3 these objects plot on different sides of the anorthite forsterite thermal divide suggesting that type i chondrules cannot be produced from aoas by an igneous fractionation formation of low ca pyroxene by reaction of aoas with gaseous sio and by melting of silica rich dust accreted around aoas moves bulk compositions of the aoas towards chondrules and provide possible mechanisms of transformation of refractory materials into chondrules or chondrule precursors the rare occurrences of low ca pyroxene in aoas may indicate that either aoas were isolated from the hot nebular gas before condensation of low ca pyroxene or that condensation of low ca pyroxene by reaction between forsterite and gaseous sio was kinetically inhibited if the latter is correct then the common occurrences of pyroxene rich type i chondrules may require either direct condensation of low ca pyroxenes or sio2 from fractionated nebular gas or condensation of gaseous sio into chondrule melts copyright 2004 elsevier ltd

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

amoeboid olivine aggregates aoas in the efremovka leoville and vigarano cv3 chondrites a record of condensate evolution in the solar nebula abstract amoeboid olivine aggregates aoas in the efremovka leoville and vigarano cv3 chondrites were studied using petrographic and microanalytical techniques to evaluate the origins of these inclusions and their relationships to other chondrite components our data support the idea that the inclusions formed by dust growth in the solar nebula as condensates and that they preserve a record of mineralogical and chemical evolution spinel rich nodules and ribbons in aoas often show group ii trace element patterns and formed either by fractional condensation or by condensation following partial vaporization the dominant olivine component in aoas can be explained as a condensate produced in the same way but with fractionation occurring at a lower temperature other spinel rich material in aoas appears to be a vaporization residue ca rich pyroxene anorthite patches in aoas show chemical signatures similar to those observed for spinel or melilite bearing regions but with lower refractory element and higher mg and si contents supporting the idea that the patches formed by reaction between cai like material and mg si rich gas different trace element patterns group ii and non group ii refractory are sometimes found for al rich regions ca pyroxene anorthite or melilite bearing in a given aoa suggesting that aoas agglomerated from materials that formed under different conditions and that they did not subsequently homogenize aoas appear to have originated in a system with near canonical solar composition and a low pressure gas 10 6 10 4 bar over a range of temperatures mainly 1200 1384 k under somewhat non equilibrium conditions relative to predicted equilibrium condensates most aoas show an apparent paucity of metal possibly reflecting differential density dependent sorting between olivine and metal grains during aggregate formation trace element data are consistent with a simple model involving melting and igneous fractionation of aoa like olivine to produce type i chondrule olivine but other data suggest that open system melting of chondrules occurred in a different gaseous environment than that which produced aoas

lime silicates in amoeboid olivine aggregates in carbonaceous chondrites indicator of nebular and asteroidal processes mno feo ratios in olivine from amoeboid olivine aggregates aoas reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites lime low iron mn enriched olivine was identified in aoas from y 81020 co3 05 kaba cv 3 1 and in y 86009 cv3 y 86751 cv3 nwa 1152 cr cv3 but was not identified in aoas from efremovka cv3 1 3 4 or allende cv 3 6 according to thermodynamic models of nebular condensation lime olivine is stable at lower temperatures than mn poor olivine and at low oxygen fugacities dust enrichment 95 in our study whereas efremovka aoas are slightly fe rich fo 92 aoa olivines from kaba are mostly forsteritic but rare fe rich olivine precipitated from an aqueous fluid a combination of precipitation of fe rich olivine and diffusion of fe into primary olivine grains resulted in iron rich compositions fo97 59 in allende aoas variations from fine grained nonporous matrix toward higher porosity and coarser lath like matrix olivine can be divided into six stages represented by 1 y 81020 efremovka nwa 1152 2 y 86751 lithology b 3 y 86009 4 kaba 5 y 86751 lithology a 6 allende these stages are inferred to represent general degree of metamorphism although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain

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

oxygen silicon and mn cr isotopes of fayalite in the kaba oxidized cv3 chondrite constraints for its formation history the iron rich olivine end member fayalite occurs in the matrix chondrules ca al rich inclusions cais silicate aggregates and dark inclusions in the kaba and mokoia oxidized cv3 chondrites in most occurrences fayalite is associated with magnetite and troilite to help constrain the origin of the fayalite fa98 100 we measured oxygen and silicon isotopic compositions and mn cr systematics in fayalite from two petrographic settings of the kaba meteorite one setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine grained magnetite and sulfide while the other setting consists of small fayalite magnetite sulfide assemblages within or at the surface of type i barred or porphyritic olivine chondrules oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line and is distinct from that in chondrule forsterites which are enriched in 16o 17o 4 oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites silicon isotopes suggest that forsterite is 1 amu heavier than adjacent fayalite within kaba chondrules however we were unable to confirm large silicon isotopic differences among fayalites reported previously the mn cr data for big kaba fayalites give an initial 53mn 55mn ratio of 2 07 0 17 10 6 consistent with literature results on mokoia chondrule fayalites the combined data suggest that fayalites in both petrographic settings formed at about the same time 9 7 ma after the formation of cais our data indicate that those fayalite magnetite troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body the formation site and mechanism for the big fayalite laths is less clear but the petrographic setting indicates that they did not form in situ none of the models that have been suggested for formation of these fayalites is entirely satisfactory