the kaidun chondrite breccia petrology oxygen isotopes and trace element abundances abstract oxygen isotope and trace element data for 13 samples of the kaidun chondritic breccia reaffirm the complex polymict nature of this unique meteorite bulk kaidun samples most closely resemble cr chondrites but the matrix is ci like two separated clasts are cr like but have some properties that resemble cm two clasts are enstatite chondrites one el and one eh one clast is an aubrite like metal rich impact melt and one clast is a unique layered olivine bearing pyroxenite with the isotopic composition of an aubrite yet although each clast resembles a known meteorite group all deviate in some respect from the norms for those groups collectively kaidun has sampled materials not yet represented in the world meteorite collections and which greatly extend the definitions of known meteorite groups phyllosilicates in kaidun span a very wide range in composition and vary from clast to clast suggesting that the aqueous alteration experienced by the clasts predated assembly of the kaidun parent body

labile trace elements in carbonaceous chondrites a survey we report radiochemical neutron activation analysis data for co au ga rb sb ag se cs te zn cd bi tl and in ordered by increasing putative volatility in primary nebular processes in 42 c2 c6 chondrites all but three from antarctica from these and literature data for 19 additional chondrites cl normalized concentrations of the nine most volatile elements ag in are quite constant in most meteorites trace element trends in 39 antarctic and 22 non antarctic carbonaceous chondrites are similar no evidence exists for substantial alteration by weathering of samples in antarctica nor do the data reflect modification by open system parent body metamorphism at 500 c volatile element concentrations and siderophile ratios au co and ga co define continua which correlate at statistically significant levels carbonaceous chondrites sample not a few compositionally distinct parents but rather a compositional continuum in which parent materials forming under more oxidizing conditions incorporated lesser complements of volatiles essentially unfractionated from cosmic composition this may well reflect the range of formation conditions temperature duration and water rock ratios represented by oxygen isotope variations during preterrestrial aqueous alteration of parent materials

indicators of aqueous alteration in cm carbonaceous chondrites microtextures of a layered mineral containing fe s o and ni a petrographic and transmission electron microscopy study of the mighei murchison and murray cm carbonaceous chondrites shows that much of the cm matrix material was probably produced by aqueous alteration of olivine pyroxene sulfide and metal the amount of cm matrix appears to be proportional to the degree of alteration as suggested by mcsween 1979 and microtextures of pcp poorly characterized phase provide evidence of the progressive alteration pcp is divided into two major types one occurs in chondrules and aggregates and consists largely of an fe ni s o phase type i and the other occurs in matrix and consists of the fe ni s o phase and cronstedtite in various proportions type ii microtextures of pcp suggest that it resulted from a three stage alteration process 1 1 type i was produced by alteration of kamacite in chondrules and aggregates presumably early in the parent body regolith 2 2 as the alteration advanced olivine and pyroxene were converted to serpentine type i pcp separated from chondrules and aggregates into the matrix during regolith gardening simultaneously the fe ni s o phase reacted with si released by alteration of olivine and pyroxene producing well formed platy cronstedtite and coherent intergrowths of the fe ni s o phase and cronstedtite the fe ni s o phase also recrystallized into platy and pod like crystals fe s ni cr and p were leached out of type i pcp and were deposited as small grains of fe ni sulfides magnetite chromite and a mineral unidentified containing fe ni cr and p as a result pcp came to consist primarily of the fe ni s o phase and cronstedtite i e type ii pcp 3 3 during continued alteration the well formed crystals of the fe ni s o phase cronstedtite and their intergrowths in type ii pcp were replaced by poorly formed fibers in comparison to other cm chondrites mighei murchison and murray are relatively unaltered their matrices retain abundant amounts of the fe ni s o phase and cronstedtite commonly as distinctive pcp grains which account for a large proportion of the fe in these meteorite matrices in more altered cm chondrites much of the fe ni s o phase was probably consumed to produce cronstedtite magnetite and sulfides with further alteration cronstedtite itself reacted with the serpentine to form ferroan serpentine thus the cm matrix was increasingly enriched in mg with alteration and pcp was increasingly degraded and intimately mixed with the magnesian phyllosilicates

hydrogen isotopic composition of ci and cm like clasts from meteorite breccias sampling unknown sources of carbonaceous chondrite materials abstract volatile rich ci and cm like clasts occur in different brecciated achondrite and chondrite groups the ci like clasts in heds polymict ureilites as well as ordinary cr and cb chondrites have a similar mineralogy indicating a similar alteration history however when viewed in detail their mineral chemistry shows some minor differences between the clasts from different meteorite groups for cm like clasts found in hed meteorites the clasts are based on their mineralogy clearly fragments of cm chondrites to be able to decipher whether ci or cm like clasts from different meteorite groups are related to certain meteorite classes known to contain volatiles we obtained d h ratios of several clasts from the meteorite groups mentioned above and compared them with those of ci and cm chondrites as well as to unique carbonaceous chondrites such as bells essebi and tagish lake considering the d values cm like clasts in heds span a similar range compared to bulk values of cm chondrites further indicating that cm like clasts are fragments of cm chondrites for ci like clasts a clear distinction can be made while ci like clasts in heds and ordinary chondrites show a very similar range in their d signatures compared to common ci chondrites meaning that these clasts are likely related to ci chondrites the ci like clasts in polymict ureilites are enriched in d up to 3000 a similarly high enrichment is found for the ci like clasts in cr chondrites thus although the ci like clasts in ureilites and cr chondrites likely experienced similar alteration histories as the ci like clasts found in the other meteorite types these clasts probably formed in a different region than the ci chondrites and thus are more accurately referred to as c1 clasts overall the existence and isotopic signatures of the c1 clasts in several meteorite groups proves the existence of additional primitive volatile rich material in the early solar system besides the matter we study as the ci cm and cr chondrites this material was distributed throughout the solar system very early and might have played an important role for the volatile inventory of the terrestrial planets

aragonite breunnerite calcite and dolomite in the cm carbonaceous chondrites high fidelity recorders of progressive parent body aqueous alteration abstract carbonate minerals in cm carbonaceous chondrite meteorites along with the silicates and sulphides with which they are intergrown provide a detailed record of the nature and evolution of parent body porosity and permeability and the chemical composition temperature and longevity of aqueous solutions fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered cm2 5 to completely hydrated cm2 0 all of them contain calcite whereas aragonite occurs only in the cm2 5 cm2 2 meteorites and dolomite in the cm2 2 cm2 0 all of the aragonite crystals and most of the calcite and dolomite grains formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of h2o rich ice aragonite was the first carbonate to precipitate in the cm2 5 to cm2 2 meteorites and grew from magnesium rich solutions in the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix the strong correlation between the petrologic subtype of a meteorite the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces is because aragonite was dissolved in the more altered meteorites on account of their higher permeability and or greater longevity of the aqueous solutions dolomite and breunnerite formed instead of aragonite in some of the cm2 1 and cm2 2 meteorites owing to higher parent body temperatures the pore spaces that remained after precipitation of aragonite dolomite and breunnerite cements were occluded by calcite following completion of cementation the carbonates were partially replaced by phyllosilicates and sulphides calcite in the cm2 5 cm2 2 meteorites was replaced by fe rich serpentine and tochilinite followed by mg rich serpentine in the cm2 1 and cm2 0 meteorites dolomite breunnerite and calcite were replaced by fe rich serpentine and fe ni sulphide again followed by mg rich serpentine the difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered cms were derived this transition from fe rich to mg rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of fe rich primary minerals followed by the more resistant mg rich anhydrous silicates almost all of the cms examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite dedolomitization the ca and co2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates and ions may have been transported over metre plus distances through high permeability conduits that were created by impact fracturing

carbonates in cm2 chondrites constraints on alteration conditions from oxygen isotopic compositions and petrographic observations high precision measurements of the oxygen isotopic compositions of carbonates calcite and dolomite from five cm2 chondrites are presented and put into context of the previously determined mineralogic alteration index mai which places these meteorites into an alteration sequence the carbonate oxygen isotopic compositions range from 20 0 to 35 7 for 18o 8 0 to 17 7 for 17o and 0 7 to 2 7 for 17o carbonate 17o values are inversely correlated with mai and track the evolution of fluid composition from higher to lower 17o values with increasing alteration on the cm parent body similar 17o values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir however reversed calcite dolomite fractionations 18odol 18occ indicate that the fluid was subject to processes such as freeze thaw or evaporation that fractionated isotopes in a mass dependent way consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves the data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed system two reservoir models but that a slightly larger matrix water fractionation factor may apply in the context of this model more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid rock interaction the data can also be reconciled with fluid flow models in a restricted region of the parent body which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies in this case the model results for a 9 km radius body and our data place the location of the cm chondrite formation in a 100 m thick zone 1 km from the surface the size of this zone could be increased if the model parameters were adjusted

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

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

aqueous alteration in carbonaceous chondrites mass balance constraints on matrix mineralogy abstract bulk chemical compositions of matrix material in antarctic cm chondrites and other non antarctic cm and ci chondrites have been determined using microprobe defocused beam techniques these are used along with the results of previously published mineralogical studies to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices results of these calculations indicate differing amounts of pcp a mixture of approximately 25 tochilinite and 75 cronstedtite and serpentines mg rich and fe rich varieties in varying proportions or intermediate compositional varieties additional sulfide phases are also probably necessary to account for excess ni and s fe si ratios for matrices of individual meteorites range from 1 21 to 2 77 corresponding to pcp pcp serf ratios of 0 16 to 0 58 progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite then cronstedtite and mg rich serpentine and finally fe rich serpentine and sulfides cm matrix clearly did not behave as an isolated system during alteration ci chondrite matrices appear to contain little if any pcp this may be a natural consequence of the absence of chondrule associated metal from which pcp forms in the unaltered precursor material these data provide a more quantitative picture of low temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from tem studies alone

hidden secrets of deformation impact induced compaction within a cv chondrite the cv3 allende is one of the most extensively studied meteorites in worldwide collections it is currently classified as s1 essentially unshocked using the classification scheme of stoffler et al 1991 however recent modelling suggests the low porosity observed in allende indicates the body should have undergone compaction related deformation in this study we detail previously undetected evidence of impact through use of electron backscatter diffraction mapping to identify deformation microstructures in chondrules aoas and matrix grains our results demonstrate that forsterite rich chondrules commonly preserve crystal plastic microstructures particularly at their margins that low angle boundaries in deformed matrix grains of olivine have a preferred orientation and that disparities in deformation occur between chondrules surrounding and non adjacent matrix grains we find heterogeneous compaction effects present throughout the matrix consistent with a highly porous initial material given the spatial distribution of these crystal plastic deformation microstructures we suggest that this is evidence that allende has undergone impact induced compaction from an initially heterogeneous and porous parent body we suggest that current shock classifications stoffler et al 1991 relying upon data from chondrule interiors do not constrain the complete shock history of a sample

episodic carbonate precipitation in the cm chondrite alh 84049 an ion microprobe analysis of o and c isotopes we have determined the o and c isotope compositions of dolomite grains and the c isotope compositions of calcite grains in the highly altered cm1 chondrite alh 84049 using secondary ion mass spectrometry sims chemically zoned dolomite constitutes 0 8 volume percent vol of the sample and calcite 0 9 vol thirteen separate dolomite grains have 13c values that range from 37 to 60 2 18 o values from 25 to 32 3 and 17 o values from 10 to 16 3 vsmow intragrain 13 c values in dolomite vary up to 10 the 13 c values of three calcite grains are distinct from those of dolomite and range from 10 to 13 2 pdb calcite and dolomite appear to record different precipitation episodes carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for cm chondrites our results imply that bulk carbonate c and o isotope analyses may oversimplify the history of carbonate precipitation multiple generations of carbonates with variable isotope compositions exist in alh 84049 and perhaps in many cm chondrites this work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for cm chondrite carbonates less altered cm meteorites with more homogeneous c isotope compositions however may be suitable for bulk carbonate analyses but detailed carbonate petrologic and isotopic characterization of individual samples is advised

compositions and taxonomy of 15 unusual carbonaceous chondrites abstract we used instrumental neutron activation analysis and petrography to determine bulk and phase compositions and textural characteristics of 15 carbonaceous chondrites of uncertain classification acfer 094 type 3 0 ungrouped cm related belgica 7904 mildly metamorphosed anomalous cm like chondrite possibly a member of a new grouplet that includes wisconsin range wis 91600 dhofar 225 and yamato 86720 dar al gani dag 055 and its paired specimen dag 056 anomalous reduced cv3 like dag 978 type 3 ungrouped dominion range 03238 anomalous magnetite rich co3 1 elephant moraine 90043 anomalous magnetite bearing co3 graves nunataks 98025 type 2 or type 3 ungrouped grosvenor mountains gro 95566 anomalous cm2 with a low degree of aqueous alteration hammadah al hamra hah 073 type 4 ungrouped possibly related to the coolidge loongana c l 001 grouplet lewis cliff lew 85311 anomalous cm2 with a low degree of aqueous alteration northwest africa 1152 anomalous cv3 pecora escarpment pca 91008 anomalous metamorphosed cm queen alexandra range 99038 type 2 ungrouped sahara 00182 type 3 ungrouped possibly related to hah 073 and or to c l 001 and wis 91600 mildly metamorphosed anomalous cm like chondrite possibly a member of a new grouplet that includes belgica 7904 dhofar 225 and y 86720 many of these meteorites show fractionated abundance patterns especially among the volatile elements impact volatilization and dehydration as well as elemental transport caused by terrestrial weathering are probably responsible for most of these compositional anomalies the metamorphosed cm chondrites comprise two distinct clusters on the basis of their 17o values approximately 4 for pca 91008 gro 95566 dag 978 and lew 85311 and approximately 0 for belgica 7904 and wis 91600 these six meteorites must have been derived from different asteroidal regions

r chondrite bulk chemical compositions and diverse oxides implications for parent body processes abstract r chondrites are among the most oxidized chondrite groups they also have the highest 17o values known in whole rock meteorites we analyzed r chondrites six antarctic four hot desert by instrumental neutron activation analysis data for one of the former and three of the latter show large weathering effects but the remainder show only moderate scatter and permit us to determine trends and mean compositions for the group bulk r chondrite compositions are similar to those in h and l chondrites but the concentrations of several volatiles especially se and zn are higher the more volatile the element the higher the enrichment in r chondrites relative to h and l petrologic types in r chondrites extend as low as 3 6 we determined olivine compositional distributions and studied opaque oxides in 15 r chondrite thin sections including a newly discovered r4 clast in bencubbin adding to the diversity of chondritic clasts in this polymict breccia and an r clast in cm2 murchison opaque oxides in r chondrites include nearly pure magnetite al rich chromite magnetite chromite solid solution nearly pure chromite and ilmenite this diverse set of opaque phases reflects differing aqueous alteration conditions the least equilibrated r chondrites contain nearly pure magnetite but the spinels in metamorphosed r chondrites contain additional components e g cr2o3 and al2o3 and some minor cations the nio content in olivine correlates with the magnetite component in magnetite chromite solid solution in equilibrated r chondrites and is a function of the degree of oxidation the absence of metallic fe in a 881988 and lap 031156 indicates a high degree of oxidation the relatively low feo fa35 olivine in these rocks in part reflects the conversion of fe2 to fe3 and its partitioning into magnetite oxidation trends in r chondrites are affected by both aqueous alteration and thermal metamorphism the differing degrees of oxidation in this group reflect differences in local environments on the parent asteroid

modal mineralogy of cm2 chondrites by x ray diffraction psd xrd part 1 total phyllosilicate abundance and the degree of aqueous alteration abstract cm carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids using position sensitive detector x ray diffraction psd xrd and a pattern stripping technique we quantify the modal mineralogy of cm2 chondrites mighei murray murchison nogoya and cold bokkeveld there is a narrow range in the combined modal volume vol of the most abundant phases mg serpentine 25 33 and fe cronstedtite 43 50 cold bokkeveld is anomalous in containing more mg serpentine 49 59 than fe cronstedtite 19 27 even including cold bokkeveld the range in modal total phyllosilicate is 73 79 average 75 total phyllosilicate abundance provides a non ambiguous measure of the degree of aqueous alteration and indicates that these meteorites have all experienced essentially the same degree of aqueous alteration this reflects pervasive hydration of matrix across cm2 samples apparent differences in the alteration of chondrules observed in petrographic studies represent various stages in the progression towards complete hydration of all components but are not manifest in significant differences in modal mineralogy for all samples there is a limited range in olivine 6 9 and pyroxene 5 abundances modal abundances of the remaining identified phases also show a limited range calcite 0 1 3 gypsum 0 1 6 magnetite 1 1 2 4 pentlandite 0 2 1 and pyrrhotite 1 3 8 as expected we observe a strong negative correlation in the modal abundance of anhydrous fe mg silicates olivine pyroxene and total phyllosilicate mg serpentine fe cronstedtite consistent with the idea that phyllosilicate is forming by aqueous alteration of the anhydrous components the negative correlation in the modal abundance between mg serpentine and fe cronstedtite indicates a mineralogic transformation of fe cronstedtite to mg serpentine by fluid driven recrystallisation or b that these meteorites had different initial abundances of olivine and pyroxene the observed positive correlation in the relative proportion of mg serpentine with increasing total phyllosilicate abundance reflects the evolution of increasingly mg rich phyllosilicate during aqueous alteration fe cronstedtite is the dominant phyllosilicate while cm chondrule olivines are forsteritic and will form mg serpentine during aqueous alteration this implies that matrix olivine was more fe rich than chondrule olivine prior to aqueous alteration

clasts in the cm2 carbonaceous chondrite lonewolf nunataks 94101 evidence for aqueous alteration prior to complex mixing clasts in the cm2 carbonaceous chondrite lonewolf nunataks lon 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration and the timing of alteration relative to incorporation of clasts into the host the provenance of the clasts and the mechanism by which they were incorporated and mixed with their host material are also considered results show that at least five distinct types of clasts occur in lon 94101 of which four have been aqueously altered to various degrees and one is largely anhydrous the fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host further the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources two of the clasts one containing approximately 46 vol carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body ies possibly by flow of aqueous solutions

collisional facilitation of aqueous alteration of cm and cv carbonaceous chondrites abstract cm chondrites exhibit a strong correlation between the degree of alteration and the extent of particle alignment i e the strength of the petrofabric it seems likely that the s1 shock stage of essentially every cm and the high matrix abundance 70 vol of these samples ensured that the shock waves that produced cm petrofabrics by collapsing matrix pores and squeezing chondrules into pore spaces were significantly attenuated and were too weak to damage olivine crystal lattices random collisions on the cm body produced petrofabrics and created fractures in the target rocks subsequent impact mobilization of water caused hydrated phases to form preferentially in the more fractured regions those with the strongest petrofabrics the less deformed less fractured cm regions experienced lower degrees of aqueous alteration many cv3 chondrites also have petrofabrics roughly half are from the oxidized bali like subgroup cv3 oxb roughly half are from the reduced subgroup cv3 r and none is from the oxidized allende like subgroup cv3 oxa which is less altered than cv3 oxb nearly all cvs with petrofabrics are s3 s4 and nearly all cvs that lack petrofabrics are s1 oxidized cvs have much higher porosities typically 20 28 than reduced cvs 0 6 8 facilitating more extensive aqueous alteration the cv3 r chondrites formed from low porosity material that inhibited oxidation during alteration the oxidized cv subgroups formed from higher porosity materials the cv3 oxb samples were shocked became extensively fractured and developed petrofabrics the cv3 oxa samples were not shocked and never developed petrofabrics when water was mobilized both sets of porous cv chondrites became oxidized the more fractured cv3 oxb subgroup was more severely altered

generation of abnormal trace element abundances in antarctic eucrites by weathering processes abstract based on rees antarctic eucrites can be divided into two groups those showing normal trace element characteristics e g similar to juvinas and those showing abnormal trace element abundances many antarctic eucrite polymict eucrite and basaltic clast samples show the abnormal trace element abundances with ree patterns exhibiting positive ce anomalies sometimes negative ce anomalies positive eu anomalies and low abundances of the remainder of the rees with the lrees generally being at lower relative abundances than the hrees most samples of crystalline clasts from the polymict eucrites lew85300 lew85302 and lew85303 show the abnormal patterns while the glassy matrixes of these meteorites show normal patterns exterior samples generally show more abnormal patterns larger anomalies greater depletions than interior samples from the same meteorites comparison of all basaltic eucrite literature data combined with our data shows that positive and negative ce anomalies and positive eu anomalies are found in about 61 of antarctic eucrite analyses and are virtually unknown in non antarctic eucrite analyses further positive ce anomalies and positive eu anomalies are commonly associated with antarctic eucrites having low ree concentrations consideration of mineral melt partition coefficients shows that it is unlikely that ce anomalies are magmatic features from the hed parent body cerium anomalies on earth are generally restricted to the weathering zone where the relative ease of oxidizing ce to the 4 state allows for fractionation of ce from the 3 rees we believe the unusual ree patterns of abnormal antarctic eucrites arise from weathering effects generated in or on the antarctic ice our suggested scenario involves formation of melt water and its equilibration with the atmosphere which promotes dissolution of ree rich phosphates and oxidation of ce tetravalent ce can then be fractionated from the trivalent ree in solution the details of the weathering process are unclear and will require detailed chemical and sem investigations of eucrites for their elucidation we predict that rapidly chilled eucrites with glassy rather than crystalline mesostases will be more likely to survive the antarctic environment without alteration of their ree patterns occasional s se and k enrichments are likely due to weathering in the antarctic environment as well but these enrichments are not well correlated with ce anomalies

mineralogy of tagish lake an ungrouped type 2 carbonaceous chondrite in this paper we describe the recovery handling and preliminary mineralogical investigation of the tagish lake meteorite tagish lake is a type 2 carbonaceous chondrite which bears similarities to ci1 and cm chondrite groups but is distinct from both abundant phyllosilicates as well as chondrules however sparse and common olivine grains in the matrix preclude any other classification the bulk density of tagish lake 1 67 g cc which is far lower than ci or cm chondrites 2 2 2 3 and 2 6 2 9 g cc respectively or any other meteorite for that matter we have identified two lithologies a dominant carbonate poor lithology and a less abundant carbonate rich lithology the meteorite is a breccia at all scales we have noted similarities between tagish lake and some clasts within the enigmatic meteorite kaidun possibly there are genetic relationships here worth exploring in the paper we describe a clast of cm1 material within tagish lake which is very similar to a major lithology in kaidun

sulfur isotopic composition of fe ni sulfide grains in ci and cm carbonaceous chondrites in situ secondary ion mass spectrometry analyses of 32 s 33 s and 34 s in iron nickel sulfide grains in two ci1 chondrites and six cm chondrites were performed the results show a wider range of both enrichment and depletion in 34 s relative to troilite from the canyon diablo meteorite cdt than has been observed in previous studies all data points lie within error of a single mass dependent fractionation line sulfides from ci1 chondrites show 34 scdt from 0 7 to 6 8 while sulfide grains in the cm1 chondrite are generally depleted in heavy sulfur relative to cdt 34 s from 2 9 to 1 8 cm2 chondrites contain sulfide grains that show enrichment and depletion in 34 s 34 scdt from 7 0 to 6 8 sulfates forming from sulfide grains during aqueous alteration on the chondrite parent body are suggested to concentrate light sulfur leaving the remaining sulfide grains enriched in the heavy isotopes of sulfur the average degree of enrichment in 34 s in cm chondrite sulfides is broadly consistent with previously suggested alteration sequences

abundances of presolar silicon carbide grains in primitive meteorites determined by nanosims abstract it has been suggested that the matrices of all chondrites are dominated by a common material with ivuna like ci abundances of volatiles presolar grains and insoluble organic matter iom e g alexander 2005 however matrix normalized abundances of presolar silicon carbide sic grains estimated from their noble gas components show significant variations in even the most primitive chondrites huss and lewis 1995 huss et al 2003 in contradiction to there being a common chondrite matrix material here we report presolar sic abundances determined by nanosims raster ion imaging of iom extracted from primitive members of different meteorite groups we show that presolar sic abundance determinations are comparable between nanosims instruments located at three different institutes between residues prepared by different demineralization techniques and between microtomed and non microtomed samples our derived sic abundances in cr chondrites are comparable to those found in the ci chondrites 30 ppm and are much higher than previously determined by noble gas analyses the revised higher cr sic abundances are consistent with the crs being amongst the most primitive chondrites in terms of the isotopic compositions and disordered nature of their organic matter similar abundances between cr1 cr2 and cr3 chondrites indicate aqueous alteration on the cr chondrite parent body has not progressively destroyed sic grains in them a low sic abundance for the reduced cv3 rbt 04133 can be explained by parent body thermal metamorphism at an estimated temperature of 440 c minor differences between primitive members of other meteorite classes which did not experience such high temperatures may be explained by prolonged oxidation at lower temperatures under which sic grains formed outer layers of sio2 that were not thermodynamically stable leading to progressive degassing destruction of sic

the fall recovery orbit and composition of the tagish lake meteorite a new type of carbonaceous chondrite the preatmospheric mass of the tagish lake meteoroid was about 200 000 kilograms its calculated orbit indicates affinity to the apollo asteroids with a semimajor axis in the middle of the asteroid belt consistent with a linkage to low albedo c d and p type asteroids the mineralogy oxygen isotope and bulk chemical composition of recovered samples of the tagish lake meteorite are intermediate between cm and ci meteorites these data suggest that the tagish lake meteorite may be one of the most primitive solar system materials yet studied

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

hydrogen isotopic composition of water from fossil micrometeorites in howardites we have measured the hydrogen isotopic composition d h ratios of the water from 13 carbonaceous chondritic microclasts ccms size 1 mm trapped in two howardites kapoeta and yamato 793497 early in the evolution of solar system the division into tochilinite rich magnetite rich olivine poor magnetite rich olivine rich ccm types is corroborated by the hydrogen isotopic compositions both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite rich ccms represent cm2 chondritic matter in contrast there is no good match between the isotopic and mineralogical properties of the magnetite rich ccms and the known groups of carbonaceous chondrites suggesting that magnetite rich ccms represent a new kind of chondritic matter not yet sampled in meteorite collections this demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete the study of micro clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids the average hydrogen isotopic composition of water belonging to ccms d h 152 0 4 8 x 10 6 1 sigma m is similar to that of antarctic micrometeorites amms d h 161 2 3 8 x 10 6 1 sigma m the similarity in terms of mineralogy and hydrogen isotopic composition between ccms and amms demonstrates that the composition of the micrometeorites has not been modified over the whole history of the solar system it indicates that the composition of the micrometeorite flux onto earth has been and is dominated by a mixture of cm2 like magnetite rich olivine poor magnetite rich olivine rich carbonaceous chondritic matter exemplified by ccms found in howardites because ccms have not suffered atmospheric entry they provide an abundant source of pristine micrometeorites the average d h ratio of the whole population of ccms is identical within errors to that of the earth 149 3 x 10 6 the match between the ccms d h ratio and that of the earth is especially remarkable because 1 three different populations of ccms are needed to make the d h ratio of the earth 2 there is no single carbonaceous chondrite group for which a similar match exists this observation suggests that ccms population might be representative of the late veneer agent s that delivered water to the earth

aqueous alteration in cr chondrites meteorite parent body processes as analogue for long term corrosion processes relevant for nuclear waste disposal aqueous alteration of carbonaceous chondrites is one of the fundamental processes on accreting planetesimals that changes pristine materials from the formation of the solar system the study of mineralogical petrological and chemical changes resulting from this alteration provides insight into the physical and chemical setting of forming planetesimals cr chondrites provide samples for all stages of aqueous alteration from type 3 to 1 entirely hydrated and are thus suited to study the alteration of pristine materials in a coherent sequence vitrification is a common way to store and stabilize fission products and minor actinides resulting from the reprocessing of nuclear spent fuel in a nuclear boro silica glass in steel containers the waste material has to be stored safely for a period of at least 105 106 years in a clay rich geological repository laboratory experiments being too short to follow the long term evolution of these materials we analyzed the mineralogical petrological and chemical changes in a series of cr chondrites renazzo cr2 al rais cr2 and gro 95577 cr1 to serve as analogues rims of secondary materials around metal grains in contact to the fine grained matrix serve as analogue to the interface between steel containment and the surrounding clay rich geological layer while chondrule glassy mesostasis is used as a proxy of the nuclear glass with increasing degree of aqueous alteration in the sequence renazzo al rais gro 95577 the size of the rims increase fe rich alteration rims are 10 m in thickness around metal grains in the fine grained matrix in renazzo in al rais multi layered structures of interchanging fe s and p ca rich layers appear with a thickness of up to 30 m in the highly altered gro 95577 extensive inner and external rims of secondary phases reach up to 200 m into the surrounding matrix in chondrules metal in contact with the altered mesostasis shows similar trends but with thinner alteration rims altered mesostasis is similar to the surrounding fine grained matrix consisting of serpentine and smectite however the chemistry has not completely equilibrated as indicated by e g a remaining high al contents 2 2 wt fine grained matrix and altered mesostasis show a trend towards decreasing fe with progressing alteration matrix is also the main source of s in the alteration system most of the dissolved species stays close to the source the main source for fe ni p and also ti cr and mn is the altered metal these findings confirm the model for long term corrosion of nuclear disposal sites alteration rims of iron oxides form around the steel containments whereas dissolved species fe al and si diffuse into the surroundings

importance of infragravity waves for the generation of washover deposits abstract washover deposits are among the most remarkable sedimentary signatures of dune breaching and coastal inundation overwash of the barrier leading to washover development can be related either to storms or tsunamis but the distinction between these two processes based on the depositional record is still a matter of debate hence studying recent well documented washover deposits combining sedimentological and hydrodynamic investigations is a key approach to identify the processes governing the washover deposition and to analyze their sediment record within the washover during the winter of 2013 2014 the coasts of the bay of biscay experienced an exceptional wave climate leading to dune breaching and washover deposition at many locations including the gatseau spit south of oleron island france field observations aerial photographs topographic measurements gpr investigations trenches and cores grain size analysis and magnetic fabric measurements were carried out and complemented with hydrodynamic modeling of tides storm surges short waves and infra gravity waves in order to compute maximum runup during the winter of 2013 2014 such an interdisciplinary approach led to propose that 1 large overwash events occurring during the winter of 2013 2014 were mainly driven by infra gravity ig waves combined with high tides 2 the washover elongated outer shape was strongly controlled by the antecedent morphology of the back barrier area 3 one overwash flow driven by one ig wave is recorded by one normally graded lamina composed of quartz sand at base and heavy minerals enriched sand at top 4 dominant settling of the particles from a suspension during sedimentation occurred in the end of each overwash flow in response to flow velocity decrease 5 laminae can be grouped in laminasets generally composed of thinning upward laminae related to overwash flow depth decrease during falling tide and to erosion of basal laminae by highest overwash flows occurring more likely around the high tide 6 the number of laminasets in a proximal position is in the same order of magnitude as the number of modeled highest runup events offering an almost complete record of overwash events that occurred during the winter 2013 2014

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

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

carbonate compositions in cm and ci chondrites and implications for aqueous alteration abstract carbonate minerals in fourteen cm chondrites and two ci chondrites have been analyzed by electron microprobe to provide a better understanding of the aqueous processes that affected carbonaceous chondrite parent bodies calcites in cm chondrites and dolomites and magnesites in ci chondrites display the compositions expected of stable phases formed at low temperatures dolomites in cm chondrites identified here for the first time in five members of the group have small amounts of excess ca which may reflect metastable growth the distribution of fe between dolomite and coexisting serpentine differs in the two chondrite groups if the distributions reflect an approach to chemical equilibrium then the difference implies higher alteration temperatures for the ci group than the cm group in agreement with the results of previously published oxygen isotope thermometry and mineral solubility modeling of the alteration process dolomite fe contents are relatively uniform in the two chondrite groups dolomite mn contents by contrast vary widely the variations may reflect transport controlled coprecipitation of mn resulting from a heterogeneous distribution of the element in the anhydrous precursor material if this interpretation is correct then the altering fluids were essentially immobile while hydration reactions proceeded on the meteorite parent bodies the near closed system character of the alteration process long known from bulk chemical analyses of the meteorites is a direct consequence of the limited mobility of dissolved species

progressive aqueous alteration of cm carbonaceous chondrites abstract cm chondrites are aqueously altered rocks that contain 9 wt h 2 o i e indigenous water bound in phyllosilicates also present are clumps of serpentine tochilinite intergrowths previously called poorly characterized phases or pcp pentlandite and ni bearing pyrrhotite we studied 11 cm chondrites that span the known range from least altered to most altered we used various petrologic properties many previously identified that provide information regarding the degree of aqueous alteration there are no known unaltered or slightly altered cm chondrites e g rocks containing numerous chondrules with primary igneous glass some cm properties result from processes associated with early and intermediate stages of the alteration sequence i e hydration of matrix alteration of chondrule glass and production of large pcp clumps other petrologic properties reflect processes active throughout the alteration sequence these include oxidation of metallic fe ni alteration of chondrule phenocrysts changes in pcp composition reflecting an increase in the phyllosilicate sulfide ratio and changes in carbonate mineralogy reflecting the development of dolomite and complex carbonates at the expense of ca carbonate on the basis of these parameters we propose a numerical alteration sequence for cm chondrites because there are no known cm samples that display only incipient alteration the least altered sample was arbitrarily assigned to subtype 2 6 the most altered cm chondrites currently classified cm1 are assigned to subtype 2 0 these highly altered rocks have essentially no mafic silicates they contain chondrule pseudomorphs composed mainly of phyllosilicate however their bulk compositions are cm like and they are closer in texture to other c2 chondrites than to ci1 chondrites which lack chondrule pseudomorphs using several diagnostic criteria we assigned petrologic subtypes 0 1 to every cm chondrite in our study que 97990 cm2 6 murchison cm2 5 kivesvaara cm2 5 murray cm2 4 2 5 y 791198 cm2 4 que 99355 cm2 3 nogoya cm2 2 cold bokkeveld cm2 2 que 93005 cm2 1 lap 02277 cm2 0 met 01070 cm2 0 the proposed cm numerical alteration sequence improves upon the existing scheme of browning et al 1996 in that it does not require a complicated algorithm applied to electron microprobe data to determine the average matrix phyllosilicate composition the new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values new neutron activation analyses of que 97990 que 93005 met 01070 murchison and crescent together with literature data confirm the compositional uniformity of the cm group different degrees of alteration among cm chondrites do not lead to resolvable bulk compositional differences this suggests that the textural differences among individual cm chondrites reflect progressive alteration of similar hypothetical cm3 0 starting materials in different regions of the same parent body with minimal aqueous transport of materials over appreciable e g meters distances

p o rich sulfide phase in cm chondrites constraints on its origin on the cm parent body cm chondrites are a group of primitive meteorites that have recorded the alteration history of the early solar system we report the occurrence chemistry and oxygen isotopic compositions of p o rich sulfide phase in two cm chondrites grove mountains grv 021536 and murchison this p o rich sulfide is a polycrystalline aggregate of nanometer size grains it occurs as isolated particles or aggregates in both cm chondrites these grains in the matrix and in type i chondrules from murchison were partially altered into tochilinite however grains enclosed by ca carbonate are much less altered this p o rich sulfide in murchison is closely associated with magnetite feni phosphide brezinaite cr3s4 and eskolaite cr2o3 in addition to sulfur as the major component this sulfide contains 6 3 wt o 5 4 wt p and minor amounts of hydrogen analyses of oxygen isotopes by sims resulted in an average 18o value of 22 5 and an average 17o value of 0 2 9 2 2 limited variations in both chemical compositions and electron diffraction patterns imply that the p o rich sulfide may be a single phase rather than a polyphase mixture several features indicate that this p o rich sulfide phase formed at low temperature on the parent body most likely through the alteration of feni metal a close association with other low temperature alteration products b the presence of hydrogen c high 17o values and the presence in altered mesostasis of type i chondrules and absence in type ii chondrules the textural relations of the p o rich sulfide and other low temperature minerals reveal at least three episodic alteration events on the parent body of cm chondrites 1 formation of p o rich sulfide during sulfur rich aqueous alteration of p rich feni metal 2 formation of ca carbonate during local carbonation and 3 alteration of p o rich sulfide and formation of tochilinite during a period of late stage intensive aqueous alteration

the carbon and oxygen isotopic composition of meteoritic carbonates the 13c12c and 18o16o isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on co2 released by the action of h3po4 on whole rock samples carbonates from ci cm and cr carbonaceous chondrites exhibit a range in 18o of ca 15 20 5 to 35 1 relative to smow limited data from co2 water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17o isotopic compositions therefore they are truly enriched in 13c with 13c between 23 7 and 80 7 relative to pdb large internal variations in 13c and 18o were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present the abundance 13c and 18o of carbonate in cm2 chondrites may be related to the extent of aqueous alteration of the meteorites carbonates in ci and cr chondrites have a median 13c ca 50 to 60 whereas 13c of cm meteorites lie in the range 40 to 50 although exceptions exist in both sets of samples cv3 and co3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of co2 on acid treatment which might be from carbonate dissolution but which is not enriched in 13c exhibiting 13c values ca 0 10 the exception to this is bishunpur with 13c ca 23 5 the difference in 13c of the ci cm and cr vs cv co and ordinary chondrite carbonates may be a result of the progressive enrichment in 13c of percolating fluids brought about by increasing solubilization of exotic 13c enriched grains

the isotopic composition of volatiles in the unique bench crater carbonaceous chondrite impactor found in the apollo 12 regolith abstract projectiles striking the moon have modified its crust and delivered volatile elements to its interior and surface direct evidence of impactor origins is recorded by the rare occurrence of sub cm sized meteorite fragments identified in apollo samples and lunar meteorites the bench crater meteorite is a millimetre sized carbonaceous chondrite collected in regolith on the rim of bench impact crater at the apollo 12 landing site transmission electron microscopy has previously shown that bench crater contains abundant hydrated silicates establishing the survivability of hydrated material impacting the lunar surface to provide further information on the volatile inventory of the bench crater meteorite we report here the isotope compositions of hydrogen nitrogen carbon and oxygen this is the first direct isotopic analysis of meteoritic material delivered to the lunar surface and provides context for volatile and organic element signatures in lunar regolith samples and the survivability of volatile material delivered to planetary surfaces during impact bombardment the bench crater meteorite is characterised by d values ranging between 36 40 and 200 40 and bulk average 13 c of 13 30 and 15 n of 40 36 all uncertainties at the 2 confidence level the oxygen isotope compositions measured in situ in matrix silicates and magnetite in bench crater are consistent with those measured in matrix and magnetite in ci and cm chondrite falls altogether these new h c n and o isotope data coupled to mineralogical and geochemical observations suggest that bench crater may have been derived from an asteroidal parent body not represented in the terrestrial meteorite collection this is a crucial outcome in the current context of sample return missions to carbonaceous asteroids and more broadly for investigating the flux of material delivered to the earth moon system through time

carbonates in cm chondrites complex formational histories and comparison to carbonates in ci chondrites cm chondrites are primitive solar system materials that have undergone high degrees of aqueous alteration resulting in the formation of secondary minerals including carbonates two different carbonate minerals calcite aragonite and dolomite together constitute 1 4 2 8 vol of cm chondrites in contrast ci chondrites contain four different carbonate minerals calcite aragonite dolomite breunnerite and siderite ci chondrites have abundant dolomite a mineral that seems to be absent in the most aqueously altered cm chondrites in this study carbonates in seven cm chondrites y 791198 lapaz icefield 04796 cold bokkeveld nogoya queen alexandra range 93005 allan hills 83100 and meteorite hills 01070 were studied petrographically and by electron microprobe the results indicate that carbonate formation in cm chondrites differs from that in ci chondrites and is more complex than previously recognized our studies of cm chondrites indicate that 1 carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition 2 at some stage ca and mg activities in the environment were high enough to form metastable dolomite and 3 dolomites disappeared in the most aqueously altered cm chondrites

the nanoscale mineralogy of fe ni sulfides in pristine and metamorphosed cm and cm ci like chondrites tapping a petrogenetic record we have sampled sulfide grains from one pristine cm2 chondrite yamato y 791198 one thermally metamorphosed cm2 chondrite y 793321 and two anomalous metamorphosed cm ci like chondrites y 86720 and belgica b 7904 by the focused ion beam fib technique and studied them by analytical transmission electron microscopy tem our study aims at exploring the potential of sulfide assemblages and microstructures to decipher processes and conditions of chondrite petrogenesis complex exsolution textures of pyrrhotite crystallographic nc type with n 6 troilite and pentlandite occur in grains of y 791198 and y 793321 additionally polycrystalline 4c pyrrhotite pentlandite magnetite aggregates occur in y 791198 pointing to diverse conditions of gas solid interactions in the solar nebula coarser exsolution textures of y 793321 grains indicate higher long term average temperatures in the 100 c range compared to y 791198 and other cm chondrites sulfide mineralogy of y 86720 and b 7904 is dominated by aggregates of pure troilite and metal indicating metamorphic equilibration at sulfur fugacities fs2 of the iron troilite buffer absence of magnetite in equilibrium with sulfide and metal in y 86720 indicates higher peak temperatures compared with b 7904 in which coexistence of troilite metal and magnetite constrains metamorphic temperature to less than 570 c nc pyrrhotite occurs in both meteorites as nm wide rims on troilite grains and together with frequent anhydrite indicates a retrograde metamorphic stage at higher fs2 slightly above the fayalite magnetite quartz pyrrhotite buffer fine grained troilite olivine intergrowths in both meteorites suggest the pre metamorphic presence of tochilinite serpentine interlayer phases pointing to mineralogical cm affinity pseudomorphs after euhedral pyrrhotite crystals in y 86720 in turn suggest ci affinity as do previously published o isotopic data of both meteorites

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

absorption bands near three micrometers in diffuse reflectance spectra of carbonaceous chondrites comparison with asteroids abstract we measured infrared diffuse reflectance spectra of several carbonaceous chondrites in order to obtain additional information on the surface materials of their presumed parent bodies c type asteroids the presence and intensity of absorption bands near 3 m in the reflectance spectra are due to the presence and abundance of hydrates and or hydroxyl ions the absorption features of the 3 m hydration bands of carbonaceous chondrites were compared with those of asteroids 1 ceres and 2 pallas they are commonly classified into separate subtypes g and b type the spectral shapes of pallas and renazzo cr2 chondrite around the 3 m absorption band are an excellent match this result may suggest that the amount of hydrous minerals in the surface material of pallas is smaller than that in the cm2 or ci chondrites and the hydrous minerals on the surface of pallas may be similar to those found in renazzo the spectral features around the 3 m band of ceres are different from those of carbonaceous chondrites studied in this paper

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

low temperature aqueous alteration on the cr chondrite parent body implications from in situ oxygen isotope analyses the presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early solar system however the process of aqueous alteration is still poorly understood renazzo like carbonaceous cr chondrites are particularly well suited for the study of aqueous alteration samples range from being nearly anhydrous to fully altered essentially representing snapshots of the alteration process through time we studied oxygen isotopes in secondary minerals from six cr chondrites of varying hydration states to determine how aqueous fluid conditions including composition and temperature evolved on the parent body secondary minerals analyzed included calcite dolomite and magnetite the o isotope composition of calcites ranged from 18o 9 to 35 dolomites from 18o 23 to 27 and magnetites from 18o 18 to 5 calcite in less altered samples showed more evidence of fluid evolution compared to heavily altered samples likely reflecting lower water rock ratios most magnetite plotted on a single trend with the exception of grains from the extensively hydrated chondrite mil 090292 the mil 090292 magnetite diverges from this trend possibly indicating an anomalous origin for the meteorite if magnetite and calcite formed in equilibrium then the relative 18o fractionation between them can be used to extract the temperature of co precipitation isotopic fractionation in al rais carbonate magnetite assemblages revealed low precipitation temperatures 60 c assuming that the cr parent body experienced closed system alteration a similar exercise for parallel calcite and magnetite o isotope arrays yields global alteration temperatures of 55 to 88 c these secondary mineral arrays indicate that the o isotopic composition of the altering fluid evolved upon progressive alteration beginning near the al rais water composition of 17o 1 and 18o 10 and becoming increasingly 16o enriched toward a final fluid composition of 17o 1 2 and 18o 15

fabric analysis of allende matrix using ebsd fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution electron backscatter diffraction ebsd has allowed fabrics in these fine grained materials to be visualized in detail for the first time our data reveal that allende a cv3 chondrite possesses a uniform planar short axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction interference between this matrix fabric and the larger more rigid components such as dark inclusions dis and calcium aluminium rich inclusions cais has lead to the development of locally oriented and intensified matrix fabrics in addition dis possess fabrics that are conformable with the broader matrix fabric these results suggest that dis were in situ prior to the deformational shortening event responsible for these fabrics thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid kojima and tomeoka 1996 fruland et al 1978 moreover both dis and allende matrix are highly porous 25 corrigan et al 1997 mobilizing a highly porous di during impact induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable we favor a model that involves allende dis cais and matrix accreting together and experiencing the same deformation events

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

fluid flow and chemical alteration in carbonaceous chondrite parent bodies abstract carbonaceous chondrites are a class of primitive meteorites that likely represent early solar system materials thus providing critical insights about planetary evolution however evidence shows that they have undergone considerable processing elucidating the complex formation history of aqueously altered chondrite groups has been a principal objective of cosmochemistry fulfilling this objective requires understanding the nature of the water rock interactions involved many current alteration models are generally founded upon an assumption of uniform alteration in a closed system in these models different chondrite groups must originate on separate parent bodies contrary to this common view we show that chondrite data are consistent with non uniform alteration in a convecting system so that multiple chondrite groups can form within a single body

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

an oxygen isotope dichotomy in cm2 chondritic carbonates a sims approach abstract we present petrologic and secondary ion mass spectrometry sims oxygen isotope analyses of ca carbonate within a group of paired antarctic cm2 chondrites the carbonates can be grouped into two isotopically and morphologically distinct populations type 1 grains small matrix grains possess average 18 o of 33 7 2 3 1 and average 17 o of 0 81 0 90 1 type 2 grains calcite aggregates possess distinct oxygen isotopic compositions average 18 o of 19 4 1 5 1 and average 17 o of 1 98 0 9 1 these differences are interpreted to indicate that the two populations of calcite formed under different conditions at different times the carbonates have textural features that suggest an extraterrestrial origin the data presented here fall within error of a previously measured array for carbonates from cm falls benedix et al 2003 the presence of two generations of carbonate suggests carbonate formation in two discrete events on the parent body of these meteorites the oxygen isotopic data presented here deviate from prior bulk carbonate measurements undertaken for these meteorites most likely this deviation is because bulk carbonate analyses included vein carbonate which formed during terrestrial weathering

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

the cm carbonaceous chondrite regolith diepenveen a carbonaceous chondrite was recovered immediately after the fall near the village of diepenveen in the netherlands on october 27 1873 but came to light only in 2012 analysis of sodium and poly aromatic hydrocarbon content suggests little contamination from handling diepenveen is a regolith breccia with an overall petrology consistent with a cm classification unlike most other cm chondrites the bulk oxygen isotopes are extremely 16o rich apparently dominated by the signature of anhydrous minerals distributed on a steep slope pointing to the domain of intrinsic cm water a small subset plots closer to the normal cm regime on a parallel line 2 lower in 17o different lithologies in diepenveen experienced varying levels of aqueous alteration processing being less aqueously altered at places rather than more heated the presence of an agglutinate grain and the properties of methanol soluble organic compounds point to active impact processing of some of the clasts diepenveen belongs to a cm clan with 5 ma cre age longer than most other cm chondrites and has a relatively young k ar resetting age of 1 5 ga as a cm chondrite diepenveen may be representative of samples soon to be returned from the surface of asteroid 162173 ryugu by the hayabusa2 spacecraft

oxygen isotope constraints on the alteration temperatures of cm chondrites abstract we report a systematic oxygen isotopic survey of ca carbonates in nine different cm chondrites characterized by different degrees of alteration from the least altered known to date paris 2 7 2 8 to the most altered alh 88045 cm1 our data define a continuous trend that crosses the terrestrial fractionation line tfl with a general relationship that is indistinguishable within errors from the trend defined by both matrix phyllosilicates and bulk o isotopic compositions of cm chondrites this bulk matrix carbonate bmc trend does not correspond to a mass dependent fractionation i e slope 0 52 as it would be expected during fluid circulation along a temperature gradient it is instead a direct proxy of the degree of o isotopic equilibration between 17 18 o rich fluids and 16 o rich anhydrous minerals our o isotopic survey revealed that for a given cm no carbonate is in o isotopic equilibrium with its respective surrounding matrix this precludes direct calculation of the temperature of carbonate precipitation however the o isotopic compositions of alteration water in different cms inferred from isotopic mass balance calculation and direct measurements define another trend cmw for cm water parallel to bmc but with a different intercept the distance between the bmc and cmw trends is directly related to the temperature of cm alteration and corresponds to average carbonates and serpentine formation temperatures of 110 c and 75 c respectively however carbonate o isotopic variations around the bmc trend indicate that they formed at various temperatures ranging between 50 and 300 c with 50 of the carbonates studied here showing precipitation temperature higher than 100 c the average 17 o and the average carbonate precipitation temperature per chondrite are correlated revealing that all cms underwent similar maximum temperature peaks but that altered cms experienced protracted carbonate precipitation event s at lower temperatures than the least altered cms our data suggest that the 17 o value of ca carbonates could be a reliable proxy of the degree of alteration experienced by cm chondrites

oxygen isotopes in separated components of ci and cm meteorites oxygen isotopic compositions have been determined for whole rock and separated components from the ci chondrites alais ivuna and orgueil and from the atypical cm chondrites bells and essebi the composition of ci matrix is in agreement with previous measurements and is systematically different from that of cm matrix probably due to warmer and wetter conditions of aqueous alteration for ci matrix although bells matrix is like that of other cm chondrites the matrix of essebi falls in the ci field essebi also contains abundant magnetite like cis and unlike other cms the isotopic compositions of magnetite in alais ivuna orgueil and essebi define a trend with 17o of 1 3 to 1 8 distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite the magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more 16o depleted than the earth

replacement of olivine by serpentine in the queen alexandra range 93005 carbonaceous chondrite cm2 reactant product compositional relations and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous alteration abstract isovolumetric replacement of euhedral and anhedral olivine by serpentine produced both centripetal and meshwork textures in the cm2 chondrites alh 81002 and nogoya the compositions of these textural varieties of serpentine are uniform within narrow limits within each previously studied meteorite independent of the composition of olivine being replaced and different between the two meteorites in que 93005 cm2 coarse olivines of widely varying compositions fo are replaced in a texturally similar manner by compositionally uniform serpentine mg 0 73 0 05 fe 0 27 0 05 3 si 2 o 5 oh 4 the narrow compositional range of serpentine replacing coarse olivine indicates that the aqueous solution from which the serpentine formed was compositionally uniform on scales at least as large as the meteorite 2 5 cm in longest dimension isovolumetric textures and compositional observations constrain elemental redistribution from coarse olivine to serpentine and to surrounding phases during serpentinization regardless of olivine s composition isovolumetric replacement of coarse olivines by serpentine of the observed composition released more mg and si from olivine than was required to form the serpentine excess mg and si released by olivine destruction and not retained in serpentine were exported from the replaced volume olivines with different fa fo proportions contributed different amounts of fe and mg to the serpentine ferroan olivines released more fe than required to form the serpentines replacing them so some of the fe released from ferroan olivine was exported from the replaced volumes forsteritic olivines released less fe than required to form the serpentines replacing them so some fe was imported into the replaced volumes augmenting the small amount of fe released from forsteritic olivine in que 93005 fo 83 8 is the threshold composition between fe exporting and fe importing behavior in individual olivine serpentine pairs which released exactly the amount of fe required to form serpentine of the observed uniform composition compositions of serpentines isovolumetrically replacing olivines and threshold olivine compositions in que 93005 differ from the corresponding values in nogoya solvent and solute species diffused through the serpentine between the olivine serpentine interface and the aqueous solution outside the isovolumetrically replaced volume in que 93005 some of the fe released from ferroan olivine in excess of the amount required to form serpentine reacted with s sourced from outside the pseudomorphs to form fe sulfide decorating the margins of the pseudomorphs of serpentine after fayalitic olivine such fe sulfide decorated outlines after fayalitic olivine do not occur in alh 81002 or nogoya indicating different fe and s mass transfer regimes in different cm2 chondrites mg fe si and s in the aqueous solution including the excess mg and si exported from all serpentine pseudomorphs after olivine of any composition were available to be incorporated into other phases spatially separate from the pseudomorphs after olivine including regularly interstratified serpentine tochilinite serpentines that replaced coarse olivines in que 93005 and alh 81002 are less magnesian than those in nogoya indicating that the nogoya aqueous alteration environment was more evolved toward mg rich solutions this easily located and characterized phase assemblage may be potentially useful for characterizing clasts of varying degrees of alteration in brecciated and heterogeneous cm chondrites and future returned samples from mineralogically similar asteroids

fe ni metal and sulfide minerals in cm chondrites an indicator for thermal history abstract cm chondrites were subjected to aqueous alteration and in some cases to secondary metamorphic heating the effects of these processes vary widely and have mainly been documented in silicate phases herein we report the characteristic features of fe ni metal and sulfide phases in 13 cm and 2 cm related chondrites to explore the thermal history of these chondrites the texture and compositional distribution of the metal in cm are different from those in unequilibrated ordinary and co chondrites but most have similarities to those in highly primitive chondrites such as ch cr and acfer 094 we classified the cm samples into three categories based on metal composition and sulfide texture fe ni metal in category a is kamacite to martensite category b is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite opaque mineral assemblages of category c are typically kamacite ni co rich metal and pyrrhotite these categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration the characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration category a cm chondrites are unheated whereas those in category b experienced small degrees of secondary heating cms in category c were subjected to the most severe secondary heating process thus opaque minerals can provide constraints on the thermal history for cm chondrites

shock and annealing in the amphibole and mica bearing r chondrites mil 11207 r6 and lap 04840 r6 contain hornblende and phlogopite mil 07440 r6 contains accessory titan phlogopite and no hornblende all three meteorites have been shocked mil 11207 contains extensive sulfide veins pyroxene that formed from dehydrated hornblende and an extensive network of plagioclase glass mil 07440 contains chromite plagioclase assemblages chromite veinlets and blebs pincer shaped plagioclase patches but no sulfide veins lap 04840 contains olivine grains with chromite bleb laden cores and opaque free rims rare grains of pyroxene that formed from dehydrated hornblende and no sulfide veins these meteorites appear to have been heated to maximum temperatures of approximately 700 900 c under conditions of moderately high ph2o perhaps 250 500 bars all three samples underwent postshock annealing during this process olivine crystal lattices healed giving the rocks the appearance of shock stage s1 and diffusion of fe and s from thin sulfide veins to coarse sulfide grains caused the veins to disappear in mil 07440 and lap 04840 this latter process apparently also occurred in most s1 s2 ordinary chondrites of high petrologic type the pressure temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers as suggested in the literature a giant impact or a series of smaller impacts would then have been required to excavate the hornblende and biotite bearing rocks and bring them closer to the surface it was in that latter location where the samples were shocked deposited in a hot ejecta blanket of low thermal diffusivity and annealed

gems like material in the matrix of the paris meteorite and the early stages of alteration of cm chondrites abstract the paris meteorite is a weakly altered cm chondrite that has been discovered recently hewins et al 2014 its matrix offers the opportunity to search for well preserved pristine pre accretional material as well as to study the earliest stages of aqueous alteration in the cm parent body the study of the matrix of paris has been conducted by analytical transmission electron microscopy on focused ion beam sections extracted from matrix areas showing different degrees of aqueous alteration the least altered matrix sample consists of amorphous silicate grains a few hundreds of nm in size separated from one another by an abundant porosity the amorphous silicates enclose numerous fe sulfide nanograins and their average composition is close to the chondritic composition they share many similarities with gems glass with embedded metal and sulfides grains present in chondritic porous interplanetary dust particles and with primitive type 3 0 carbonaceous chondrites this first discovery of gems like texture in a cm chondrite suggests that gems grains could have been the building blocks of the cm matrices in more aqueously altered samples pronounced microstructural heterogeneities were detected at the micrometer scale the matrix consists mostly of a mixture of amorphous material and fe rich spongy to fine fibrous poorly crystalline phyllosilicates the porosity fraction is significantly reduced and the mixed amorphous fibrous material frequently forms a continuous groundmass the close association between these two material types suggests a replacement mechanism due to aqueous alteration chemical compositions correlate strongly with the microstructure the amorphous material has a composition close to the chondritic value while the fine fibrous phyllosilicate material is fe enriched this fe enrichment is found to be continuous from weakly to more heavily altered areas in which the fibrous morphology is coarser and better crystalline cronstedtite with intercalated tochilinite is also found but in pore spaces this chemical evolution concomitant with the maturation of the phyllosilicates demonstrates that the early aqueous fluids that interacted with silicates in the matrix were enriched in fe this composition is probably the consequence of the preferential dissolution of metal and iron sulfides during the first stages of alteration the enrichment of phyllosilicates in mg seen in more altered cm chondrites is not observed in paris

correlated alteration effects in cm carbonaceous chondrites three parameters are proposed to determine the relative extent of alteration in cm chondrites the mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to mg serpentine and increases with increasing alteration to calculate values of this index an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual cm chondrites the second parameter is the volume percent of isolated matrix silicates which decreases with progressive alteration due to mineral hydration finally the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds these parameters define the first cm alteration scale that relies on multiple indicators of progressive alteration the following relative order of increasing alteration is established by this model murchison bells pollen murray mighei nogoya cold bokkeveld the relative degree of aqueous processing cochabamba and boriskino experienced is less precisely constrained although both fall near the middle of this sequence a comparison between the mineralogic alteration index and literature values for the whole rock chemistry of cm chondrites reveals several correlations a positive nearly linear correlation between bulk h content and progressive cm alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to mg serpentine the abundance of trapped planetary 36ar decreases systematically in progressively altered cm chondrites suggesting the wholesale destruction of primary noble gas carrier phase s by aqueous reactions because low temperature fluid rock reactions are generally associated with large isotopic mass fractionation factors we also compared our model predictions with 18o values for bulk cm samples although some of these data are poorly resolved the order of increasing 18o values approximates the order of increasing alteration predicted by our model parameters multiple correlations between diverse alteration parameters strongly suggest that a different cm chondrites experienced similar kinds of processes and conditions and b cm materials experienced in situ alteration on the cm parent body or bodies

an american on paris extent of aqueous alteration of a cm chondrite and the petrography of its refractory and amoeboid olivine inclusions paris is the least aqueously altered cm chondrite identified to date classified as subtype 2 7 however literature data indicate that some regions of this apparently brecciated meteorite may be subtype 2 9 the suite of cais in paris includes 19 spinel pyroxene inclusions 19 spinel inclusions 8 spinel pyroxene olivine inclusions 43 pyroxene inclusions 8 pyroxene olivine inclusions and 3 hibonite bearing inclusions both simple and complex inclusions are present some have nodular banded or distended structures no melilite was identified in any of the inclusions in the present suite but other recent studies have found a few rare occurrences of melilite in paris cais because melilite is highly susceptible to aqueous alteration it is likely that it was mostly destroyed during early stage parent body alteration two of the cais in this study are part of compound cai chondrule objects their presence suggests that there were transient heating events probably associated with chondrule formation in the nebula after chondrules and cais were admixed also present in paris are a few amoeboid olivine inclusions aoi consisting of relatively coarse forsterite rims surrounding fine grained porous zones containing diopside and anorthite the interior regions of the aois may represent fine grained rimless cais that were incorporated into highly porous forsterite rich dustballs these assemblages were heated by an energy pulse that collapsed and coarsened their rims but failed to melt their interiors

aqueous alteration of chondrules from the murchison cm carbonaceous chondrite replacement pore filling and the genesis of polyhedral serpentine forsterite and clinoenstatite in type iab chondrules from the murchison cm carbonaceous chondrite have been partially serpentinized and the mechanisms of their alteration reveal crystallographic and microstructural controls on the reaction of silicate minerals with parent body aqueous solutions grains of forsterite were altered in two stages narrow veinlets of fe rich serpentine formed first and by the filling of sheet pores most of these pores were oriented parallel to 010 and 001 and had been produced by earlier fracturing and or congruent dissolution in the second stage the subset of veinlets that were oriented parallel to 001 was widened accompanying the replacement of forsterite by mg fe serpentine this reaction proceeded most rapidly parallel to 001 and crystallographic controls on the trajectory of retreating vein walls created fine scale serrations murchison clinoenstatite grains have a skeletal appearance due to the presence of abundant veinlets and patches of phyllosilicate two alteration stages can again be recognized with initial water mineral interaction producing tochilinite rich veinlets by the filling of 001 parallel contraction cracks pores then formed by congruent dissolution that was guided principally by orthopyroxene lamellae and they were subsequently filled by submicrometer sized crystals of polyhedral serpentine this finding that murchison forsterite and clinoenstatite grains have been altered demonstrates that aqueous processing of magnesium silicate minerals started much earlier in cm parent body history than previously believed our results also show that the occurrence of polyhedral serpentine can be used to locate former pore spaces within the parent body

the paris cm chondrite secondary minerals and asteroidal processing abstract we report a petrographic and mineralogical survey of paris a new cm chondrite considered to be the least altered cm identified so far hewins et al 2014 compared to other cms paris exhibits 1 a higher concentration of fe ni metal beads with nickel contents in the range 4 1 8 1 wt 2 the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains and 3 ubiquitous tochilinite cronstedtite associations with higher feo sio2 and s sio2 ratios in addition paris shows the highest concentration of trapped 36ar reported so far for a cm chondrite hewins et al 2014 in combination with the findings of previous studies our data confirm the reliability of 1 the alteration sequence based on the chemical composition of tochilinite cronstedtite associations to quantify the fluid alteration processes and 2 the use of cr content variability in type ii ferroan chondrule olivine as a proxy of thermal metamorphism in contrast the scales based on 1 the fe3 content of serpentine in the matrix to estimate the degree of aqueous alteration and 2 the chemical composition of fe ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of paris it also appears that the amount of trapped 36ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites for both aqueous alteration and thermal metamorphism considering paris our data suggest that this chondrite should be classified as type 2 7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism these results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of cm chondrites

microstructure of calcite in the cm2 carbonaceous chondrite lon 94101 implications for deformation history during and or after aqueous alteration the microstructure of calcite in the cm2 carbonaceous chondrite lon 94101 has been characterized using electron backscatter diffraction ebsd analysis to reconstruct the parent body deformation history during and or after aqueous alteration the results suggest that at least two events of calcite crystallization have taken place during aqueous alteration and at least three episodes of deformation are recorded by the calcite the first event of calcite crystallization produced calcite grains scattered throughout the matrix and the second event formed a calcite vein via localized fluid flow the first episode of deformation is recorded in the crystallographic preferred orientations of the calcite grains and occurred via a directed stress probably induced by compaction in shallow crustal levels of the parent body the second episode of deformation is recorded in an e twin microstructure and it suggests a deformation induced via directed stress by impact processing also in shallow crustal levels the third episode of deformation generated subgrains in the calcite vein and in some calcite grains and fragmented and disrupted the calcite vein this could have been a result of a relatively forceful deformation event perhaps when the meteorite was released from its parent body this study shows that carbonate microstructures in carbonaceous chondrites is a powerful and versatile tool for reconstructing the history of deformation during and or after aqueous alteration

cm chondrites exhibit the complete petrologic range from type 2 to 1 we have characterized the most phyllosilicate rich members of the cm chondrite group based upon petrographic and compositional factors we conclude that these particular meteorites have experienced pervasive aqueous alteration far beyond that witnessed by typical cms the lack of anhydrous silicates cai and except as relicts chondrules merits the classification of type cmi for three meteorites eet 83334 alh 88045 and one kaidun lithology still other cms notably alh 83100 eet 90047 and yamato 82042 are clearly intermediate between types 1 and 2 brecciated cm chondrites like cold bokkeveld contain all of these varied lithologies the cm chondrites thus exhibit the complete petrologic range from 2 through 1 our results show that progressive aqueous alteration on the parent cm asteroid s was locally accompanied by significant increases in temperature to a peak of 450 c for the kaidun lithology fo2 and locally degree of chemical leaching all well beyond the conditions recorded by typical cm2s the most altered cms also are commonly deformed displaying a distinct lineation probably due to static rather than dynamic forces

mn cr ages of dolomites in ci chondrites and the tagish lake ungrouped carbonaceous chondrite abstract 53 mn 53 cr ages have been measured for dolomites in three ci chondrites and an ungrouped c2 chondrite tagish lake for an accurate age determination we utilized in contrast to previous works a matrix matched carbonate standard to appropriately correct the observed mn cr ratios the mn cr data of the dolomites most likely represent isochrons with chronological significances and the obtained ages are concentrated in 4563 8 4562 5 million years ago 4 4 5 7 million years after the birth of the solar system the ages of the ci and tagish lake dolomites are very similar to those of carbonates in cm chondrites although ci and cm chondrites are likely to have different alteration histories the similar carbonate ages in various carbonaceous chondrites suggest that the aqueous activities in hydrous asteroids initiated almost concurrently if this is the case the most plausible heat source for the aqueous alteration is the decay energy of short lived radionuclides especially 26 al we carried out a model calculation for the thermal history of the ci chondrite parent body assuming that 26 al is the dominant heat source by two different approaches to estimate the accretion time of the ci chondrite parent body we conclude that it accreted 3 0 3 9 million years after the birth of the solar system this is also likely to be the case for the cm chondrite and tagish lake parent bodies because the result of the calculation is not sensitive to the ice abundance therefore we stress that near the condensation front of water ice meteorite parent bodies with abundant ice accreted almost simultaneously at 3 4 million years after cai formation combining our mn cr ages of the dolomites with those of breunnerites reported previously the youngest one is 4554 6 million years ago we found that the aqueous alteration in the ci chondrite parent body lasted for at least 9 million years therefore the parent body should be large enough 50 km in radius so that the internal heat had been kept for 9 million years

53mn 53cr systematics of carbonates in cm chondrites implications for the timing and duration of aqueous alteration abstract cm chondrites contain carbonates and other secondary minerals such as phyllosilicates sulfides sulfates oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid we report in situ mn cr isotope measurements in the highly aqueously altered cm2 1 chondrites que 93005 and alh 83100 using secondary ion mass spectrometry cameca ims 1270 ion microprobe the 53 cr excesses are correlated with the 53 mn 55 mn ratio and result from the in situ decay of 53 mn a short lived radioisotope with a half life of 3 7 ma if we assume that carbonate grains in samples que 93005 and alh 83100 are cogenetic then the excesses define initial 53 mn 55 mn ratios 53 mn 55 mn 0 of 4 1 1 2 10 6 and 5 1 1 7 10 6 respectively these values are comparable to those in carbonates from other cm chondrites as reported in the literature initial 53 mn 55 mn ratios for calculated model isochrones for individual carbonate grains range from 3 8 1 4 10 6 to 4 8 2 1 10 6 for que 93005 and from 3 1 1 6 10 6 to 1 3 0 5 10 5 for alh 83100 a possible interpretation for the ranges in 53 mn 55 mn 0 could be that alteration in individual cm chondrites was episodic and occurred over an extended period of time however isochrones based on the entire set of carbonate grains in each of the cm chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in cm chondrites of different subtypes and that alteration on the cm parent asteroid started contemporaneously with or shortly after cai formation and lasted at least 4 ma

mineralogy of iron sulfides in cm1 and ci1 lithologies of the kaidun breccia records of extreme to intense hydrothermal alteration the polymict kaidun microbreccia contains lithologies of c type chondrites with euhedral iron sulfide crystals of hydrothermal origin our fib tem study reveals that acicular sulfides in a cm1 lithology are composed of fe rich pyrrhotite with nonintegral vacancy superstructures nc pyrrhotite troilite and pentlandite all showing distinct exsolution textures based on phase relations in the fe ni s system we constrain the temperature of formation of the originally homogeneous monosulfide solid solution to the range of 100 300 c in some crystals the exsolution of pentlandite and the microtextural equilibration was incomplete probably due to rapid cooling we use thermodynamic modeling to constrain the physicochemical conditions of the extreme hydrothermal alteration in this lithology unless the cm1 lithology was sourced from a large depth in the parent body internal pressure 85 bar or the temperatures were in the lower range of the interval determined the water was likely present as vapor previously described light 34s compositions of sulfides in kaidun s cm1 lithology are likely due to the loss of 34s enriched h2s during boiling platy sulfide crystals in an adjacent intensely altered ci1 lithology are composed of fe poor monoclinic 4c pyrrhotite and nc pyrrhotite and probably formed at lower temperatures and higher fs2 relative to the cm1 lithology however a better understanding of the stability of fe poor pyrrhotites at temperatures below 300 c is required to better constrain these conditions

clastic matrix in eh3 chondrites patches of clastic matrix 15 to 730 m in size constitute 4 9 vol of eh3 yamato y 691 and 11 7 vol of eh3 allan hills alh 81189 individual patches in y 691 consist of 1 25 vol relatively coarse opaque grain fragments and polycrystalline assemblages of kamacite schreibersite perryite troilite some grains with daubreelite exsolution lamellae niningerite oldhamite and caswellsilverite 2 30 vol relatively coarse silicate grains including enstatite albitic plagioclase silica and diopside and 3 an inferred fine nebular component 45 vol comprised of submicrometer size grains clastic matrix patches in alh 81189 contain relatively coarse grains of opaques 20 vol kamacite schreibersite perryite and troilite and silicates 30 vol enstatite silica and forsterite as well as an inferred fine nebular component 50 vol the o isotopic composition of clastic matrix in y 691 is indistinguishable from that of olivine and pyroxene grains in adjacent chondrules both sets of objects lie on the terrestrial mass fractionation line on the standard three isotope graph some patches of fine grained matrix in y 691 have distinguishable bulk concentrations of na and k inferred to be inherited from the solar nebula some patches in alh 81189 differ in their bulk concentrations of ca cr mn and ni the average compositions of matrix material in y 691 and alh 81189 are similar but not identical matrix in alh 81189 is much richer in mn 0 23 0 05 versus 0 07 0 02 wt and appreciably richer in ni 0 36 0 10 versus 0 18 0 05 wt than matrix in y 691 each of the two whole rocks exhibits a petrofabric probably produced by shock processes on their parent asteroid

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

multiple precursors of secondary mineralogical assemblages in cm chondrites we report a petrographic and mineralogical survey of tochilinite cronstedtite intergrowths tcis in paris a new cm chondrite considered to be the least altered cm identified to date our results indicate that type i tcis consist of compact tochilinite cronstedtite rims surrounding fe ni metal beads thus confirming kamacite as the precursor of type i tcis in contrast type ii tcis are characterized by complex compositional zoning composed of three different fe bearing secondary minerals from the outside inwards tochilinite cronstedtite and amakinite type ii tcis present well developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors the results demonstrate that type ii tcis formed by pseudomorphism of the anhydrous silicates olivine and pyroxene hence there is no apparent genetic relationship between type i and type ii tcis in addition the complex chemical zoning observed within type ii tcis suggests that the alteration conditions evolved dramatically over time at least three stages of alteration can be proposed characterized by alteration fluids with varying compositions 1 fe and s rich fluids 2 s poor and fe and si rich fluids and 3 s and si poor fe rich fluids the presence of unaltered silicates in close association with euhedral type ii tcis suggests the existence of microenvironments during the first alteration stages of cm chondrites in addition the absence of mg bearing secondary minerals in paris tcis suggests that the mg content increases during the course of alteration

accretional and alterational differences in a carbonaceous chondrite parent body evidence from the nwa 5491 cv3 meteorite the nwa 5491 cv3 meteorite is a cvoxa subtype and composed of two substantially different units titled upper and lower units in the cm size range with original accreted material and also subsequent alteration produced features based on the large chondrules in the upper unit and the small chondrules plus cais in the lower unit they possibly accreted material from different parts of the solar nebula and or at different times whereas substantial changes happened in the nebula s composition differences are observed in the level of early fragmentation too which was stronger in the upper units during later alteration oxidizing fluids possibly circulated only in the upper unit mechanical fragmentation and resorption were also stronger there in the last phase of the geological history these two rock units came into physical contact but impact driven shock effects were not observed the characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body

the paradox between low shock stage and evidence for compaction in cm carbonaceous chondrites explained by multiple low intensity impacts abstract petrographic analysis of eight cm carbonaceous chondrites eet 96029 lap 031166 lon 94101 met 01072 murchison murray sco 06043 que 93005 by electron imaging and diffraction and x ray computed tomography reveals that six of them have a petrofabric defined by shock flattened chondrules with the exception of murchison those cms that have a strong petrofabric also contain open or mineralized fractures indicating that tensional stresses accompanying the impacts were sufficient to locally exceed the yield strength of the meteorite matrix the cms studied span a wide range of petrologic subtypes and in common with rubin 2012 we find that the strength of their petrofabrics increases with their degree of aqueous alteration this correspondence suggests that impacts were responsible for enhancing alteration probably because the fracture networks they formed tapped fluid reservoirs elsewhere in the parent body two meteorites that do not fit this pattern are met 01072 and murchison both have a strong petrofabric but are relatively unaltered in the case of met 01072 impact deformation is likely to have postdated parent body aqueous activity the same may also be true for murchison but as this meteorite also lacks fractures and veins its chondrules were most likely flattened by multiple low intensity impacts multiphase deformation of murchison is also revealed by the microstructures of calcite grains and chondrule defined petrofabrics as revealed by x ray computed tomography the contradiction between the commonplace evidence for impact deformation of cms and their low shock stages most belong to s1 can be explained by most if not all having been exposed to multiple low intensity i e

investigation of organo carbonate associations in carbonaceous chondrites by raman spectroscopy abstract carbonates record information regarding the timing nature and conditions of the fluids circulating through asteroid parent bodies during aqueous alteration events determining carbonate abundances and their relationships with organic matter improves our understanding of the genesis of major carbonaceous components in chondritic materials in this study five cm2 carbonaceous chondrites cm2 2 nogoya cm2 3 jbilet winselwan cm2 5 murchison cm2 santa cruz and cm2tii wisconsin range 91600 were studied with raman spectroscopy carbonates were identified in these meteorite samples by the distinctive raman band in the 1100 cm 1 region representing the symmetric stretching vibration mode 1 of the co 3 2 anion carbonates identified in the meteorite samples are all calcite with the exception of a single dolomite grain in nogoya the v 1 positions of the cm calcites are 2 3 cm 1 higher than in pure calcite which suggests that they contain significant impurity cations typical graphitic first order d and g bands were identified in the meteorite matrix as well as in 25 of the analyzed carbonate grains from the raman results we postulate that the carbonates might not have formed under equilibrium conditions from a single fluid the first generation of carbonate is interpreted to have formed from highly oxidized fluids that led to the oxidation of organic matter om and produced carbonates that are om barren the second generation of carbonate was formed from a more evolved aqueous fluid with the presence of om the raman parameters of the organics in carbonates clearly deviate from the matrix om which suggests that the carbonate organics contain very different carbonaceous components that are distinct from the typical amorphous om of the cm matrix the occurrence of different generations of carbonate in close proximity may be partly responsible for the wide range in estimated ages of carbonates in carbonaceous chondrites reported in previous studies

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

replacement of olivine by serpentine in the carbonaceous chondrite nogoya cm2 abstract coarse chondrule and isolated olivine in some cm chondrites is replaced by serpentine in both centripetal and meshwork replacement textures locally preserved textures formed by partial replacement of coarse olivine by serpentine in the carbonaceous chondrite nogoya cm2 establish unique associations between each individual mass of serpentine and the specific olivine from which that serpentine formed electron probe microanalyses show that the composition of serpentine replacing coarse olivine is uniform throughout all analyzed volumes of nogoya and is independent of the composition of the olivine being replaced if as previously proposed late stage alteration fluids were mg rich because fe source minerals were depleted in earlier stages then the uniform mg rich composition of the serpentine replacing large silicate grains during advanced stages of alteration may indicate diffusional homogenization of the aqueous solutions over progressively larger spatial scales enabled by long timescales and previously proposed stagnant or slow moving fluids the range of olivine compositions replaced in nogoya is even larger than previously reported from alh 81002 cm2 this militates against hypotheses of strong primary mineral control on the compositions of alteration products at least at advanced stages of alteration the serpentine formed by olivine replacement in nogoya is more magnesian than the counterpart serpentine replacing all anhydrous primary silicates in alh 81002 this intermeteorite heterogeneity of replacement serpentine composition between alh 81002 and nogoya indicates that the aqueous solutions in which the olivine serpentine replacement reactions occurred were of different compositions in the two different cm parent body volumes sampled by alh 81002 and nogoya the more magnesian character of serpentines in nogoya than in alh 81002 indicates that the nogoya aqueous alteration environment was even more highly evolved toward mg rich solutions than the environment indicated by the composition of the serpentine in alh 81002 persistence of primary silicate remnants within centripetal and meshwork serpentine indicates that either the aqueous alteration episodes in the parent body volumes represented by individual meteorites were too short to allow complete replacement of olivine by serpentine or one or more reactants most likely water were completely consumed before the coarse primary silicate was completely replaced seemingly incompatible arguments for and against primary mineral control of serpentine composition during cm chondrite alteration may be reconciled by considering the different grain sizes and reaction timescales that likely existed in different textural settings

constraining the pressure threshold of impact induced calcite twinning implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and or after aqueous alteration but the shock pressure threshold at which the twins develop is unknown accordingly the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins this was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets brecciation fracturing and calcite e twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of 110 to 480 mpa which is at least a factor of ten higher than the 10 mpa that is considered to be required to produce calcite twins in low strain rate terrestrial settings these pressures are equivalent to shock stage s1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies

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

aragonite in the murray cm2 carbonaceous chondrite implications for parent body compaction and aqueous alteration the matrix of the cm2 carbonaceous chondrite murray contains rare micrometer sized prismatic crystals of aragonite that formed during late stage parent body aqueous alteration the aragonite was identified by x ray microanalysis coupled with electron backscatter diffraction ebsd tem selected area electron diffraction and cathodoluminescence spectroscopy the sixteen crystals found all occur within loose and elongate submillimeter sized clusters and one cluster is present in each of the two thin sections studied orientation determinations using ebsd show that the c axes of aragonite crystals within each cluster lie roughly in a plane itself aligned approximately parallel to the long axis of the host cluster aragonite is inferred to have crystallized after calcite but before completion of static impact related compaction the clusters developed by growth of aragonite within films of aqueous fluids that had a relatively high mg ca ratio these fluids were focused within zones of high porosity and permeability along a weak compactional fabric in the matrix and this fabric is also likely to have influenced the orientations of aragonite crystals as they grew these results suggest that aragonite probably occurs in most of those carbonaceous chondrites that have undergone moderate degrees of parent body aqueous alteration and may provide further insights into the evolution of pore fluid compositions and volumes and the chronology of asteroidal evolution

refractory inclusions in the prototypical cm chondrite mighei abstract refractory inclusions in the mighei cm2 meteorite are very abundant 35 being found in three thin sections of the meteorite the most common are pyroxene rimmed spinel rich inclusions that occur in a range of structural and textural types some inclusions are simple aggregates of crystals or rimmed crystals ranging from loose clusters of spinel grains to distended chains of pyroxene rimmed spinel knots others are more compact and may have experienced recrystallization or melting these include nodular spinel inclusions with polygonal granular textured spinel interiors and pyroxene rims ribbons of spinel in some cases with hibonite and or perovskite that are again rimmed by pyroxene and refractory spherules four inclusions are complex aggregates of several texturally distinct objects each collectively mighei inclusions are similar to those in murchison but appear to be somewhat more altered volatility fractionated and unfractionated trace element patterns similar to those found in refractory inclusions in other carbonaceous chondrites are represented in mighei inclusions and in addition some mighei inclusions have volatility fractionated pattern types not previously found in refractory inclusions in three of the complex aggregates fundamentally different trace element patterns are present within different portions of each inclusion meaning that these objects accreted from genetically different components that never equilibrated with one another subsequent to accretion bulk inclusion magnesium isotopic compositions show minor mass fractionation effects fmg values range from 2 3 to 7 5 but the average fmg value is indistinguishable from normal 1 8 1 2 there are no correlations between magnesium isotopic composition and textural type or trace element pattern type the diverse inclusion types in mighei represent a variety of origins and histories textures suggest that the nodular and banded inclusions may have experienced solid state recrystallization whereas the porous and chain like varieties and complex aggregates have not magnesium isotopic compositions and trace element abundances suggest that the mighei inclusions are unlikely to be evaporation residues and are more likely to be the products of condensation or recondensation processes

bright carbonate veins on asteroid 101955 bennu implications for aqueous alteration history introduction carbonaceous asteroids formed early in solar system history and experienced varying degrees of aqueous water rock and thermal alteration most models of the evolution of these asteroids suggest that aqueous alteration was driven by hydrothermal convection however it is debated whether this alteration occurred in a chemically closed or open system the bulk chemical compositions of the carbonaceous chondrite meteorites imply that the system was closed models predict that large scale fluid flow in an open system took place on at least some asteroids in this scenario fluids would have flowed through fractures from the interior and minerals would have precipitated into these fractures forming veins rationale global spectral observations by the osiris rex origins spectral interpretation resource identification and security regolith explorer spacecraft have shown that carbon bearing materials including organics and or carbonates are widespread on the surface of near earth asteroid 101955 bennu to understand the composition of these carbon bearing materials and their implications for bennu s aqueous alteration history we examined visible near infrared spectra obtained at resolutions of 4 m in the region around the mission s primary sampling site nightingale we compared these spectra with laboratory spectra of organics and carbonates broadband panchromatic images of the nightingale region at pixel scales of 1 4 cm allowed us to search for surface features that could contextualize the spectral data results on the basis of the wavelength position and shape of spectral absorption features the closest matches to a subset 15 of spectra collected in the nightingale region are the carbonate minerals calcite dolomite and magnesite with calcite being the most prevalent these minerals are found in the highly aqueously altered cm and ci group carbonaceous chondrites proposed meteorite analogs of bennu bright centimeters thick roughly meter long veins are apparent in images of some boulders near the nightingale site the veins have albedos of 10 to at least 19 much brighter than bennu s average of 4 4 these albedos are consistent with the veins being filled by carbonates with minor abundances of opaque materials such as magnetite and organics the albedos are too high for the veins to be dominantly composed of organics or any other material that has been identified on bennu except carbonates there are only a few bright vein filling materials found in meteorites and carbonate is the only one of these that is plausibly present on bennu linear mixture modeling suggests that only a small fraction of a spectrometer field of view conclusion the observed bright veins on bennu are most likely dominantly composed of carbonates the detection of these meter sized putative carbonate veins indicates that extensive water flow occurred in a chemically open hydrothermal system on the larger parent asteroid of bennu before the catastrophic disruption of the parent asteroid led to bennu s reaccumulation as a rubble pile kinetic modeling assuming a calcite vein composition shows that a hydrothermal system kilometers in size with alteration likely taking place over thousands to millions of years would have been needed to create veins of the dimensions preserved in the constituent boulders of bennu some cm and ci group meteorites have very small micrometers to millimeters carbonate veins or presumed vein fragments veins on the scale that we observe on bennu are not present in meteorites it is possible that such veins are present on other carbonaceous asteroids that have not been observed by spacecraft we predict that the sample of bennu s surface material that is planned to be returned to earth by the osiris rex spacecraft will contain carbonates and that they will be distributed with a structure and scale distinct from those that occur in meteorites