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

elephant moraine 96029 a very mildly aqueously altered and heated cm carbonaceous chondrite implications for the drivers of parent body processing elephant moraine eet 96029 is a cm carbonaceous chondrite regolith breccia with evidence for unusually mild aqueous alteration a later phase of heating and terrestrial weathering the presence of phyllosilicates and carbonates within chondrules and the fine grained matrix indicates that this meteorite was aqueously altered in its parent body features showing that water mediated processing was arrested at a very early stage include a matrix with a low magnesium iron ratio chondrules whose mesostasis contains glass and or quench crystallites and a gehlenite bearing calcium and aluminium rich inclusion eet 96029 is also rich in fe ni metal relative to other cm chondrites and more was present prior to its partial replacement by goethite during antarctic weathering in combination these properties indicate that eet 96029 is one of the least aqueously altered cms yet described cm2 7 and so provides new insights into the original composition of its parent body following aqueous alteration and whilst still in the parent body regolith the meteorite was heated to 400 600 c by impacts or solar radiation heating led to the amorphisation and dehydroxylation of serpentine replacement of tochilinite by magnetite loss of sulphur from the matrix and modification to the structure of organic matter that includes organic nanoglobules significant differences between samples in oxygen isotope compositions and water hydroxyl contents suggests that the meteorite contains lithologies that have undergone different intensities of heating eet 96029 may be more representative of the true nature of parent body regoliths than many other cm meteorites and as such can help interpret results from the forthcoming missions to study and return samples from c complex asteroids

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

incompletely transformed serpentine type phyllosilicates in the matrix of antarctic cm chondrites abstract recent hrem high resolution electron microscopy investigation has shed light on the characterization of matrix phyllosilicates in cm chondrites in examining the antarctic cm chondrite yamato 793321 by hrem mineral grains which show an ed electron diffraction pattern consisting of both olivine and serpentine spots show disordered and defect structures in patchy hrem images these data are interpreted as representing an intermediate phase in the transformation from serpentine to olivine this is the first description of such an intermediate phase in belgica 7904 phyllosilicates seem to have been almost completely transformed to olivine it is suggested that it formed by heating of matrix phyllosilicates preliminary heating experiments on murchison also a cm chondrite showed that the matrix phyllosilicates transformed to a phase which gave an ed pattern and hrem image similar to those for yamato 793321 the results of these heating experiments support the formation of the intermediate phase by transformation from phyllosilicate to olivine during a heating event at a late stage

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

modeling aqueous alteration of cm carbonaceous chondrites results from an inorganic geochemical modeling study support a scenario in which low temperature aqueous alteration of an anhydrous cm asteroidal parent body and melt water from h20 and co ices produces the altered assemblage observed in cm carbonaceous chondrites chrysotile greenalite tochilinite cronstedtite and minor calcite and magnetite we consider a range of possible precursor mineral assemblages varying with respect to the fe oxidation state of the initial anhydrous phases the aqueous solutions produced by this alteration are generally strongly basic and reducing and a large quantity of h2 and possible ch4 gas can be released during aqueous alteration

the paris meteorite the least altered cm chondrite so far the paris chondrite provides an excellent opportunity to study cm chondrules and refractory inclusions in a more pristine state than currently possible from other cms and to investigate the earliest stages of aqueous alteration captured within a single cm bulk composition it was found in the effects of a former colonial mining engineer and may have been an observed fall the texture mineralogy petrography magnetic properties and chemical and isotopic compositions are consistent with classification as a cm2 chondrite there are 45 vol high temperature components mainly type i chondrules with olivine mostly fa0 2 mean fa0 9 with granular textures because of low mesostasis abundances type ii chondrules contain olivine fa7 to fa76 these are dominantly of type iia but there are iiab and iib chondrules ii a b chondrules with minor highly ferroan olivine and iia c with augite as the only pyroxene the refractory inclusions in paris are amoeboid olivine aggregates aoas and fine grained spinel rich ca al rich inclusions cais the cai phases formed in the sequence hibonite perovskite grossite spinel gehlenite anorthite diopside fassaite and forsterite the most refractory phases are embedded in spinel which also occurs as massive nodules refractory metal nuggets are found in many cai and refractory platinum group element abundances pge decrease following the observed condensation sequences of their host phases mn cr isotope measurements of mineral separates from paris define a regression line with a slope of 53mn 55mn 5 76 0 76 106 if we interpret cr isotopic systematics as dating paris components particularly the chondrules the age is 4566 44 0 66 myr which is close to the age of cai and puts new constraints on the early evolution of the solar system eleven individual paris samples define an o isotope mixing line that passes through cm2 and co3 falls and indicates that paris is a very fresh sample with variation explained by local differences in the extent of alteration the anhydrous precursor to the cm2s was co3 like but the two groups differed in that the cms accreted a higher proportion of water paris has little matrix 47 plus 8 fine grained rims and is less altered than other cm chondrites chondrule silicates except mesostasis cai phases submicron forsterite and amorphous silicate in the matrix are all well preserved in the freshest domains and there is abundant metal preserved metal alteration stage 1 of palmer and lauretta 2011 metal and sulfide compositions and textures correspond to the least heated or equilibrated cm chondrites category a of kimura et al 2011 the composition of tochilinite cronstedtite intergrowths gives a pcp index of 2 9 cronstedtite is more abundant in the more altered zones whereas in normal highly altered cm chondrites with petrologic subtype 2 6 2 0 based on the s sio2 and feo sio2 ratios in pcp or tochilinite cronstedtite intergrowths rubin et al 2007 cronstedtite is destroyed by alteration the matrix in fresh zones has ci chondritic volatile element abundances but interactions between matrix and chondrules occurred during alteration modifying the volatile element abundances in the altered zones paris has higher trapped ne contents more primitive organic compounds and more primitive organic material than other cms there are gradational contacts between domains of different degree of alteration on the scale of 1 cm but also highly altered clasts suggesting mainly a water limited style of alteration with no significant metamorphic reheating

formation age and genesis of the gongchangling neoarchean banded iron deposit in eastern liaoning province constraints from geochemistry and shrimp zircon u pb dating abstract the gongchang iron deposit a typical algoma type banded iron formation bif deposit located in the eastern block of the north china craton ncc is hosted in neoarchean metamorphic rocks including amphibolite biotite leptynite garnet biotite schist plagioclase hornblende schist garnet chlorite schist muscovite quartz schist and actinolite magnetite quartzite geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite respectively shrimp u pb dating of zircons from the plagioclase hornblende schist shows that the gongchangling iron deposit was formed at 2554 12 ma major elements and ree data of the ores indicate that the gongchangling bif precipitated from seawater and the ultimate iron and silica origins may have been related to nearby volcanic activity the 18o values of quartz from the bifs range between 9 1 and 12 9 suggesting that exotic high 18o fluids were involved during metamorphism the pyrite have 34s values of range between 3 5 and 14 7 suggesting that mineralization were originally from metamorphism comparison of the mineralization age of the gongchangling bif with those of other local bifs in north china and other cratons throughout the world indicates that the gongchangling bif formed coevally with the global bif mineralization at 2 5 ga taken together these data suggest that the gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations

shock melts in que 94411 hammadah al hamra 237 and bencubbin remains of the missing matrix we have studied the cb carbonaceous chondrites queen alexandra range que 94411 hammadah al hamra hh 237 and bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone iron nickel metal and chondrule silicates are the main components in these meteorites these high temperature components are held together by shock melts consisting of droplets of dendritically intergrown fe ni metal sulfide embedded in silicate glass which is substantially more feo rich 3040 wt than the chondrule silicates feo 5 wt fine grained matrix material which is a major component in most other chondrite classes is extremely scarce in que 94411 and hh 237 and has not been observed in bencubbin this material occurs as rare hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts and cl we infer that hydrated fine grained material compositionally similar to these matrix lumps was originally present between the fe ni metal grains and chondrules but was preferentially shock melted other shock related features in que 94411 hh 237 and bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments

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

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

metal saturated sulfide assemblages in nwa 2737 evidence for impact related sulfur devolatilization in martian meteorites nwa 2737 a martian meteorite from the chassignite subclass contains minute amounts 0 010 0 005 vol of metal saturated fe ni sulfides these latter bear evidence of the strong shock effects documented by abundant fe nanoparticles and planar defects in northwest africa nwa 2737 olivine a ni poor troilite fe s 1 0 0 01 sometimes cr bearing up to 1 wt coexists with micrometer sized taenite tetrataenite type native ni fe alloys ni fe 1 and fe os ir ru alloys a few hundreds of nanometers across the troilite has exsolved flame like pentlandite fe fe ni 0 5 0 6 chalcopyrite is almost lacking and no pyrite has been found as a hot desert find nwa 2737 shows astonishingly fresh sulfides the composition of troilite coexisting with ni fe alloys is completely at odds with chassigny and nahkla sulfides pyrite metal deficient monoclinic type pyrrhotite it indicates strongly reducing crystallization conditions close to iw several log units below the fo2 conditions inferred from chromites compositions and accepted for chassignites fmq 1 log unit it is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures shock related s degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages with concomitant loss of magnetic properties as documented in some other martian meteorites

ferrous silicate spherules with euhedral iron nickel metal grains from ch carbonaceous chondrites evidence for supercooling and condensation under oxidizing conditions the ch carbonaceous chondrites contain a population of ferrous fe fe mg approximate to 0 1 0 4 silicate spherules chondrules about 15 30 mum in apparent diameter composed of cryptocrystalline olivine pyroxene normative material sio2 rich glass and rounded to euhedral fe ni metal grains the silicate portions of the spherules are highly depleted in refractory lithophile elements cao al2o3 and tio2 0 04 wt and enriched in feo mno cr2o3 and na2o relative to the dominant volatile poor magnesian chondrules from ch chondrites the fe fe mg ratio in the silicate portions of the spherules is positively correlated with fe concentration in metal grains which suggests that this correlation is not due to oxidation reduction or both of iron feosil fe met during melting of metal silicate solid precursors rather we suggest that this is a condensation signature of the precursors formed under oxidizing conditions each metal grain is compositionally uniform but there are significant intergrain compositional variations about 8 18 wt ni 0 09 wt cr and a sub solar co ni ratio the precursor materials of these spherules were thus characterized by extreme elemental fractionations which have not been observed in chondritic materials before particularly striking is the fractionation of ni and co in the rounded to euhedral metal grains which has resulted in a co ni ratio significantly below solar the liquidus temperatures of the euhedral fe ni metal grains are lower than those of the coexisting ferrous silicates and we infer that the former crystallized in supercooled silicate melts the metal grains are compositionally metastable they are not decomposed into taenite and kamacite which suggests fast postcrystallization cooling at temperatures below 970 k and lack of subsequent prolonged thermal metamorphism at temperatures above 400 500 k

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

widespread evidence for high temperature formation of pentlandite in chondrites abstract by investigating the compositional and textural evolution of sulfides within a wide range of relatively pristine aqueously altered and thermally metamorphosed chondrites we constrain the equilibration temperatures of sulfide minerals and compare them to the metamorphic history of their host meteorite sulfides in mighei like carbonaceous chondrites are complex as they equilibrated mostly between 100 and 135 c but some may have equilibrated at temperatures up to 600 c this is consistent with some cm chondrite sulfides forming at high temperature during chondrule cooling and others during low temperature aqueous alteration and or annealing karoonda like carbonaceous chondrite sulfides equilibrated between 500 and 230 c which is consistent with formation during cooling and annealing after thermal metamorphism sulfides in the ll chondrites equilibrated between 600 and 230 c and are consistent with formation during chondrule cooling for semarkona ll3 00 and during cooling after thermal metamorphism for the equilibrated samples types 4 6 sulfides in the rumuruti like r chondrites equilibrated between 600 and 500 c and are consistent with formation after thermal metamorphism the sulfides within the brachinite equilibrated between 600 and 400 c consistent with formation during cooling after thermal metamorphism contrary to the assertion that pentlandite is solely the product of low temperature aqueous alteration in many chondrite groups this study suggests that most sulfides in chondrites are formed at or upon cooling from high temperature the evaluation of a single mineral system within samples that retain petrographic context is vital to the interpretation of formation and alteration processes recorded by small extraterrestrial samples such as those that have been returned by the spacecraft missions stardust and hayabusa and will be returned by osiris rex and hayabusa2

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

asiras airborne radar resolves internal annual layers in the dry snow zone of greenland 1 we present data from the european space agency s airborne sar interferometric radar altimeter system asiras flown during the cryovex 2004 field calibration validation campaign and new high resolution depth profiles of snow density measured in the field by neutron scattering we combine these data to calculate the depth of internal reflecting horizons in the asiras data the high resolution density data allow us to identify annual layers in the snow density profile and correlate their peaks with the reflecting horizons we use the thickness of the annual layers combined with the density profile to determine the spatial and temporal pattern of snow accumulation along the radar track for a period of 6 years from 1995 2002 our mean annual accumulation rate is 0 47 0 09 ma 1 water equivalent in agreement with the value determined from a core taken in this location in 1992 similarly our inter annual variability shows the same trends as recent model estimates over the entire ice sheet because asiras was designed to mimic as closely as possible the characteristics of the sar interferometric radar altimeter siral the principal payload of cryosat the detection of internal layering with asiras illuminates the possibility of detecting internal layers with a space borne radar and thus the possible application of this technique to the dry snow zones of antarctica greenland and smaller ice bodies

the origins of i type spherules and the atmospheric entry of iron micrometeoroids the earth s extraterrestrial dust flux includes a wide variety of dust particles that include feni metallic grains during their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed i type spherules these particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 au this article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials the results indicate that iron micrometeoroids experience peak temperatures 300 800 k higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of 50 m the lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates which enhances their abundance among micrometeorites by a factor of 2 the abundance of i types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite derived mms arriving at earth furthermore carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the i type spherule flux events involving such objects therefore cannot be recognized from i type spherule abundances in the geological record

short duration thermal metamorphism in cr chondrites cr chondrites are considered as one of the most primitive classes of meteorites most of them experienced a mild aqueous alteration and show no evidence of significant effect of thermal metamorphism we present here a search for low degree metamorphic effects in cr chondrites we studied 15 cr chondrites using different metamorphic indicators 1 structure and ni content of metal grains 2 hydration state of matrix 3 structure and composition of organic matter the different metamorphic indicators show that two of the analyzed cr chondrites gra 06100 and gro 03116 experienced thermal metamorphism indeed all of the metal grains in gra 06100 and half of the metal grains in gro 03116 show ni rich phases the matrix of gra 06100 is almost completely dehydrated and the matrix of gro 03116 is partially dehydrated raman spectra of organic matter in these two meteorites are clearly different from those obtained for organic matter in the other cr chondrites which resemble raman spectra of organic matter in unmetamorphosed cm2 meteorites ir spectra of insoluble organic matter extracted from gra 06100 and gro 03116 show lower carbonyl abundance and higher ch2 ch3 ratio with respect to organic matter of unmetamorphosed chondrites the other cr chondrites analyzed here lack these characteristics and only show a few metal grains with ni rich inclusions our results also show that the metamorphic effects observed in gra 06100 and gro 03116 are different from those observed in type 3 chondrites which experienced long duration metamorphism of radiogenic origin we infer that thermal processing in these two crs extended over a short duration and was triggered by impacts

opaque assemblages in cr2 graves nunataks gra 06100 as indicators of shock driven hydrothermal alteration in the cr chondrite parent body we have studied the petrologic characteristics of sulfide metal lodes polymineralic fe ni nodules and opaque assemblages in the cr2 chondrite graves nunataks gra 06100 one of the most altered cr chondrites unlike low petrologic type cr chondrites alteration of metal appears to have played a central role in the formation of secondary minerals in gra 06100 differences in the mineralogy and chemical compositions of materials in gra 06100 suggest that it experienced higher temperatures than other cr2 chondrites mineralogic features indicative of high temperature include 1 exsolution of ni poor and ni rich metal from nebular kamacite 2 formation of sulfides oxides and phosphates 3 changes in the co ni ratios and 4 carbidization of fe ni metal the conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600 c opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides fe rich carbonates including endmember ankerite and the sulfide silicate phosphate scorzalite we suggest that these materials formed via impact driven metamorphism mineralogic features indicative of impact metamorphism include 1 the presence of sulfide metal lodes 2 the abundance of polymineralic opaque assemblages with mosaic like textures and 3 the presence of suessite initial shock metamorphism probably resulted in replacement of nebular fe ni metal in chondrules and in matrix by ni rich co rich fe metal al ti cr rich alloys and fe sulfides while subsequent hydrothermal alteration produced accessory oxides phosphates and fe carbonates an extensive network of sulfide metal veins permitted effective exchange of siderophile elements from pre existing metal nodules with adjacent chondrules and matrix resulting in unusually high fe contents in these objects

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

fe ni metal in primitive chondrites indicators of classification and metamorphic conditions for ordinary and co chondrites we report the results of our petrological and mineralogical study of fe ni metal in type 3 ordinary and co chondrites and the ungrouped carbonaceous chondrite acfer 094 fe ni metal in ordinary and co chondrites occurs in chondrule interiors on chondrule surfaces and as isolated grains in the matrix isolated ni rich metal in chondrites of petrologic type lower than type 3 10 is enriched in co relative to the kamacite in chondrules however ni rich metal in type 3 15 3 9 chondrites always contains less co than does kamacite fe ni metal grains in chondrules in semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and ni rich regions metal in other type 3 chondrites is composed of fine to coarse grained aggregates of kamacite and ni rich metal resulting from metamorphism in the parent body we found that the number density of ni rich grains in metal number of ni rich grains per unit area of metal in chondrules systematically decreases with increasing petrologic type thus fe ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites among the known ordinary and co chondrites semarkona is the most primitive the range of metamorphic temperatures were similar for type 3 ordinary and co chondrites despite them having different parent bodies most fe ni metal in acfer 094 is martensite and it preserves primary features the degree of metamorphism is lower in acfer 094 a true type 3 00 chondrite than in semarkona which should be reclassified as type 3 01

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

the thermometry of enstatite chondrites a brief review and update due to the discoveries in antarctica the number of known enstatite chondrites has doubled in the last few years and many rare or previously unknown types have been collected most notably many el3 and eh3 chondrites we have applied the five major enstatite chondrite thermometers to the new and previously known enstatite chondrites the thermometers being 1 kamacite quartz enstatite oldhamite troilite kqeot 2 oldhamite 3 alabandite niningerite 4 sphalerite and 5 phosphide metal measured temperatures based on the kqeot and oldhamite systems are 800 c 1000 c with the type 3 enstatite chondrites having values similar to those of type 4 6 it seems likely that these temperatures relate to events prior to parent body metamorphism such as nebula condensation or chondrule formation and were not significantly reset by later events measured temperatures for alabandite niningerite metal phosphide and sphalerite in eh chondrites increase from 300 c 400 c to 600 c 800 c with petrographic indications of increasing metamorphism in contrast measured temperatures for all el chondrites including the most heavily metamorphosed are generally 400 c apparently el chondrites cooled more slowly than the eh chondrites regardless of metamorphism experienced measured temperatures for the alabandite niningerite metal phosphide and sphalerite are actually closure temperatures for the last thermal event suffered by the meteorite and the fast cooling rates indicated are most consistent with processes occurring in thick regoliths

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

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

an early solar system magnetic field recorded in cm chondrites abstract we present a paleomagnetic study of seven cm carbonaceous chondrites cm chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system as such cm chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism the ferromagnetic assemblage of cm chondrites is composed of a mixture of primary metallic iron pyrrhotite and magnetite the remanent properties are usually dominated by secondary pyrrhotite paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin trending component of magnetization in the seven studied cm chondrites in each meteorite this component is homogeneous in direction at least at the cm scale and is therefore post accretional we interpret this stable component as a pre terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few t 2 1 5 t considering the timescale and intensities of primordial magnetic fields both internally generated fields from a putative dynamo and external fields generated in the protoplanetary disk may have been recorded by cm chondrites it is presently difficult to discriminate between the two hypotheses regardless cm chondrites likely contain the oldest paleomagnetic record yet identified

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

primitive feni metal grains in ch carbonaceous chondrites formed by condensation from a gas of solar composition some feni metal grains similar to 150 mu m in apparent diameter in ch carbonaceous chondrites are concentrically zoned in ni similar to 5 10 wt co 0 2 0 4 wt and cr 0 3 0 8 wt silicon is present at the similar to 0 1 wt level these observations are consistent with predicted gas solid condensation from a gas of solar composition at temperatures of similar to 1370 1270 k and total pressure of similar to 10 4 bar estimates of feni metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula compositionally similar feni metal grains have also been reported in cr and bencubbin like chondrites because feni metal is highly susceptible to secondary alteration i e metamorphism melting oxidation the observed feni metal condensates in ch bencubbin like and cr chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections

oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules feo poor type i porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet elusive mechanism two settings are generally considered for their formation i a nebular setting where primordial solids were melted e g by shock waves propagating through the gas and ii a collisional planetary setting here we report a method combining high current electron microprobe x ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors we find that these olivine crystals are ca al ti poor relative to host olivine crystals their variable 17o even in individual chondrule is inconsistent with derivation from planetary interiors as previously argued from 120 triple junctions also exhibited by the chondrules studied herein this indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions we propose that porphyritic chondrules formed during gas assisted melting of nebular condensates comprising relict olivine grains with varying 17o values and ca al ti rich minerals such as those observed within amoeboid olivine aggregates incomplete melting of chondrule precursors produced ca al ti rich melts cat melts allowing subsequent crystallization of ca al ti rich host olivine crystals via epitaxial growth on relict olivine grains incoming mgo and sio from the gas phase induced i the dilution of cat melts as attested by the positive al ti correlation observed in chondrule olivine crystals and ii buffering of the o isotope compositions of chondrules as recorded by the constant 17o values of host olivine grains the o isotopic compositions of host olivine grains are chondrule specific suggesting that chondrules formed in an array of environments of the protoplanetary disk with different 17o values possibly due to variable solid gas mixing ratios

iron and iron nickel sulfides in chondritic interplanetary dust particles abstract we report the results of a study of iron and iron nickel sulfides in chondritic anhydrous and hydrated interplanetary dust particles idps the idps were characterized using chemical and mineralogical techniques including energy dispersive x ray analyses and transmission electron microscopy it is widely believed that the first sulfur containing mineral to form in the early solar nebula should have been stoichiometric fes troilite and that pyrrhotite is of secondary origin all examined interplanetary dust particles both anhydrous and hydrous types contain more pyrrhotite than troilite suggesting that the former was a common possibly predominant nebular condensate phase pyrrhotite could form from sulfidation of troilite after the latter had exfoliated from substrate metal grains in contrast to anhydrous idps hydrated ones contain ni rich sulfides including pentlandite which are probably of secondary origin

northwest africa 5958 a weakly altered cm related ungrouped chondrite not a ci3 northwest africa nwa 5958 is a carbonaceous chondrite found in morocco in 2009 preliminary chemical and isotopic data leading to its initial classification as c3 0 ungrouped have prompted us to conduct a multitechnique study of this meteorite and present a general description here the petrography and chemistry of nwa 5958 is most similar to a cm chondrite with a low degree of aqueous alteration apparently under oxidizing conditions and evidence of a second limited alteration episode manifested by alteration fronts the oxygen isotopic composition with delta o 17 4 3 parts per thousand is more o 16 rich than all cm chondrites indicating along with other compositional arguments a separate parent body of origin we suggest that nwa 5958 be reclassified as an ungrouped carbonaceous chondrite related to the cm group