metal troilite magnetite assemblage in shock veins of sixiangkou meteorite the first natural occurrence of metal troilite magnetite assemblage composed of fe ni metal and magnetite dendrites and a groundmass of troilite was identified in the shock veins of the sixiangkou l6 chondrite which contain abundant high pressure minerals this assemblage suggests a liquid miscibility among metal fes and iron oxide and subsequently quenching under pressure components of magnetite could be in origin related to chromite that was embedded and dissolved in an fe ni s liquid cr2o3 dissociated from chromite was mainly incorporated into garnet and magnesiowustite in the fine grained matrix of shock veins in which chromium behaves as a lithophile element at the p t conditions experienced by the shock veins the occurrence of metal troilite magnetite assemblage suggests that the shock veins were still under pressure at temperatures from 900 to 950degreesc during solidification of fe ni s or fe ni s o liquid hence indicating a long duration of high pressure in the shock veins copyright c 2002 elsevier science ltd

thermal and impact histories of reheated group iva ivb and ungrouped iron meteorites and their parent asteroids the microstructures of six reheated iron meteoritesotwo iva irons maria elena 1935 fuzzy creek one ivb iron ternera and three ungrouped irons hammond babb s mill blake s iron and babb s mill troost s iron owere characterized using scanning and transmission electron microscopy electron probe microanalysis and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids maria elena and hammond were heated below approximately 700 750 c so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite both meteorites retained a record of the original widmanstitten pattern the other four which show no trace of their original microstructure were heated above 600 700 c and recrystallized to form 10 20 lm wide homogeneous taenite grains on cooling kamacite formed on taenite grain boundaries with their close packed planes aligned formation of homogeneous 20 lm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 c or approximately 1 h at 1300 c all six irons contain approximately 5 10 lm wide taenite grains with internal microprecipitates of kamacite and nanometer scale m shaped ni profiles that reach approximately 40 ni indicating cooling over 100 10 000 yr un decomposed high ni martensite a2 in taeniteothe first occurrence in ironsoappears to be a characteristic of strongly reheated irons from our studies and published work we identified four progressive stages of shock and reheating in iva irons using these criteria cloudy taenite m shaped ni profiles in taenite neumann twin lamellae martensite shock hatched kamacite recrystallization microprecipitates of taenite and shock melted troilite maria elena and fuzzy creek represent stages 3 and 4 respectively although not all reheated irons contain evidence for shock it was probably the main cause of reheating cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays if the reheated irons that we studied are representative the iva irons may have been shocked soon after they cooled below 200 c at 4 5 gyr in an impact that created a rubblepile asteroid with fragments from diverse depths the primary cooling rates of the iva irons and the proposed early history are remarkably consistent with the pb pb ages of troilite inclusions in two iva irons including the oldest known differentiated meteorite blichert toft et al 2010

the distinct morphological and petrological features of shock melt veins in the suizhou l6 chondrite the morphology and petrology of distinct melt veins in the suizhou l6 chondrite have been investigated using scanning electron microscopy electron microprobe analyses and raman spectroscopy synchrotron energy dispersive diffraction and transmission electron microscopy it is found that the melt veins in the suizhou meteorite morphologically are the simplest straightest and thinnest among all shock veins known from meteorites at first glance these veins look like fine fractures but petrologically they are solid melt veins of chondritic composition and consist of fully crystalline materials of two distinct lithological assemblages with no glassy material remaining the suizhou melt veins contain the most abundant high pressure mineral species when compared with all other veins known in chondrites thus these veins in suizhou are classified as shock veins all rock forming and almost all accessory minerals in the suizhou shock veins have been transformed to their high pressure polymorphs and no fragments of the precursor minerals remain in the veins among the 11 high pressure mineral phases identified in the suizhou veins three are new high pressure minerals namely tuite after whitlockite xieite and the cf phase after chromite on the basis of transformation of plagioclase into maskelynite it is estimated that the suizhou meteorite experienced shock pressures andmore shock temperatures up to 22 gpa and 1000 c respectively shearing and friction along shock veins raised the temperature up to 1900 2000 c and the pressure up to 24 gpa within the veins hence phase transition and crystallization of high pressure minerals took place only in the suizhou shock veins fast cooling of the extremely thin shock veins is regarded as the main reason that up to 11 shock induced high pressure mineral phases could be preserved in these veins less

eutectic metal troilite fe mn na phosphate al free chromite assemblage in shock produced chondritic melt of the yanzhuang chondrite an assemblage with feni metal troilite fe mn na phosphate and al free chromite was identified in the metal troilite eutectic nodules in the shock produced chondritic melt of the yanzhuang h6 meteorite electron microprobe and raman spectroscopic analyses show that a few phosphate globules have the composition of na bearing graftonite fe mn na 3 po4 2 whereas most others correspond to mn bearing galileiite na fe mn 4 po4 3 and a possible new phosphate phase of na2 fe mn 17 po4 12 composition the yanzhuang meteorite was shocked to a peak pressure of 50 gpa and a peak temperature of approximately 2000 c all minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus the volatile elements p and na released from whitlockite and plagioclase along with elements cr and mn released from chromite are concentrated into the shock produced fe ni s o melt at high temperatures during cooling microcrystalline olivine and pyroxene first crystallized from the chondritic melt metal troilite eutectic intergrowths and silicate melt glass finally solidified at about 950 1000 c on the other hand p mn and na in the fe ni s o melt combined with fe and crystallized as fe mn na phosphates within troilite while cr combined with fe and crystallized as al free chromite also within troilite

origin of a unique impact melt rock the l chondrite ramsdorf we have studied a unique impact melt rock the ramsdorf l chondrite using optical and scanning microscopy and electron microprobe analysis ramsdorf contains not only clast poor impact melt begemann and wlotzka 1969 but also a chondritic portion 60 g with what appears at low magnification to be a normal well defined chondritic texture however detailed studies at high magnification show that 90 vol of the crystals in the chondritic portion were largely melted by the impact the chondrules lack normal microtextures and are ghosts of the original features the only relics from the precursor chondrules are olivine crystals which have the highest melting temperature 1620 c pyroxene rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine rich melts fine grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts probably because pyroxene rich melts are significantly more viscous complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of ramsdorf formed directly from petrologic type 3 4 material by strong shock we infer that ramsdorf was largely melted by shock pressures of 75 90 gpa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture

metal silicate interaction in quenched shock induced melt of the tenham l6 chondrite abstract the metal silicate microstructures in the shock induced melt pockets of the tenham l6 chondrite have been investigated by analytical transmission electron microscopy the melt areas formed under high pressure high temperature dynamic shock conditions consist of spherical fe ni metal iron sulfide globules embedded in a silicate glass matrix showing that the melt was quenched at high cooling rate the fe ni fraction in the globules is two phase composed of a bcc phase 5 wt ni and an fcc phase 49 wt ni indicating that fractional crystallisation of the metal occurred during the fast cooling the metal fraction also contains appreciable amounts of non siderophile elements mostly si mg and o suggesting that these elements were trapped in the metal either as alloying components or as tiny silicate or oxide inclusions in the iron sulfide fraction the na content is high 3 wt suggesting chalcophile behaviour for na during the shock event the composition of the silicate glass reflects non equilibrium melting of several silicate phases olivine pyroxene and plagioclase moreover the feo content is high compared to the feo contents of the unmelted silicates some fe redistribution took place between metal and silicate liquids during the shock event the silicate glass also contains tiny iron sulfide precipitates which most probably originated by exsolution during quench suggesting that the molten silicate retained significant amounts of s dissolved at high temperature and high pressure based on these observations we suggest that non equilibrium phenomena may be important in determining the compositions of metal and silicate reservoirs during their differentiation

the halite bearing zag and monahans 1998 meteorite breccias shock metamorphism thermal metamorphism and aqueous alteration on the h chondrite parent body zag and monahans 1998 are h chondrite regolith breccias comprised mainly of light colored metamorphosed clasts dark clasts that exhibit extensive silicate darkening and a halite bearing clastic matrix these meteorites reflect a complex set of modification processes that occurred on the h chondrite parent body the light colored clasts are thermally metamorphosed h5 and h6 rocks that were fragmented and deposited in the regolith the dark clasts formed from light colored clasts during shock events that melted and mobilized a significant fraction of their metallic fe ni and troilite grains the clastic matrices of these meteorites are rich in solar wind gases parent body water was required to cause leaching of chondritic minerals and chondrule glass the fluids became enriched in na k cl br al ca mg and fe evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated grains of halite and in the case of monahans 1998 halite with sylvite inclusions precipitated at low temperatures 100 c in the porous regolith in both meteorites fluid inclusions were trapped inside the halite crystals primary fluid inclusions were trapped in the growing crystals secondary inclusions formed subsequently from fluid trapped within healed fractures

ubiquitous brecciation after metamorphism in equilibrated ordinary chondrites our petrographic study of 42 h and l chondrites of types 4 6 revealed 8 that contain rare chondrules or clasts with olivines and pyroxenes having aberrant fe fe mg ratios for example a chondrule in slovak h5 contains olivine with fa 17 22 elsewhere olivine contains fa 17 18 although these chondrites are not obviously brecciated their ingredients could not have experienced the same degree of metamorphism these rocks must be fragmental breccias that were lithified after peak metamorphism regolith breccias and postmetamorphic fragmental breccias both polymict and monomict may constitute a majority of ordinary chondrites comparison of chondritic and achondritic breccias indicates that chondritic fragmental breccias are often very difficult to recognize because they resemble the fine grained mixtures of chondrules and mineral and chondrule fragments from which they formed in addition the chemical and textural uniformity throughout most of the ordinary chondrite bodies ensures that mixtures of materials from different regions are well disguised contrary to the authors claim the van schmus wood classification of ordinary chondrites does not identify rocks with similar thermal histories the petrologic type of a chondrite is only a measure of the average degree of metamorphism experienced by its ingredients thus attempts to correlate physical properties with petrologic types must fail unless breccia and shock properties are considered the extent of brecciation in ordinary chondrites and most achondrites indicates that meter sized unbrecciated rocks must be rare on the surfaces of their parent bodies

comment on petrologic evidence for low temperature possibly flood evaporitic origin of carbonates in the alh84001 meteorite by paul h warren

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

abee and related eh chondrite impact melt breccias abstract the abee eh chondrite is an impact melt breccia about 80 90 of the chondrules were melted many of those that remain have been partly resorbed euhedral enstatite phenocrysts are abundant many have nucleated on the surfaces of relict chondrules and now protrude into kamacite globules euhedral laths of graphite essentially identical to those produced magmatically on earth occur throughout the rock abee experienced two episodes of impact melting with an intervening period of brecciation the first impact melting event produced a relatively homogeneous rock with abundant melt upon cooling the rock was shattered by bombarding meteoroids igneous oldhamite rich dark inclusions were introduced at this time a second impact event produced melt which engulfed the abee fragments and dark inclusions primarily digesting the small ones heat lost to the surrounding clastic debris caused the groundmass melt to quench previously described diamonds in abee containing typical solar system isotopic compositions of n and xe were probably produced from graphite by shock several other eh chondrites may also be impact melt breccias the most extensively melted of these is adhi kot which contains shocked but largely unmelted chondrule rich clasts as well as chondrule free sulfide and silica rich clasts that also appear to be impact products

observations of impact induced molten metal silicate partitioning observations of molten mid ocean ridge basalt morb molybdenum mo interactions produced by shock experiments provide insight into impact and differentiation processes involving metal silicate partitioning analysis of fragments recovered from experiments achieving morb liquid shock pressures from 0 8 to 6 gpa revealed significant changes in the composition of the morb and mo due to reaction of the silicate and metal liquids on a short time scale 13 s the feo concentration of the shocked liquid de creases systematically with increasing pressure in fact the most highly shocked liquid 6 gpa contains only 0 1 wt feo compared to an initial concentration of 9 wt in the morb we infer from the presence of micrometer sized fe si and mo rich metallic spheres in the shocked glass that the fe and si oxides in the morb were reduced in an estimated oxygen fugacity of 10 17 bar and subsequently alloyed with the mo the in situ reduction of feo in the shocked molten basalt implies that shock induced reduction of impact melt should be considered a viable mechanism for the formation of metallic phases similar metallic phases may form during impact accretion of planets and in impacted material found on the lunar surface and near terrestrial impact craters in particular the minute isolated fe particles found in lunar soils may have formed by such a process furthermore the metallic spheres within the shocked glass have a globular texture similar to the textures of metallic spheroids from lunar samples and the estimated slow cooling rate of 140 c s for our spheres is consistent with the interpretation that the lunar spheroids formed by slow cooling within a melted target

first finding of impact melt in the iie netschaevo meteorite about half of the iie nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature the presence of preserved chondrules has been reported for two iie meteorites so far netschaevo and mont dieu which represent the most primitive silicate material within this group in this study silicate inclusions from two samples of netschaevo were examined both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse grained olivine and pyroxene set in a fine grained groundmass that consists of new crystals of olivine and a glassy appearing matrix this texture does not correspond to the description of the previously examined pieces of netschaevo which consist of primitive chondrule bearing angular clasts detailed petrographic observations and geochemical analyses suggest that the investigated samples of netschaevo consist of quenched impact melt this implies that netschaevo is a breccia containing metamorphosed and impact melt rock imr clasts and that collisions played a major role in the formation of the iie group

nature of the h chondrite parent body regolith evidence from the dimmitt breccia the dimmitt h chondrite regolith breccia consists of in vol 40 h4 and h5 chondrite clasts 3 impact melt rock clasts 0 5 shocked h chondrite clasts 1 5 exotic clasts including carbonaceous and ll5 chondrites and 55 gas rich matrix the ll5 clast is the best documented example of an ordinary chondrite in a host of a different compositional group the matrix contains unequilibrated material which differs from typical h3 material in having little 0 2 vol fine grained opaque silicate matrix and having 20 of the olivines with compositions in the range fa21 24 about 10 15 of this unequilibrated material is probably derived from graphite magnetite rich chondrites and 2 from h3 0 3 5 chondrites the absence of h3 clasts suggests that most of the unequilibrated material was derived from unconsolidated type 3 components many exotic clasts may have been derived from planetesimals that accreted to the h chondrite parent body prior to regolith development one slowly cooled melt rock clast formed beneath a 500 m thick melt breccia pile on the floor of a large impact crater and was later excavated by additional impacts incorporated into the regolith and consolidated with other components to form the dimmitt breccia

40ar 39ar dating of plagioclase grain size separates from silicate inclusions in iab iron meteorites and implications for the thermochronological evolution of the iab parent body abstract in order to better constrain the thermochronological evolution of the iab parent body we performed a 40ar 39ar age study on individual silicate inclusions of the iab irons caddo county campo del cielo landes and ocotillo in contrast to earlier studies several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the ar ar ages in nearly all inclusions we found significantly different ar ar ages among the separates caddo county 4 472 0 02 4 562 0 02 ga campo del cielo 2 4 362 0 04 4 442 0 03 ga landes 2 4 412 0 05 4 522 0 04 ga ocotillo 4 382 0 04 4 462 0 03 ga these ages were calculated using the new 40k decay constant published by mundil r renne p r min k and ludwig k r 2006 resolvable miscalibration of the 40ar 39ar geochronometer eos trans agu 87 fall meet suppl abstract v21a 0543 the ages did not systematically correlate with the respective grain size of the separate as expected i e smaller grains did not necessarily show younger ages due to later closure to ar diffusion or easier re opening of the system in the course of a reheating event compared to larger grains based on the large range of ar ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the iab parent body while some grains remained in a hot deep environment that allowed ar diffusion over an extended time period in some cases combined with grain coarsening others cooled significantly earlier near surface through the k ar blocking temperature these different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by benedix et al benedix g k mccoy t j keil k and love s g 2000 a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body meteorit planet sci 35 1127 1141 due to rapid cooling after the impact some of the age differences among the grains could be preserved based mainly on our caddo county ar ar age information the iab parent body was destroyed by impact and reassembled between 4 5 and 4 47 ga however iab silicate ar ar ages should depend much more on the pre and post impact cooling rate and burial depth than on the time of the actual impact this is supported by a compilation of our and literature iab and winonaite ar ar ages ranging smoothly from the time of accretion of the chondritic iab parent body down to the time of its final cooling through the k ar blocking temperature after impact and reassembly instead of showing a peak in ar ar ages at the time of the destructive impact

clues to the origin of metal in almahata sitta el and eh chondrites and implications for primitive e chondrite thermal histories abstract enstatite e chondrites are a group of texturally highly variable meteorites formed under strongly reducing conditions giving rise to unique mineral and chemical characteristics e g high abundances of various sulfides and si bearing metal in particular the abundant metal comprises a range of textures in e chondrites of different petrologic type but available in situ siderophile trace element data on metal are limited nine samples of e chondrites from the recent almahata sitta fall one eh3 two el3 4 two el6 two el impact melt rocks imr two eh imr were investigated in this study in addition to st mark s eh5 and grein 002 el4 5 with a focus on the nature of their metal constituents special attention was given to metal silicate intergrowths mssi that occur in many primitive e chondrites which have been interpreted as post accretionary asteroidal impact melts or primitive nebular condensates this study shows that siderophile trace element systematics in e chondrite metal are independent of petrologic type of the host rock and distinct from condensation signatures three basic types of siderophile trace element signatures can be distinguished indicating crystallization from a melt thermal equilibration upon metamorphism complete melting and exsolution of schreibersite perryite sulfide textural and mineral chemical constraints from el3 4s are used to evaluate previously proposed formation processes of mssi impact melting vs nebular condensation and elucidate which other formation scenarios are feasible it is shown that post accretionary in situ impact melting or metallic melt injection forming mssi on the thin section scale and nebular condensation are unlikely formation processes this leads to the conclusion that mssis are pre accretionary melt objects that were formed during melting processes prior to the accretion of the primitive e chondrites the same can be concluded for metal nodules in the eh3 chondrite examined the pre accretionary origin of mssis in e chondrites is consistent with a growing body of evidence for early differentiation followed by impact disruption of early formed planetesimals in all major chondrite types

origin of kamacite schreibersite and perryite in metal sulfide nodules of the enstatite chondrite sahara 97072 eh3 perryite fe ni x si p y schreibersite fe ni 3p and kamacite afeni are constituent minerals of the metal sulfide nodules in the sahara 97072 eh3 enstatite chondrite meteorite we have measured concentrations of ni cu ga au ir ru and pd in these minerals with laser ablation inductively coupled plasma mass spectrometry icp ms we also measured their fe ni p si and co concentrations with electron microprobe in kamacite ratios of ru ir pd ir and pd ru cluster around their respective ci values and all elements analyzed plot near the intersection of the equilibrium condensation trajectory versus ni and the respective ci ratios in schreibersite the pd ru ratio is near the ci value and perryite contains significant cu ga and pd we propose that schreibersite and perryite formed separately near the condensation temperatures of p and si in a reduced gas and were incorporated into fe ni alloy upon further cooling sulfidation of fe in kamacite resulted in the formation of additional perryite at the sulfide interface still later transient heating re melted this perryite near the fe fes eutectic temperature during partial melting of the metal sulfide nodules the metal sulfide nodules are pre accretionary objects that retain ci ratios of most siderophile elements although they have experienced transient heating events

shock induced black veins and organic compounds in ordinary chondrites abstract a detailed study of barbotan h5 and charsonville h6 and a partial study of aa pervomaisky l6 indicates that shock induced black veins differ from the host part of chondrites by the following peculiarities 1 1 smaller quantities of na and quite possible fe ni co and in some cases ir 2 2 the presence of a major portion of glass and melted structures of troilite metal mixture 3 3 different composition of metal troilite ca poor pyroxene feldspar and some grains of olivine 4 4 the presence of organic compounds organic matter in h chondrites is identical and corresponds to paraffins and in l chondrite to aliphatic hydrocarbons and carbonyl and n containing compounds their presence in black veins may be related to two possible sources 1 1 contamination or 2 2 cosmic origin the character of distribution and disposition of organic compounds in the close pores of veins and their chemical interrelation with chemical group of chondrites allows speculation about a cosmic origin of organic matter related to shock metamorphism

the gao guenie impact melt breccia sampling a rapidly cooled impact melt dike on an h chondrite asteroid the gao guenie h5 chondrite that fell on burkina faso march 1960 has portions that were impact melted on an h chondrite asteroid at 300 ma and through later impact events in space sent into an earth crossing orbit this article presents a petrographic and electron microprobe analysis of a representative sample of the gao guenie impact melt breccia consisting of a chondritic clast domain quenched melt in contact with chondritic clasts and an igneous textured impact melt domain olivine is predominantly fo80 82 the clast domain contains low ca pyroxene impact melt grown pyroxene is commonly zoned from low ca pyroxene in cores to pigeonite and augite in rims metal troilite orbs in the impact melt domain measure up to 2 mm across the cores of metal orbs in the impact melt domain contain 7 9 wt of ni and are typically surrounded by taenite and ni rich troilite the metallography of metal troilite droplets suggest a stage i cooling rate of order 10 c s 1 for the superheated impact melt the subsolidus stage ii cooling rate for the impact melt breccia could not be determined directly but was presumably fast an analogy between the ni rim gradients in metal of the gao guenie impact melt breccia and the impact melted h6 chondrite orvinio suggests similar cooling rates probably on the order of 5000 40 000 c yr 1 a simple model of conductive heat transfer shows that the gao guenie impact melt breccia may have formed in a melt injection dike 0 5 5 m in width generated during a sizeable impact event on the h chondrite parent asteroid

a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body abstract we have studied iab iron meteorites and their silicate bearing inclusions to elucidate the origin of their parent body we have divided iab irons into five categories which best describe the inclusions and other properties of the irons these are 1 sulfide rich irons such as mundrabilla that contain a significant volume fraction of troilite with or without associated silicates 2 irons with non chondritic silicate inclusions such as basaltic inclusions in caddo county 3 irons with rounded troilite and or graphite inclusions sometimes containing embedded angular silicate inclusions 4 irons with angular silicate inclusions similar to those of the winonaites where silicates may comprise up to 40 vol of the rock and 5 irons with phosphate bearing inclusions such as the brianite bearing iab san cristobal similarities in mineralogy mineral and oxygen isotopic compositions suggest that iab irons and winonaites are almost certainly from a common parent body iiicd irons may also have originated on this parent body although evidence supporting this suggestion is less convincing previous models for the origin of iab irons propose that they are either products of selective impact melting in the megaregoliths of a chondritic asteroid or products of partial melting and fractional crystallization during formation of a sulfur rich core the former model suffers from the fact that impact is incapable of causing preferential melting of metal and sulfide on a large scale and pooling of these selective melts into sizable bodies the latter model does not readily explain the occurrence of often fragmentary angular silicate clasts in a metallic fe ni host we propose a new model for the origin of iab irons that incorporates aspects of both previous models we suggest that a chondritic possibly heterogeneous parent body was heated by a non collisional heat source which caused partial but not complete melting of the body after peak temperatures were reached and cooling and crystallization had begun a major impact catastrophically fragmented this body and the debris gravitationally reassembled scrambling the parent body lithologies we suggest that the impact caused extensive mixing of molten metallic fe ni and solid silicate material resulting in the formation of iab irons with silicate inclusions whereas winonaites represent metamorphosed rocks and partial melt residues of the precursor silicate lithologies high temperatures of some of the reassembled debris and deep burial resulted in continued fractionation and formation of the 2

first discovery of stishovite in an iron meteorite the first occurrence of stishovite in an iron meteorite muonionalusta group iva is reported the mineral occurs intimately mixed with amorphous silica forming tabular grains up to 3 mm wide with a hexagonal outline it was identified using x ray diffraction and raman microspectroscopy the unit cell parameters of stishovite are a 4 165 3 a and c 2 661 6 a and its chemical composition is nearly pure sio2 raman spectra show relatively sharp bands at 231 and 754 cm 1 and a broad band with an asymmetric shape and a maximum around 500 cm 1 the rare grains are found within troilite nodules together with chromite daubreelite and schreibersite from their composition and morphology and by comparisons with silica inclusions in e g the gibeon iva iron we conclude that these rare grains represent pseudomorphs after tridymite the presence of stishovite in muonionalusta is suggested to reflect shock metamorphic conditions in the iva parent asteroid during a cosmic impact event

shock experiments with the h6 chondrite kernouve pressure calibration of microscopic shock effects shock recovery experiments were carried out on samples of the h6 chondrite kernouve at shock pressures of 10 15 20 25 30 35 45 and 60 gpa and preheating temperatures of 293 k low temperature experiments and 920 k high temperature experiments using a calculated equation of state of kernouve pressure pulse durations of 0 3 to 1 2 s were estimated the shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine orthopyroxene oligoclase and troilite the following pressure calibration is proposed for silicates 1 undulatory extinction of olivine gpa 2 weak mosaicism of olivine from 10 15 gpa to 20 25 gpa 3 onset of strong mosaicism of olivine at 20 25 gpa 4 transformation of oligoclase to diaplectic glass completed at 25 30 gpa low temperature experiments and at 20 25 gpa high temperature experiments 5 onset of weak mosaicism in orthopyroxene at 30 35 gpa low temperature experiments and at 25 30 gpa high temperature experiments and 6 recrystallization or melting of olivine starting at 45 60 gpa low temperature experiments and at 35 45 gpa high temperature experiments and completed above 45 60 gpa in the high temperature experiments troilite displays distinct differences between the samples shocked at low and high temperatures in the low temperature experiments the following effects can be observed in troilite 1 undulatory extinction up to 25 gpa 2 twinning up to 45 gpa 3 partial recrystallization from 30 to 60 gpa and 4 complete recrystallization 35 gpa whereas in the high temperature experiments troilite shows 1 complete recrystallization from 10 up to 45 gpa and 2 melting and crystallization above 45 gpa localized shock induced melting is observed in samples shocked to pressures 15 gpa in the high temperature experiments and 30 gpa for the low temperature experiments in the form of feni metal and troilite melt injections and intergrowths and as pockets and veins of whole rock melt obviously the onset and abundance of shock induced localized melting strongly depends on the initial temperature of the sample

mineralogy and petrology of the abee enstatite chondrite breccia and its dark inclusions the abee e4 enstatite chondrite breccia consists of clasts many rimmed by metallic fe ni dark inclusions and matrix the clasts and matrix were well equilibrated by thermal metamorphism as evidenced by uniform mineral compositions recrystallized chondrules low mno content of enstatite and high abundance of orthoenstatite the clasts acquired their metal rich rims prior to this metamorphic episode the occurrence in abee of relatively unmetamorphosed dark inclusions clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation 18 19 indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia the dark inclusions are an unusual kind of enstatite chondritic material distinguished from the clasts and matrix by their relative enrichments in ree 21 23 low relative abundances of kamacite total metallic fe ni and silica lower niningerite total sulfide ratios high relative abundances of oldhamite and martensite smaller euhedral enstatite more heterogeneous enstatite and metallic fe ni more calcic enstatite and more nickeliferous schreibersite we propose the following model for the petrogenesis of the abee breccia the maximum metamorphic temperature of breccia parent material was 840 c the minimum temperature of formation of abee niningerite and perhaps near 950 1000 c the fe ni s eutectic temperature euhedral enstatite crystals in metallic fe ni and sulfide rich areas grew at these metamorphic temperatures into pliable metal and sulfide breccia parent material was impact excavated from depth admixed with dark inclusions and rapidly cooled 700 to 200 c in about 2 hours 15 during this cooling clast and matrix material acquired thermal remanent magnetization random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix a subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random the abee breccia was later consolidated possibly by shock or by shallow burial and very long period low temperature 215 c metamorphism

shock features in iron nickel metal and troilite of l group ordinary chondrites shock metamorphic features in opaque minerals feni metal and troilite of 22 l chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate based shock classification scheme stoffler et al 1991 and the peak shock and postshock thermal history recorded in these minerals unshocked to weakly shocked s1 s3 l chondrites contain feni metal and troilite that display textures related to normal slow cooling they may also contain rare disequilibrium shock features which suggest localized departures from equilibrium shock conditions above shock stage s3 selected melting of feni metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample at these higher shock levels the abundance of other shock induced features such as polycrystalline kamacite sheared and fizzed troilite coarse grained pearlitic plessite polycrystalline troilite and polymineralic melt veins serve as textural criteria that can be used to establish peak shock conditions minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage thereby providing additional information about the postshock thermal histories of l chondrites at the highest shock levels recorded in l chondrites s6 and above melting and chemical homogenization of feni metal produces flattened ni profiles that may partially to completely obscure any evidence for an earlier slow cooling history all of these features serve as aids for shock classifying l chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism

possible impact induced refractory lithophile fractionations in el chondrites abstract literature data show that refractory lithophile elements in most chondrite groups are unfractionated relative to ci chondrites the principal exception is the el chondrite group whose observed falls all of which are type 6 are depleted in ca and light ree in contrast literature data and our new inaa data on el3 pca 91020 el3 mac 88136 and el4 grein 002 show that some replicates of these samples have nearly flat ree patterns unlike those of el6 chondrites other replicates exhibit fractionated ree patterns similar to those of el6 chondrites petrographic examination shows that many el6 and some el3 and el4 chondrites are impact melt breccias or contain impact melted portions we suggest that the same impact processes that formed these breccias and produced melt are responsible for the observed bulk compositional fractionations in refractory lithophile elements i e el6 chondrites were produced from initially unequilibrated el3 material when large amounts of impact heat were deposited plagioclase and or oldhamite cas the major ree carriers in enstatite chondrites may have been melted and then transported appreciable 10 cm distances el6 chondrites represent the residuum that is depleted in ree particularly in lree and ca unlike the case for el chondrites our new inaa data on alh 84170 eet 87746 and sah 97096 all eh3 show some scatter but are consistent with the eh group having uniform refractory lithophile abundances

cat mountain a meteoritic sample of an impact melted asteroid regolith cat mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule bearing clasts of l5 material these clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years probably within a melt breccia lens in the bottom of a large 1 km diameter crater on an l chondrite asteroid noble gas isotopes indicate that the sample was involved in at least two different impact events approximately 880 and 20 myr ago following the 4 55 ga accretion of primitive chondritic material the 880 ma event is responsible for the impact breccia texture of the sample and the 20 ma event reduced the sample to a meter sized object we also infer that another impact occurred between 880 and 20 ma possibly the 500 ma event recorded in many other l chondrites to jettison the material from the asteroid belt into an orbit that evolved into an earth crossing trajectory the shock metamorphic processes that occurred at 880 ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases this reduced the reflectance of the l5 material and decreased the amplitude of its spectral absorption features these characteristics are consistent with the spectral characteristics of some c class asteroids and suggest that some dark asteroids that appear to belong to the c class could be covered with shocked ordinary chondrite material if one assumes that cat mountain came from the same asteroid as other l chondrites with the same cosmic ray exposure age then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m furthermore the structural integrity of cat mountain and other l chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere however in a regime where the asteroid is the target of impact fragmentation rather than the projectile the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy in this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material

origin of rapidly solidified metal troilite grains in chondrites and iron meteorites inclusions of troilite and metallic fe ni 0 2 4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites cooling rates in the interval 1400 950 c calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10 7 to 104 c sec in 8 of these chondrites which are breccias containing some normal slow cooled metal grains the inclusions solidified before they were incorporated into the breccias their cooling rates of 1 300 c sec indicate cooling by radiation or by conduction in contact with cold silicate or hot silicate volumes only 6 40 mm in size this is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body by impact and were not formed at depth from an internally derived melt in ramsdorf rose city and shaw which show extensive reheating to 1000 c fe fes textures in melted areas are coarser and indicate cooling rates of 10 1 to 10 4 c sec during solidification this metal may have solidified inside hot silicate volumes that were 10 300 cm in size as shaw and rose city are breccias of unmelted and melted material their melted metal did not necessarily cool through 1000 c within a few m of the surface shock melted fine grained irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of 105 c sec their kamacite and taenite compositions may result from annealing at 250 c of metallic glass or exceedingly fine grained quench products

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

mineralogy petrology and distribution of meteorites at the whitecourt crater alberta canada the whitecourt meteorite impact crater alberta canada is a rare example of a well preserved small impact structure with which thousands of meteorite fragments are associated as such this crater represents a unique opportunity to investigate the effect of a low energy impact event on an impacting iron bolide excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types the meteorites distribution and internal and external characteristics support a low altitude breakup of the impactor which caused atmospherically ablated regmaglypted meteorites to fall close to the crater and avoid impact related deformation in contrast shrapnel fragments sustained deformation at macro and microscales resulting from the catastrophic disruption of the impactor the impactor was significantly fragmented along pre existing planes of weakness including kamacite lamellae and inclusions resulting in a bias toward low mass 100 g fragments meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite consistent with other low ni iiiab iron meteorites considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity lower mass impactor than previously inferred

shock metamorphism of enstatite chondrites abstract we have extended the shock metamorphism classification scheme of stoffler et al 1991 to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites orthopyroxene exhibits the following sequence of progressive shock effects shock stage s1 unshocked sharp optical extinction s2 very weakly shocked undulose extinction s3 weakly shocked development of clinoenstatite lamellae parallel to 100 s4 moderately shocked weak mosaicism s5 strongly shocked strong mosaicism as in the stoffler et al 1991 scheme stage s5 is characterized principally by the solid state transformation of crystalline plagioclase into maskelynite most el3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles el5 and el6 chondrites are all shock stage s2 because opaque veins are rare in s2 ordinary chondrites it is plausible that the centimeter size kamacite veins in atlanta and blithfield and the 1 6 cm long oldhamite rich vein in jajh deh kot lalu all el6 formed when their hosts were shocked to s3 s5 levels because removal of shock stage s3 s5 features including repair of shock damaged orthopyroxene requires levels of metamorphism comparable to those experienced by petrologic type 5 to 6 chondrites we infer that proto el6 material was shocked to s3 s5 levels prior to peak metamorphism and shocked again to stage s2 after metamorphism overall enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites

the role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams abstract by integrating the results of recent studies on the concentration and flux of si in european forest soils podzols and acid brown soils with published evidence on the amounts of si cycled through forest vegetation we conclude that phytoliths must be the principal immediate source and sink of silica in soil solution although mineral weathering is the ultimate source during the growing season forest vegetation takes up most of the si made available by weathering of soil minerals and by phytolith dissolution and converts it into phytoliths phytoliths will be the principal source of si leached from the soil during winter rains and spring snowmelt this work accounts for the plateau concentrations of si observed in laboratory equilibrations of bs horizon soils for the maximum si concentration reported for b horizons in the field for a deep sink for si in bs horizons and for the fact that the concentrations of si in soil drainage water and in streams all fall in or below the range expected for phytolith solubility concentrations of si in surface horizons can exceed these levels during droughts the common assumption that the concentration of si in soil solution is a result of a dynamic balance between si released by mineral weathering and that lost by leaching is untenable for bs horizons

metallic minerals thermal histories and parent bodies of some xenolithic ordinary chondrite meteorites we have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes using reflected light microscopy and electron probe analysis in weston and fayetteville which both contain solar flare tracks and solar wind gases kamacite taenite and tetrataenite ordered feni and troilite show a variety of textures on a wood plot of central ni content vs dimension taenite analyses scatter as if metal grains cooled at rates of 10 1000 and 1 100 k myr respectively through 700 k although metal in an h6 clast in fayetteville plots coherently with a cooling rate of 50 k myr we propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted and were not subsequently heated above 650 k we predict a similar history for all gas rich ordinary chondrites by contrast metallic minerals throughout bhola and mezo madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to 600 k of 0 1 and 1 k myr respectively we conclude that host and xenoliths in both chondrites were slowly cooled after compaction thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 k in different environments according to the conventional onion shell model for h l or ll chondrite parent bodies material of petrologic types 3 5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately but if peak metamorphic temperatures were reached during not after accretion as seems plausible maximum metamorphism may have occurred in planetesimals 10 km in radius cooling through 700 k may then have occurred in larger bodies that accreted from these planetesimals iron meteorites mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies

carlisle lakes and allan hills 85151 members of a new chondrite grouplet carlisle lakes and allan hills 85151 alh85151 are members of a new grouplet of chondrites characterized by feo rich silicates olivine fa 38 41 mol unusual oxygen isotopic composition and abundant 35 vol recrystallized matrix material yamato 75302 is not as well characterized but is probably also a member of this grouplet siderophile elements in carlisle lakes and alh85151 are fractionated relative to ci chondrites refractory and common siderophiles are at 1 ci ga and as are lower 0 6 0 9 ci and au is very low 0 17 ci and 0 36 ci in carlisle lakes and alh85151 respectively the low au abundance may indicate that au resided mainly in fine grained metal particles that were suspended in the nebular gas during the time these chondrites agglomerated the oxidation state of chondrites may be controlled by the presence of oxidizing agents e g magnetite in agglomerated matrix material the high matrix abundance that characterizes this new grouplet may thus be responsible for the high oxidation state of these meteorites

spade an h chondrite impact melt breccia that experienced post shock annealing the low modal abundances of relict chondrules 1 8 vol and of coarse i e 200 m size isolated mafic silicate grains 1 8 vol in spade relative to mean h6 chondrites 11 4 and 9 8 vol respectively show spade to be a rock that has experienced a significant degree of melting various petrographic features e g chromite plagioclase assemblages chromite veinlets silicate darkening indicate that melting was caused by shock plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains during crystallization plagioclase developed igneous zoning low ca pyroxene that crystallized from the melt or equilibrated with the melt at high temperatures acquired relatively high amounts of cao metallic fe ni cooled rapidly below the fe ni solvus and transformed into martensite subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite ambiguities exist in the shock stage assignment of spade the extensive silicate darkening the occurrence of chromite plagioclase assemblages and the impact melted characteristics of spade are consistent with shock stage s6 low shock stage s2 is indicated by the undulose extinction and lack of planar fractures in olivine this suggests that spade reached a maximum prior shock level equivalent to stage s6 and then experienced post shock annealing probably to stage s1 these events were followed by a less intense impact that produced the undulose extinction in the olivine characteristic of shock stage s2 annealing could have occurred if spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket spade firmly establishes the case for post shock annealing this may have been a common process on oc asteroids

formation of large metal nodules in ordinary chondrites literature data show that many large 2 30 mm metallic fe ni nodules in ordinary chondrites are depleted in refractory siderophiles e g re os ir ru pt and rh and cu relative to fine grained metal but are not depleted in normally refractory w and mo common siderophiles e g ni and co and volatile siderophiles e g au as and sb it was recently proposed that metamorphic diffusion could form the metal nodules and that partitioning of siderophile elements between kamacite and taenite could account for the nodules unusual compositions however the nodules have the same compositional trends as metal nodules and veins in shocked meteorites that clearly formed by impact related processes if large metal nodules formed during metamorphism type 6 chondrites should contain abundant large nodules instead the mean metal grain size in h6 chondrites is 0 12 0 15 mm far smaller than any of the nodules it seems likely that the metal nodules formed by impact vaporization oxidation of w and mo to form volatile oxides concomitant partial oxidation of re and os rapid fractional condensation of refractory siderophiles transport of the residual vapor condensation of cu s and se into vugs and fractures to form sulfide and condensation of the remaining siderophiles to form metal nodules and veins during vapor cooling and condensation or during subsequent parent body annealing w mo re and os oxides were reduced

smyer h chondrite impact melt breccia and evidence for sulfur vaporization smyer is an h chondrite impact melt breccia containing 20 vol 0 5 to 13 mm thick silicate rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension at the interface between some of the melt veins and chondritic clasts there are troilite rich regions consisting of unmelted crushed 0 2 to 140 m size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins troilite fills every available fracture in the silicates including some as thin as 0 1 m little metallic fe ni is present in these regions the fes fe modal ratio ranges from 25 1 to 500 1 far higher than the eutectic weight ratio of 7 5 1 the texture of these regions indicates that the sulfide formed from a fluid of very low viscosity the moderately high viscosity 0 2 poise and large surface tension of liquid fes its inability to wet silicate grain surfaces at low oxygen fugacities and the supereutectic fes fe ratios in the troilite rich regions indicate that the fluid was a vapor it seems likely that during the shock event that melted smyer many silicates adjacent to the melt veins were crushed upon release of shock pressure some of the troilite evaporated and dissociated molecules of s2 were transported and condensed into fractures and around tiny silicate grains there they combined with fe from small adjacent metallic fe ni grains to form troilite the ni content at the edges of some of these metal grains increased significantly co from these ni rich grains diffused into nearby kamacite impact induced s volatilization may have played a major role in depleting the surface of 433 eros and other chondritic asteroids in s

chromite plagioclase assemblages as a new shock indicator implications for the shock and thermal histories of ordinary chondrites abstract chromite in ordinary chondrites oc can be used as a shock indicator a survey of 76 equilibrated h l and ll chondrites shows that unshocked chromite grains occur in equant subhedral and rounded morphologies surrounded by silicate or intergrown with metallic fe ni and or troilite some unmelted chromite grains are fractured or crushed during whole rock brecciation others are transected by opaque veins the veins form when impacts cause localized heating of metal troilite intergrowths above the fe fes eutectic 988 c mobilization of metal troilite melts and penetration of the melt into fractures in chromite grains chromite plagioclase assemblages occur in nearly every shock stage s3 s6 oc the assemblages range in size from 20 300 m and consist of 0 2 20 m size euhedral subhedral anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition plagioclase has a low impedance to shock compression heat from shock melted plagioclase caused adjacent chromite grains to melt chromite grains crystallized from this melt those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in al 2 o 3 than unmelted matrix chromite grains in the same meteorite chromite veinlets typically 0 5 2 m thick and 10 300 m long occur typically in the vicinity of chromite plagioclase assemblages the veinlets formed from chromite plagioclase melts that were injected into fractures in neighboring silicate grains chromite crystallized in the fractures and the residual plagioclase rich melt continued to flow eventually pooling to form plagioclase rich melt pockets chromite rich chondrules consisting mainly of olivine plagioclase normative mesostasis and 5 15 vol chromite occur in many shocked oc and oc regolith breccias but they are absent from primitive type 3 oc they may have formed by impact melting chromite plagioclase and adjacent mafic silicates during higher energy shock events the melt was jetted from the impact site and formed droplets due to surface tension crystallization of these droplets may have commenced in flight prior to landing on the parent body surface chromite plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock stage s1 oc of petrologic type 5 and 6 that i examined although these rocks contain unstrained olivine with sharp optical extinction most possess other shock indicators such as extensive silicate darkening numerous occurrences of metallic cu polycrystalline troilite and opaque veins it seems likely that these rocks were shocked to levels at least as high as shock stage s3 and then annealed by heat generated during the shock event during annealing the olivine crystal lattices healed but other shock indicators survived published ar ar age data for some s1 oc indicate that many shock and annealing events occurred very early in the history of the parent asteroids the common occurrence of shocked and annealed oc is consistent with collisions being a major mechanism responsible for metamorphosing oc

petrogenesis of acapulcoites and lodranites a shock melting model abstract acapulcoites are modeled as having formed by shock melting cr like carbonaceous chondrite precursors the degree of melting of some acapulcoites was low enough to allow the preservation of 3 6 vol relict chondrules shock effects in acapulcoites include veins of metallic fe ni and troilite polycrystalline kamacite fine grained metal troilite assemblages metallic cu and irregularly shaped troilite grains within metallic fe ni while at elevated temperatures acapulcoites experienced appreciable reduction because graphite is present in some acapulcoites and lodranites it seems likely that carbon was the principal reducing agent reduction is responsible for the low contents of olivine fa 4 14 mol and low ca pyroxene fs 3 13 mol in the acapulcoites the observation that in more than two thirds of the acapulcoites the fa value is lower than the fs value in contrast to the case for equilibrated ordinary chondrites the low feo mno ratios in acapulcoite olivine 16 18 compared to 32 38 in equilibrated h chondrites the relatively high modal orthopyroxene olivine ratios e g 1 7 in monument draw compared to 0 74 in h chondrites and reverse zoning in some mafic silicate grains lodranites formed in a similar manner to acapulcoites but suffered more extensive heating loss of plagioclase and loss of an fe ni s melt acapulcoites and lodranites experienced moderate post shock annealing presumably resulting from burial beneath material of low thermal diffusivity the annealing process repaired damaged olivine crystal lattices lending acapulcoites and lodranites the appearance of unshocked i e shock stage s1 rocks any high pressure phases that may have formed during initial shock reverted to their low pressure polymorphs during annealing some samples were subsequently shocked again several acapulcoites reached shock stage s2 levels alh 84190 reached s3 and the lodranite mac 88177 reached s5

impact melt products of chondritic material a large variety of objects in chondritic meteorites formed as a result of impact melting shock veins metal troilite mixtures metal and sulfide nodules melt pockets vugs agglutinates glassy and crystalline spherules and numerous types of melt rock clasts the type of object produced in an impact event is related to the interaction of the shock waves with the particular target rock impact melting has also affected large portions of several chondrite breccias many of these contain vugs and patches of silicate glass some workers have suggested that various iron meteorites including groups iab iiicd and iie as well as several ungrouped irons were formed from individual impact melt pools in chondritic regoliths the eagle station trio pallasites and several ungrouped meteorites bencubbin weatherford enon acapulco and allan hills a77081 may have formed from impact melted chondritic material it is also possible that the parent magmas of ureilites were derived from impact melted metal rich carbonaceous chondrite material

formation of feldspathic and metallic melts by shock in enstatite chondrite reckling peak a80259 the enstatite chondrite reckling peak rkp a80259 contains feldspathic glass kamacite troilite and unusual sets of parallel fine grained enstatite prisms that formed by rapid cooling of shock melts metallic fe ni and troilite occur as spherical inclusions in feldspathic glass reflecting the immiscible fe ni s and feldspathic melts generated during the impact the fe ni s and feldspathic liquids were injected into fractures in coarse grained enstatite and cooled rapidly resulting in thin 10 m semicontinuous to discontinuous veins and inclusion trails in host enstatite whole rock melt veins characteristic of heavily shocked ordinary chondrites are conspicuously absent raman spectroscopy shows that the feldspathic material is a glass elevated mgo and sio2 contents of the glass indicate that some enstatite and silica were incorporated in the feldspathic melt metallic fe ni globules are enclosed by sulfide and exhibit nienrichment along their margins characteristic of rapid crystallization from a fe ni s liquid metal enclosed by sulfide is higher in si and p than metal in feldspathic glass and enstatite possibly indicating lower o fugacities in metal sulfide than in silicate domains fine grained elongate enstatite prisms in troilite or feldspathic glass crystallized from local pyroxene melts that formed along precursor grain boundaries but most of the enstatite in the target rock remained solid during the impact and occurs as deformed coarsegrained crystals with lower cao al2o3 and feo than the fine grained enstatite reckling peak a80259 represents an intermediate stage of shock melting between unmelted e chondrites and whole rock shock melts and melt breccias documented by previous workers the shock petrogenesis of rkpa80259 reflects the extensive impact processing of the enstatite chondrite parent bodies relative to those of other chondrite types

the galim ll eh polymict breccia evidence for impact induced exchange between reduced and oxidized meteoritic material galim is a polymict breccia consisting of a heavily shocked shock stage s6 ll6 chondrite galim a and an impact melted eh chondrite galim b relict chondrules in galim b served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt many of the reduced phases typical of eh chondrites e g si bearing metallic fe ni ti bearing troilite are absent galim b was probably shock melted while in contact with a more oxidized source namely galim a during this event si was oxidized from the metal and ti was oxidized from troilite galim a contains shock veins and recrystallized unzoned olivine the absence of evidence for reduction in galim a may indicate that the amount of ll material greatly exceeded that of eh material shock metamorphism may have taken place on the ll parent body shock induced redox reactions such as those inferred for the galim breccia appear to be restricted mainly to asteroids because the low end tail of their relative velocity distribution permits mixing of intact disparate materials including accretion of projectiles of different oxidation states whereas the peak of the distribution leads to high equilibration shock pressures allowing impact induced exchange between previously accreted disequilibrated materials galim probably formed by a two stage process 1 accretion to the ll parent body of an intact eh projectile at low relative velocities and 2 shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities

impact melting in the cumberland falls and mayo belwa aubrites abstract six chondritic clasts in the cumberland falls polymict breccia were examined four texturally resemble ordinary chondrites ocs and two are impact melt breccias containing shocked oc clasts adjacent to a melt matrix the six chondritic clasts are probably remnants of a single oc projectile that was heterogeneously shocked when it collided with the cumberland falls host mayo belwa is the first known aubrite impact melt breccia it contains coarse enstatite grains exhibiting mosaic extinction the enstatite grains are surrounded by a melt matrix composed of 3 16 m size euhedral and subhedral enstatite grains embedded in sodic plagioclase numerous vugs ranging from a few micrometers to a few millimeters in size constitute 5 vol of the meteorite they occur nearly exclusively within the mayo belwa matrix literature data show that some vugs are lined with bundles of acicular grains of the amphibole fluor richterite this phase has been reported previously in only two other enstatite meteorites abee and st sauveur both of which are eh chondrite impact melt breccias it seems likely that in mayo belwa volatiles were vaporized during an impact event and formed bubbles in the melt as the melt solidified the bubbles became cavities plagioclase and fluor richterite crystallized at the margins of these cavities via reaction of the melt with the vapor

sulfide rich metallic impact melts from chondritic parent bodies sacramento wash 005 saw 005 meteorite hills 00428 met 00428 and mount howe 88403 how 88403 are s rich fe ni rich metal meteorites with fine metal structures and homogeneous troilite we compare them with the h metal meteorite lewis cliff 88432 phase diagram analyses suggest that saw 005 met 00428 and how 88403 were liquids at temperatures above 1350 c tridymite in how 88403 constrains formation to a high temperature and low pressure environment the morphology of their metal troilite structures may suggest that met 00428 cooled the slowest saw 005 cooled faster and how 88403 cooled the quickest saw 005 and met 00428 contain h chondrite like silicates and saw 005 contains a chondrule bearing inclusion that is texturally and compositionally similar to h4 chondrites the compositional and morphological similarities of saw 005 and met 00428 suggest that they are likely the result of impact processing on the h chondrite parent body saw 005 and met 00428 are the first recognized iron and sulfide rich meteorites which formed by impact on the h chondrite parent body which are distinct from the iie iron meteorite group the morphological and chemical differences of how 88403 suggest that it is not from the h chondrite body although it likely formed during an impact on a chondritic parent body

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

simulation experiments for shocked primitive materials in the solar system abstract shock recovery experiments were conducted on porous mixtures initial porosity 35 5 in silicate metal and silicate metal sulfide systems in an attempt to simulate impact phenomena of unconsolidated porous materials similar to ordinary chondrites in a wide range of shock pressures from 20 to 70 gpa the textures and chemical compositions of shocked samples were investigated in detail features such as grain deformation fracture density decrease and heterogeneous meltings are found at

ordinary chondrite metallography part 2 formation of zoned and unzoned metal particles in relatively unshocked h l and ll chondrites we studied the metallography of fe ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of p free fe ni alloy cooling experiments described in reisener and goldstein 2003 two types of fe ni metal particles were observed in the chondrites zoned taenite kamacite particles and zoneless plessite particles which lack systematic ni zoning and consist of tetrataenite in a kamacite matrix both types of metal particles formed during metamorphism in a parent body from homogeneous p poor taenite grains the phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles polycrystalline taenite particles transformed to zoned taenite kamacite particles by kamacite nucleation at taenite taenite grain boundaries during cooling monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process the varying proportions of zoned taenite kamacite particles and zoneless plessite particles in types 46 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles the distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of fe ni alloy phase transformations during cooling the distribution cannot be explained by hot accretion autometamorphism post metamorphic brecciation or shock processing

the nature and significance of stacking faults in wadsleyite natural β mg fe 2sio4 from the peace river meteorite abstract the phase spinelloid polymorph of mg fe 2 sio 4 makes up a major part of the transition of the earth s mantle naturally occurring mg fe 2 sio 4 wadsleyite from the peace river meteorite was found to carry a variety of stacking faults the nature of which have been studied using high resolution transmission electron microscopy the faults lie on 010 and are generally of a complex nature best described in terms of various stacking sequences of the component spinelloid units the stacking faults locally transform the cation distribution so that in the plane of the fault the structure is that of spinel the development of such stacking faults is consequently a significant feature of the martensitic transformation of spinel to phase the possible occurrence of transformation enhanced plasticity associated with this inversion is discussed and the probable deformation mechanisms of mg fe 2 sio 4 are outlined

the evolution of partially differentiated planetesimals evidence from iron meteorite groups iab and iiicd group iab irons probably come from a parent body that partially differentiated into a sulfur rich molten core and a primitive silicate outer layer in which only minor or localized melting occurred although the complexity of meteorites from this parent body or bodies irons with and without massive silicate inclusions stony meteorites like winona sulfide rich meteorites like mundrabilla etc implies a complicated history the partial differentiation model is consistent with most properties of these meteorites the behavior of five model parent bodies of different composition during heating to 1400 k the temperature recorded in many i ab silicates shows that meteorites like iab irons can form by this process sulfide rich parent bodies differentiate into a liquid s rich core and a primitive metal bearing mantle around 1230 k but in parent bodies lower in sulfide abundance this process may be delayed until minor melting of silicates occurs 1370 30 k in most cases any silicate melt that may have formed resolidified in situ the sulfide melt fractionally crystallized along the fe ni fes cotectic some melt was injected into silicates to form copiapo type breccias considerations of graphite metal silicate equilibrium and the retention of noble gases suggest that brecciated iabs formed at depth inside a moderately large a few tens to a hundred kilometers parent body the correlation of i xe ages with ni content is interpreted as the result of an excess 129xe component associated with the s rich melt which increases with melt evolution

impact melt rock clasts in the hvittis enstatite chondrite breccia implications for a genetic relationship between el chondrites and aubrites the hvittis enstatite chondrite breccia contains 5 2 vol dark clasts which appear to be impact melt rocks the largest clast clast a is 2 5 cm in diameter it has an igneous texture no chondrules and is depleted in metallic fe ni and sulfide its fine grain size low ni kamacite and schreibersite and rare martensite resulted from rapid cooling the abundance of melt rock clasts in hvittis suggests that the breccia resided at its parent body surface however the apparent absence of el chondrites with solar wind implanted rare gases suggests that the el parent body was too small to develop a substantial regolith the bulk composition of clast a is very similar to that of average aubrites except for a sevenfold enrichment in s cr and mn these elements are concentrated in the sulfides it thus is possible that the clast was derived from one of the sulfide rich regions of the hvittis matrix if so then aubrites may be genetically related to el chondrites

a composite fe ni fes and enstatite forsterite diopside glass vitrophyre clast in the larkman nunatak 04316 aubrite origin by pyroclastic volcanism we studied the mineralogy petrology and bulk trace element oxygen and noble gas isotopic compositions of a composite clast approximately 20 mm in diameter discovered in the larkman nunatak lar 04316 aubrite regolith breccia the clast consists of two lithologies one is a quench textured intergrowth of troilite with spottily zoned metallic fe ni which forms a dendritic or cellular structure the approximately 30 m spacings between the fe ni arms yield an estimated cooling rate of this lithology of approximately 25 30 c s 1 the other is a quench textured enstatite forsterite diopside glass vitrophyre lithology the composition of the clast suggests that it formed at an exceptionally high degree of partial melting perhaps approaching complete melting and that the melts from which the composite clast crystallized were quenched from a temperature of approximately 1380 1400 c at a rate of approximately 25 30 c s 1 the association of the two lithologies in a composite clast allows for the first time an estimation of the cooling rate of a silicate vitrophyre in an aubrite of approximately 25 30 c s 1 while we cannot completely rule out an impact origin of the clast we present what we consider is very strong evidence that this composite clast is one of the elusive pyroclasts produced during pyroclastic volcanism on the aubrite parent body wilson and keil 1991 we further suggest that this clast was not ejected into space but retained on the aubrite parent body by virtue of the relatively large size of the clast of approximately 20 mm our modeling taking into account the size of the clast suggests that the aubrite parent body must have been between approximately 40 and 100 km in diameter and that the melt from which the clast crystallized must have contained an estimated maximum range of allowed volatile mass fractions between approximately 500 and approximately 4500 ppm

a transmission electron microscopy tem investigation of opaque phases in shocked chondrites shock defects in the most common silicate minerals of chondrites olivine pyroxenes and feldspars have been investigated in detail but there have been almost no studies of the shock defects in other components like metal and sulfide this probably stems from the fact that these latter phases are opaque in the optical microscope the same reason explains why veins and melt pockets which are constituted of microcrystalline or glassy phases i e isotropic are also poorly documented we have investigated such phases by analytical transmission electron microscopy atem in two shocked chondrites tenham l6 and gaines county h5 we have characterized shock defects in troilite very similar to those occurring in silicates i e a mosaic texture and sets of straight and very narrow 10 nm lamellae of amorphized fes there are many small regions in shocked chondrites that are composed of very fine grained 1 m mixtures of metal and sulfide or of various silicates they must result from local melting followed by a rapid cooling that prevented grain growth we have determined the chemical compositions and the volume proportions of the tiny grains in these veins and melt pockets which has allowed their temperature and pressure t p history to be partially deciphered finally we have observed a dense network of very narrow fractures down to 10 nm in the olivine and enstatite grains these fractures are systematically filled with an amorphous or cryptocrystalline material that stems from the melt pockets and was injected when the fractures were opened by the rarefaction wave this material was then quenched at the contact with the colder crystalline rims

impact cratering on the h chondrite parent asteroid 1 this paper reports petrological data for lapaz icefield 02240 03922 031125 031173 031308 04462 and 04751 which are meteoritic samples of clast rich impact melt rocks from the h chondrite parent asteroid the size distribution and metallographic characteristics of fe ni metal in the melts indicate very rapid 1 to 40 c s cooling in the temperature range between 1500 and 950 c when the clast melt mixtures were thermally equilibrating cooling slowed to values between 10 3 and 10 2 c s in the temperature range between 700 and 400 c when the melt rocks were cooling to their surroundings these data suggest that the rocks cooled near the surface of the h chondrite asteroid within suevitic impact deposits integrating these data with the petrologic characteristics of other h chondrite melt rocks and their radioisotopic ages indicates that the h chondrite asteroid suffered at least one large impact event while still cooling from endogenous metamorphism at 4500 ma this impact must have degraded the asteroid s integrity but did not cause shattering impact events in the era between 4100 and 3600 ma produced melt volumes large enough to allow segregation of metal and troilite from silicate melts possibly within continuous impact melt sheets contained in craters the impact record after 3600 ma does not display such assemblages which suggests a decrease in the rate of large impact events or a catastrophic size reduction of the h chondrite parent asteroid at around this time

mineralogy petrology chronology and exposure history of the chelyabinsk meteorite and parent body three masses of the chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the chelyabinsk parent body the samples exhibit little to no postentry oxidation via mossbauer and raman spectroscopy indicating their fresh character but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result detailed studies show three distinct lithologies indicative of a genomict breccia a light colored lithology is ll5 material that has experienced thermal metamorphism and subsequent shock at levels near s4 the second lithology is a shock darkened ll5 material in which the darkening is caused by melt and metal troilite veins along grain boundaries the third lithology is an impact melt breccia that formed at high temperatures 1600 c and it experienced rapid cooling and degassing of s2 gas portions of light and dark lithologies from chel 101 and the impact melt breccias chel 102 and chel 103 were prepared and analyzed for rb sr sm nd and ar ar dating when combined with results from other studies and chronometers at least eight impact events e g 4 53 ga 4 45 ga 3 73 ga 2 81 ga 1 46 ga 852 ma 312 ma and 27 ma are clearly identified for chelyabinsk indicating a complex history of impacts and heating events finally noble gases yield young cosmic ray exposure ages near 1 ma these young ages together with the absence of measurable cosmogenic derived sm and cr indicate that chelyabinsk may have been derived from a recent breakup event on an neo of ll chondrite composition

thermal history of the shock melted antarctic ll chondrites from the yamato 79 collection abstract the sr and rare gas isotopic compositions and abundances of lithophile trace elements k rb sr ba and rees were determined for a series of shock melted yamato 79 ll chondrites to investigate their late thermal history and the chemical features of shock processes all meteorites show similarities in shock ages 1 2 ga as confirmed by rb sr internal isochron and k ar dating rare gas compositions as well as cosmic ray exposure ages 28 ma petrographie textures and sampling sites in antarctica these results indicate that all of these meteorites are part of the same fall the 1 2 ga shock event caused a severe partial to total melting followed by recrystallization of olivine and clinopyroxene vesiculation shock induced alkali homogenization and local isotopic equilibration or perturbation of the rb sr system the degrees of shock effects are variable from specimen to specimen and from portion to portion even in a single specimen model calculations of fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body from these model calculations and the evidence provided by cosmogenic rare gas compositions it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth 2 m shielded from cosmic rays in an impact crater by the 1 2 ga event the parent body was fragmented to meter size stones by an additional collision at 28 ma resulting in the formation of the parent material of the yamato 79 shocked chondrites

shock melted material in the krymka ll3 1 chondrite behavior of the opaque minerals six large millimeter to centimeter size regions of one specimen of the krymka ll3 1 ordinary chondrite show evidence of having been completely or nearly completely shock melted in situ a phenomenon rarely observed in primitive chondrites the shock pressure nominally in the range of 75 90 gpa could only have been 30 35 gpa in a porous material like fine grained matrix the melted regions have an igneous texture and their silicates are zoned and unequilibrated large metal troilite intergrowths formed in these regions the metal has a nickel content corresponding to martensite and the troilite contains up to 4 2 wt nickel melting must have been very short and cooling very fast 100 c h at high temperature the metal contains up to 0 7 wt phosphorus abundant chromite crystals and sodium iron phosphate glass globules are found in troilite the differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions surprisingly high co concentrations up to 13 wt were found in some metal grains in or at the periphery of melted regions they likely resulted from sulfurization of metal by sulfur vapor produced during the shock after solidification at least one other shock led to mechanical effects in the melted regions

rotational spectral variations of asteroid 8 flora implications for the nature of the s type asteroids and for the parent bodies of the ordinary chondrites abstract the new interpretive calibrations olivine pyroxene abundance spectral signature of chondritic metal grains have been applied to visible and near infrared spectral reflectance data for the typical s type asteroid 8 flora chondritic metal grains do not exhibit the steeply reddened spectral signature previously associated with a nickel iron nife metal phase the conditions required to produce a s type spectral curve from a chondritic material are much more restrictive and improbable than previously supposed five mineralogic and patrologic parameters have been derived for the surface material of flora from the analysis of the spectral reflectance and rotational spectral variations of this object these include bulk mineralogy nife metal olivine pyroxene mafic mineral abundance ol py 2 8 metal nature abundant 50 and coarse grained in the substrate pyroxene composition orthopyroxene f s 50 and or abundant clinopyroxene and spatial mineralogic variations olivine pyroxene ratio and pyroxene composition do not covary in a chondritic manner all of the diagnostic mineralogic properties of the flora surface material indicate that this is a differentiated body none of the derived surface properties unambiguously support a chondritic type material and several contradict any type of undifferentiated assemblage flora is most probably the residual core of an intensely heated thermally evolved and magmatically differentiated planetesimal which was subsequently disrupted the present surface samples layers formed at and near the core mantle boundary in the parent body several lines of evidence suggest that the s type asteroids as a class are predominantly the core fragments of disrupted differentiated planetesimals a main belt source of the ordinary chondrites remains elusive

twentieth century sea level an enigma changes in sea level relative to the moving crust are associated with changes in ocean volume mostly thermal expansion and in ocean mass melting and continental storage t steric t eustatic t recent compilations of global ocean temperatures by levitus and coworkers are in accord with coupled ocean atmosphere modeling of greenhouse warming they yield an increase in 20th century ocean heat content by 2 1023 j compared to 0 1 1023 j of atmospheric storage which corresponds to greenhouse 2000 3 cm the greenhouse related rate is accelerating with a present value greenhouse 2000 6 cm century tide records going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century we take historic 18 cm century the intergovernmental panel on climate change attributes about 6 cm century to melting and other eustatic processes leaving a residual of 12 cm of 20th century rise to be accounted for the levitus compilation has virtually foreclosed the attribution of the residual rise to ocean warming notwithstanding our ignorance of the abyssal and southern oceans the historic rise started too early has too linear a trend and is too large melting of polar ice sheets at the upper limit of the intergovernmental panel on climate change estimates could close the gap but severe limits are imposed by the observed perturbations in earth rotation among possible resolutions of the enigma are a substantial reduction from traditional estimates including ours of 1 5 2 mm y global sea level rise a substantial increase in the estimates of 20th century ocean heat storage and a substantial change in the interpretation of the astronomic record

the l3 6 chondritic regolith breccia northwest africa nwa 869 i petrology chemistry oxygen isotopes and ar ar age determinations northwest africa nwa 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons it is an l3 6 chondrite and probably represents the largest sample of the rare regolith breccias from the l chondrite asteroid it contains unequilibrated and equilibrated chondrite clasts some of which display shock darkening impact melt rocks imrs both clast free and clast poor are strongly depleted in fe ni metal and sulfides an unequilibrated microbreccia two different light inclusions and two different sio2 bearing objects were found although the matrix of this breccia appears partly clastic it is not a simple mixture of fine grained debris formed from the above lithologies but mainly represents an additional specific lithology of low petrologic type we speculate that this material stems from a region of the parent body that was only weakly consolidated one imr clast and one sio2 bearing object show 17o values similar to bulk nwa 869 suggesting that both are related to the host rock in contrast one light inclusion and one imr clast appear to be unrelated to nwa 869 suggesting that the imr clast is contaminated with impactor material 40ar 39ar analyses of a type 4 chondrite clast yield a plateau age of 4402 7 ma which is interpreted to be the result of impact heating other impact events are recorded by an imr clast at 1790 36 ma and a shock darkened clast at 2216 40 ma demonstrating that nwa 869 escaped major reset in the course of the event at approximately 470 ma that affected many l chondrites