the classification and complex thermal history of the enstatite chondrites we have carried out instrumental neutron activation analyses of 11 enstatite chondrites and electron microprobe analyses of 17 enstatite chondrites most of which were previously little described we report here the third known eh5 chondrite lew 88180 and an unusual el6 chondrite lew 87119 new data on four el3 chondrites alh 85119 eet 90299 pca 91020 and mac 88136 which is paired with mac 88180 and mac 88184 the second el5 chondrite til 91714 and an unusual metal rich and sulfide poor el3 chondrite lew 87223 the often discussed differences in mineral composition displayed by the eh and el chondrites are not as marked after the inclusion of the new samples in the database and the two classes apparently experienced a similar range of equilibration temperatures however texturally the el chondrites appear to have experienced much higher levels of metamorphic alteration than eh chondrites of similar equilibration temperatures most of the petrologic type criteria are not applicable to enstatite chondrites and unlike the ordinary chondrites texture and mineralogy reflect different aspects of the meteorite history we therefore propose that the existing petrologic type scheme not be used for enstatite chondrites we suggest that while textural type reflects peak metamorphic temperatures the mineralogical type reflects equilibration during postmetamorphic probably regolith processes unlike the ordinary chondrites and eh chondrites el chondrites experienced an extensive low temperature metamorphic episode there are now a large number of enstatite meteorite breccias and impact melts and apparently surface processes were important in determining the present nature of the enstatite chondrites

geochemical evolution and tectonic significance of boninites and tholeiites from the koh ophiolite new caledonia the central chain ophiolites in new caledonia are fragments of a supra subduction zone ssz ophiolite now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick middle triassic to upper jurassic volcaniclastic sequence most of the fragments were formed by a single tholeiitic magmatic episode but one of these the koh ophiolite was formed by two tholeiitic magmatic episodes separated by boninites the first event in the koh ophiolite formed cumulate gabbros dolerites plagiogranites and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements lree and abnormally low tio2 0 5 at mg 60 shortly after their eruption these tholeiitic lavas were overlain by a high ca boninitic unit with a basal section of boninite pillows flows and breccias and an upper section of boninitic dacites and tuffs the last magmatic phase involved eruption of evolved tholeiitic basalts as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite this second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts babb erupted during the early rifting history of modern back arc basins the boninitic volcanics belong to a high ca series with slightly lower sio2 al2o3 and tio2 compared to those from modern island arc systems and they lack the positive zr spike relative to adjacent rare earth elements ree in normalised element variation patterns these boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40 60 extension of the initial basin crust and eruption of the upper tholeiites the dominance of babb like tholeiites throughout the central chain ophiolites in new caledonia the restricted occurrence of boninites and the stratigraphy and chemistry of the koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation we suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre the occurrence of a thick blanket of calc alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the koh boninites were not associated with the initiation of subduction a close modern analogy for the koh ophiolite exists on the hunter ridge protoisland arc between southernmost vanuatu new hebrides island arc and the fijian islands there high ca boninites lacking positive zr spikes occur together with low ti tholeiites and more typical babb tholeiites where the southern spreading centre of the north fiji basin is propagating into the protoarc crust of the hunter ridge

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

the origin and nebular history of the metal phase of ordinary chondrites abstract we present new inaa results for bulk metal from h and ll chondrites both have characteristic elemental patterns similar to l chondrite metal in particular increasing abundances of w and ga from unequilibrated to equilibrated chondrites but a reverse variation of v and cr abundances these characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material the similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor similar to ci or cm material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites co ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups this suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions the chondrites that accreted earliest have preserved their melting characteristics while those that accreted later reequilibrated with the ambient gas at different temperatures the differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies the initial metal composition established during the melting stage 13 wt ni co ni 0 031 is inferred from the metal in highly unequilibrated chondrites the accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction h chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups ll chondrites were accreted at the lowest temperature and are most oxidized compositionally the metal component that would account for the fe si fractionation among h l and ll chondrites is different from the metal formed by melting indicating that the different fe s ratios of h l and ll chondrites were established before their accretion

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

chemical and physical studies of type 3 chondrites v the enstatite chondrites abstract we report instrumental neutron activation analysis determinations of 19 major minor and trace elements in three enstatite chondrites based on these and literature data on the bulk and mineral composition of enstatite chondrites we discuss the history of the type 3 or unequilibrated enstatite chondrites and their relationship with the other enstatite chondrites the type 3 enstatite chondrites have e chondrite lithophile element abundances and their siderophile element abundances place them with the eh chondrites well resolved from the el chondrites moderately volatile chalcophile elements are at the low end of the eh range and cr appears to be intermediate between eh and el we suggest that the type 3 enstatite chondrites are eh chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock produced sulfurous liquids the oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas 30 higher than the others or with the loss of a plagioclase rich liquid the mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration or in some instances with shock effects

the compositional classification of chondrites ii the enstatite chondrite groups we present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important st sauveur and discuss the classification of enstatite chondrites the enstatite chondrites can be divided into two compositionally distinct sets in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1 5 2 0 higher than in the other a well resolved compositional hiatus separates these two sets the differences in composition are as great as those between the groups of ordinary chondrites and therefore it appears best to treat these sets as separate groups by analogy with the symbols used for ordinary chondrites we propose to designate the high fe high siderophile group eh and the low fe low siderophile group el known members of the eh group belong to petrologic types 4 and 5 whereas all el members are petrologic type 6 within the eh group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis two physical properties show only modest overlap between the eh and el groups cosmic ray ages for eh chondrites are 0 5 7 ma while those for el chondrites are 4 18 ma relative to bjurbole i xe formation intervals are 1 3 0 6 ma for eh chondrites and 2 9 0 5 ma for el chondrites the weight of the chemical and physical evidence indicates that the eh and el groups formed separate bodies at similar distances from the sun the available evidence for shallowater and happy canyon two strongly recrystallized silicate rich meteorites containing 40 mg g fe ni indicates that the former is an enstatite clan chondrite altered by loss of sulfide and plagioclase rich melts whereas the latter is intermediate in composition between el chondrites and the chondritic silicates in the pine river iab anomalous meteorite

recognition characterization and implications of high grade silicic ignimbrite facies from the paleoproterozoic bijli rhyolites dongargarh supergroup central india abstract a controversy regarding the distinction between the highly welded lava like ignimbrites sometimes showing strongly rheomorphic characters and the extensive silicic lava flow has been overwhelming in the recent literature however a rethinking after walker 1983 has brought into light the concept of grade referring to the degree and extent of welding between the pyroclasts various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites which differentiate them from lava here a comprehensive study on early proterozoic acid magmatic rocks forming lower part of the dongargarh supergroup central india has been made to suggest extensive occurrence of high grade welded rheomorphic tuffs the possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences despite having overall monolithologic look various units bear distinction on account of their nature of welding enrichment of phenocrysts and degree of stretching the presence of vitroclastic texture melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits based on these characters four facies a b c and d from bottom to the top respectively have been identified from field studies around salekasa facies a and b represent clast supported matrix supported welded pyroclastic flow deposits facies c represents extremely welded thinly laminated rheomorphic tuffs while lava like tuffs with an autobreccia carapace is represented by facies d a complete gradation of facies a b to d through c exists high to extremely high grade nature of welding in these deposits suggests a low column height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting

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

compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body abstract chemical compositions of eleven enstatite chondrites 1 eh5 4 eh3 3 el3 i el5 and 2 el6 and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported the abundances of nonvolatile lithophile elements such as al sc and mg increase from eh to el while those of siderophile and moderately volatile elements decrease in the sequence of eh5 to eh3 el3 and el5 6 the continuity in compositions of enstatite chondrites and in particular the inverse variations of moderately volatile element abundances with petrographic type between eh and el groups demonstrate that enstatite chondrites have been derived from a common parent body the enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region metal was effectively accreted into the core where eh5 chondrites were derived and the abundances of metal decreased as accretion proceeded in complement silicate abundances gradually increased in the layers that accreted later the lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes furthermore it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites it is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating the chondrule formation process formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period the mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism the metamorphic trend from eh3 to eh5 resulted from internal heating whereas the trend from el3 to el6 was due to an external heat source it seems very likely that the eh chondrites were metamorphosed during accretion thus cooled rather rapidly activities of the early sun could serve as an energy source for the external heating

the origin of oldhamite in unequilibrated enstatite chondrites abstract we report the results of an ion probe study of rare earth elements rees in six unequilibrated enstatite chondrites oldhamite is confirmed as the major but not the only ree carrier in these meteorites data are also provided for other minerals enstatite niningerite and alabandite that carry detectable amounts of rees although the variety of ree patterns in oldhamite is not as extreme as in hibonite there are seven distinct ree patterns four of which form a continuum that characterize most oldhamite grains from e chondrites two of these patterns f and g were each found in only one weathered grain there is no correlation between ree pattern and oldhamite petrologic setting our trace element data are used to evaluate earlier work and a number of elements zr y sc rees sr se and probably sb and cs including both refractory and volatile species under oxidizing conditions are found enriched in oldhamite a comparison of ree patterns with the results of equilibrium condensation calculations indicates that nebular processes were important in determining the composition of the oldhamite precursors whereas metamorphic processes also affected oldhamite compositions enrichments of chalcophiles are not necessarily due to interaction of oldhamite with nebular gas at low temperatures as previously suggested oldhamite like hibonite was formed from melts whose trace element compositions were determined by high temperature transient events in the solar nebula

pyroxene structures cathodoluminescence and the thermal history of the enstatite chondrites in order to explore the thermal history of enstatite chondrites we examined the cathodoluminescence cl and thermoluminescence tl properties of 15 eh chondrites and 21 el chondrites including all available petrographic types both textural types 3 6 and mineralogical types alpha delta the cl properties of el3 alpha and eh3 alpha chondrites are similar enstatite grains high in mn and other transition metals display red cl while enstatite with low concentrations of these elements show blue cl a few enstatite grains with greater than 5 wt feo display no cl in contrast the luminescent properties of the metamorphosed eh chondrites are very different from those of metamorphosed el chondrites while the enstatites in metamorphosed eh chondrites display predominantly blue cl the enstatites in metamorphosed el chondrites display a distinctive magenta cl with blue and red peaks of approximately equal intensity in their spectra the tl sensitivities of the enstatite chondrites correlate with the intensity of the blue cl and unlike other meteorite classes are not simply related to metamorphism the different luminescent properties of metamorphosed eh and el chondrites cannot readily be attributed to compositional differences but x ray diffraction data suggests that the enstatite in eh5 gamma delta chondrites is predominantly disordered orthopyroxene while enstatite in el6 beta chondrites is predominantly ordered orthopyroxene the difference in thermal history of metamorphosed el and eh chondrites is so marked that the use of single petrographic types is misleading and separate textural and mineralogical types are preferable our data confirm earlier suggestions that metamorphosed eh chondrites underwent relatively rapid cooling and the metamorphosed el chondrites cooled more slowly and experienced prolonged heating in the orthopyroxene field

trace element distributions within ordinary chondrite chondrules implications for chondrule formation conditions and precursors the concentrations of thirty six lithophile elements in the major silicate phases of chondrules have been determined by ion microprobe two types of porphyritic chondrule olivines normal and incompatible rich have been identified the incompatible rich olivines which have forsterite contents that are greater than fo98 appear to have formed under highly reducing conditions the olivine and low ca pyroxene grains in porphyritic chondrules have relatively unfractionated l chondrite normalized ree abundances compared to expectations from equilibrium distribution coefficients this is consistent with porphyritic chondrules having formed at cooling rates of the order of 1000 c h or faster the alkali metal na k rb and cs to al ratios in chondrule glasses vary by two orders of magnitude the lower values possibly being the result of volatile loss from chondrules no correlation was found between alkali metal depletions in chondrule glasses and enrichments in the surrounding rims but any such correlation may have been destroyed by the diffusive redistribution of these elements in rims that is likely to occur even under mild metamorphic conditions the refractory lithophiles with the exception of mg are all highly incompatible in silicate melts and are as a result concentrated in the chondrule glasses the refractory lithophiles in the glass are in most cases unfractionated from one another suggesting that neither vapor phase nor crystal liquid fractionation played a major role in the formation of chondrules or their precursors based largely on the observation that the refractory lithophiles are concentrated in the chondrule glasses it is suggested that previously reported correlations between these elements in bulk chondrule analyses are due to chondrules themselves as opposed to refractory condensates having been their own immediate precursors a simple monte carlo simulation using the compositions of the major chondrule silicate phases reported here supports this hypothesis it is argued that the upper limit of about one observed in the atomic na al ratios of chondule glasses is not due to an albitic precursor but simply reflects that for na to be incorporated into the structure of the melt it must take part in the coupled substitution naalo2 sio2

chondrules in the qingzhen type 3 enstatite chondrite possible precursor components and comparison to ordinary chondrite chondrules abstract the qingzhen eh3 fall appears to be the least equilibrated enstatite chondrite mineral compositions vary widely from chondrule to chondrule and in some cases from grain to grain within individual chondrules as in ordinary chondrite oc chondrules most elements show appreciable ranges in concentration porphyritic olivine pyroxene pop chondrules are richer than porphyritic pyroxene pp and radial pyroxene rp chondrules in refractory lithophiles and siderophiles unlike oc chondrules qingzhen s rp and pp chondrules have similar bulk compositions elements in each of the following sets intercorrelate significantly probably because these elements were present in the same nebular components 1 1 fe co ni ir au 2 2 ca eu se 3 3 al sc hf 4 4 na ree 5 5 cl br several additional elements show no correlations set 1 elements were probably derived from a metal component as in oc chondrules set 3 elements which occur in higher concentrations in olivine bearing pop chondrules suggest the existence of a refractory lithophile rich olivine rich component similar to the one previously inferred for oc chondrules set 2 elements suggest a precursor component rich in oldhamite and set 4 elements suggest another refractory bearing component components 2 and 4 were not precursors of oc chondrules for some chondrules interelement ratios of refractory lithophiles such as ca al ti sc and ree are similar to ci ratios this indicates that these elements were probably derived from silicates we suggest that the earliest solid phases were refractory silicates that were sulfurized before chondrule formation some of these sulfurized components e g one rich in oldhamite were abundant in the region where the qingzhen chondrules formed

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

itqiy a metal rich enstatite meteorite with achondritic texture itqiy is a unique coarse grained metal rich enstatite meteorite that was found in the western sahara and consists of two rocks together weighing 4 72 kg which are both completely coated with fusion crust we report results from our electron microprobe and instrumental neutron activation analysis techniques itqiy consists of subhedral equigranular millimeter sized enstatite 25 vol of millimeter sized kamacite and a few tiny intergrowths of sulfides and kamacite relic chondrules are absent pyroxene fs0 2 is chemically similar to enstatite in el chondrites but the metal is closer in composition to that in eh chondrites sulfides resemble those in e chondrites but their compositions are distinct from those in both el and eh chondrites itqiy clearly formed under very reducing conditions but it does not appear to have formed from eh or el chondrites two thermal events can be distinguished silicate compositions including rare earth element abundances indicate loss of partial melt and slow cooling heterogeneous sulfides indicate a subsequent reheating and quenching event which may have been due to shock as many enstatite grains show shock stage s3 features

oxygen isotopic compositions of enstatite chondrites and aubrites oxygen isotopic compositions have been measured for ten eh chondrites eight el chondrites and nine aubrites the mean 18o and 17o values are 5 31 and 2 74 5 57 and 2 96 5 24 and 2 70 respectively within the eh group more equilibrated meteorites are fractionated toward higher heavy isotope abundances than the less equilibrated meteorites small departures of individual meteorites from a common mass fractionation occur for some of the eh samples the aubrites may be genetically related to either the eh or el group or to both almost all of the measured isotopic compositions are indistinguishable from the terrestrial oxygen isotope fractionation line suggesting a genetic relationship among the earth the moon and the enstatite the meteorites

chondrules as products of dust collisions with totally molten droplets within a dust rich nebular environment an experimental investigation abstract chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting if barred olivine bo chondrules formed by nearly total melting at their respective liquidi i e over a wide temperature range it is surprising that there are so few totally glassy chondrules the occurrence of fine grained accretionary rims on chondrules suggests mineral dust in the chondrule forming environment which might have collided with totally molten droplets to produce chondrule textures we have therefore conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500 c h and cause nucleation our experiments reproduce the majority of common chondrule textures the production of textures in our experiments is a function of the temperature at which dust is encountered the number of dust grains encountered and to a lesser extent the mineralogy of the encountered mineral grains crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters we have produced some less commonly discussed types of chondrule textures e g barred olivine porphyritic olivine pyroxene bo pop by dust seeding that have not been reproduced by the incomplete melting of starting materials the reproduction of bo pop and excentroradial textures by dust collisions and the scarcity of natural totally glassy chondrules suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures e g feo rich chondrules from these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs however we have reproduced the very fine grained microporphyritic textures characteristic of type ia chondrules only by incomplete melting such chondrules could not have reached average internal temperatures above their liquidus temperatures the conclusion that at least some chondrules formed from total melts that collided with mineral dust during cooling indicates that chondrule formation occurred within a region of the nebula rich in mineral dust

chemical characteristics of metal phases of the richardton h5 chondrite abstract the magnetic fraction was separated from the richardton h5 chondrite with a hand magnet and was leached with concentrated hf at high temperature for different periods the resulting metallic fractions and the untreated magnetic fraction were analyzed using mossbauer spectroscopy and instrumental neutron activation analysis inaa leaching in concentrated hf for 1 5 5 min was effective in removing silicates from the magnetic fraction while leaving the metal grains intact leaching with increasing time selectively dissolved kamacite with only little attack of the taenite grains the mossbauer spectroscopy shows that martensite phase iron is present in the richardton metals and is dissolved in hf less easily than kamacite but more easily than taenite the distributions of trace siderophile elements among the different metal phases of richardton were obtained based on the inaa data siderophile elements except for co are enriched in the taenite fraction but with different concentration ratios between taenite and kamacite suggesting that kamacite and taenite were not formed by either oxidation reduction of fe of the fe ni metal or by condensation from the nebula or by metal solid differentiation a more likely explanation is that kamacite and taenite have equilibrated by low temperature diffusion it seems that most trace siderophile elements were once dissolved in the fe ni metal and later distributed among the metal phases by diffusion however ir os and ru may still partly exist as tiny separate grains in the feni metal and their distributions among the metal phases are not completely equilibrated

cosmic ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy 21ne cosmic ray exposure ages have been calculated from literature data for 201 h 203 l and 38 ll chondrites corrected for shielding differences when possible the distributions of exposure ages again show the familiar peaks at 4 5 and 20 myr for the h s but no outstanding events for the l s and ll s if the l chondrite distribution is interpreted as a series of discrete events then at least 6 peaks between 1 and 35 myr are needed to model it the observations that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases suggest that the parent bodies have been fragmented and reassembled into a megabreccia for the h chondrites both large and small peaks contain about 15 solar gas bearing meteorites which could mean that surface material has been mixed to depths represented by the largest event on the order of a kilometer in contrast only 3 of the l s contain solar wind which may be related to breakup of their parent planet those l s with especially low radiogenic he u th he ages 1 ae tend to have low exposure ages their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes with significant solar heating

the trace element chemistry of cas in enstatite chondrites and some implications regarding its origin abstract samples of the mineral oldhamite cas were extracted from enstatite chondrites and analyzed by inaa prior to extraction the petrologic setting of the grains was studied microscopically and their minor element contents determined by microprobe analyses minor element contents of cas are known to vary and correlate with petrologic type indicating secondary redistribution during metamorphism for this reason samples were chosen that displayed a range of minor element contents the trace element contents determined in this study follow a similar pattern the most primitive samples of cas studied contain virtually the entire inventory of the host meteorite s lree and eu plus 30 50 of the hree inventory in less primitive samples the lree are less enriched although eu remains highly concentrated in cas several other elements including lithophiles ba cs cr hf and sc and chalcophiles sb and zn are most enriched in more primitive cas the high concentrations of refractory elements several of which have a tendency to form sulfides at high temperatures in a gas slightly more reducing than solar lend support to the suggestion that cas originated at high temperatures in a reduced region of the nebula the high concentrations of volatile cs sb and zn indicate that with decreasing temperature cas continued to interact with the nebular gas which therefore must have had a low oxygen fugacity at low temperatures

noble gases in enstatite chondrites ii the trapped component the trapped noble gas record of 57 enstatite chondrites e chondrites has been investigated basically two different gas patterns have been identified dependent on the petrologic type all e chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases q and a subsolar component range of elemental ratios for e4 6 chondrites 36ar 132xe 582 270 and 36ar 84kr 242 88 e3 chondrites usually contain q gases but also a composition with lower 36ar 132xe and 36ar 84kr ratios which we call sub q 36ar 132xe 37 0 18 0 and 36ar 84kr 41 7 18 1 the presence of either the subsolar or the sub q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites we therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in e chondrites trapped solar noble gases have been observed only in some e3 chondrites about 30 of each group eh3 and el3 chondrites amounting to 9 of all analyzed e chondrites show the solar signature notably only one of those meteorites has been explicitly described as a regolith breccia

first known el5 chondrite evidence for dual genetic sequence for enstatite chondrites the enstatite chondrites form two compositionally distinct groups the eh and el chondrites and four petrologic types types 3 6 where each type represents increasing degrees of metamorphic alteration1 2 prior to the recovery of the antarctic meteorite reckling peak a80259 the subject of the present report all known el chondrites were petrologic type 6 whereas all known eh chondrites were types 3 5 it has long been appreciated that the variations in bulk composition preclude a simple conversion of eh4 material into el6 material3 7 however complex models involving simultaneous variations in bulk composition and petrologic type have been discussed1 5 8 9 and may be implied by other classification schemes in common use that of anders keil type i intermediate type ii and that of van schmus and wood e3 e4 e5 e6 we report here the discovery of the first el5 chondrite an el5 breaks the eh3 4 eh5 el6 sequence and indicates that the enstatite chondrites constitute two discrete isochemical metamorphic sequences eh3 5 and el5 6

plagioclase rich objects in the ungrouped e3 chondrite lewis cliff 87234 petrology and aluminum magnesium isotopic data

129i 127i variations among enstatite chondrites abstract this paper presents the 129i 127i composition for 11 different enstatite chondrites there is a well defined hiatus in iodine isotopic compositions separating the enstatite chondrites the hiatus correlates with a similar hiatus defined by variations in chemical composition both major and minor elements eh 4 5 chondrites have 129i 127i ratios 20 higher 4 m y older than el 6 chondrites the enstatite chondrites are the third meteorite group joining silicates in iab irons and c3v carbonaceous chondrites in which 129i 127i correlates with chemical composition within each enstatite chondrite group the variations in initial iodine isotopic composition is less than 4 comparable to the experimental resolution the 20 difference between eh and el may indicate a maximum iodine isotopic heterogeneity between the eh and el nebular reservoirs alternatively if eh and el chondrites evolved from a homogeneous iodine isotopic reservoir then the el group must have evolved 4 0 m y longer than the eh group and either in an environment with a much lower i xe ratio than presently observed or one which was open to the loss of radiogenic 129xe the latter case may imply a nebular environment and would therefore be consistent with the open system evolution required to produce the chemical hiatus the duration of subsequent evolution in the high i xe environment must have been 1 m y and probably represents thermal metamorphism in two different parent bodies

metal phases of l chondrites their formation and evolution in the nebula and in the parent body abstract metal phases of six three equilibrated and three unequilibrated l chondrites were studied by inaa sem and mossbauer spectroscopy characteristics retained in the bulk metals of unequilibrated chondrites abundant carbon high contents of cr v mn and low contents of w mo and ga compared to metals of equilibrated chondrites less enrichment of w than mo and fractionation of co from ni demonstrate that chondrite metals are not nebular condensates all those characteristics can be well explained by melting chondrite metals are not melting remnants of previously condensed metals rather they were produced by reduction of ci or cm like material during the melting process the complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the low temperature diffusion products and must have been developed in the chondrite parent body the difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body thus while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity the crystallization of eoc silicates resulted from an external heating the internal metamorphism was mild 400 600 c and long whereas the external heating was intense with a maximum temperature in range of 800 950 c and short tetrataenite is present not only in uocs but also in eocs suggesting that the external heating occurred during the internal metamorphism i e within 100 myr of chondrite formation

evidence for relict grains in chondrules of qingzhen an e3 type enstatite chondrite abstract petrographic and chemical studies of the qingzhen chondrite strongly suggest that it is the most highly unequilibrated type 3 enstatite chondrite recognized so far qingzhen contains abundant well defined chondrules some of which were incompletely molten during the chondrule formation process the relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic fe which appear to be the in situ reduction product of the fayalitic component of the olivine the reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatite chondrite material we believe that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula provided fluctuations in the degree of oxidation of the nebular gas existed at such locations reheating of this material under more reducing conditions would lead to the observed reduction of the olivine igneous olivines within chondrules always contain detectable amounts of cao while relict olivines are essentially cao free this seems to suggest that the relict olivines did not originate during a previous igneous process of chondrule formation and might represent condensation products from the early solar nebula

noble gases in e chondrites noble gas data are reported for 12 e chondrites combined with literature data they show that k ar ages are 4 ae for 14 out of 18 meteorites yet u th he ages are often shorter perhaps due to late mild reheating cosmic ray exposure ages differ systematically between types 4 and 6 with e4 s mostly below 16 myr and e6 s above 30 myr this may mean that the e chondrite parent body contains predominantly a single petrologic type on the 1 km scale of individual impacts in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites the heavy noble gases consist of at least two primordial components the usual planetary component 36ar132xe 80 and a less fractionated subsolar component 2700 36ar132xe 3800 the latter is found in highest concentration in the e4 chondrite south oman 36ar 760 10 8cc g 36ar132xe 2700 the isotopic compositions of both components are similar to typical planetary values indicating that some factor other than mass controlled the noble gas elemental ratios the heavy xe isotopes occasionally show some of the lowest 134xe132xe and 136xe132xe ratios measured in bulk chondrites suggestive of nearly fission free xe e g 136xe132xe 0 3095 0 0020 amounts of planetary gas in e4 e6 chondrites fall in the range for ordinary chondrites of types 4 6 but in contrast to the ordinary chondrites fail to correlate with petrologic type or volatile trace element contents another unusual feature of e chondrites is that primordial ne is present even in most 4 s and 5 s 20nep 1 to 7 10 8cc g with an isotopic composition consistent with planetary ne analyses of mineral separates show that the planetary gases are concentrated in an hf and hcl insoluble mineral similar to phase q the poorly characterized hno3 soluble carrier of primordial gases in carbonaceous and ordinary chondrites the subsolar gases on the other hand are located in an hcl and hno3 resistant phase possibly enstatite or a minor phase included in enstatite much of the 129xer 50 for e4 s 70 for e6 s is in hcl resistant but hf soluble sites suggestive of a silicate a similar subsolar component may be responsible for the high 36ar132xe ratios of some c3 s unequilibrated ordinary chondrites and the unique aubrite shallowater the planet venus also has a high arkr ratio well above the planetary range and hence may have acquired its noble gases from an e chondrite like material similar to south oman

intensive parameters of enstatite chondrite metamorphism abstract the enstatite oldhamite geothermometer eog developed by larimer and buseck 1974 makes use of the five phase assemblage kamacite quartz enstatite oldhamite troilite to determine metamorphic temperatures in enstatite chondrites a re evaluation of the thermodynamic foundation of the eog shows that several of the assumptions and approximations can be significantly improved with new thermodynamic data the geothermometer has been modified to reflect among other things the solution behavior of si in kamacite the solution behavior of a calcium silicate component in enstatite and metal sulfide fusion as a function of temperature and composition an analysis of the eog shows that the activity coefficient of si in kamacite and the ca rich component of enstatite are the model s most important parameters for si the activity coefficient data of sakao and elliot 1975 have been used the activity coefficient of a ca rich component of enstatite presents a unique problem since data on this parameter do not exist for one pyroxene systems and two pyroxene solution models do not provide unique activity coefficients in the absence of diopside the model therefore has been formulated using camgsi 2 o 6 as the ca rich component of enstatite and assumes that it behaves as a symmetric regular margules solution the margules interaction parameter w g is then used as an adjustable constant and is varied such that the resulting temperatures fall between minimum and maximum temperatures determined independently temperature minima were determined from the enstatite diopside solvus of carlson 1986 and indicate that the el6 chondrites have been metamorphosed to temperatures exceeding 900 1000 c the temperature of the enstatite plagioclase quartz eutectic 1090 c appropriate for el6 plagioclase compositions was used as the metamorphic maxima with these constraints a w g value of 52 kj mole was estimated as appropriate within the context of the model for el6 metamorphic conditions el6 temperatures determined via this method range from 981 c for yilmia to 1068 c for blithfield temperature estimates of greater than 1000 c for the highly equilibrated el6 chondrites require the formation of a metal sulfide liquid during metamorphism liquid fractionation of this predominantly fe fes liquid could explain the siderophile chalcophile and volatile element depletion in the el6 class relative to ci and eh chondrites liquid fractionation however is not required by the model and is consistent with a moderate cooling rate which is fast enough to lock in high cao contents in enstatite but slow enough to re equilibrate troilite and alabandite to low temperatures the oxygen fugacity determined in this study shows the enstatite chondrites to have equilibrated at extremely low f o 2 this as well as the activity of si that can be extracted from the eog indicates that the enstatite chondrites that contain sinoite si 2 n 2 o equilibrated under high nitrogen fugacities high f n 2 values imply that sinoite bearing e chondrites either formed in a nebula of high pressure f n 2 10 1 at 1050 c or more likely are products of planetary metamorphism