implications of the carbonaceous chondrite mn cr isochron for the formation of early refractory planetesimals and chondrules abstract an excellent 53 mn 53 cr isochron for bulk ci cm co cv cb and ungrouped c3 chondrites seems to suggest that each carbonaceous chondrite group acquired its mn cr ratio 4568 1 myr ago this age is indistinguishable from the age of ca al rich inclusions cais which is considered to be the start of the solar system t 0 however carbonaceous chondrites were not assembled until at least 1 5 5 myr after t 0 to judge by the 207 pb 206 pb and 26 al 26 mg ages of the chondrules within them and by the fact that they were not melted by heat from the decay of 26 al presumably therefore these meteorites inherited their bulk mn cr isochron from precursor materials which experienced mn cr fractionation at t 0 as a possible physical mechanism for how the isochron was established initially and later inherited by the carbonaceous chondrites we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements and hence had variably low mn cr the planetesimals and the undepleted high mn cr primitive dust from which they were made shared the same initial e 53 cr and therefore evolved on an isochron we suggest that later impact disruption of the planetesimals produced dusty debris which became mixed in various proportions with unprocessed high mn cr dust before accreting to the carbonaceous chondrite parent bodies with mixing in a closed system the isochron was unchanged we infer that some debris rich material was converted to chondrules prior to accretion the chondrules could have been formed by flash melting of the mixed dust or could instead have been made directly by the impact splashing of molten planetesimals or by condensation from impact generated vapor plumes

the rupture history of the mw 8 0 jalisco mexico earthquake of 1995 october 9 summary finite fault broad band teleseismic p waveform inversion was applied to the large mw 8 0 jalisco mexico earthquake of 1995 october 9 the earthquake hypocentre was located at shallow depths just near the boundary zone between two lithospheric plates rivera and cocos and was felt with a maximum intensity of 9 mm on the pacific coast it was the first earthquake of magnitude greater than 6 to occur in this region since the two great jalisco earthquakes ms 8 0 and 7 6 of 1932 for inversion we used vertical records from 14 digital stations situated 30d to 90d from the epicentre we used the fixed rake of the thrust type focal mechanism given by the preliminary harvard cmt the low angle dipping fault plane was taken as a 180 90 km2 rectangle divided into 162 subfaults of 10 10 km2 each subfault had five intermediate point sources along strike and five intermediate point sources downdip the size of the fault plane was changed from an initial size of 110 60 km2 estimated by resco as an aftershock area to the final size of 180 90 km2 which included all significant displacements along the fault the hypocentre was embedded 40 km from the left edge of the 180 km long fault 40 km down dip from the top of the fault which was situated practically along the trench axis a boxcar source time function of width 2 0 s was used for each discrete rupture interval of each subfault the rupture velocity was taken to be 2 8 km s 1 which is approximately 80 per cent of the shear wave speed in the layer containing the hypocentre the following results on the rupture history were obtained 1 the rupture duration was about 55 s and slip occurred within an area of about 180 90 km2 along the middle american trench in the depth interval from 9 to 33 km 2 there were two main stages in the rupture history during the first 18 s two main asperities at a distance of about 30 km from the earthquake hypocentre were ruptured with maximum slip up to 610 cm at a depth of 12 15 km then during the next 37 s rupturing was observed to the north along the trench the slip distribution obtained shows that the faulting originating near the boundary between the rivera and cocos plates went mainly along the rivera north american plate boundary and is consistent with the subduction of the rivera plate beneath north america

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

chromium isotope anomaly in an impactite sample from the el gygytgyn structure russia evidence for a ureilite projectile the 3 6 ma 18 km diameter el gygytgyn impact structure arctic russia is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks previous analyses of impactites from el gygytgyn showed minor enrichments of the siderophile elements including ir which together with distinct cr enrichments gave rise to speculation that an achondritic projectile was involved we studied the major and trace element composition in samples from the new icdp drill core obtained near the center of the structure as well as the chromium isotopic composition of an impact glass sample collected on the surface several suevitic breccias from the upper part of the suevite sequence in the drill core show higher cr and ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples however it is difficult to unambiguously establish a meteoritic component from trace element data as input from rare mafic target rocks is a possibility in contrast the cr isotopic composition of the impact glass sample yielded a nonterrestrial e 54 cr value of 0 72 0 31 2 std err this negative e 54 cr is different from known carbonaceous chondrite values e 54 cr of 0 95 to 1 65 but is nearly identical to reported values for ureilites approximately 0 77 the value is however also within analytical error of eucrites approximately 0 38 and ordinary chondrites approximately 0 42 given the chemical signatures found in previous analyses of el gytgytgyn impactites and the similarity of our cr isotopic data to ureilites we suggest that the impacting asteroid could have been an f type asteroid of mixed composition similar to the recent almahata sitta fall in sudan

manganese chromium systematics in sulfides of unequilibrated enstatite chondrites we measured with a secondary ion mass spectrometer mn cr ratios and cr isotopes in individual grains of mn bearing sulfides i e sphalerites zns alabandites mns and niningerites mgs in nine unequilibrated enstatite chondrites uecs the goals were to determine whether live 53mn half life 3 7 ma was incorporated in these objects at the time of their isotopic closure and to establish whether mn cr systematics in sulfides in uecs can be used as a high resolution chronometer to constrain formation time differences between these meteorites sulfide grains analysed in four of these uecs mac 88136 el3 mac 88184 el3 mac 88180 el3 and indarch eh4 have clear 53cr excesses these 53cr excesses can be very large 53cr 52cr ranges up to 18 400 the largest 53cr excess measured so far and in some grains are well correlated with the mn cr ratios thus they were most likely produced by the in situ decay of 53mn in the meteorite samples in the remaining five meteorites no detectable excesses of 53cr were found and only upper limits on the initial 53mn 55mn ratios could be established the four meteorites with 53cr excesses show variations in the inferred 53mn 55mn ratios in various sulfide grains of the same meteorite the mn cr systematics in these sulfides were disturbed during and or after the decay of 53mn by varying degrees of reequilibration provided 53mn was homogeneously distributed in the region of the early solar system where these objects formed the data suggest that the time of the last isotopic equilibration of sulfides in el chondrites occurred at least 3 ma after a similar episode in eh chondrites

trapped nitrogen in individual chondrules nature of chondrule precursors and clues to formation mechanisms 1 nitrogen amount and isotopic composition have been measured in individual chondrules of six ordinary chondrites oc and two each of carbonaceous cc and enstatite chondrites ec the n ppm 15nt corrected for cosmogenic contribution for the chondrules have ranges of 0 8 to 61 97 to 164 19 to 140 21 to 31 and 2 to 77 27 to 116 among oc ec and cc respectively for oc while n contents of chondrules fall in the range of bulk oc 15n spans a wide range in contrast to the narrow range 25 to 14 for bulk for cc chondrules n contents are lower while 15n is different and heavier than bulk in contrast ec chondrules and bulk have comparable n and 15n values neither n nor 15nt of oc and cc chondrules show any size expected for nebular exchange or chemical mineralogical dependence ruling out the dominance of secondary effects of nebular or parent body origin the range of 15nt among oc and cc chondrules represents 15nt heterogeneity of chondrule precursors and their survival after a high temperature chondrule forming event on the basis of 15n of the chondrules we propose the presence of at least three n components carried by phase q presolar diamonds hl and insoluble organic matter iom the precursors of chondrules from oc and cc are different than the bulk oc and cc which represents the different environment for their formation compared to the bulk oc and cc however for ec chondrules precursors as well as the formation environment are the same as those of enstatite chondrites

composition and origin of clasts and inclusions in the abee enstatite chondrite breccia abstract the concentrations of 25 major minor and trace elements have been determined in four clasts a metal rich inclusion and two dark metal poor inclusions from the abee enstatite chondrite the clasts are heterogeneous displaying 2 fold enrichments or depletions in some elements the data suggest that there are two generations of metal one with low the other with high concentrations of refractory siderophiles the other elemental patterns can be understood in terms of variations in the abundance of major minerals we infer that sc and mn are located largely in the niningerite fe mg s v in the troilite fes and rare earth elements in the oldhamite cas heterogeneities among the clasts are probably primary resulting from the accretion agglomeration process although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence in the dark inclusions the concentrations of the rare earths eu excepted are 4 higher than mean eh levels this infers enhanced amounts of cas the dark inclusions are low in siderophiles sc mn k na and al implying low amounts of metal niningerite and feldspar the origin of the dark inclusions is unclear they do not appear to be the result of a simple single stage process

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

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

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

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

manganese chromium isotope systematics of enstatite meteorites abstract we present results of a study of the 53 mn 53 cr isotope systematics in the enstatite chondrites and achondrites aubrites the goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short lived radionuclide 53 mn our earlier work lugmair and shukolyukov 1998 shukolyukov and lugmair 2000a has shown that the asteroid belt bodies are characterized by essentially the same initial 53 mn abundance however we have found the presence of a gradient in the abundance of the radiogenic 53 cr between the earth moon system mars and the asteroid vesta if this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated this can be explained either by an early volatility controlled mn cr fractionation in the nebula or by an original radially heterogeneous distribution of 53 mn the enstatite chondrites are suggested to form in the inner zones of the solar nebula much closer to the sun than the ordinary chondrites therefore their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53 mn we have studied the bulk samples of the eh4 chondrites indarch and abee and the el6 chondrite khairpur although these meteorites have essentially the same mn cr ratio as the ordinary chondrites the relative abundance of the radiogenic 53 cr is three times smaller than in the ordinary chondrites because these meteorites are primitive undifferentiated and no mn cr fractionation had occurred within their parent bodies this difference is a strong argument in favor of an initially heterogeneous distribution of 53 mn in the early inner solar system this finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula using the characteristic 53 cr excess of the enstatite chondrites and the observed gradient their place of origin falls at about 1 4 au or somewhat closer to the sun i e 1 0 1 4 au we also present chronological results for the enstatite chondrites and achondrites the absolute 53 mn 53 cr ages of the eh4 chondrites are old 4565 ma the el6 chondrite khairpur is 4 5 ma younger which is in good agreement with the 129 i 129 xe data from the literature the age of the aubrite pena blanca spring appears to be similar to those of the enstatite chondrites while that of the aubrite bishopville is at least 10 ma younger which is also in agreement with the 129 i 129 xe data the results from bulk samples of aubrites indicate that the last mn cr fractionation in their parent body occurred 4563 ma ago and imply an evolution of the mn cr isotope system in an environment with an higher than chondritic mn cr ratio for several millions of years

the redox state feo content and origin of sulfur rich magmas on mercury 1 mercury surface space environment geochemistry and ranging messenger orbital observations of mercury have revealed elevated s abundances ca s and mg s correlations and a low upper limit for ferrous iron in surface silicates these data indicate the presence of ca and or mg sulfides in volcanic rocks and a low oxygen fugacity fo2 in their parental magmas we have evaluated coupled fo2 and fs2 values and feo contents in mercury s magmas from silicate sulfide equilibria and empirical models for silicate melts and metallurgical slags the evaluated fo2 at 1700 1800 k is 4 5 to 7 3 log10 units below the iron wustite buffer these values correspond to 0 028 0 79 wt feo implying that fe must be also present in sulfides and metal and are also consistent with the composition of the partial melt of an enstatite chondrite this derived upper limit for feo is substantially lower than the limits obtained from reflectance measurements of mercury s surface materials the low fo2 and feo values provide new constraints for igneous processes on mercury as well as the formation evolution and internal structure of the innermost planet

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

geochemistry of 2 63 2 49 ga impact spherule layers and implications for stratigraphic correlations and impact processes thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long distance stratigraphic correlation of precambrian successions seven formations in western australia and south africa clustered around the archean proterozoic boundary each contain a single spherule layer all of which were deposited between ca 2 49 and 2 63 ga analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component etc in all six layers based on pge ratios and or cr isotopic composition the amount of etc varies from marginally detectable to clear and obvious it generally amounts to ca a few percent both pge ratios and cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition in contrast all spherule layers older than ca 3 0 ga that have been analyzed were produced by carbonaceous chondritic impactors normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in pge ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation the geochemical data also provide a means to test proposed intra and intercontinental stratigraphic correlations cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2 63 ga in contrast clear geochemical differences between the middle layers both deposited ca 2 54 ga suggest they were not produced by the same impact event the youngest banded iron formation hosted layers on both continents have also been correlated but no geochemical data are available on the african layer to test this at present

manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites whole chondrule mn cr isochrons are presented for chondrules separated from the chainpur ll3 4 and bishunpur ll3 1 meteorites the chondrules were initially surveyed by instrumental neutron activation analysis ll chondrite normalized mn cr mn fe and sc fe served to identify chondrules with unusually high or low mn cr ratios and to correlate the abundances of other elements to sc the most refractory element measured a subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x ray spectrometer prior to high precision cr isotopic analyses 53cr 52cr correlates with 55mn 52cr to give initial 53mn 55mn i 9 4 1 7 10 6 for chainpur chondrules and 53mn 55mn i 9 5 3 1 10 6 for bishunpur chondrules the corresponding chondrule formation intervals are respectively tlew 10 1 ma for chainpur and 10 2 ma for bishunpur relative to the time of igneous crystallization of the lewis cliff lew 86010 angrite because mn sc correlates positively with mn cr for both the chainpur and bishunpur chondrules indicating dependence of the mn cr ratio on the relative volatility of the elements we identify the event dated by the isochrons as volatility driven elemental fractionation for chondrule precursors in the solar nebula thus our data suggest that the precursors to ll chondrules condensed from the nebula 5 8 2 7 ma after the time when initial 53mn 55mn i 2 8 0 3 10 5 for calcium aluminum rich inclusions cais our preferred value determined from data for a mineral separates of type b allende cai br1 b spinels from efremovka cai e38 and c bulk chondrites mn cr formation intervals for meteorites are presented relative to average i mn 53mn 55mn ch 9 46 10 6 for chondrules mn cr ratios for radiogenic growth of 53cr in the solar nebula and later reservoirs are calculated relative to average i mn 53cr i 9 46 0 08 10 6 0 23 0 08 for chondrules inferred values of mn cr lie within expected ranges thus it appears that evolution of the cr isotopic composition can be traced from condensation of cais via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids measured values of 53cr for individual chondrules exhibit the entire range of values that has been observed as initial 53cr values for samples from various planetary objects and which has been attributed to radial heterogeneity in initial 53mn 55mn in the early solar system estimated 55mn 52cr 0 42 0 05 for the bulk earth combined with 53cr 0 for the earth plots very close to the chondrule isochrons so that the earth appears to have the mn cr systematics of a refractory chondrule thus the earth apparently formed from material that had been depleted in mn relative to cr contemporaneously with condensation of chondrule precursors if as seems likely the earth s core formed after complete decay of 53mn there must have been little differential partitioning of mn and cr at that time

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

a cr isotopic study of the bon accord nio body in the barberton greenstone belt south africa abstract an unusual oxide body enriched in ni and the platinum group elements was found in the north western part of the barberton greenstone belt south africa in the 1920s with mineralogy and geochemistry which are extremely anomalous and do not conform to that of any other ni rich crustal rocks it has been interpreted as either an archaean paleo meteorite or of terrestrial deep mantle origin cr isotopic ratios were used to evaluate the merits of these two possibilities isotopic analyses returned values for e 53 of 0 00 epsilon units within a 2 s error relative to a terrestrial standard we interpret this information together with trace element data from the surrounding peridotites as pointing to a terrestrial origin for the body and conclude that the current dataset is not inconsistent with a deep mantle origin as a remnant of core formation with subsequent transport to and emplacement in the lithosphere by a mantle plume

variable tl pb and cd concentrations and isotope compositions of enstatite and ordinary chondrites evidence for volatile element mobilization and decay of extinct 205pb new tl pb and cd concentration and tl pb isotope data are presented for enstatite as well as l and ll type ordinary chondrites with additional cd stable isotope results for the former all three chondrite suites have tl and cd contents that vary by more than 1 2 orders of magnitude but pb concentrations are more uniform as a result of terrestrial pb contamination model calculations based on pb isotope compositions indicate that for more than half of the samples more than 50 of the measured pb contents are due to addition of modern terrestrial pb in part this is responsible for the relatively young and imprecise pb pb ages determined for eh l and ll chondrites which are hence only of limited chronological utility in contrast four particularly pristine el chondrites define a precise pb pb cooling age of 4559 6 ma the enstatite chondrites ecs have highly variable e114 110cd of between about 3 and 70 due to stable isotope fractionation from thermal and shock metamorphism furthermore nearly all enstatite meteorites display e205tl values from 3 3 to 0 8 while a single anomalous sample is highly fractionated in both tl and cd isotopes the majority of the ecs thereby define a correlation of e205tl with e114 110cd which suggests that at least some of the tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205tl from 205pb decay considering l chondrites most e205tl values range between 4 and 1 while two outliers with e205tl 10 are indicative of stable isotope fractionation considering only those l chondrites which are least likely to feature pb contamination or stable tl isotope effects the results are in accord with the former presence of live 205pb on the parent body with an initial 205pb 204pb 1 5 1 4 10 4 which suggests late equilibration of the pb tl system 26 113 ma after carbonaceous chondrites ccs the ll chondrites display highly variable e205tl values from 12 5 to 14 9 also indicative of stable isotope effects however the data for three pristine ll3 ll4 chondrites display an excellent correlation between e205tl and 204pb 203tl this defines an initial 205pb 204pb of 1 4 0 3 10 4 equivalent to a 205pb 205tl cooling age of 55 12 24 ma 31 67 ma after ccs

hydrothermal eruptions at el humazo domuyo geothermal field argentina insights into the eruptive dynamics and controls abstract hydrothermal eruptions are explosive events driven by the sudden and violent vaporization of the water present in many hot water geothermal fields worldwide over recent years thermodynamical conditions of fluids and petrophisical geomechanical features of host rocks were mentioned as responsible of the wide range of eruptive styles and deposits of hydrothermal eruptions nevertheless geological and historical records show that pdc and fallout processes are underestimated as common processes linked to hydrothermal eruptions this paper analyzes from a multi disciplinary approach virtual outcrops modeling sedimentology petrographic and xdr studies facies modeling eye witness source analysis and stratigraphy the pristine holocene historic hydrothermal eruptive record of el humazo manifestation domuyo geothermal field patagonia argentina different eruptive scenarios are characterized by ballistic ejection pdc and or fallout mechanisms with discernible tefra composition and granulometry associated with both excavational and or aggradational landforms we show how diverse eruptive dynamics are controlled by pre eruptive internal and external factors such as reservoir energy depth of the explosion fragmentation levels physical properties and structural anisotropy of the host rocks and eruptive center migrations we consider that this case study may be useful not only for the application in the assessment of the volcanic hazard associated with the domuyo geothermal field but also contributes to the knowledge of the eruptive dynamics in other geothermal systems with similar characteristics

i xe ages of enstatite chondrites abstract in order to elucidate the early thermal history of enstatite chondrite parent bodies we determined 129i 129xe whole rock ages of enstatite chondrites 5 eh 2 el one eh impact melt relative to the shallowater reference meteorite 4562 3 0 4 ma all errors are 1 i xe ages of both el6 chondrites lon 94100 4 38 0 60 ma and neuschwanstein 3 87 0 73 ma negative sign indicates ages younger than shallowater agree well with data of other el6 chondrites lon 94100 displayed a second isochron at lower temperatures equivalent to a younger age of 5 25 1 17 ma perhaps reflecting different retention temperatures of respective carrier phases during sequential cooling the enstatite chondrites abee eh4 indarch eh4 eet 96135 eh4 5 and st marks eh5 encompass a i xe age range of 0 57 1 05 ma eet 96135 1 to 0 45 0 72 ma abee again in agreement with previously reported ages of eh chondrites only the age of st marks differs strongly from previously reported younger ages now being more in accordance with other members of the eh clan the eh3 chondrite sahara 97096 showed the youngest i xe age of 7 87 0 46 ma distinctly younger than other i xe ages of eh chondrites including other eh3s due to the apparent high retention temperature of the i xe system in enstatite estimated 800 c this young age implies a later resetting of the i xe system by a severe thermal likely impact induced event the eh impact melt lap 02225 records a similarly young thermal event though no isochron relationship could be established the data fall within an apparent i xe age range of 5 to 15 ma similar to sahara 97096 overall eh chondrite parent body experienced a thermal history determined by a complex interplay between impact disturbances and parent body metamorphism

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

testing an integrated chronology i xe analysis of enstatite meteorites and a eucrite we have determined initial 129i 127i ratios for mineral concentrates of four enstatite meteorites and a eucrite in the case of the enstatite meteorites the inferred ages are associated with the pyroxene rich separates giving pyroxene closure ages relative to the shallowater standard of indarch eh4 0 04 0 67 ma khairpur el6 4 22 0 67 ma khor temiki aubrite 0 06 ma and itqiy enstatite achondrite 2 6 2 6 ma negative ages indicate closure after shallowater no separate from the cumulate eucrite asuka a 881394 yielded a consistent ratio though excess 129xe was observed in a feldspar separate suggesting disturbance by thermal metamorphism within 25 ma of closure in shallowater iodine 129 ages are mapped to the absolute pb pb time scale using the calibration proposed by gilmour et al 2006 who place the closure age of shallowater at 4563 3 0 4 ma comparison of the combined 129i pb data with associated 53mn ages for objects that have been dated by both systems indicates that all three chronometers evolved concordantly in the early solar system the enstatite chondrites are offset from the linear array described by asteroid belt objects when 53mn ages are plotted against combined 129i pb data supporting the suggestion that 53mn was radially heterogeneous in the early solar system

nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from zn isotopes enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites aubrites they are the most reduced group of all meteorites the oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line which suggests some genetic links between these meteorites and the earth as well for this study we measured the zn isotopic composition of 25 samples from the following groups aubrites main group and shallowater el chondrites eh chondrites and happy canyon impact melt breccia we also analyzed the zn isotopic composition and elemental abundance in separated phases metal silicates and sulfides of the eh4 el3 and el6 chondrites the different groups of meteorites are isotopically distinct and give the following values parts per thousand aubrite main group 7 08 delta zn 66 0 37 eh3 chondrites 0 15 delta zn 66 0 31 eh4 chondrites 0 15 delta zn 66 0 27 eh5 chondrites delta zn 66 0 27 0 09 n 1 el3 chondrites 0 01 delta zn 66 0 63 the shallowater aubrite 1 48 delta zn 66 2 36 el6 chondrites 2 26 delta zn 66 7 35 and the impact melt enstatite chondrite happy canyon delta zn 66 0 37 the aubrite pena blanca spring delta zn 66 7 0 parts per thousand and the el6 north west forrest delta zn 66 7 35 are the isotopically lightest and heaviest samples respectively known so far in the solar system in comparison the range of zn isotopic composition of chondrites and terrestrial samples 1 5 delta zn 66 1 parts per thousand is much smaller luck et al 2005 herzog et al 2009 eh and el3 chondrites have the same zn isotopic composition as the earth which is another example of the isotopic similarity between earth and enstatite chondrites the zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between el3 and el6 chondrites is due to volatilization probably during thermal metamorphism aubrites show strong elemental depletion in zn compared to both eh and el chondrites and they are enriched in light isotopes delta zn 66 down to 7 04 parts per thousand this is the opposite of what would be expected if zn elemental depletion was due to evaporation assuming the aubrites started with an enstatite chondrite like zn isotopic composition evaporation is therefore not responsible for volatile loss from aubrites on earth zn isotopes fractionate very little during igneous processes while differentiated meteorites show only minimal zn isotopic variability it is therefore very unlikely that igneous processes can account for the large isotopic fractionation of zn in aubrites condensation of an isotopically light vapor best explains zn depletion and isotopically light zn in these puzzling rocks mass balance suggests that this isotopically light vapor carries zn lost by the el6 parent body during thermal metamorphism and that aubritcs evolved from an el6 like parent body finally zn isotopes suggest that shallowater and aubrites originate from distinct parent bodies

early archean spherule beds chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type three early archean spherule beds from barberton south africa have anomalous cr isotope compositions in addition to large ir anomalies confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites the extra terrestrial components in beds s2 s3 and s4 are estimated to be approx l 50 60 and 15 30 respectively these beds are probably the distal and possibly global ejecta from major large body impacts these impacts were probably much larger than the cretaceous tertiary event and all occurred over an interval of approx 20 m y implying an impactor flux at 3 2 ga that was more than an order of magnitude greater than the present flux