ava analysis reveals in situ sediment diagenesis at the costa rican decollement complete sediment subduction at the costa rica subduction zone makes this convergent margin an ideal place to investigate the effects of tectonic deformation in situ we present a seismic reflection study along a line located 3 km landward of the middle american trench and oriented parallel to the strike of the decollement the ocean bottom hydrophone obh seismic data include large offsets and incidence angles at the reflectors we derive the p and s waves velocity distribution below the decollement using a p wave analysis of amplitude with reflection angle the investigation shows that there are unexpected large lateral velocity variations at a scale of only a few 100 m the shear wave velocity in the uppermost subducted sediment varies between 300 and 700 m s while the variation of the compressional wave velocity is in a range of 1700 to 2000 m s the variation of the vp vs ratio between 2 8 and 5 2 can only be explained by variations of the pore fluid pressure the modelled velocities correspond to a normalised pore fluid pressure ratio in the range between 0 02 and 0 93 the most reasonable explanation for these observations is the localised presence of fluids which are released during diagenesis by smectite to illite transformation during this process which takes place in three discrete steps the interlayer water of the smectite is added to the pore fluid and the permeability of the sediment is decreased both effects lead to the formation of small overpressured cells

diffusion kinetics of lutetium in diopside and the effect of thermal metamorphism on lu hf systematics in clinopyroxene abstract we report new experimental data for lu diffusion in diopside from 1150 to 1250 c at 1 bar pressure and oxygen fugacity corresponding to the iron wustite buffer the primary experimental dataset defines the following arrhenius relation for lu diffusion parallel to the b crystallographic axis d lu b 1 37 1 2 10 13 exp 208 7 15 2 kj mol 1 rt m 2 s where t is in kelvin we find that lu diffusion in diopside is significantly faster than that of hf which could lead to spurious lu hf ages via decoupling of 176lu from 176hf if the system was subjected to protracted periods of high temperature the extent to which lu hf ages are compromised during initial cooling depends on the peak temperature grain size cooling rate and the nature of lu partitioning between clinopyroxene and its surrounding phase s however in geologically realistic scenarios the effects of diffusion on lu hf ages during initial cooling do not produce lu hf ages that differ from the age corresponding to the closure temperature of hf by more than a few million years lu hf systematics in clinopyroxene are far more susceptible to diffusion during reheating events that occur after radiogenic 176hf has already accumulated in the host phase for a given thermal pulse the lu hf age varies as a function of grain size temperature the timing and duration of reheating and the partitioning of lu between clinopyroxene and its surrounding phase s the data collected and numerical model developed in this study have been used to simulate the effect of thermal metamorphism on the internal lu hf systematics of the eucrites millbillillie and piplia kalan although our model results do not offer a comprehensive solution to the apparent pre solar lu hf ages observed in many ancient meteorites we argue that the 4 9 ga pyroxene whole rock lu hf ages of these specific samples are at least partially if not primarily due to diffusive loss of lu from the constituent minerals to a partial melt phase during high temperature metamorphism

147sm 143nd and 87rb 87sr ages of the eucrite piplia kalan abstract prompted by the finding that the eucrite piplia kalan could have retained pu fission xe earlier than the eucrites so far studied and hence be very ancient we have measured a precise internal 147 sm 143 nd isochron for this meteorite the age and initial nd ratio relative to chur are 4 570 0 023 ga and 1 3 0 7 u respectively a rb sr whole rock clast isochron for this meteorite corresponds to an age of 3 963 0 119 ga and initial 87 sr 86 sr ratio of 0 69902 3 but initial 87 sr 86 sr ratio calculated for an age of 4 57 ga is 0 698956 25 which is indistinguishable from 0 698970 15 reported for the angrites lew and ador dated at 4 5578 0 0005 ga these results indicating that piplia kalan could have formed within only a few million years of the earliest condensates in the solar system are strongly supported by the recent discovery of live 26 al in it

thermoluminescence constraints on the metamorphic shock and brecciation history of basaltic meteorites abstract induced thermoluminescence measurements have been performed on eighteen eucrites thirteen howardites plus seven splits from the bholghati howardite six diogenites and fifteen mesosiderites in order to help clarify their metamorphic shock and brecciation history the tl sensitivities of the basaltic meteorites reflect their bulk composition and the effects of 1 igneous processes diogenites and mesosiderites and 2 the mixing of materials of different compositions e g diogenitic and eucritic during brecciation howardites can be observed two previously unclassified mesosiderites lew87006 and eet87500 are assigned to class a and b respectively on the basis of their tl sensitivities the eucrites show a 15 fold range of tl sensitivities which correlate with petrographie indicators of metamorphic intensity we therefore describe a petrologic type scale based on tl and petrographie evidence where the least metamorphosed eucrites are type 1 and most equilibrated are type 8 we see no evidence for two discrete metamorphic environments corresponding to the equilibrated and unequilibrated eucrites the temperature of the dominant tl peak observed for basaltic meteorites and experiments in which four eucrites with diverse petrographie properties were annealed at various temperatures in the laboratory suggests the metamorphic equilibration temperatures for most basaltic meteorites were 800 c this estimate is generally consistent with palaeotemperatures determined from two pyroxene olivine pyroxene and oxygen isotope equilibria assuming this temperature was typical of conditions during metamorphism then burial depths for type 5 eucrites were 350 m and

mesosiderite formation on asteroid 4 vesta by a hit and run collision collision and disruption processes of protoplanetary bodies in the early solar system are key to understanding the genesis of diverse types of main belt asteroids mesosiderites are stony iron meteorites that formed by the mixing of howardite eucrite diogenite like crust and molten core materials and provide unique insights into the catastrophic break up of differentiated asteroids however the enigmatic formation process and the poorly constrained timing of metal silicate mixing complicate the assignment to potential parent bodies here we report the high precision uranium lead dating of mesosiderite zircons by isotope dilution thermal ionization mass spectrometry to reveal an initial crust formation 4 558 5 2 1 million years ago and metal silicate mixing at 4 525 39 0 85 million years ago the two distinct ages coincide with the timing of the crust formation and a large scale reheating event on the eucrite parent body probably the asteroid vesta this chronological coincidence corroborates that vesta is the parent body of mesosiderite silicates mesosiderite formation on vesta can be explained by a hit and run collision 4 525 4 million years ago that caused the thick crust observed by nasa s dawn mission and explains the missing olivine in mesosiderites howardite eucrite diogenite meteorites and vestoids mesosiderite meteorites may originate from a hit and run impact on the parent asteroid of eucrite meteorites probably vesta as mesosiderite zircon u pb ages are found to coincide with those for eucrites

igneous and metamorphic enclaves in the s type deddick granodiorite lachlan fold belt se australia petrographic geochemical and nd sr isotopic evidence for crustal melting and magma mixing the deddick granodiorite a mafic s type pluton in the lachlan introduction fold belt contains abundant enclaves most are derived from enclaves in granites and volcanic rocks can provide high grade metasediments of pelitic psammitic composition and petrogenetic information not readily available from their migmatites are common among these discrete fragments of host rocks didier 1973 didier barbarin 1991 for melanosomes rich in cordierite and garnet are restites from partial example metasedimentary enclaves can place constraints melting however compositional and nd sr isotopic data indicate on the age composition and deformational history of they are not restite in equilibrium with the melt component of the magma source regions within continental crust fleming host magma but may have formed part of a spectrum of diverse 1991 steele et al 1991 enclaves formed by acmagma source lithologies alternatively they may be accidental cumulation of early phenocrysts autoliths document xenoliths regardless of their origin data for these enclaves suggest the early evolution of the host magma whereas others their precursors could represent deeply buried age equivalents of the may represent commingled magmas and illustrate the ubiquitous ordovician turbidites of the lachlan fold belt three possible role of hybridization didier 1987 types of microgranular enclaves with igneous textures are disenclave suites in the relatively mafic s type granitoids tinguished the most common type are small and rounded mafic of se australia are often diverse including abundant enclaves of tonalitic composition they carry xenocrysts derived from enclaves of metasedimentary origin and microgranular the host magma and some contain high mg pyroxene derived from enclaves phillips et al 1981 price 1983 chen et al a mafic magma isotopic data form an array with end from 6 to 1989 wyborn et al 1991 the metasedimentary en 12 and sr sr from 0 7130 to 0 7167 host rock 10 and claves are lithologically diverse including gneissic 0 715 respectively these enclaves formed as globules of hybrid migmatitic types and biotite rich surmicaceous types mafic magma commingled with and contaminated by the more apart from their potential as indicators of crustal structure felsic host magma other enclave types were derived from disrupted and composition they are of interest as possible fragments syn plutonic dykes having a distinct isotopic composition and from of source material and or refractory residue from which a cogenetic marginal facies of the host peraluminous melts were generated and extracted by

u pb systematics of the unique achondrite ibitira precise age determination and petrogenetic implications abstract ibitira is an unbrecciated equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites we present the first combined high precision u and pb isotopic data for this unique meteorite the 238 u 235 u value of 137 777 0 013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites this value results in corrections of 1 1 ma for pb pb dates calculated using the previously assumed invariant 238 u 235 u value of 137 88 using the determined 238 u 235 u value the 7 most radiogenic pb isotopic analyses for acid leached pyroxene rich and whole rock fractions yield an isochron pb pb age of 4556 75 0 57 ma in excellent agreement with the results of mn cr chronology which give the ages of 4557 4 2 5 ma and 4555 9 3 2 ma using the u corrected pb pb age of d orbigny as a time anchor along with the previously proposed thermal history of ibitira and our closure temperature estimates for pb diffusion the pb pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism the metamorphism may have been caused by burial of ibitira lava under successive lava flows and if so the pb pb age should post date the crystallization by a short time interval the pb isotopic data for acid leachates suggest partial re equilibration of pb between plagioclase and phosphate perhaps during an impact event at 4 49 ga as recorded by k ar systematics the whole rock 238 u 204 pb indicates that compared to ci chondrites ibitira is less depleted in pb than in some alkali elements despite a lower condensation temperature of pb than the alkali elements the restricted pb depletion may reflect preferential concentration of metals with high fluid melt partition coefficients including pb and zn as a result of fluid exsolution and migration within the parent magma we discuss the implications of the u pb systematics for the origin and differentiation of the parent body

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

puxe dating of eucrites abstract we report the results of xe analyses of the eucrites bereba cachari caldera camel donga chervony kut ibitira jonzac juvinas milbillillie moore county padvarninkai pasamonte pomozdino sioux county and vetluga and the howardite petersburg the obtained data together with data from the literature were utilized for the calculation of pu xe ages using a modified method where the concentration of the reference element nd that is used in the traditional pu xe dating is replaced by that of spallation 126 xe originating from rare earth elements the fraction of spallation 126 xe from ree in total spallation 126 xe is calculated via the ba ree ratios that are very similar in different samples of a given meteorite in comparison with the traditional pu xe dating the present method avoids problems of sampling heterogeneity and uncertainties in absolute instrument sensitivity and can be used when the concentration of nd in a sample is not known the pu xe ages for different samples of individual meteorites show good reproducibility ages of the eucrites bouvante chervony kut ibitira juvinas moore county pasamonte pca82502 pomozdino of the howardite petersburg and of the angrite lew86010 are old and most likely date the time of crystallization at 4 56 ga almost complete retention of fission xe in these meteorites is in a stark contrast with the apparent 1 retention of radiogenic 129 xe the eucrites bereba cachari caldera camel donga jonzac millbillillie neuvo laredo padvarninkai sioux county stannern and vetluga are distinctly younger this can be interpreted in different ways the ages may represent crystallization and or metamorphic ages or may simply reflect the different extent of diffusive fission xe losses during later events comparison of the pu xe ages with those from the sm nd and pb pb isotope chronometers yields in most cases a reasonable agreement in some meteorites however the pb pb chronometer appears to have been reset during secondary events while the pu xe isotope system is seen to be more resistant to such events

experimental geochemistry of pu and sm and the thermodynamics of trace element partitioning the partitioning of pu and sm between diopside liquid and whitlockite liquid has been investigated experimentally to evaluate the geochemical coherence of pu and the light rees pu d sm d is 2 for both diopsidic pyroxene and whitlockite this small amount of fractionation would be decreased further if pu were compared to ce or nd our experimental results thus validate the suggestion that pu behaves as a lree during igneous processes in reducing environments our data and the data of ray et al 1983 indicate that temperature rather than melt composition is the most important control on elemental partitioning this is true even though we demonstrate that additions of only 1 2 wt of p2 o 5 to the diopside anorthite albite system change pu d cpx by a factor of two our data suggest that p 2o 5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

the geologic history of vesta inferred from combined 207pb 206pb and 40ar 39ar chronology of basaltic eucrites abstract eucrites represent samples of mafic crust of the parent body likely vesta and record a complex geologic history involving magmatism metamorphism impact processing and metasomatism for better understanding of the geologic history we present combined 207pb 206pb and 40ar 39ar chronology of four basaltic eucrites showing distinct petrologic features agoult an equilibrated eucrite with granulitic textures camel donga a brecciated equilibrated eucrite enriched in metallic iron dag 380 a partially equilibrated monomict eucrite and nwa 049 a metasomatized polymict eucrite comprising mostly unequilibrated clasts the 207pb 206pb dating in combination with the acid washing technique was applied to pyroxene rich and plagioclase rich fractions while the 40ar 39ar dating was performed mainly on plagioclase grains using the laser incremental heating method the pb isotopic data for acid leaching residues are more radiogenic than those for acid washes reflecting efficient removal of non radiogenic pb components by the acid washing among the residues however only those of the agoult and camel donga plagioclase rich fractions yielded isochrons rather than errorchrons with 207pb 206pb ages of 4532 39 0 87 ma and 4515 43 0 42 ma respectively the trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase on the other hand the ar isotopic data for agoult and camel donga yielded plateau ages with weighted means of 4494 9 ma and 3749 25 ma in contrast to those for dag 380 and nwa 049 that did not return any plateau the 207pb 206pb and 40ar 39ar ages of agoult are interpreted to reflect the time of the u pb and k ar system closure during a prolonged thermal metamorphism by combining the ages reported here and in the literature with the calculated closure temperatures the cooling rate of agoult is determined to be 11 2 c ma this cooling rate corresponds to a burial depth of 10 20 km indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed the 20 ma younger 207pb 206pb ages of camel donga than agoult are considered to reflect delayed burial metamorphism in the source region whereas the plateau 40ar 39ar age would record the time of an impact causing brecciation and efficient ar degassing of the buried crust furthermore the model 207pb 206pb dates calculated for acid washed pyroxenes imply that the burial metamorphism of agoult and dag 380 were broadly contemporaneous and also that nwa 049 was metasomatized by fluids within the uppermost crust later than 4 4 ga

samarium neodymium evolution of meteorites we have obtained sm nd data on two differentiated meteorites ibitira a eucrite with distinct basaltic texture and with evidence of crystallization and morristown a group 3a mesosiderite as well as on acapulco an unclassified meteorite with chondritic chemical composition and a highly recrystallized texture we have demonstrated the presence of in situ decay of short lived 146 sm in these meteorites with initial abundance of 146 sm 144 sm from 0 009 to 0 007 for the different meteorites precisely defined 147 sm 143 nd internal isochrons were obtained yielding ages of 4 60 0 03 ae for acapulco 4 47 0 02 ae for morristown and 4 46 0 02 ae for ibitira the corresponding initial 143 nd 144 nd values obtained were 0 2 0 9 2 1 0 7 and 1 6 0 8 u epsilon units detailed consideration of the coupled 147 sm 143 nd and 146 sm 142 nd systematics and of the elevated initial 143 nd 144 nd values indicates the presence of inconsistencies in the data for the two sm nd parent daughter pairs in spite of the precise isochrons we present a model which provides an interpretation of the paired sm nd systematics in terms of an impulsive disturbance the abundance of phosphate with large concentrations of rees controls the ree economy plagioclase with low ree concentrations is assumed to be susceptible to exchange pyroxene is considered to be undisturbed because of the low diffusion coefficients of trace elements the experimental results can be reasonably explained by this model the results indicate that three meteorites studied some with very low ree concentrations including a mesosiderite are relatively ancient objects formed within the first 50 to 100 m y of the solar system by planetary differentiation and impact processes and were subjected to late metamorphism the time of late metamorphism is substantially younger than 4 4 ae and possibly as recent as 3 9 ae this metamorphism has resulted in partial very limited reequilibration between plagioclase and phosphate resulting in the observed shifts in initial 143 nd 144 nd sm nd model ages and the 142 nd 144 nd abundances but preserving the major temporal structures of early original formation and later disturbances these results show the need for care in the interpretation of the parent daughter systematics even when they appear well behaved and the mineral phases are viewed as highly resistant to mild metamorphism

noble gas retention chronologies for the st severin meteorite abstract 40 ar 39 ar and 129 xe 128 xe analyses were performed on two lithologies light and dark of the st severin ll6 chondrite for the light and dark fractions respectively we obtained 40 ar retention ages of 4 38 and 4 42 ae and 129 xe retention ages of 8 4 and 15 2 myr after bjurbole the two methods give age differences of opposite sense and by both methods the differences are significant both the 40 ar and the 129 xe ages are interpreted as dating relaxation of metamorphic conditions these two chronometers are decoupled however and do not date the same events 40 ar 39 ar reflect chondritic metamorphisrn on a 10 8 yr time scale the 129 xe 128 xe ages reflect isotopic closure at higher temperatures and earlier times

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

pu xe u xe u pb chronology and isotope systematics of ancient zircons from western australia the presence of xenon isotopes from in situ spontaneous fission of short lived 244 pu has been confirmed in a suite of 16 hadean detrital zircons from western australia in order to investigate the effects of xenon loss on estimates of the inferred pu u ratio we have irradiated the zircons with thermal neutrons to generate xe from 235 u neutron fission 131 xe 134 xe and 132 xe 134 xe ratios have been used to calculate the relative contributions from spontaneous fission of 244 pu and 238 u and neutron fission of 235 u and hence compare nominal pu u ratios and xenon retention ages u xe ages are typically lower than the pb pb ages indicating that xenon loss is common we show how the ternary mixing diagram can be used to place constraints on the timing and extent of this loss and to generate a corresponding pu u xe isochron although the zircons investigated in this study were extracted from the same metasedimentary unit the timing of xenon loss is variable this suggests that the loss may be the result of variable degrees of metamictization from grain to grain inferred pu u o ratios show a general decrease with the discordancy between pb pb and u xe ages for the least discordant samples we infer pu u o 0 008 which is close to the widely adopted chondritic value while we cannot completely exclude the effects of pu u fractionation in magmatic and other processes between formation of the earth and crystallisation of the zircons we conclude that they have been relatively small bfactor 2 and reflect the compatible behaviour of pu and u in the 4 valence state 2007 elsevier b v all rights reserved

old sm nd ages for cumulate eucrites and redetermination of the solar system initial 146sm 144sm ratio article i nfo sm 142 nd systematics short lived 146 sm 142 nd and long lived 147 sm 143 nd chronometers have been measured in three cumulate eucrites binda moore county and moama the two major mineral phases plagioclase and pyroxene present in these achondrites are characterized by a wide range of sm nd ratios that allows well resolved sm nd isochrons this group of meteorites thus is suitable to better constrain the initial 146 sm 144 sm ratio of the solar system binda and moore county give concordant ages of 4544 88 and 4542 85 ma respectively with initial 143 nd 144 nd ratios slightly higher to within error of chondritic these ages are in agreement with most of the radiometric ages determined on basaltic eucrites a best estimate for the solar system initial 146 sm 144 sm ratio is obtained using the five point regression line determined for binda the 146 sm 144

noble gases 81kr kr exposure ages and 244pu xe ages of six eucrites bereba binda camel donga juvinas millbillillie and stannern noble gases including radioactive 81kr t1 2 2 1 105 yr have been measured for the cumulate eucrite binda and five noncumulate eucrites bereba camel donga juvinas millbillillie and stannern some of which have been repeatedly analyzed concentrations of 81kr which range from 4 2 10 14 to 3 2 10 13 cm3 stp g roughly correlate with abundances of the main target elements sr y and zr 81kr kr cosmic ray exposure ages are obtained as 24 5 0 6 ma 21 0 1 6 ma 36 6 1 4 ma 10 6 0 8 ma 20 8 0 5 ma and 35 1 0 7 ma for bereba binda camel donga juvinas millbillillie and stannern respectively all the measured eucrites have 244pu derived fission xe on the basis of fission 136xe concentrations 244pu abundances are calculated to range from 0 40 ppb for binda to 1 08 ppb for juvinas following the method proposed by shukolyukov and begemann 1996a 244pu xe ages relative to the absolute crystallization age of 4 5578 ga for angra dos reis are determined as 46 18 ma bereba 29 34 ma binda 51 16 ma camel donga 10 23 ma juvinas 8 24 ma fine grained portion of millbillillie 51 21 ma coarse grained portion of millbillillie and 124 13 ma stannern which correspond to absolute ages ranging from 4 434 ga stannern to 4 566 ga fine grained portion of millbillillie millbillillie is a mixture of materials with different 244pu xe ages stannern s unusually young 244pu xe age might reflect a secondary shock disturbance but otherwise it tends to confirm a distinctive mode of igneous petrogenesis in comparison to most other eucrites the 244pu xe ages of the cumulate eucrites obtained so far binda this work moore county shukolyukov and begemann 1996a do not differ systematically from those of noncumulate eucrites the four eucrites with the older 244pu xe ages bereba binda juvinas and the fine grained portion of millbillillie show significant amounts of radiogenic 129xe 0 6 2 3 10 12 cm3 stp g after correction for fission 129xe which imply earlier retention of radiogenic 129xe from extinct 129i than other eucrites with young 244pu xe ages

ree patterns of eucrites and their genetic implications abstract rare earth element ree abundances in eucrites were precisely determined on juvinas pasamonte cachari stannem and antarctic eucrites y 74450 63 d g y 75015 20 e f y 75011 73 y 790007 61 f y 75011 84 d 1 and y 790007 61 e 4 the last two samples are eucrite clasts from polymict eucrites the other antarctic eucrite samples are matrix from polymict eucrites like the stannern eucrite all of the antarctic eucrites have relatively high ree abundances ree abundances in two clast samples are higher than those in the stannern and matrix samples of the antarctic eucrites when ree abundances in the antarctic eucrites and the stannem eucrite are normalized by the ree abundance suite of the juvinas or pasamonte eucrites highly linear ree patterns are obtained for the juvinas or pasamonte normalized ree patterns of the antarctic and stannern eucrites the inclination of the linear ree patterns increases with increasing ree abundances these observations suggest that the eucrites with larger ree abundances such as the stannern and antarctic eucrites were produced as residual liquid in a fractional crystallization process from the main group of eucrites e g the juvinas eucrite on the other hand the moore county cumulate eucrite is considered to be a solid phase formed in this process

60fe in eucrites additional evidence for the existence in the early solar system of live60fe has been found in the basaltic achondrite juvinas the relative abundance of 60fe 56fe of x0223c 4 10 10 at the time of meteorite solidification is one order of magnitude lower than that previously observed for the eucrite chervony kut this difference corresponds to a 4 7 ma time interval between closure of the60fe 60ni isotopic system in the two meteorites the factor two higher initial 60ni 58ni ratio in juvinas is consistent with this time difference and a common bulk fe ni ratio in the two meteorites using the60fe 60ni isotope system as a chronometer various models for the evolution of the eucrite parent body of different degrees of complexity are discussed regardless of the detailed differences between these models a short time interval of only a few million years between planetary differentiation and basaltic crust formation is indicated

differentiated achondrites asuka 881371 an angrite and divnoe noble gases ages chemical composition and relation to other meteorites abstract we present a study of the noble gas isotopic abundances in two achondrites divnoe and the asuka 881371 angrite for divnoe we also performed chemical analyses of the major elements and of some minor and trace elements that are relevant for the comparison with other meteorite types and for the interpretation of the noble gas data based on the cosmic ray produced noble gases an exposure age of 5 4 0 7 ma for asuka 881371 was obtained this ejection time from the parent asteroid differs from those of the other three angrites angra dos reis 55 5 ma lew 86010 17 6 ma and lew 87051 0 2 ma whereas the u th 4 he and k 40 ar gas retention ages of asuka 881371 are 3750 1000 ma and 3910 500 ma respectively we derive for the 244 pu 136 xe system a formation age of 4533 40 ma which is contemporaneous with that of the other angrites angra dos reis and lew 86010 for divnoe the abundance patterns of the major lithophile siderophile refractory and volatile elements the isotopic abundance pattern for primordial trapped xe and the oxygen isotopes indicate that this unique meteorite is related to the brachinite achondrites assuming no pre exposure to cosmic rays on the parent body divnoe s asteroid ejection event occurred 17 2 2 3 ma ago

history of lunar meteorites queen alexandra range 93069 asuka 881757 and yamato 793169 based on noble gas isotopic abundances radionuclide concentrations and chemical composition we investigated the characteristics and history of lunar meteorites queen alexandra range 93069 yamato 793169 and asuka 881757 based on the abundances of all stable noble gas isotopes the concentrations of the radionuclides 10be 26al 36cl and 81kr and the abundances of mg al k ca fe cl sr y zr ba and la based on the solar wind and cosmic ray irradiations que 93069 is the most mature lunar meteorite studied up to now the 40ar 36ar ratio of the trapped component is 1 87 0 16 this ratio corresponds to a time when the material was exposed to solar and lunar atmospheric volatiles 400 ma ago on the other hand yamato 793169 and asuka 881757 contain very little or no solar noble gases which indicates that these materials resided in the top layer of the lunar regolith only briefly or not at all for all lunar meteorites we observe a positive correlation of the concentrations of cosmic ray produced with trapped solar noble gases the duration of lunar regolith residence for the lunar meteorites was calculated based on cosmic ray produced 21ne 38ar 78kr 83kr and 126xe and appropriate production rates that were derived based on the target element abundances and the shielding indicator 131xe 126xe for que 93069 yamato 793169 and asuka 881757 we obtained 1000 400 ma 50 10 ma and 1 ma respectively both asuka 881757 and yamato 793169 show losses of radiogenic 4he from u and th decay and yamato 793169 also 40ar loss from k decay for asuka 881757 we calculate a k ar gas retention age of 3100 600 ma and a 244pu 136xe fission age of 4240 170 ma this age is one of the oldest formation ages ever observed for a lunar basalt the exposure history of que 93069 after ejection from the moon was derived from the radionuclide concentrations ejection 0 16 0 03 ma ago duration of moon earth transit 0 15 0 02 ma and fall on earth 0 015 ma ago this ejection event is distinguished temporally from those which produced the other lunar meteorites we conclude that six to eight events are necessary to eject all the known lunar meteorites

megaregolith insulation and the duration of cooling to isotopic closure within differentiated asteroids and the moon ages determined for extraterrestrial samples by the sm nd and rb sr techniques are commonly assumed to record igneous crystallization events because in solid silicates nd and sr diffuse at exceedingly slow rates however we find that for coarse grained igneous cumulate rocks from the moon or from a large thoroughly brecciated asteroid this assumption may not be reliable the moon and at least one asteroid the parent body of the eucrite diogenite and howardite meteorites appear to have been largely molten at or about the time they formed we have modeled global cooling of the moon and large r 40 250 km asteroids starting at or near the solidus a crucial factor in determining the prevailing interval ic of cooling between igenous crystallization and isotopic closure for any given depth in the crust is the extent to which the body is insulated by a regolith megaregolith layer of porous fragmental impact debris given plausible assumptions regarding the thicknesses of such layers on the moon and the eucrite parent asteroid and regarding the radius of the eucrite asteroid our results indicate that deep crustal regions tend to remain above the nd and sr isotopic closure temperature for intervals that are long in comparison to the precision of modern nd and sr based age measurements and in comparison to suggested chronologic scenarios of global differentiation ic intervals of as long as 100 m y may be common among available samples of primordial deep crustal cumulates from both bodies chronologies for the gross solidification of the moon and the eucrite asteroid should allow for the possibility that any single age for a coarse grained plutonic or cumulate textured rock might be many tens of millions of years younger than the igneous crystallization age

in situ pb pb dating of silica rich northwest africa nwa 6594 basaltic eucrite and its constraint on thermal history of the vestan crust

martian xenon components in shergotty mineral separates locations sources and trapping mechanisms isotopic signatures and concentrations of xenon have been measured in shergotty mineral separates by laser step heating martian atmosphere and martian interior xenon are present as is a spallation component martian atmospheric xenon is 510 times more concentrated in opaque minerals magnetite ilmenite and pyrrhotite and maskelynite than in pyroxenes perhaps reflecting grain size variation this is shown to be consistent with shock incorporation a component consisting of solar xenon with a fission contribution similar to components previously identified in martian meteorites and associated with the martian interior is best defined in the pyroxene dominated separates this component exhibits a consistent 129xe 129xe 132xe 1 2 excess over solar planetary 129xe 132xe 1 04 we suggest that gas present in the melt perhaps a mixture of interior xenon and martian atmosphere was incorporated into the pyroxenes in shergotty as the minerals crystallized

plutonium uranium and rare earths in the phosphates of ordinary chondrites the quest for a chronometer abstract the distributions of pu u and the ree in particular nd in single crystals of the calcium phosphates merrillite and apatite of ordinary chondrites were studied to establish whether 244 pu can be used to determine time differences in meteorite formation uranium was measured by an induced fission track technique and the ree by secondary ion mass spectrometry limits on pu concentrations at the time of track retention were inferred from fossil track measurements it is argued here that most previous estimates of pu concentrations obtained from fossil tracks in phosphates failed to correct adequately for spallation induced tracks and were probably not correct plutonium is preferentially enriched in merrillite relative to apatite in contrast u concentrations are on the average 15 times higher in apatite than in merrillite there are grain to grain variations in the same phosphate phase from a given meteorite for both elements on the other hand in the merrillite of a given meteorite the ree concentrations are remarkably constant typical grain to grain variations for type 5 and 6 chondrites are in most cases less than 10 average nd concentrations in the merrillite of 15 different chondrites range from 96 to 164 ppm ree abundances in apatite show more variations average nd concentrations in the apatite of 13 chondrites range from 8 to 54 ppm and the ratios of nd concentrations in merrillite relative to apatite range from 2 7 to 11 despite the higher affinity of both pu and the ree for merrillite than for apatite there is no quantitative correlation between the abundances of these elements in merrillite grains from a given ordinary chondrite because of this lack of geochemical coherence between pu and any of the ree it is concluded that 244 pu cannot be used to determine the relative formation times of chondrites bulk measurements that avoid the problems associated with fossil track measurements are discussed however they have so far proven to be of little utility

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

impact melting of the cachari eucrite 3 0 gy ago both the host phase and glass veins of the cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their 39ar 40ar gas retention ages cachari is chemically similar to other non cumulate eucrites the vesicular glass veins vary from pure glass to devitrified glass to areas that are substantially crystalline the glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of cachari but some differences probably due to preferential dissolution occur along melt contacts the glass formed by shock melting of cachari host or of rock identical to it 39ar 40ar data for the host and glass suggest distinctly different ages of 3 04 07 gy and 3 47 04 gy respectively the time of glass formation which may also be the time of brecciation is most likely given by the 3 0 gy age of the host the higher age for the glass is interpreted to represent incomplete ar degassing during the 3 0 gy event due to the greater resistance to ar diffusion shown by the glass compared to the host event ages significantly younger than 4 5 gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body

the puerto lapice eucrite puerto lapice is a new eucrite fall castilla la mancha spain 10 may 2007 in this paper we report its detailed petrography magnetic characterization mineral and bulk chemistry oxygen and noble gas isotope systematics and radionuclide data study of four thin sections from two different specimens reveal that the meteorite is brecciated in nature and it contains basaltic and granulitic clasts as well as recrystallized impact melt and breccia fragments shock veins are ubiquitous and show evidence of at least three different shock events bulk chemical analyses suggest that puerto lapice belongs to the main group of basaltic eucrites although it has a significantly higher cr content oxygen isotopes also confirm that the meteorite is a normal member of the hed suite noble gas abundances show typical patterns with dominant cosmogenic and radiogenic contributions and indicate an average exposure age of 19 2 ma and a pu fission xe age well within typical eucrite values cosmogenic radionuclides suggest a preatmospheric size of about 20 30 cm in diameter