a waveform migration for the investigation of p wave structure at the top of d beneath northern siberia we present the first waveform migration for the lower mantle focusing on the lowermost 400 km of the mantle beneath northern siberia we migrate vertical component p wave seismograms recorded in western europe for earthquakes located in the kuril islands region we focus on the time window between the direct p wave and the pcp wave reflected at the core mantle boundary and we investigate the origin of pcp precursors the data selected for their good signal to noise ratio are deconvolved by the source time function determined by the beam of the p wave we observe the signature of a finite size scattering object located around 75 n 85 e at an apparent depth of 2650 km with respect to the iasp91 model its scale length is probably of the order of 100 km this object is characterized by an anisotropic scattering diagram and may be related to the presence of an ancient subducted plate near the core mantle boundary

siderophile geochemistry of ureilites a record of early stages of planetesimal core formation abstract new bulk compositional data including trace siderophile elements such as ir os au and ni are presented for 25 ureilites without exception ureilites have siderophile abundances too high to plausibly have formed as cumulates ureilites undoubtedly underwent a variety of smelting by which c was oxidized to co gas while olivine feo was reduced to fe metal however pressure buffered equilibrium smelting is not a plausible model for engendering the wide range 75 96 mol of mafic silicate core mg among ureilites the smelting reaction produces too much co gas even supposing a disequilibrium process with the smelt gas leaking out of the mantle none of the ureilites least of all the ureilite with the most reduced highest olivine core mg alh84136 has the high fe metal abundance predicted by the smelted cores model in principle the fe metal generated by smelting could have been subsequently lost but siderophile data show that ureilites never underwent efficient depletion of fe metal ureilites display strong correlations among siderophile ratios such as au ir ni ir co ir as ir se ir and sb ir ureilite siderophile depletion patterns loosely resemble siderophile fractionations presumably nebular in origin among carbonaceous chondrites however zn for an element of moderate volatility is anomalously high in ureilites a tight correlation between au and ni extrapolates to the low ni au side of the compositional range of carbonaceous chondrites from this mismatch mild but nonetheless significant depletions of refractory siderophile elements such as ir and os and moderate depletions of strongly siderophile weakly chalcophile elements such as ni and au we infer that the ureilite siderophile fractionations are largely the result of a non nebular process i e removal of s rich metallic melt possibly with minor entrainment of fe metal several lines of trace element evidence indicate that melt porosity during ureilite anatexis was at least moderate the ureilite pattern of very mild depletions of extremely siderophile elements but much deeper depletions of moderately siderophile chalcophile elements suggests that asteroidal core formation probably occurs in two discrete stages in general separation of a considerable proportion several wt of s rich metallic melt probably occurs long before and at a far lower temperature than separation of the remaining s poor fe metal apart from the fe metal itself only extremely siderophile elements wait until the second stage to sequester mainly into the core

microdistributions and petrogenetic implications of rare earth elements in polymict ureilites polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites including plagioclase enstatite feldspathic melt clasts and dark inclusions this paper investigates the microdistributions and petrogenetic implications of rare earth elements rees in three polymict ureilites elephant moraine eet 83309 eet 87720 and north haig focusing particularly on the mineral and lithic clasts not found in monomict ureilites as in monomict ureilites olivine and pyroxene are the major heavy h ree carriers in polymict ureilites they have light l ree depleted patterns with little variation in ree abundances despite large differences in major element compositions the textural and ree characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body simple ree modeling shows that the most common melt clasts in polymict ureilites can be produced by 20 30 partial melting of chondritic material leaving behind a ureilitic residue the plagioclase clasts as well as some of the high ca pyroxene grains probably represent plagioclase pyroxene rock types on the ureilite parent body however the variety of ree patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event multiple processes probably including shock melting and different sources are required to account for all the ree characteristics observed in lithic and mineral clasts the c rich matrix in polymict ureilites is lree enriched like that in monomict ureilites the occurrence of ce anomalies in c rich matrix dark inclusions and the presence of the hydration product iddingsite imply significant terrestrial weathering a search for 26mg excesses from the radioactive decay of 26al in the polymict ureilite eet 83309 was negative

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

light rare earth element enrichments in ureilites a detailed ion microprobe study this paper explores the possible origin of the light rare earth element lree enrichments observed in some ureilites a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites the spatial microdistributions of rees in c rich matrix areas of the three ureilites with the most striking v shaped whole rock ree patterns kenna goalpara and novo urei were investigated using the ion imaging capability of the ion microprobe all olivines and clinopyroxenes measured have lree depleted patterns with little variation in ree abundances despite large differences in their major element compositions from ureilite to ureilite furthermore we searched for but did not find any minor mineral phases that carry lrees the only exception is one ti rich area 20 m in lewis cliff lew 85400 with a major element composition similar to that of titanite ree abundances in this area are high ranging from la 400 ci to lu 40 ci in contrast all ion microprobe analyses of c rich matrix in kenna goalpara and novo urei revealed large lree enrichments in addition c rich matrix areas in the three polymict ureilites elephant moraine eet 83309 eet 87720 and north haig which have less pronounced v shaped whole rock ree patterns show smaller but distinct lree enrichments the c rich matrix in antarctic ureilites tends to have much lower lree concentrations than the matrix in non antarctic ureilites there is no obvious association of the lrees with other major or minor elements in the c rich areas ion images further show that the lree enrichments are homogeneously distributed on a microscale in most c rich matrix areas of kenna goalpara and novo urei these observations suggest that the lrees in ureilites most probably are absorbed on the surface of fine grained amorphous graphite in the c rich matrix it is unlikely that the lree enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body we favor lree introduction by terrestrial contamination

the chromium isotopic composition of almahata sitta abstract nine ureilitic fragments of the anomalous polymict ureilite almahata sitta have been analyzed for their cr isotopic compositions all the samples including both nonmagnetic and magnetic portions show essentially the same e54cr deficit 0 77 0 10 relative to the terrestrial cr standard this contrasts with the variable positive 54cr anomalies observed for carbonaceous chondrites but agrees with the values measured for eucrites diogenites and mesosiderites trinquier et al 2007 this implies that contrary to previous suggestions based on o isotopes ureilites were not derived from any known carbonaceous chondrite parent body instead the almahata sitta parent body may have accreted in a nebular region environment similar to that of the howardite eucrite and diogenite hed parent body in addition the lack of variation in e54cr combined with variable o isotopic compositions in the meteorite fragments suggests that whatever process es caused the o isotopic heterogeneity of the solar system was probably not responsible for heterogeneity in e54cr the samples show resolvable variations in e53cr 0 15 0 41 that are correlated with mn cr ratios suggesting that live 53mn was present at the time of formation of almahata sitta the isochron yields an initial 53mn 55mn value of 3 1 1 1 10 6 corresponding to an age of 4563 6 2 2 ma when related to u pb and mn cr data for the d orbigny angrite this age is consistent with the mn cr and al mg ages of two other polymict ureilites goodrich et al 2010 magmatic activity on the ureilites parent body seems to have postdated the formation of refractory inclusions by approximately 4 5 ma

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

spectral reflectance properties of ureilites abstract the 0 35 2 6 m reflectance spectra of 18 ureilites have been examined in order to improve our understanding of the spectral reflectance properties of this meteorite class across this spectral range ureilite spectra are characterized by a steep rise in reflectance over the 0 3 to approximately 0 7 m range low overall reflectance 25 and weak mafic iron silicate absorption bands in the 1 and 2 m region the weakness of these bands and the low reflectance are attributed to the presence of dispersed graphite and related carbonaceous phases metal and possibly shock wavelength positions of the mafic silicate absorption bands span a range of values but are consistent with the presence of pyroxene and olivine ureilite spectra generally exhibit blue slopes across the 0 7 2 6 m interval and exhibit many overall similarities to some carbonaceous chondrites the weak features and spectral diversity of ureilites make reflectance spectroscopy based identification of a ureilite parent body challenging as terrestrial alteration of ureilites is prevalent spectral studies of falls are most useful for determining the spectral properties of likely parent bodies

volatilization induced by impacts recorded in zn isotope composition of ureilites ureilites are ultramafic achondrites formed by igneous processes early in the history of the solar system ureilites are heavily depleted in volatile elements probably by evaporation at the surface of the ureilite parent body however the moderately volatile element zn is an exception and its abundance is similar to cl volatilization can fractionate isotopes in a mass dependent manner therefore comparing the isotope composition of volatile elements in ureilites may better constrain the physical conditions during ureilite formation and the mechanism of origin of the variation of zn content for this study we measured the zn isotopic composition of 11 ureilites with a large range of zn content the elemental abundance of zn is negatively correlated with delta zn 66 which may reflect that isotopic fractionation occurred by evaporation during the heating event on the ureilite parent body simple rayleigh distillation predicts isotopic fractionations much larger than what is actually observed therefore such a model cannot account for the observed zn isotope fractionation in ureilites we propose that the observed isotopic fractionation can be produced by evaporation in a diffusion limited regime in addition the isotopic composition of zn is shown to be correlated with the shock grade of the ureilites this may indicate that the heating events responsible for the depletion in light isotopes in some ureilites were impacts c 2010 elsevier b v all rights reserved

mineralogy of coexisting pyroxenes in magnesian ureilites and their formation conditions abstract the occurrence of orthopyroxene in two magnesian ureilites found in antarctica yamato y 791538 and lew85440 has been confirmed by electron probe microanalysis and by single crystal x ray diffraction the first coexisting equilibrium pigeonite orthopyroxene pair has been found in y791538 the lew85440 orthopyroxene coexists with minor augite the orthopyroxenes and olivines in y791538 y74659 and lew85440 have almost identical compositions but the ca contents of their coexisting clinopyroxenes differ considerably these phase assemblages can be explained by crystallization of mafic silicates with a similar composition oxygen fugacity and oxygen isotope anomaly at different temperature above 1220 c the high temperature crystallization of refractory mg rich silicates and the possible preservation of the oxygen isotope anomaly of precursor carbonaceous chondrites like materials may be consistent with their formation processes probably through local melting of more matrices than mg rich silicates and crystal growth induced by the planetesimal scale collision

ureilites trace element clues to their origin carbonaceous vein separates from kenna and havero as well as bulk kenna were analyzed by rnaa for ag au bi br cd cs ge in ir ni pd os rb re sb se te tl u and zn the data are reviewed together with four earlier chicago analyses of bulk ureilites linear regressions confirm the presence of two metal components with the following cl normalized ratios ir ni 14 6 1 ge ni 5 4 2 4 au ni 2 3 0 9 the high ir component is enriched in vein separates and hence belongs to veins the lowir component belongs to the ultramafic rock vein material is enriched in all elements analyzed by us except zn and accounts for most of the c noble gases and presumably siderophiles in the meteorite most of the properties of ureilites apparently can be explained by the cumulate model of berkley et al 1980 with certain modifications comparison of ureilites with three other ultramafic rocks from different planets earth s mantle lunar dunite and chassigny suggests that the ureilite parent body had a primitive chondritic composition similar to c3v chondrites but richer in metal and carbon it melted causing depletion of incompatibles to a mean abundance of 0 02 cl and incomplete segregation of metal fes and c fractional crystallization or melting of metal in the presence of s and c apparently can explain the fractionations of ir re ni au and perhaps ge obviating the need for extraneous sources of vein metal or unusual parent body compositions noble gases from the parent material may have been retrapped in carbon during magmatism provided the system was closed

primary igneous carbon in ureilites petrological implications the ureilite meteorites are carbonaceous olivine pyroxene achondrites they typically contain up to 4 wt carbon carbonaceous matrix as graphite diamond and lonsdaleite shock degradation has effectively obliterated primary textures in the carbonaceous matrix of previously described ureilites a factor that has hampered efforts to explain the origin of this material in contrast the antarctic ureilite alha78019 displays perfectly preserved primary textures in the carbonaceous matrix characterized by euhedral graphite blades intergrown with fe ni metal and sulfide diamonds are absent this petrographic feature suggests that most graphite in ureilites originated by crystallization from a c rich metallic phase assuming that fo2 is controlled by c co co2 reactions the compositions of silicates and metals in ureilites imply a two stage redox history the noble gases and rare earths of ureilites are discussed in light of this model

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

seismic migration processing of p sv converted phases for mantle discontinuity structure beneath the snake river plain western united states we experiment with backprojection migration processing of teleseismic receiver functions from the snake river plain srp broadband seismic experiment previous analyses of data from this experiment have used a common midpoint cmp stacking approach a method widely applied for analysis of p sv converted phases receiver functions to obtain high resolution imaging of upper mantle discontinuities the cmp technique assumes that all p sv conversions are produced by flat lying structures and may not properly image dipping curved or laterally discontinuous interfaces in this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions we first perform synthetic experiments that illustrate the potential improvement of migration processing over cmp stacks application of the migration processing to the srp data set shows most of the major features as in the original cmp work but with a weaker 410 km discontinuity and a more intermittent discontinuity at 250 km apparent depth random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images these tests show that a 20 km elevation of the 660 km discontinuity directly beneath the snake river plain is robust but that the variations in 410 km discontinuity topography that we observe are not stable upon resampling bright spots near 250 km apparent depth are robust upon resampling but interpretation of these features is complicated by possible sidelobe artifacts from topside moho reverberations

origin and evolution of the ureilite parent magmas multi stage igneous activity on a large parent body analyses of olivine and pigeonite cores interpreted as cumulate crystals in eight low shock ureilites show well defined correlations between fe x where x is one of the minor elements mn cr ca al ti p or ni and fe mg ratios for the lithophile minor elements these trends are linear with positive slope and pass through or near the origin indicating various degrees of feo reduction of the parent magmas the trends shown by p and ni are consistent with this interpretation and additionally require equilibrium crystallization of 20 27 mole metal graphite was most likely the reducing agent which implies that the ureilites equilibrated at different pressures because the graphite fo2 buffer is pressure dependent there is no evidence for fractionation of olivine or pryroxene in any of these trends the ca ti and al trends show significant scatter indicating that there must have been multiple parent magmas with very similar mg mn and mgcr ratios but different mgal mgca and mgti ratios cao al2o3 ratios vary by a factor of 3 among the eight magmas and are superchondritic 14 32 molar similar to lunar mare basalts we propose the following model for generation and crystallization of ureilite parent magmas upm chondritic material undergoes 10 25 partial melting leaving a plagioclase depleted residue the magma or magmas form a differentiated crust on the ureilite parent body upb with mafic cumulates and plagioclase cumulates spatially separated later remelting 10 of the mafic cumulates produced the upm siderophile elements indicate that the source region of the upm had not experienced segregation of metal such as would occur during core formation in the source region the pressure must have been at least 0 5 1 kb because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron the graphite bearing upm were emplaced at various levels in the crust where they were reduced to various extents dependent on depth ureilites are early olivine pigeonite cumulates from these magmas compositions of interstitial low ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower mnmg crmg camg almg and namg ratios than the upm and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites this interstitial material may be derived by mixing a shock melt of nearby olivine pigeonite cumulates with residual primary liquid our model predicts that the upb had a differentiated crust did not have a core and was at least 235 km in radius the lack of a ureilite like body among spectrally analyzed asteroids suggests that either the spectral signature of the upb is not that of ureilites themselves perhaps of carbon free feldspathic material instead that the upb is not intact or that ureilites did not originate in the asteroid belt

sinking of the dharwar basin south india implications for archaean tectonics abstract archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite gneiss masses the late archaean bababudan greenstone belt dharwar craton south india contains a low grade volcanosedimentary basin sequence the dharwar supergroup which unconformably overlies a previously eroded basement made up mainly of granite and gneiss fabric trajectories kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks as well as strike slip shearing this is attributed to the progressive sagduction gravity sinking of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing we propose a deformation model for the bababudan belt that may explain common features of many greenstone belts such as decollements and superimposed upright fold patterns which are attributed to the progressive sinking of the supracrustals into their basement the results of our study and other field investigations in the crust of the dharwar craton as well as structural evidence from other cratons zimbabwe and pilbara raise the possibility that a unique continental tectonic regime existed during the archaean controlled by the emplacement of dense volcanic traps mantle underplating and thermal blanketing of the continental crust this tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity i e rayleigh taylor instabilities between the volcanic traps and the rest of the continental crust it allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating the diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large scale accretion events of the continental crust perhaps in hot spot environments

trace element analysis of ureilites new constraints on their petrogenesis abstract six ureilites alha77257 alha81101 alh82130 pca82506 kenna and novo urei were analyzed using neutron activation analysis for ca sc cr mn fe co ni zn ga ree w re os ir and au we examined bulk samples as well as acid treated samples in the bulk samples the refractory siderophiles concentrations range from approximately 0 1 to 1 0 times ci chondrites while the volatile siderophiles range from about 0 07 to 0 3 times ci chondrites rare earth elements rees in ureilites are quite depleted and display light and heavy rare earth enrichments the antarctic meteorites display either much less pronounced v shaped patterns or no enrichment in the light rare earths at all in terms of the new trace element results ureilites do not fall into the coherent groups that other workers have defined by chemical or petrographic characteristics trace elements do provide additional constraints on the models for the petrogenesis of ureilites in particular the siderophile element abundances call for simplified models of chemical processing rather than the complex multistage processing called for in silicate fractionation models ree concentrations on the other hand imply multistage processing to produce the ureilites none of the ureilite petrogenesis models extant account for the trace element data these new data and the considerations of them with respect to the proposed ureilite petrogenesis models indicate that the direction of modelling should be toward contemplation of mixtures and how the components we observe in ureilites behave under such conditions

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

transition zone velocity gradients and the 520 km discontinuity stacks of long period global digital seismograph network gdsn seismograms at 110 to 180 epicentral distance reveal precursors to ss that result from underside reflections off upper mantle seismic discontinuities the 410 and 660 km discontinuities are obvious in these stacks but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410 and 660 km reflections these sidelobes result from the limited bandwidth of the gdsn instrument responses and the effect of crustal reverberations on the ss reference phase the crustal effects can be minimized by restricting the records to oceanic bounce points where the 6 km thick crust has little effect on the long period waveforms over 2000 long period transverse component seismograms with oceanic ss bounce points recorded by the gdsn from 1976 to 1991 are manually edited aligned on ss and then stacked using a new procedure that weights the records by data quality the resulting image shows a clear reflection from a 520 km discontinuity that cannot be explained as a sidelobe artifact confirming earlier results of shearer 1990 1991 and revenaugh and jordan 1991 by stacking along the expected travel time curves for discontinuity phases the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time the features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces using geometrical ray theory and assuming a constant velocity versus density scaling relationship i fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion the inverse problem is addressed by performing a deconvolution of the profile with the ss reference phase after a correction for attenuation followed by a direct mapping of reflectivity versus time into velocity versus depth velocity depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models except that the ss precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660 km discontinuity estimated discontinuity shear impedance changes are 6 7 1 1 at 420 km 2 9 0 7 at 519 km and 9 9 1 5 at 663 km the impedance change near 520 km is consistent with current mineral physics results for the olivine to phase change and places constraints on the fraction of olivine in the transition zone

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

chondrules matrix and coarse grained chondrule rims in the allende meteorite origin interrelationships and possible precursor components bulk compositions of 20 chondrules 13 coarse grained chondrule rims and two composite samples of matrix material from cv3 allende were determined by instrumental neutron activation analysis three rare nonporphyritic cv chondrules from allende leoville and vigarano were analyzed by broad beam electron microprobe analysis chondrule precursor components deduced from chondrule compositions are characterized by 1 refractory and semi refractory lithophiles with low feo 2 common siderophiles and chalcophiles 3 common to slightly volatile lithophiles si cr mn and modal pyroxene with moderately high feo feo mgo 4 moderately volatile siderophiles and 5 alkalies these precursor components can be explained by plausible processes occurring in the solar nebula the precursor components of coarsegrained rims are apparently the same as those of chondrules these components differ from those inferred previously for oc eh or cm co chondrules in general the compositions of coarse grained rims are more similar to the mean chondrule composition than to those of the chondrules they enclose rim siderophile contents are generally 2 5x greater than those in the chondrules and 0 5 to 2x those in bulk allende rims as a group are more homogeneous than chondrules suggesting that rims formed from finer grained precursors several chondrules and rims have fractionated rare earth patterns and may have been affected by metamorphism the enclosure of some coarse grained rims by fine grained matrix like rims that are almost certainly of nebular origin indicates that coarse grained rims were also formed in the nebula two matrix samples are similar in composition to some coarse grained rims suggesting that matrix was derived from similar compositional reservoirs of nebular dust chondrules with coarse grained rims were reheated in space following the accretion of dusty coatings this restricts plausible chondrule forming mechanisms to those that allow multiple nebular heating events

elemental mapping by dawn reveals exogenic h in vesta s regolith using dawn s gamma ray and neutron detector we tested models of vesta s evolution based on studies of howardite eucrite and diogenite hed meteorites global fe o and fe si ratios are consistent with hed compositions neutron measurements confirm that a thick diogenitic lower crust is exposed in the rheasilvia basin which is consistent with global magmatic differentiation vesta s regolith contains substantial amounts of hydrogen the highest hydrogen concentrations coincide with older low albedo regions near the equator where water ice is unstable the young rheasilvia basin contains the lowest concentrations these observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites observed as clasts in some howardites and subsequent removal or burial of this material by large impacts

osmium isotope systematics of ureilites abstract the 187 os 188 os for 22 ureilite whole rock samples including monomict augite bearing and polymict lithologies were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body or bodies the re os ratios of most ureilites show evidence for a recent disturbance probably related to re mobility during weathering and no meaningful chronological information can be extracted from the present data set the ureilite 187 os 188 os ratios span a range from 0 11739 to 0 13018 with an average of 0 1258 0 0023 1 similar to typical carbonaceous chondrites and distinct from ordinary or enstatite chondrites the similar mean of 187 os 188 os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites from the narrow range of the 187 os 188 os distribution in ureilite meteorites it is further concluded that re was not significantly fractionated from os during planetary differentiation and was not lost along with the missing ureilitic melt component the lack of large re os fractionations requires that re os partitioning was controlled by a metal phase and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body

primary magmatic carbon in ureilites evidence from cohenite bearing metallic spherules metallic spherules containing cohenite fe ni metal sulfides and rare phosphorus bearing minerals occur as inclusions within olivine and pigeonite grains in five low shock ureilites the bulk compositions mineralogy and textures of these spherules indicate that they represent fe ni c s alloys that existed as droplets of immiscible metallic liquid in the magma s from which ureilites crystallized at magmatic temperatures these alloys were carbon saturated and probably coexisted with graphite in the magma this is consistent with the occurrence of graphite crystals as inclusions in olivine in some ureilites and supports the view that graphite in ureilites was a primary magmatic component graphite could have been a residue from the solid source materials that melted to produce the ureilite parent magma s or could have crystallized from the magma s the metallic liquids separated into immiscible fe c liquids and fe s liquids after they were trapped within silicates the fe c liquids contained 4 5 wt c and 3 7 wt ni cohenite was the liquidus phase and started to crystallize at 1200 1225 c forming euhedral and subhedral crystals at the eutectic 1175 c iron containing 7 9 ni and 1 8 c crystallized along with cohenite at lower temperatures the iron exsolved cohenite and possibly iron

53mn 53cr and 26al 26mg ages of a feldspathic lithology in polymict ureilites abstract we report 53 mn 53 cr and 26 al 26 mg isotopic data obtained by in situ sims analysis for feldspathic clasts in polymict ureilites dag 319 and dag 165 the analyzed clasts belong to the albitic lithology the most abundant population of indigenous feldspathic materials in polymict ureilites and are highly fractionated igneous assemblages of albitic plagioclase fe rich pyroxenes phosphates ilmenite silica and fe mn k p ti enriched glass glass in dag 165 clast 19 has extremely high and variable 55 mn 52 cr ratios 500 58 000 and shows correlated 53 cr excesses up to 1500 clearly indicating the presence of live 53 mn at the time of formation the slope of the well correlated isochron defined by glass and pyroxenes from this clast corresponds to 53 mn 55 mn 2 84 0 10 10 6 2 data for less 55 mn 52 cr enriched glasses from dag 319 clast b1 as well as phosphates from several other clasts are consistent with this isochron the 53 mn 55 mn ratio obtained from the isochron implies that these clasts are 0 70 0 18 ma younger than the d orbigny angrite corresponding to the absolute age of 4563 72 0 22 ma plagioclase in dag 319 clast b1 has a fairly constant 27 al 24 mg ratio of 900 and shows resolvable 26 mg excesses of 2 the slope of the isochron defined by pyroxene and plagioclase in this clast is 3 0 1 1 10 7 2 corresponding to a time difference of 5 4 0 3 0 5 ma after cai assuming the canonical initial 26 al 27 al ratio of 5 10 5 and an age 0 5 0 3 0 5 ma younger than d orbigny its absolute age relative to d orbigny is 4563 9 0 4 0 5 ma in agreement with the 53 mn 53 cr age from clast 19 these data provide the first high precision age date 5 4 ma after cai for ureilites giving a minimum estimate for the age of differentiation of their parent asteroid interpretation of this age for the thermal and physical history of that asteroid depends on a number of currently unknown or model dependent parameters including its size bulk composition and oxidation state and the petrologic relationship between the feldspathic clasts and main group ureilites

uthpb and smnd isotopic systematics of the goalpara ureilite resolution of terrestrial contamination the u th pb and sm nd isotopic systematics of mineral separates from the goalpara ureilite were studied after applying acid leaching techniques the separates were successively leached with dilute hbr and hno3 in order to remove any secondary pb components in the meteorite as well as to separate a light ree rich component which has been known to be dissolved using a similar acid leaching procedure the leachates contained large amounts of terrestrial pb as well as significant amounts of u th sm and nd whereas the residues were highly depleted in all five of these elements between 0 1 and 0 001 times ci chondritic abundance the u pb th pb and sm nd isotopic systematics can be interpreted as mixtures of highly depleted ureilite and terrestrial contamination in all fractions for this reason precise ages could not be obtained from any of these systems the lead isotopic compositions in the residues are considered mixtures of terrestrial pb and primordial meteoritic pb canon diablo troilite ph with a small amount of radiogenic pb however on a u pb concordia diagram the residues indicate a formation age as old as 4 55 ga assuming the u pb data represent a mixture of terrestrial and ureilite u and pb similarly sm nd systematics of the residues form a mixing line between the composition of 4 55 gy old material and an average terrestrial crustal composition yielding a 3 74 ga pseudo isochron furthermore the acid leachates show terrestrial signatures of thu and smnd ratios and pb and nd isotopic ratios therefore it is doubtful that the so called light ree rich component in ureilites is indigenous and that the implied metasomatic event on the ureilite parent body is real

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

crustal structure of the eastern snake river plain determined from ray trace modeling of seismic refraction data ray trace travel time modeling of the seismic refraction record sections for a profile from near soda springs idaho to near mckay idaho was used to derive a crustal model across the eastern snake river plain esrp the derived crustal model is consistent with the velocity structure interpreted from a profile along the axis of the esrp the interpretation also indicates that significant lateral inhomogeneities exist in the upper crust beneath the esrp when compared with the upper crust beneath the adjoining northern rocky mountain and basin and range provinces the most prominent features of the crustal structure inferred by the ray trace modeling are as follows 1 the northwest margin of the esrp was modeled as a fault structure downthrown on the srp side se with an offset of greater than 4 km the southeast margin conversely appears to be downwarped with possible minor faulting paleozoic sedimentary rocks are present beneath the cenozoic volcanics of the esrp possibly to and beyond 40 km from the southeastern margin of the esrp near blackfoot idaho 2 the modeling indicates no abrupt variation in the depth to the top of the lower crustal layer near the margins of the esrp 3 an intermediate 6 5 km s layer occurring beneath the esrp interpreted from a refraction profile located along the axis of the esrp was found to be localized within the esrp margins this layer is interpreted as a pervasive intrusion of higher velocity material from the upper mantle into the highly fractured upper crustal layer in this region 4 a density model of the crust across the esrp was prepared with the densities selected using the interpreted seismic velocities as a constraint the gravity field calculated from this model resulted in a good match to observed gravity data over the eastern snake river plain

explosive volcanism and the graphite oxygen fugacity buffer on the parent asteroid s of the ureilite meteorites the 39 ureilite meteorites are in many respects primitive objects they are highly carbonaceous averaging 3 0 wt c mostly as graphite yet their consistently coarse grained textures and plagioclase depleted compositions imply that they are igneous despite diverse mafic silicate mg mg fe and pyroxene olivine ratios plagioclase is totally absent among 35 monomict unbrecciated ureilites even among 4 polymict brecciated ureilites the average al concentration is only 0 28 0 04 wt we suggest that the ureilites are residues from partial melting of graphite rich source materials which formed sufficiently deep in a relatively large low f o2 asteroid or more likely asteroids for graphite to be stable even at magmatic t the al rich basaltic magmas that ascended from the residues were nearly quantitatively lost from the asteroid during pyroclastic eruptions graphite entrained in the ascending magmas was oxidized by an extremely pressure sensitive reaction into cox the expansion of which into the vacuous environment of the asteroid engendered pyroclastic exit velocities greatly in excess of the asteroid s escape velocity carbon would oxidize long before the attainment of magmatic temperatures in small asteroids but we infer that basalt dissipative explosive volcanism was common on all c rich asteroids that were roughly 100 200 km in radius and heated to widespread deep melting and most unmetamorphosed chondritic meteorites are c rich based on the ureilites consistently modest depletions of light rare earth elements siderophile elements and feni metal we infer that the melt fraction was removed mostly by equilibrium batch partial melting late stage ureilite evolution involved an abrupt increase in the cooling rate implying a drop in environmental temperature and a slight amount of graphite oxidation implying a drop in pressure the simplest model to account for this non sequitur and for the conversion of some of the graphite into diamonds is catastrophic impact disruption of the primary asteroid the remarkably efficient removal of the final traces of melt from the ureilites is difficult to explain by any model but possibly was caused by thermal migration along steep thermal gradients engendered after the catastrophic disruption

the snake river plain experiment revisited relationships between a farallon plate fragment and the transition zone we reconsider the analysis of the 93 passcal oregon srp experiment data the techniques applied to the processing of pds waves which are p to s conversions at seismic discontinuities at depth d in the receiver region differ from those employed by dueker sheehan 1997 we perform global move out corrections and migration taking advantage of the alignment of seismicity along the great circle described by the array south america and the kuril and aleutians islands we do not detect a deep signature of a mantle plume the 410 km discontinuity exhibits a fragmented aspect which prevents us from reliably estimating the transition zone thickness the 660 km discontinuity is clearly more visible and largely deflected in a large part of the profile we associate this anomalous behaviour and the noisy aspect of the discontinuities with the presence of a farallon plate fragment

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

sm nd and rb sr isotopic systematics of ureilites abstract sm nd and rb sr isotopic data are reported for seven ureilites on the basis of sm nd isotopic systematics these ureilites are divided into three groups 1 alh82130 pca82506 and meta78008 2 kenna novo urei and alha77257 3 lew85440 group 1 whole rock samples of these ureilites are highly depleted assemblages 147 sm 144 nd 0 33 0 35 having sm nd model ages consistent with 4 55 ga their rb sr isotopic systematics however require that radiogenic sr present day 87 sr 86 sr 0 716 0 721 was introduced into these rocks at some time later than 4 55 ga group 2 whole rock samples of kenna novo urei and alha77257 and a pyroxene separate from kenna plot on a sm nd isochron with an age of 3 74 0 02 ga and an initial 143 nd 144 nd ratio of 0 50945 2 whole rock samples of these ureilites are heterogeneous mixtures of an unidentified lree enriched component and a lree depleted olivine pyroxene assemblage the lree enriched component is volumetrically minor but has lree abundances sufficiently high to dominate whole rock sm and nd abundances and sm nd ratios our best estimate indicates that it has nd 8ppm and 147 sm 144 nd 0 115 variation in the quantity of lree enriched component present in whole rock samples accounts for most of the variation in their sm nd ratios second order variation is due to varying olivine pyroxene ratios the possibility that 3 74 ga is the crystallization age of kenna novo urei and alha77257 cannot be ruled out however we favour the interpretation that the 3 74 ga isochron records an event in which lree enriched material a metasomatic fluid which was the precursor of the present lree enriched component reacted with a preexisting highly depleted 147 sm 144 nd 0 51 ultramafic assemblage the sr isotopic compositions of eleven samples of kenna whole rocks and various separated fractions are virtually identical 87 sr 86 sr 0 7086 regardless of rb sr ratio there is a positive correlation between sr concentration and degree of lree enrichment in these samples suggesting that the lree enriched component is also sr rich the sr data for kenna could be explained by introduction of radiogenic sr along with the lree enriched material at 3 74 ga and a subsequent rb free history all present rb would have to be terrestrial contamination in this interpretation the sr isotopic compositions of novo urei and alha77257 are not however the same as that of kenna group 3 lew85440 neither has a model sm nd age of 4 55 ga nor plots on the 3 74 ga isochron of the kenna group it might have had an evolution similar to that of the kenna group but involving different times and or isotopic compositions if some samples of group 1 ureilites contain a lree enriched component then this component must have been generated and mobilized on the ureilite parent body or bodies at least twice in the interval 4 55 3 74 ga there is no apparent correlation between oxygen isotopic systematics and sm nd or rb sr isotopic systematics

shrimp u pb zircon dating of archean core complex formation and pancratonic strike slip deformation in the east pilbara granite greenstone terrain sensitive high resolution ion microprobe shrimp u pb dating of zircons from granitic rocks in the east pilbara granite greenstone terrain has provided time constraints for main tectonic events in the shaw granitoid complex and has shown that deformation was intricately related to granitoid intrusion the earliest recognized deformation activity occurred within the split rock shear zone srsz probably between 3470 and 3420 ma the timing geometry and kinematics of this shear zone suggest that it was a midcrustal detachment in a core complex type extensional setting it was active during the intrusion of synkinematic sheeted granodiorite of the north shaw suite and brittle extensional faulting within the upper crust samples of gray migmatic gneiss and a diorite that occur structurally below the srsz were dated at 3451 1 ma and 3463 2 ma respectively indicating that melting and metamorphism took place in the lower units during extension in the middle to upper crust the 3470 3420 ma extensional event is most consistent with extensional collapse possibly as a result of delamination or convectional thinning of part of the lithosphere activity on the pancratonic strike slip mulgandinnah shear zone msz was constrained by the crystallization age of a synkinematic granitic dyke to have occurred at 2934 2 ma the msz and activity on the sholl shear zone of the western pilbara at 2960 ma smith et al 1998 are consistent with a regional nw se directed compressional stress field at this time steepening of greenstone belt stratigraphy has been attributed to diapirism but probably occurred after a 3300 3200 ma compressional event and following deposition of the youngest clastic sediments in the eastern pilbara stratigraphic sequence diapiric processes did not play a major role during the early evolution of the eastern pilbara craton from 3470 to 3300 ma

182hf 182w chronometry and early differentiation of the ureilite parent body abstract eight samples spanning the bulk chemical range of monomict ureilites have been analyzed for their w isotopic composition and hf w in order to elucidate the differentiation history of the ureilite parent body the ureilites studied here show sub chondritic hf and w concentrations and low hf w ratio and all show clear 182 w deficits relative to bulk chondrites the w isotope data provide evidence that the ureilite parent body differentiated within 1 to 2 million years after the start of the solar system such early accretion and differentiation is comparable to the parent bodies of various magmatic iron meteorites as well as the hed parent body however the trace element fractionations and by inference melting processes are very different an early smelting partial melting event coupled with metal silicate segregation however can reproduce the observed hf w data in ureilites and is consistent with petrological and other chemical evidence for a missing basaltic component and with primitive oxygen and noble gases compositions

interplay between deformation and magmatism during doming of the archaean shaw granitoid complex pilbara craton western australia abstract the shaw granitoid complex one of the classic granitoid domes of the archaean pilbara craton western australia evolved from a plutonic complex into a 60 km diameter steep sided dome during migmatisation of the deep crust consistent stratigraphic younging of greenstones away from the dome and underlying outward dipping domical foliations indicate that the present exposure is a crustal section the section passes downward through progressively older greenstones into a near concordant high level sheeted granite complex the ca 3470 ma north shaw suite nss to a younger sill dyke complex of leucogranite the pilga leucogranite farther below the leucogranite grades via diatexite and a network of leucogranitic dykes into the lowermost unit of migmatitic orthogneisses which have protolith ages that overlap with the nss a semi continuous solid state domal foliation extends downward from the greenstones through the nss into the migmatitic orthogneiss but is less evident in the intervening pilga leucogranite a set of nnw trending upright folds with axial planar leucogranite dykes exist in both the pilga leucogranite and underlying orthogneisses but not in the nss or greenstones field relations indicate that much of the solid state dome concordant foliation developed in a sub horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses emplacement occurred along a sub horizontal high strain zone probably at a rheological boundary between the relatively rigid nss and softer underlying migmatitic orthogneisses the sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds these leucogranite magmas locally extended into the overlying deformed nss where they also acquired a weak domical fabric suggesting syn doming emplacement although presently estimated to be 3445 3410 ma old stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 ma the contrasting strain patterns above and below the pilga leucogranite are not expected for horizontal tectonic models invoking thrusting core complex formation or cross folding but are similar to one observed for gravity driven vertical tectonic models thermal softening initiated deformation and then deformation localised melts at a specific rheological interface once established this interface controlled the locus of pilga leucogranite intrusion during dome evolution demonstrating the interplay between deformation and magmatism

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

crustal velocity structure across the eastern snake river plain and the yellowstone swell teleseismic receiver functions are used to estimate the crustal structure beneath a 36 station 500 km long nw oriented linear array centered on the eastern snake river plain and crossing the yellowstone hotspot swell 250 km sw of yellowstone structure derived previously for this region from wide angle reflection data is used as an initial model and this structure explains most features observed in our receiver functions based on a combination of forward and inverse modeling our data require several modifications to the initial structure 1 moho depth is 42 km beneath most of the snake river plain shallows to 37 km to either side and thickens abruptly to 47 km beneath sw wyoming 2 a midcrustal layer interpreted previously as a 9 km thick gabbroic sill is flat topped across the entire 90 km width of the snake river plain and 3 a low velocity layer is found beneath and southeast of the snake river plain which probably is partially molten lower most crust using the seismic structure of the crust to estimate the crustal load upon the mantle and assuming local isostasy we calculate that mantle beneath the yellowstone swell is approximately uniformly as buoyant as 12 million year old ocean mantle and more buoyant than the adjacent wyoming mantle by an amount equivalent of 1 5 km of elevation the transition between these regions of greatly different mantle occurs across a major paleozoic boundary that now separates the basin and range from the rocky mountains

origin and history of ureilitic material in the solar system the view from asteroid 2008 tc3 and the almahata sitta meteorite asteroid 2008 tc3 approximately 4 m diameter was tracked and studied in space for approximately 19 h before it impacted earth s atmosphere shattering at 44 36 km altitude the recovered samples 680 individual rocks comprise the meteorite almahata sitta ahs approximately 50 70 of these are ureilites ultramafic achondrites the rest are chondrites mainly enstatite ordinary and rumuruti types the goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid almahata sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts however we calculate that the mass of fallen material was 0 1 of the pre atmospheric mass of the asteroid based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples we infer that the lost material was mostly ureilitic therefore 2008 tc3 probably contained only a few percent nonureilitic materials similar to other polymict ureilites except less well consolidated from available data for the ahs meteorite fragments we conclude that 2008 tc3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites we therefore suggest that the immediate parent of 2008 tc3 was the immediate parent of all ureilitic material sampled on earth we trace critical stages in the evolution of that material through solar system history based on various types of new modeling and re evaluation of published data we propose the following scenario 1 the ureilite parent body upb accreted 0 5 0 6 ma after formation of calcium aluminum rich inclusions cai beyond the ice line outer asteroid belt differentiation began approximately 1 ma after cai 2 the upb was catastrophically disrupted by a major impact approximately 5 ma after cai with selective subsets of the fragments reassembling into daughter bodies 3 either the upb before breakup or one of its daughters after breakup migrated to the inner belt due to scattering by massive embryos 4 one daughter after forming in or migrating to the inner belt became the parent of 2008 tc3 it developed a regolith mostly 3 8 ga ago clasts of enstatite ordinary and rumuruti type chondrites were implanted by low velocity collisions 5 recently the daughter was disrupted fragments were injected or drifted into earth crossing orbits 2008 tc3 comes from outer layers of regolith other polymict ureilites from deeper regolith and main group ureilites from the interior of this body in contrast to other models that have been proposed this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 tc3 and other polymict ureilites

formation of ureilites by nebular processes oxygen isotopic data are presented for 23 ureilites including 14 antarctic meteorites they show a much greater range in isotopic composition than those previously studied most noteworthy is the fact that unlike other achondrite groups the ureilites do not fall on a mass dependent isotopic fractionation trend but rather fall along an extrapolation of the 16o mixing line defined by components of allende and other c3 chondrites correlations between isotopic and chemical data imply that the isotopic and chemical variations are due primarily to processes in the solar nebula and not to igneous processes within a parent body the ureilite parent body accreted with isotopic and chemical heterogeneities which were not much modined by subsequent parent body processes the precursor materials were related in some way to the c3 carbonaceous chondrites

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

new constraints on the properties of the yellowstone mantle plume from p and s wave attenuation tomography 1 we estimate attenuation t for teleseismic p and s arrivals to seismometers in the yellowstone intermountain seismic array tomographically invert these data for upper mantle qp 1 and qs 1 structure and with the aid of the upper mantle velocity model of waite et al 2006 interpret the results for mantle temperature partial melt and water content because attenuation analysis is susceptible to contamination by noise we employ time and frequency domain analyses and a careful assessment of the uncertainty associated with each estimate by using the misfit between actual and predicted traces or spectra the greatest noise source is signal generated noise which affects s wave results more than p wave results s waves also appear more susceptible to the effects of plume focusing we find the upper mantle to be highly attenuative but that above 200 250 km the low velocity plume is progressively less attenuative than the adjacent mantle with decreasing depth we conclude that water dissolved in mantle minerals causes the upper mantle to be highly attenuative that the yellowstone plume is only 50 c warmer than the surrounding mantle below the north america lithosphere and that melting within this plume begins at depths of 200 250 km we attribute the lower attenuation in the partially molten plume to the dehydration of the solid matrix as water partitions into the melt the source of the upper mantle hydration is attributed to subduction including water flux from a hydrated transition zone and laramide age shallow subduction and direct hydration of north america lithosphere

mineralogy of antarctic ureilites and a working hypothesis for their origin and evolution abstract mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for antarctic ureilites y74123 y74130 y74659 y790981 alha77257 alha78019 and alh82106 extend the chemical and textural variations among ureilites beyond those known for non antarctic ureilites low ca pyroxenes possibly orthopyroxene poikilitically enclosed in olivine and coexisting with mg rich pigeonites have been identified in y74659 and alha77257 coexisting low ca pyroxene pigeonite and augite together with the very fe rich olivine in ureilites were observed in y74130 almost fe free pigeonite and diopside coexist in alh82106 no systematic differentiation trend has been reconstructed from these data but the pyroxene pairs yield formation temperatures above 1200 c a weak anticorrelation between mno and feo delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations a working hypothesis for ureilite formation involves high temperature episoded and a planetesimal scale collision which might have facilitated compaction and recrystallization of mafic silicates during the episodes fe ni s eutectic melt and a partial melt rich in ca al and fe might have been lost from a carbonaceous chondrite like source material

the 4 56 ga upb age of the met 780058 ureilite the u th pb and sm nd isotopic systematics of mineral separates from the ca rich ureilite met 78008 are reported extensive acid leaching procedures were used to eliminate terrestrial contamination because the amounts of u th pb sm and nd are extremely low in the meteorite the three residues without terrestrial pb contamination yield a pb pb age of 4 563 0 021 ga on a u pb concordia diagram the residue data corrected for an initial lead isotopic composition calculated with 11 7 for 3 ma from starting canon diablo troilite pb in indicate a 2o7pb 206pb model age of 4 563 ga the sm nd isotopic data are scattered around the 4 56 ga chondritic isochron however the sm nd model age for handpicked augite is 4 573 0 029 ga assuming an initial chur composition two separates show a positive 142nd anomaly similar to those found in other meteorites of the same age 4 5 4 6 ga these sm nd isotopic results support the pb pb and u pb ages of this meteorite therefore it is very likely that the formation age of ureilites is consistent with the so called 4 56 ga canonical age of meteorites it is also possible that extensive igneous processes for generating the ultramatic mineralogy of ureilites took place in the parent body within a few million years after formation of the solar nebula

origin of the 3500 3300 ma calc alkaline rocks in the pilbara archaean isotopic and geochemical constraints from the shaw batholith abstract calc alkaline plutonic rocks intruded at 3450ma comprise a major component of the shaw batholith in the archaean east pilbara block western australia new whole rock pb isotopic geochronology confirms the extent of these rocks but a minor plutonic phase is dated at 3338 52 ma and represents a second plutonic event of the same age as much of the nearby mt edgar batholith the sm nd isotopic systematics of the 3450ma rocks imply their derivation from a heterogeneous source which probably included a slightly older crustal component as well as a depleted mantle component the 3338 52 ma pluton includes components derived from crustal sources older than 3600 ma the geochemistry and sm nd isotopic systematics of these rocks are consistent with crustal growth in the early archaean from upper mantle sources as depleted as the modern upper mantle the shaw batholith calc alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation a suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals principally hornblende and possibly sphene magnetite and epidote and into a leucocratic component containing quartz plagioclase and k feld par these archaean calc alkaline plutonic rocks in common with rocks from many other archaean calc alkaline provinces exhibit very fractionated ree patterns with depleted hree contents a feature considered to result from equilibrium with garnet at depth in lower crustal regions the geochemistry of the pilbara archaean calc alkaline rocks is identical to the subset of modern continental margin calc alkaline plutonic rocks with fractionated ree patterns such as those from the central and eastern peninsular ranges batholith western usa the tectonic setting in which the archaean calc alkaline rocks formed is still not known this reflects both uncertainty associated with the petrogenesis and environments of modern calc alkaline rocks as well as the limited knowledge of the precise timing and relationships of plutonic depositional and tectonic events in the pilbara archaean

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

partial melting of a c rich asteroid lithophile trace elements in ureilites ureilites are among the most common achondrites and are widely believed to sample the mantle of a single now disrupted c rich body we analyzed 17 ureilite samples mostly antarctic finds and determined their incompatible trace element abundances in order to remove or reduce the terrestrial contamination which is marked among antarctic ureilites by light ree enrichment we leached the powdered samples with nitric acid the residues display consistent abundances which strongly resemble those of the pristine rocks all the analyzed samples display light ree depletions negative eu anomalies low sr eu n and zr eu n ratios which are correlated two groups of ureilites groups a and b are defined compared to group a group b ureilites which are the less numerous tend to be richer in heavy rees more light ree depleted and display among the deepest eu anomalies in addition olivine cores in group b ureilites tend to be more forsteritic mg 81 9 95 2 than in group a ureilites mg 74 7 86 1 incompatible trace element systematics supports the view that ureilites are mantle restites ree modelling suggests that their precursors were rather ree rich ca 1 8 2 x ci and contained a phosphate phase possibly merrillite the ree abundances in ureilites can be explained if at least two distinct types of magmas were removed successively from their precursors aluminous and alkali rich melts as exemplified by the almahata sitta trachyandesite alm a and al and alkali poor melts produced after the exhaustion of plagioclase from the source partial melting was near fractional group b ureilites which are probably among the least residual samples to dynamic with melt porosities that did not exceed a couple of percent group a ureilites the ureilite parent body upb was almost certainly covered by a crust formed chiefly from the extrusion products of the aluminous and alkali rich magmas it is currently uncertain whether the al and alkali poor melts produced during the second phase of melting reached the surface of the body the fact that initial silicate melting of ureilitic precursors would have produced relatively low density liquids capable of forming an external crust to the upb casts doubt on models that invoke chondritic outer layers to achondritic asteroids

comment on uthpb and smnd isotopic systematics of the goalpara ureilite resolution of terrestrial contamination by n torigoye kita k misawa and m tatsumoto

snc meteorites clues to martian petrologic evolution the shergottites nakhlites and chassigny snc meteorites are apparently cumulate mafic and ultramafic rocks that crystallized at shallow levels in the crust of their parent body the mineralogy and chemistry of these meteorites are remarkably like equivalent terrestrial rocks although their ratios of fe fe mg and certain incompatible elements and their oxygen isotopic compositions are distinctive all have crystallization ages of 1 3 b y or younger and formed from magmas produced by partial melting of previously fractionated source regions isotope systematics suggest that the snc parent body had a complex and protracted thermal history spanning most of geologic time some meteorites have been severely shock metamorphosed and all were ejected from their parent body at relatively recent times possibly in several impact events late crystallization ages complex petrogenesis and possible evidence for a large gravitational field suggest that these meteorites are derived from a large planet trapped gases in shergottite shock melts have compositions similar to the composition measured in the martian atmosphere ejection of martian meteorites may have been accomplished by acceleration of near surface spalls or other mechanisms not fully understood if snc meteorites are of martian origin they provide important information on planetary composition and evolution the bulk composition and redox state of the martian mantle as constrained by shergottite phase equilibria must be more earthlike than most current models planetary thermal models should benefit from data on the abundances of radioactive heat sources the melting behavior of the mantle and the timing of planetary differentiation calculated depletion of chalcophile elements in source regions indicates a core dominated by sulfides and paleomagnetic measurements suggest the presence of a weak magnetic field within the last several hundred thousand years concentrations of volatile elements indicate that the snc parent body was not volatile depleted and trapped atmospheric components measured in shock melts may be useful in understanding planetary degassing by providing comparisons for spectral reflectance data and viking soil analyses these meteorites may also constrain surface mineralogy and weathering mechanisms

whole mantle shear structure beneath the east pacific rise we model broadband seismograms containing triplicated s s 2 and s 3 along with scs to produce a pure path one dimensional model extending from the crust to the core mantle boundary beneath the east pacific rise we simultaneously model all body wave shapes and amplitudes thereby eliminating depth velocity ambiguities the data consist of western north american broadband recordings of east pacific rise epr affiliate transform events that form a continuous record section out to 82 and sample nearly the entire east pacific rise the best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages the 660 km discontinuity is particularly well resolved and requires a steep gradient 4 extending down to 745 km we find no discernible variation in apparent depths of the 405 and 660 km discontinuities over ridge orthogonal distances on the order of 1000 km or 20 ma lithosphere body waveform comparisons indicate that we can resolve discontinuity depths to less than 10 km providing an upper limit to transition zone topography these depth estimates in conjunction with the fan shot nature of the ray paths lower the detection limit from s 2 precursor analysis of the lateral length scale over which short wavelength topographic variation could occur and indicate the sub epr transition zone and upper mantle are remarkably homogeneous the lower mantle beneath the east pacific rise is well modeled by prem with the greatest variation occurring in scs reflecting strong heterogeneity along the core mantle boundary together these observations require that the east pacific rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients dynamically the transition zone therefore appears vertically decoupled from the overlying east pacific rise spreading system

shock post shock annealing and post annealing shock in ureilites the thermal and shock histories of ureilites can be divided into four periods 1 formation 2 initial shock 3 post shock annealing and 4 post annealing shock period 1 occurred 4 55 ga ago when ureilites formed by melting chondritic material impact events during period 2 caused silicate darkening undulose to mosaic extinction in olivines and the formation of diamond lonsdaleite and chaoite from indigenous carbonaceous material alkali rich fine grained silicates may have been introduced by impact injection into ureilites during this period about 57 of the ureilites were unchanged after period 2 during period 3 events impact induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals some ureilites experienced reduction as feo at the edges of olivine grains reacted with c from the matrix annealing may also be responsible for coarsening of graphite in a few ureilites forming euhedral appearing idioblastic crystals orthopyroxene in meteorite hills met 78008 may have formed from pigeonite by annealing during this period the rb sr internal isochron age of 4 0 ga for met 78008 probably dates the annealing event at this late date impacts are the only viable heat source about 36 of ureilites experienced period 3 events but remained unchanged afterwards during period 4 7 of the ureilites were shocked again as is evident in the polymict breccia elephant moraine eet 83309 this rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction ureilites may have formed by impact melting chondritic material on a primitive body with heterogeneous o isotopes plagioclase was preferentially lost from the system due to its low impedance to shock compression brief melting and rapid burial minimized the escape of planetary type noble gases from the ureilitic melts incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements