kink folding in an extended terrane tortilla mountains southeastern arizona structural analysis of early miocene metre scale kink folds in southeastern arizona shows that they formed in an extensional stress field and that they record horizontal extension the folds represent a previously unrecognized style of extensional fold

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

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

early stages of core segregation recorded by fe isotopes in an asteroidal mantle ureilites displays 56 fe values higher than average chondrite 29 segregation of fe sulfide melts explains the high 56 fe values in ureilites 30 formation of a core can begin at very low degrees of melting through the circulation of a fe s melt 31 through a silicate mantle 32 33 earth and planetary science letters in press 11 3 15 34 2 35 abstract 36 37 ureilite meteorites are achondrites that are debris of the mantle of a now disrupted 38 differentiated asteroid rich in carbon they provide a unique opportunity to study the differentiation 39 processes of such a body we analyzed the iron isotopic compositions of 30 samples from the ureilite 40 parent body upb including 29 unbrecciated ureilites and one ureilitic trachyandesite alm a 41 which is at present the sole large crustal sample of the upb the 56 fe of the whole rocks fall within a 42 restricted range from 0 01 to 0 11 with an average of 0 056 0 008 which is significantly 43 higher than that of chondrites we show that this difference can be ascribed to the segregation of s 44 rich metallic melts at low degrees of melting at a temperature close to the fe fes eutectic and 45 certainly before the onset of the melting of the silicates 1100 c in agreement with the marked s 46 depletions and the siderophile element abundances of the ureilites these results point to an efficient 47 segregation of s rich metallic melts during the differentiation of small terrestrial bodies 48 49

alkali feldspathic material entrained in fe s rich veins in a monomict ureilite the elephant moraine eet 96001 ureilite contains a remarkable diversity of feldspars which occur as tiny no more than 60 m maximum dimension grains within a few fe s rich now weathered to mostly fe oxide veins molar s fe ratio in the veins averages 0 08 0 02 the veins meander and feature large fluctuations in apparent width they appear to have entered this monomict breccia by a gentle percolative process not by violent impact injection the feldspars are accompanied by a diverse suite of k rich and generally also ti rich feldspathic glasses and also major proportions of silica and pyroxene which is largely fassaitic a rhnite like phase is also found and as inclusions in one of the fassaites a cr poor spinel like phase the feldspars mostly feature remarkably high k na compared to feldspars of comparable an from polymict ureilites the eet 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid the spinel included in one of the fassaites formed at remarkably high fo2 apparent oxidation state of iron 41 atom fe3 suggesting that the parent magma possibly assimilated near surface water however the fe3 was not directly measured and has conceivably been affected by terrestrial weathering also there is no assurance that this fassaite originated together with the typical feldspar we speculate that the feldspathic component was mixed into the dense fe s rich vein material and very soon thereafter the fe s rich vein material was emplaced adjacent to the eet 96001 host ureilite at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites the high k nature of the eet 96001 feldspathic component including the feldspathic glasses suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

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

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

ureilite petrogenesis a limited role for smelting during anatexis and catastrophic disruption a popular model for ureilites assumes that during anatexis in an asteroidal mantle pressure buffered equilibrium smelting partial reduction coincident with partial melting engendered their conspicuous mafic silicate core mg diversity 75 96 mol several mass balance problems arise from this hypothesis smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of fe metal and copious proportions of co gas the most serious problem concerns the yield of co gas if equilibrium smelting produced the ureilites entire 21 mol range in olivine core mg the proportion of gas within the asteroidal mantle assuming plausibly low pressure 80 bar should have reached 85 vol based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy a runaway co leaky process that can loosely be termed smelting appears to have occurred probably triggered by a major impact the runaway scenario appears likely because by le ch telier s principle co leakage would tend to accelerate the smelting process also the copious volumes of gas produced by smelting would have led to explosive mass leaky eruptions into the vacuum surrounding the asteroid loss of mass would mean diminution of interior pressure which would induce further smelting leading to further loss of mass basalt and so on such a disruptive runaway process may have engendered the ureilites distinctive reduced olivine rims but the only smelting according to this scenario was a short lived disequilibrium process that reduced only the olivine rims not the cores and the ureilites were cooling not melting during the abortive smelting episode

oxygen isotopic composition and chemical correlations in meteorites and the terrestrial planets recent models attempting to explain non mass dependent oxygen isotopic anomalies in meteorites and planets posit that they may have originated within the gas phase of the solar nebula which suggests the potential for correlations of non mass dependent oxygen isotopic anomalies with other chemical fractionations generated during cooling and condensation of the nebula we have examined three specific issues possible correlations of 17o with i oxidation state ii bulk chondrite chemistry and iii bulk planetary properties isolated grains and chondrules from several carbonaceous chondrite groups exhibit good to weak positive correlations between 17o and olivine fayalite content and whole rock ureilite samples show a positive correlation of 17o with olivine fayalite content these are consistent with oxidation of fe metal by 17o 18o enriched oxygen in the precursor materials that formed these objects in contrast oxygen isotopic and major element compositions of ferromagnesian phases in ordinary chondrite chondrules and in bulk chondrites do not show a correlation thus there is no compelling evidence that oxidation of nebular materials was tightly linked to gas species carrying anomalous o using average chondrite group compositions we demonstrate that significant negative correlations of refractory lithophile element mg and refractory siderophile element ni with 17o exist refractory inclusions cais and aoas are modally rare in many chondrite types that exhibit substantial range in 17o thus the refractory component in these chondrites must occur in a crypto component e g material dissolved in chondrule melts during chondrule formation significant positive correlations of moderately volatile elements mg with 17o are not simply explained by incomplete mixing of 16o enriched refractory grains but are broadly consistent with nebula based mechanisms of non mass dependent oxygen isotope fractionation the estimated compositions of the primitive mantles of the earth mars and 4 vesta have some compositional redox and isotopic properties that vary with heliocentric distance however 17o does not vary monotonically in this sequence indicating a decoupling of planetary o isotopic composition from other compositional characteristics

origin of ureilites inferred from a sims oxygen isotopic and trace element study of clasts in the dar al gani 319 polymict ureilite secondary ion mass spectrometer sims oxygen isotope analyses were performed on 24 clasts representing 9 clast types in the dar al gani dag 319 polymict ureilite with precisions better than 1 olivine rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the ccam carbonaceous chondrite anhydrous mineral line other igneous clasts including plagioclase bearing clasts also plot along the ccam line indicating that they were derived from the ureilite parent body upb thus we suggest that some of the plagioclase bearing clasts in the polymict ureilites represent the missing basaltic component produced by partial melting on the upb trace element concentrations mg k sc ti v cr mn fe co ni cu rb sr and ba in ureilitic plagioclase and glass from 13 clasts were obtained by using the sims high mass resolution method the trace element contents of the plagioclase generally show monotonic variations with anorthite content mol that are consistent with partial melting and fractional crystallization incompatible trace element concentrations k ti and ba are low and variable for plagioclase with an 40 indicating that the parental magmas for some clasts were derived from a depleted source we performed partial melt modeling for ci and cm precursor compositions and compared the results to the observed trace element k ba and sr abundances in the plagioclase our results indicate that 1 the upb evolved from a alkali rich carbonaceous chondritic precursor 2 parent melts of porphyritic clasts could have formed by 5 20 equilibrium partial melting and subsequent fractional crystallization and 3 parent melts of the incompatible trace element depleted clasts could be derived from fractional melting where low degree 10 partial melts were repeatedly extracted from their solid sources thus both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in dag 319 suggest that the upb underwent localized low degree partial melting events the partial melts could have been repeatedly extracted from the precursor resulting in the formation of the olivine pigeonite monomict ureilites as the final residue

interpretation of resonance frequencies recorded during hydraulic fracturing treatments hydraulic fracturing treatments are often monitored by strings of geophones deployed in boreholes instead of picking discrete events only we here use time frequency representations of continuous recordings to identify resonances in two case studies this paper outlines an interpretational procedure to identify their cause using a subdivision into source path and receiver side effects for the first case study two main resonances are observed both at depth by the downhole geophones and on the surface by two broadband arrays the two acquisition networks have different receiver and path effects yet recorded the same resonances these resonances are therefore likely generated by source effects the amplitude pattern at the surface arrays indicates that these resonances are probably due to pumping operations in the second case study selective resonances are detected by the downhole geophones resonances coming from receiver effects are either lower or higher frequency and wave propagation modeling shows that path effects are not significant we identify two possible causes within the source area namely eigenvibrations of fractures or non darcian flow within the hydraulic fractures in the first situation 15 10 m long fluid filled cracks could generate the observed resonances an interconnected fracture network would then be required corresponding to mesoscale deformation of the reservoir alternatively systematic patterns in non darcian fluid flow within the hydraulic fracture could also be their leading cause resonances can be used to gain a better understanding of reservoir deformations or dynamic fluid flow perturbations during fluid injection into hydrocarbon and geothermal reservoirs co2 sequestration or volcanic eruptions

highly siderophile elements in ureilites abstract the abundances of the highly siderophile elements hse ru pd re os ir and pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples including monomict augite bearing and polymict lithologies this report adds significantly to the quantity of available pt and pd abundances in ureilites as these elements were rarely determined in previous neutron activation studies the ci normalized hse abundance patterns of all ureilites analyzed here except alha 81101 show marked depletions in the more volatile pd with ci chondrite normalized pd os ratios excluding alha 81101 averaging 0 19 0 23 2 this value is too low to be directly derived from any known chondrite group instead the hse bulk rock abundances and hse interelement ratios in ureilites can be understood as physical mixtures of two end member compositions one component best represented by sample alha 78019 is characterized by superchondritic abundances of refractory hse rhse ru re os ir and pt but subchondritic pd rhse and is consistent with residual metal after extraction of a s bearing metallic partial melt from carbonaceous chondrite like precursor materials the other component best represented by sample alha 81101 is rhse poor and has hse abundances in chondritic proportions the genesis of the second component is unclear it could represent regions within the ureilite parent body upb in which metallic phases were completely molten and partially drained or it might represent chondritic contamination that was added during disruption and brecciation of the upb removal of carbon rich melts does not seem to play an important role in ureilite petrogenesis removal of such melts would quickly deplete the ureilite precursors in re os and as au which is inconsistent with measured osmium isotope abundances and also with literature as au data for the ureilites removal of 26 al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions

stratigraphic filtering and source penetration depth seismic exploration underneath highly heterogeneous layers such as basalt flows is possible by lowering the principal source frequency unfortunately this also reduces resolution wave localization theory is a multiple scattering theory that can be used to study stratigraphic filtering in chaotic lithologies it predicts the apparent attenuation due to scattering of a plane wave traversing a layer with high velocity fluctuations it can therefore predict the optimum principle source frequency in the trade off between loss of resolution and increased penetration depth we show how this can be done with the help of a few statistical parameters derived from a well log analysis namely the average background velocity the expected standard deviation in the velocity fluctuations the typical scale length of the heterogeneities and the thickness of the basalt layer in the likely situation that no local well logs exist a multitude of scenarios can easily be examined at low cost

jiddat al harasis 422 a ureilite with an extremely high degree of shock melting the jiddat al harasis jah 422 ureilite was found in the sultanate of oman it is classified as a ureilitic impact melt breccia the meteorite consists of rounded polycrystalline olivine clasts 35 pores 8 and microcrystalline matrix 57 clasts and matrix have oxygen isotopic values and chemical compositions major and trace elements characteristic of the ureilite group the matrix contains olivine fo83 90 low ca pyroxene en84 92wo0 5 augite en71 56wo20 31 graphite diamond fe metal sulfides chromite and felsic glass pores are partly filled by secondary fe oxihydroxide and desert alteration products pores are surrounded by strongly reduced silicates clasts consist of fine grained aggregates of polygonal olivine these clasts have an approximately 250 lm wide reaction rim in which olivine composition evolves progressively from the core composition fo79 81 to the matrix composition fo84 87 veins crossing the clasts comprise pyroxene fe oxihydroxide c phases and chromite clasts contain ca al and cr rich glass along olivine grain boundaries 1 lm wide we suggest that a significant portion of jah 422 including olivine and all the pyroxenes was molten as a result of an impact in comparison with other impact melted ureilites jah 422 shows the highest melt portion based on textural and compositional considerations clasts and matrix probably originated from the same protolith with the clasts representing relict olivine that survived but was recrystallized in the impact melt during the melt stage the high availability of feo and elevated temperatures controlled oxygen fugacity at values high enough to stabilize olivine with fo 83 87 and chromite along pores high mg compositions of silicates indicate that in a late stage or after melt crystallization feo became less available and fo2 conditions were controlled by c co co2

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

electron microscopy of pyroxene in the almahata sitta ureilite abstract we performed scanning electron microscope electron backscatter diffraction sem ebsd and focused ion beam transmission electron microscopy fib tem investigations of pyroxene in the almahata sitta ureilite sample 7 the pyroxenes mg 0 92 are present as minute individual grains 10 20 m in size showing a polycrystalline texture and they are both low ca and high ca pyroxenes although their ca contents are as low as wo2 3 the ebsd analysis shows that low ca pyroxenes are clinopyroxene p21 c the obtained pyroxene equilibration temperature 1240 1280 c is consistent with the previous studies on many ureilites in low ca pigeonite 001 augite exsolution lamellae 10 15 nm wide develop in the pigeonite host 20 45 nm wide and a similar exsolution texture was observed in augite the exsolution wavelength of pyroxene typically 30 60 nm gives the cooling rate of 0 2 5 c h 1 such a rapid cooling probably records quenching from high temperature 1240 1280 c down to 1000 c estimated from the exsolution pair due to the breakup of the ureilite parent body upb while it was still hot the pyroxene microstructure of almahata sitta is within the range of known ureilites and is most similar to that of allan hills alh a81101 with the affinity of polycrystalline texture the coarser exsolution texture of alha81101 pyroxene suggests slower cooling history than almahata sitta however direct comparison is difficult because of different pyroxene compositions because alha77257 has a similar pyroxene composition to almahata sitta and does not show visible pyroxene exsolution it should have cooled faster than almahata sitta rapid albeit variable cooling rates observed in different ureilite samples may suggest that they originated from upb fragments of different size

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

northwest africa 1500 plagioclase bearing monomict ureilite or ungrouped achondrite northwest africa nwa 1500 is an ultramafic meteorite dominated by coarse 100 500 m olivine 95 96 augite 2 3 and chromite 0 6 1 6 in an equilibrated texture plagioclase 0 7 1 8 occurs as poikilitic grains up to 3 mm in vein like areas that have concentrations of augite and minor orthopyroxene other phases are cl apatite metal sulfide and graphite olivine ranges from fo 65 73 with a strong peak at fo 68 69 most grains are reverse zoned and also have 10 30 m reduction rims in terms of its dominant mineralogy and texture nwa 1500 resembles the majority of monomict ureilites however it is more ferroan than known ureilites fo 75 and other mineral compositional parameters are out of the ureilite range as well furthermore neither apatite nor plagioclase have ever been observed and chromite is rare in monomict ureilites nevertheless this meteorite may be petrologically related to the rare augite bearing ureilites and represent a previously unsampled part of the ureilite parent body upb the mn mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation however its petrogenesis differs from that of known augite bearing ureilites in that 1 it formed under more oxidized conditions 2 plagioclase appeared before orthopyroxene in its crystallization sequence and 3 it equilibrated to significantly lower temperatures 800 1000 c from two pyroxene and olivine chromite thermometry formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the upb than previously sampled however its significantly different thermal history compared to ureilites may more plausibly be explained if it formed on a different parent body this conclusion is consistent with its oxygen isotopic composition which suggests that it is an ungrouped achondrite nevertheless the parent body of nwa 1500 may have been compositionally and petrologically similar to the upb and may have had a similar differentiation history

petrology of chromite in ureilites deconvolution of primary oxidation states and secondary reduction processes abstract ureilites are ultramafic achondrites thought to be residues of partial melting on a carbon rich asteroid they show a trend of feo variation olivine fo from 74 to 95 that suggests variation in oxidation state whether this variation was established during high temperature igneous processing on the ureilite parent body upb or preserved from nebular precursors is a subject of debate the behavior of chromium in ureilites offers a way to assess redox conditions during their formation and address this issue independent of fo we conducted a petrographic and mineral compositional study of occurrences of chromite cr rich spinel in ureilites aimed at determining the origin of the chromite in each occurrence and using primary occurrences to constrain models of ureilite petrogenesis chromite was studied in lew 88774 fo 74 2 nwa 766 fo 76 7 nwa 3109 fo 76 3 hah 064 fo 77 5 lap 03587 fo 74 9 cms 04048 fo 76 4 lap 02382 fo 78 6 and eet 96328 fo 85 2 chromite occurs in lew 88774 5 vol nwa 766 early chromite also occurs in a melt inclusion in olivine in hah 064 and in a metallic spherule in olivine in lap 02382 lap 03587 and cms 04048 contain m sized chromite pyroxene symplectic exsolutions in olivine indicating high cr valence in the primary olivine eet 96328 contains a round grain of chromite that could be a late crystallizing phase tiny chromite grains in melt inclusions in eet 96328 formed in late closed system reactions for 7 of the 8 ureilites we conclude that the relatively oxidizing conditions evidenced by the presence of primary or early chromite pertain to the period of high t igneous processing the observation that such conditions are recorded almost exclusively in low fo samples supports the interpretation that the ureilite feo variation was established during igneous processing on the upb

fe bearing phases in a ureilite fragment from the asteroid 2008 tc3 almahata sitta meteorites a combined mossbauer spectroscopy and x ray diffraction study the iron bearing phases in a ureilite fragment as 051 from the almahata sitta meteorite are studied using mossbauer spectroscopy x ray diffraction xrd and electron microprobe analysis empa as 051 has a typical ureilite texture of medium to coarse grained silicates olivine orthopyroxene and pigeonite with minor opaques fe ni metal troilite and graphite the silicate compositions determined by empa are homogeneous olivine fo90 2 orthopyroxene en86 3fs8 6wo5 1 and pigeonite en81 6fs8 9wo9 5 and are similar to those of magnesian ureilites the modal abundance of mineral phases was determined by rietveld refinement of the powder xrd data the mossbauer spectra at 295 k and 78 k are composed of two sharp well defined paramagnetic doublets superimposed on a well resolved magnetic sextet and other weak absorption features the two paramagnetic doublets are assigned to olivine and pyroxene orthopyroxene and pigeonite and the ferromagnetic sextet to kamacite magnetic hyperfine field 33 2 t in agreement with the xrd characterization the mossbauer results also show the presence of small amounts of troilite fes and cohenite fe ni co 3c using the mossbauer data the relative abundance of each fe bearing phase is determined and compared with the results obtained by xrd

planetesimal differentiation revealed by the hf w systematics of ureilites abstract determining the timescales of the accretion and chemical differentiation of meteorite parent bodies provides some of the most direct constraints on the formation of planetesimals and the earliest stages of planet formation we present high precision hf w isotope data for a comprehensive set of ureilites ultramafic mantle restites derived from a partially melted and incompletely differentiated asteroid all samples are characterized by strong 182w deficits indicating that silicate melt extraction on the ureilite parent body at 3 3 0 7 ma after cai formation postdated core formation in iron meteorite parent bodies by 2 3 ma thermal modeling of planetesimal heating by 26al decay combined with the new hf w data indicates that the ureilite parent body accreted at 1 6 ma after cai formation and therefore more than 1 ma later than iron meteorite parent bodies but more than 0 5 ma earlier that most chondrite parent bodies due to its relatively late accretion the ureilite parent body contained too little 26al to cause complete melting and therefore would have probably remained incompletely differentiated even without exhaustion of 26al by silicate melt segregation our results show that both in terms of degree of differentiation and accretion timescale the ureilite parent body is intermediate between fully differentiated and undifferentiated bodies implying that there is an inverse correlation between extent of melting and metal silicate separation versus time of accretion and differentiation

fractional melting and smelting on the ureilite parent body we investigate petrologic and physical aspects of melt extraction on the parent asteroid of the ureilite meteorites upb we first develop a petrologic model for simultaneous melting and smelting reduction of feo by c at various depths for a model starting composition determined from petrologic constraints to have been cv like except for elevated ca al 2 5 ci we determine 1 degree of melting 2 the evolution of mg 3 production of co co2 gas and 4 the evolution of mineralogy in the residue as a function of temperature and pressure we then use these relationships to examine implications of fractional vs batch melt extraction in the shallowest source regions 30 bars melting and smelting begin simultaneously at 1050 c so that mg and the abundance of low ca pyroxene initially pigeonite ultimately pigeonite orthopyroxene begin to increase immediately however in the deepest source regions 100 bars smelting does not begin until 1200 c so that mg begins to increase and low ca pyroxene pigeonite appears only after 21 melting the final residues in these two cases obtained just after the demise of augite match the end members of the ureilite mg range 94 76 in pyroxene abundance and type in all source regions production of co co2 by smelting varies over the course of melting the onset of smelting results in a burst of gas production and very high incremental gas melt ratios up to 2 5 by mass after a few s melting however these values drastically decline to 0 05 in the final increments physical modelling based on these relationships indicates that melts would begin to migrate upwards after only 1 2 melting and thereafter would migrate continuously fractionally and rapidly reaching the surface in a year in a network of veins dikes all melts produced during the smelting stage in each source region have gas contents sufficient to cause them to erupt explosively and be lost however since in all but the shallowest source regions part of the melting sequence occurs without smelting fractional melting implies that a significant fraction of upb melts may have erupted more placidly to form a thin crust 3 3 km thick for a 100 km radius body our calculations suggest that melt extraction was so rapid that equilibrium trace element partitioning may not have been attained we present a model for disequilibrium fractional melting in which ree partitioning is limited by diffusion on the upb and demonstrate that it produces a good match to the ureilite data the disequilibrium model may also apply to trace siderophile elements and might help explain the overabundance of these elements in ureilites relative to predictions from the smelting model our results suggest that melt extraction on the upb was a rapid fractional process which can explain the preservation of a primitive oxygen isotopic signature on the upb

origin of ureilite meteorites and implications for planetary accretion although ureilites have been considered bizarre meteorites our analysis of their properties suggests that they formed by simple well known processes we conclude that 1 ureilites are the sole rock type in a major fraction of their parent body 2 they are partial melt residues and not cumulates 3 removal of 90 of the al in the basalts would largely stop radioactive heating if 26al was the heat source ensuring that large volumes of ureilite like material were generated 4 they formed in a large chondritic body that had a heterogeneous o isotopic composition 5 slow diffusion of o in solid and molten silicates ensured that partial melting did not homogenize the o isotopes of the residues 6 the puzzling absence of the basaltic melts that accompanied ureilite formation probably results from ejection into space by explosive volcanism together these conclusions provide a simple plausible origin for ureilites that accounts for most of their properties ureilites are residues from the partial melting of isotopically heterogeneous chondritic material we suggest that other meteorite parent bodies and planetesimals may have experienced similar histories and that ureilites provide important constraints on the accretionary and igneous histories of planets and asteroids

feldspathic clast populations in polymict ureilites stalking the missing basalts from the ureilite parent body abstract polymict ureilites dag 164 165 dag 319 dag 665 and eet 83309 are regolith breccias composed mainly of monomict ureilite like material but containing 2 vol of feldspathic components we characterized 171 feldspathic clasts in these meteorites in terms of texture mineralogy and mineral compositions based on this characterization we identified three populations of clasts each of which appears to represent a common igneous generally basaltic lithology and whose mafic minerals show a normal igneous fractionation trend of near constant fe mn ratio over a range of fe mg ratios that extend to much higher values than those in monomict ureilites the melts represented by these populations are unlikely to be impact melts because the ubiquitous presence of carbon in polymict ureilites the regolith of the ureilite parent body implies that impact melts would have crystallized under conditions of carbon redox control and therefore have highly magnesian mafic mineral compositions with constant mn mg ratio therefore these melts appear to be indigenous products of igneous differentiation on the ureilite parent body upb complementary to the olivine pigeonite residues represented by the majority of monomict ureilites the most abundant population is characterized by albitic plagioclase in association with pyroxenes phosphates ilmenite silica and incompatible element enriched glass model calculations suggest that it formed by extensive fractional crystallization of the earliest melt s of precursor materials from which the most magnesian shallowest olivine pigeonite ureilites formed a less abundant population characterized by labradoritic plagioclase may have formed from melts complementary to more ferroan olivine pigeonite ureilites and derived from deeper in the upb the third population characterized by the presence of olivine and augite could only have formed from melts produced at greater depths in the upb than the olivine pigeonite ureilites many other feldspathic clasts cannot be positively associated with any of these three populations because their mafic mineral compositions exhibit carbon redox control however they may be products of early crystallization of basaltic melts produced on the upb before carbon was exhausted by reduction partial melting on the ureilite parent body was a fractional or incremental process melts were produced early in upb history and most likely extracted rapidly thus preserving primitive chemical and oxygen isotopic signatures in the residues

crustal differentiation in the early solar system clues from the unique achondrite northwest africa 7325 nwa 7325 the unique achondrite nwa 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase an85 93 diopsidic pyroxene en50 1 54 0 wo44 8 49 3 fs0 6 1 3 and forsteritic olivine fo97 it is al and mg rich and fe and na poor and displays very low concentrations of incompatible trace elements much below 0 3 x ci abundances for many of them it is also characterized by huge eu and sr anomalies eu eu 65 srn cen 240 although the o isotopic composition of nwa 7325 and some ureilites those with olivine cores in the range fo75 to fo88 are similar a genetic relationship between them is unlikely due to the fe poor composition of nwa 7325 it is almost certainly derived from a distinct planetesimal not previously sampled by other achondrites the low na al ga al zn al ratios as well as the low k rb and cs shown by nwa 7325 suggest a volatile depleted parent body this unique gabbro is demonstrably a cumulate but the composition of its parental melt cannot be precisely assessed however the liquid from which nwa 7325 crystallized would have been very poor in incompatible trace elements yb in the range of 0 25 to 1 5 x ci abundance with a very large positive eu anomaly such a melt cannot be the product of the early magmatic activity on a small parent body instead we propose that the parental melt to nwa 7325 formed as a consequence of the total melting of an ancient gabbroic lithology possibly upon impact in agreement with the systematics of 26al 26mg based on recent dating the crustal material that was parental to nwa 7325 must have been older than 4562 8 ma and formed possibly 4566 ma ago if this scenario is correct nwa 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied

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

pyroxene selective impact smelting in ureilites abstract a characteristic feature of ureilite meteorites is reduction of feo but the reduction is usually confined to the rims of olivine in the lar 04315 lap 03587 and almahata sitta ureilites pyroxene was extensively reduced by impact smelting in lar 04315 the impact caused nearly all of the original pigeonite to melt or otherwise become sufficiently structurally compromised to allow smelting and yet a minor proportion of the pyroxene escaped smelting and survived with its original composition en 74 1 wo 10 2 olivine mosaicism confirms that lar 04315 experienced a major shock event the smelted pyroxenes also show a distinctive patchiness in their interference colors although each grain s basic optical continuity often including twinning is still discernible they also have reduced compositions are ubiquitously porous 15 and contain sprinklings of fe metal and felsic glass for the most part the olivine underwent only very slight reduction much of the small pyroxene component of lap 03587 shows the same oddly porous texture lar 04315 also contains large traces of silica and felsic glass with a typical composition of in wt 61 sio 2 23 al 2 o 3 11 cao 3 7 na 2 o glass these two phases together form selvages that line the walls of many of the largest voids in the rock silica is a by product of pyroxene smelting the felsic glass probably derives largely from interstitial basaltic melt that predated the impact however the comparatively stiff surrounding included silica may have promoted unusually high melt retention within lar 04315 through the smelting episode one aspect of which was a major stream out through the same large voids of co x gas the impact smelted pyroxene of lap 03587 is enigmatic because this ureilite also features little shocked euhedral graphite laths and no olivine mosaicism the fine grained ureilitic component of almahata sitta appears to have likewise formed by impact smelting but with more extensive melting of pyroxene especially a ca rich pyroxene component more pulverization and melting of olivine and more displacement of both however in places the original coarse equant ureilite texture is still discernible in relict form ordinarily an impact shock melts olivine before or at least no later than pyroxene but in the case of lar 04315 and lap 03587 the great shock event evidently occurred when the material was already anatectic or very nearly so and thus the difference in melting temperature between pyroxene and olivine 300 degrees lower for pyroxene was decisive if literature inferences of extremely fast cooling rates implying shallow burial depths are accurate the proportion of co x gas generated by ureilite smelting exceeded by a very large factor of order 10 3 but possibly much greater the volume represented as porosity in the final ureilites the outflow of so much gas may have by near surface explosive expansion and jetting enhanced the thoroughness of the impact triggered catastrophic impact disruption of the parent asteroid

a northwest trending jurassic fold nappe northernmost zacatecas mexico the caopas rodeo and nazas formations exposed in the san julian uplift of northern zacatecas are distinguished principally on the basis of style and intensity of deformation they are parts of the same early mesozoic volcanogenic suite this suite was the source for overlying volcaniclastic conglomerate and sandstone la joya formation that appears transitional into succeeding late jurassic oxfordian zuloaga limestone deformation that was contemporaneous with the deposition of the lower part of the zuloaga produced an asymmetrical northwest trending fold nappe that was driven southwestward the massive quartz porphyry core of the structure caopas formation moved somewhat independently of the encasing more ductile rocks rodeo and nazas formations phyllonite developed in the nose of the nappe the upright limb was attenuated and ruptured in zones where deformation due to the nappe forming process was weaker a gently dipping foliation possibly related to an older episode of recumbent folding is preserved orientation age and location of the nappe suggest that it is a manifestation of transpressional stress developed along the mojave sonora megashear as left lateral movement carried the jurassic arc toward the southeast

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

asteroid 2008 tc 3 almahata sitta a spectacular breccia containing many different ureilitic and chondritic lithologies abstract asteroid 2008 tc3 impacted earth in northern sudan on october 7 2008 the meteorite named almahata sitta was classified as a polymict ureilite in this study 40 small pieces from different fragments collected in the almahata sitta strewn field were investigated and a large number of different lithologies were found some of these fragments are ureilitic in origin whereas others are clearly chondritic as all are relatively fresh w0 w0 1 and as short lived cosmogenic radioisotopes were detected within two of the chondritic fragments there is strong evidence that most if not all belong to the almahata sitta meteorite fall the fragments can roughly be subdivided into achondritic ureilitic 23 samples and chondritic lithologies 17 samples among the ureilitic rocks are at least 10 different lithologies a similar number of different chondritic lithologies also exist most chondritic fragments belong to at least seven different e chondrite rock types eh3 el3 4 el6 el breccias several different types of el and eh impact melt rocks and impact melt breccias some of the latter are shock darkened in addition two h group ordinary chondrite lithologies were identified and one sample of a chondrite type that is so far unique the latter has some affinities to r chondrites oxygen isotope compositions of 14 fragments provide further fundamental information on the lithological heterogeneity of the almahata sitta meteorite based on the findings presented in this study the reflectance spectrum of asteroid 2008 tc3 has to be evaluated in a new light

stable isotopes and the noncarbonaceous derivation of ureilites in common with nearly all differentiated planetary materials abstract the abundant diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents average 3 wt and have long been linked to the so called carbonaceous chondrites although this term is potentially misleading because the high petrologic type carbonaceous chondrites are if anything c poor compared to ordinary chondrites ureilite oxygen isotopic compositions i e diversely negative ccam like 17o viewed in isolation have long been viewed as confirming the carbonaceous chondritic derivation hypothesis however a very different picture emerges through analysis of a compilation of recently published high precision isotopic data for chromium titanium and nickel for ureilites and various other planetary materials ureilites have lower e62ni and far lower e50ti and e54cr than any known variety of carbonaceous chondrite on a plot of e50ti vs e54cr and similarly 17o vs e54cr ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites and the carbonaceous chondrites are separated by a wide margin from all other planetary materials i conclude that notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative 17o the ureilite precursors accreted from preponderantly noncarbonaceous sensu stricto materials despite total depletion of basaltic matter the ureilites retain moderate pyroxene olivine ratios which is an expected outcome from simple partial melting of moderate sio2 feo mgo noncarbonaceous chondritic material but would imply an additional process of major reduction of feo if the precursor material were carbonaceous chondritic the striking bimodality of planetary materials on the e50ti vs e54cr and 17o vs e54cr diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula however an alternative admittedly speculative explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system carbonaceous and materials that accreted in the inner solar system noncarbonaceous including the ureilites

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

evidence from polymict ureilite meteorites for a disrupted and re accreted single ureilite parent asteroid gardened by several distinct impactors ureilites are ultramafic achondrites that exhibit heterogeneity in mg and oxygen isotope ratios between different meteorites polymict ureilites represent near surface material of the ureilite parent asteroid s electron microprobe analyses of 500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites suggesting derivation from a single parent asteroid many ureilitic clasts have identical compositions to the anomalously high mn mg olivines and pyroxenes from the hughes 009 unbrecciated ureilite here termed the hughes cluster some polymict samples also contain lithic clasts derived from oxidized impactors the presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid in situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts using a secondary ion mass spectrometer sims with precision typically better than 0 2 0 4 2sd for 18o and 17o oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the ccam line covering the range for unbrecciated ureilites and show a good anti correlation with mineral mg sims analysis identifies one ferroan lithic clast as an r chondrite while a second ferroan clast is unlike any known meteorite an exotic enstatite grain is derived from an enstatite chondrite or aubrite and another pyroxene grain with 17o of 0 4 0 2 is unrelated to any known meteorite type ureilitic olivine clasts with mg s 85 which include the melt inclusion bearing hughes cluster ureilites thus melt was present in regions of the parent ureilite asteroid with a bulk mg 85 when the asteroid was disrupted by impact giving rise to two types of ureilites common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred one or more daughter asteroids re accreted from the remnants of the mantle of the proto ureilite asteroid polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid including impact derived material from at least six different meteoritic sources

brachinites igneous rocks from a differentiated asteroid we have done petrologic studies of brachinites allan hills alh 84025 elephant moraine eet 99402 and eet 99407 bulk geochemical studies of eet 99402 and eet 99407 ar ar studies of brachina and eet 99402 and a xe isotopic study of brachina textural mineral compositional and bulk compositional evidence show that eet 99402 and eet 99407 are paired alh 84025 eet 99402 and eet 99407 have igneous textures petrofabric analyses of alh 84025 and eet 99407 demonstrate the presence of lineations and probable foliations of olivine grains that support formation as igneous cumulates mineral minor element chemistry and bulk rock incompatible lithophile element contents of the brachinites are distinct from those of acapulcoite lodranite clan meteorites a suite of high grade metamorphic rocks and anatectic residues the differences demonstrate a higher blocking temperature of equilibration for the brachinites and that cumulus plagioclase is present in eet 99402 eet 99407 and probably alh 84025 thus indicating an igneous origin brachinites are differentiated ultramafic achondrites and are not part of a suite of primitive achondrites we infer that their parent asteroid is a differentiated body brachina has an excess of 129xe correlated with reactor produced 128xe demonstrating that short lived 129i was present at the time of formation this plus literature data attests to early formation of the brachinites within a few ma of the formation of chondrites ar ar age data show that brachina and eet 99407 were degassed about 4 13 ga ago possibly by a common impact event eet 99402 and eet 99407 show petrographic evidence for shock including possible conversion of plagioclase to maskelynite followed by devitrification brachina is unshocked making a direct association between the ar ar age and textures ambiguous

the o doherty anstey formula and localization of seismic waves angle or offset dependent effects of scattering in finely layered media can be observed and analyzed or must be compensated for in vertical seismic profiling data vsp data crosshole observations or seismic amplitude variation with offset avo measurements moreover the adequate description of multiple scattering is important for the study of seismic attenuation in sediments and for the design of inversion procedures

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

the origin of eucrites diogenites and olivine diogenites magma ocean crystallization and shallow magma chamber processes on vesta the asteroid 4 vesta is one of the very few heavenly bodies to have been linked to samples on earth the howardite eucrite diogenite hed meteorite suite this large and diverse suite of meteorites provides a detailed picture of vesta s igneous and postigneous history we have used the range of igneous rock types and compositions in the hed suite to test a series of chemical models for solidification processes following peak melting magma ocean conditions on vesta fractional crystallization cannot have been a dominant early process in the magma ocean because it leads to excessive fe enrichment in the melt models that are dominated by equilibrium crystallization cannot produce orthopyroxene cumulates diogenites our best models invoke 60 70 equilibrium crystallization of a magma ocean followed by continuous extraction of the residual melt into shallow magma chambers fractional crystallization in these magma chambers combined with continuous or periodic addition of more melt from the slowly compacting crystal mush magmatic recharge can produce all of the igneous hed lithologies noncumulate and cumulate eucrites diogenites dunites harzburgites and olivine diogenites magmatic recharge can also explain the narrow range in eucrite compositions and the variability of incompatible trace element concentrations in diogenites we predict an internal structure for vesta that permits excavation of the heds during the formation of the rheasilvia basin while remaining consistent with observations from the dawn mission and most impact models

mineralogy and cooling history of the calcium aluminum chromium enriched ureilite lewis cliff 88774 the lew 88774 ureilite is extraordinarily rich in ca al and cr and mineralogically quite different from other ureilites in that it consists mainly of exsolved pyroxene olivine cr rich spinel and c the presence of coarse exsolved pyroxene in lew 88774 is unique because pyroxene in most other ureilites is not exsolved the pyroxene has bulk wo contents of 15 20 mol and has coarse exsolution lamellae of augite and low ca pyroxene 50 m in width the compositions of the exsolved augite ca33 7mg52 8fe13 5 and host low ca pyroxene ca4 4mg75fe20 6 show that these exsolution lamellae were equilibrated at 1280 c a computer simulation of the cooling rate obtained by solving the diffusion equation for reproducing the diffusion profile of cao across the lamellae suggests that the pyroxene was cooled at 0 01 c year until the temperature reached 1160 c this cooling rate corresponds to a depth of at least 1 km in the parent body assuming it was covered by a rock like material therefore lew 88774 was held at this high temperature for 1 2 104years the proposed cooling history is consistent with that of other ureilites with coarsegrained unexsolved pigeonites lewis cliff 88774 includes abundant cr rich spinel in comparison with other ureilites the range of feo content of spinels in lew 88774 is from 1 3 wt to 21 wt fe fe mg 0 04 0 6 the cr rich and fe poor spinel in lew 88774 has less fe feo 1 3 wt than spinels in other achondrites we classify this spinel as an fe al bearing picrochromite most ureilites are depleted in ca and al but this meteorite has high ca and al concentrations in this respect as well as mineral assemblage and the presence of coarse exsolution lamellae in pyroxene lew 88774 is a unique ureilite most differentiated meteorites are poor in volatile elements such as zn but the lew 88774 spinels contain abundant zn up to 0 6 wt we note that such a high zn concentration in spinel has been observed in the carbonaceous chondrites and recrystallized chondrites this unusual ureilite has more primitive characteristics than most other ureilites

use of low frequencies for sub basalt imaging many prospective passive ocean margins are covered by large areas of basalts these basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors since high frequencies are scattered more than low frequencies we argue that the acquisition system for sub basalt targets should be modified to emphasize the low frequencies using much larger airguns and towing the source and receivers at about 20 m depth in the summer of 2001 we obtained seismic reflection data over basalt in the northeast atlantic using a system modified to enhance the low frequency energy these new data show deep reflections that are not visible on lines shot in the same places with a conventional system

microtextures and crystal chemistry of pigeonite in the ureilites alha77257 rkpa80239 y 791538 and alha81101 the microtextures of pigeonite in four ureilites allan hills alh 77257 reckling peak rkp a80239 yamato y 791538 and allan hills a81101 chosen to span a range of composition and shock level were investigated by transmission electron microscopy tem two of the samples were also investigated by single crystal x ray diffraction to determine fe2 mg cation site partitioning the low shock and compositionally homogeneous pigeonites in alha77257 and rkpa80329 wo 6 4 for both mg 86 3 and 84 3 respectively display irregularly spaced shock induced stacking faults oriented parallel to 100 and largeantiphase domains 50 100 nm antiphase domains have no preferential orientation no evidence of exsolution was observed the low shock y 791538 pigeonite is homogeneous and has higher ca and mg wo 9 4 mg 91 2 tem investigation showed spinodal decomposition indicative of incipient exsolution small antiphase domains were observed 5 nm single crystal refinement yielded r4 5 71 with fe2 mg partitioning coefficient kd 0 077 8 and tc 658 35 c alha81101 has compositionally heterogeneous pyroxenes with large local variations in wo and mg wo 4 13 mg 86 68 no compositional gradients from core to rim of grains were observed and the heterogeneity is interpreted as related to cation migration during shock in one relatively ca rich region wo 12 tem analysis showed augite pigeonite exsolution lamellae with spacing 145 20 nm results for alha77257 rkpa80239 and y 791538 support a model of rapid cooling following breakup of the ureilite parent body the presence of exsolution lamellae in alha81101 can be related to a local shock induced ca enrichment and provides no constraint on the late cooling history

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

origin of mass independent oxygen isotope variation among ureilites clues from chondrites and primitive achondrites ureilite meteorites are abundant carbon rich primitive achondrites made of coarse grained equilibrated olivine and pyroxene usually pigeonite they probably sample the baked heterogeneous melt depleted mantle of a large once chondritic parent body that was broken up catastrophically while still young and hot heterogeneity in the parent body is inferred from a considerable slope 1 variation from one meteorite to another in oxygen isotopes 2 5 17o 0 2 which correlates with both molar feo mgo range 0 03 0 35 and molar feo mno range 3 57 i e 17o correlates with the redox state no consensus has yet emerged on the cause of these correlated trends one view favors their inheritance via silicates from hot nebular preaccretion processes another invokes smelting reduction of feo by c in the hot parent body here guided mainly by similar trends among equilibrated ordinary and r chondrites studies of their unequilibrated counterparts and work on other primitive achondrites we propose a new model for ureilites in which the parent body accreted nebular ice with high 17o mg rich silicates with low 17o and varying amounts of metallic iron water from the thawing ice then oxidized the metal yielding secondary feo bearing minerals with high 17o that with metamorphism became incorporated into the ureilite silicates feo mgo feo mno and 17o correlate because they rose in unison by amounts that varied spatially depending on the local amount of metal that was oxidized we suggest that the parent body was so large radius 100 km that smelting was inhibited and that carbon played a passive role in ureilite evolution although ureilites are regarded as complicated meteorites we believe our analysis explains their mass independent oxygen isotope trend and related feo variation through well understood processes and enlightens our understanding of the evolution of early planetesimals from cold wet bodies to hot dry ones

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

comment on parent body depth pressure temperature relationships and the style of the ureilite anatexis by p h warren maps 47 209 227 ureilites are carbon rich ultramafic olivine dominantly low ca pyroxene achondrites with poorly understood petrogenesis one major problem concerns the origin of extensive variation in feo content olivine core fo values ranging from approximately 75 to 95 among the individual ureilites the two main competing hypotheses to explain this variation are 1 equilibrium smelting in which ureilite fo values were established by pressure dependent depth linked carbon redox reactions on the ureilite parent body during partial melting or 2 nebular inheritance in which the variation in feo contents was derived from ureilite precursors and was preserved during partial melting the paper parent body depth pressure temperature relationships and the style of the ureilite anatexis by warren 2012 discusses a series of topics related to ureilite petrogenesis in each case an argument is presented within the context of smelting versus nonsmelting models collectively these arguments create the impression that there are many valid arguments against smelting the purpose of this comment is to point out flaws in some of these arguments and or to show that the issues they address are independent of smelting versus nonsmelting models both equilibrium smelting and nebular inheritance simple anatexis models face challenges in explaining all the properties of ureilites but both remain viable

early petrologic processes on the ureilite parent body we present a petrographic and petrologic analysis of 21 olivine pigeonite ureilites along with new experimental results on melt compositions predicted to be in equilibrium with ureilite compositions we conclude that these ureilites are the residues of a partial melting smelting event textural evidence preserved in olivine and pigeonite record the extent of primary smelting in pigeonite cores we observe fine trains of iron metal inclusions that formed by the reduction of olivine to pigeonite and metal during primary smelting olivine cores lack metal inclusions but the outer grain boundaries are variably reduced by a late stage reduction event the modal proportion of pigeonite and percentage of olivine affected by late stage reduction are inversely related and provide an estimation of the degree of primary smelting during ureilite petrogenesis in our sample suite this correlation holds for 16 of the 21 samples examined olivine pigeonite liquid phase equilibrium constraints are used to obtain temperature estimates for the ureilite samples examined inferred smelting temperatures range from 1150 c to just over 1300 c and span the range of estimates published for ureilites containing two or more pyroxenes temperature is also positively correlated with modal percent pigeonite smelting temperature is inversely correlated with smelting depth the hottest olivine pigeonite ureilites coming from the shallowest depth in the ureilite parent body the highest temperature samples also have oxygen isotopic signatures that fall toward the refractory inclusion rich end of the carbonaceous chondrite anhydrous mineral ccam slope 1 mixing line these temperature depth variations in the ureilite parent body could have been created by a heterogeneous distribution of heat producing elements which would indicate that isotopic heterogeneities existed in the material from which the ureilite parent body was assembled

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

chromium valences in ureilite olivine and implications for ureilite petrogenesis abstract ureilites are a group of ultramafic achondrites commonly thought to be residues of partial melting on a carbon rich asteroid they show a large variation in feo content olivine fo values ranging from 74 to 95 that cannot be due to igneous fractionation and suggests instead variation in oxidation state the presence of chromite in only a few of the most ferroan fo 75 76 samples appears to support such a model microxanes analyses were used in this study to determine the valence states of cr previously unknown in olivine cores of 11 main group ureilites the goal of this work was to use a method that is independent of fo to determine the oxidation conditions under which ureilites formed in order to evaluate whether the ureilite feo variation is correlated with oxidation state and whether it is nebular or planetary in origin two of the analyzed samples lew 88774 fo 74 2 and nwa 766 fo 76 7 contain primary chromite two others lap 03587 fo 74 4 and cms 04048 fo 76 2 contain sub micrometer sized exsolutions of chromite ca rich pyroxene in olivine and one eet 96328 fo 85 2 contains an unusual chromite grain of uncertain origin no chromite has been observed in the remaining six samples fo 77 4 92 3 chromium in olivine in all eleven samples was found to be dominated by the divalent species with valences ranging from 2 10 0 02 1 to 2 46 0 04 the non chromite bearing ureilites have the most reduced cr with a weighted mean valence of 2 12 0 01 i e cr2 cr3 7 33 all low fo chromite bearing ureilites have more oxidized cr with valences ranging from 2 22 0 03 to 2 46 0 04 eet 96328 whose chromite grain we interpret as a late crystallizing phase yielded a reduced cr valence of 2 15 0 07 similar to the non chromite bearing samples based on the measured cr valences magmatic 1200 1300 c oxygen fugacities fo2 of the non chromite bearing samples were estimated to be in the range iw 1 9 to iw 2 8 assuming basaltic melt composition consistent with fo2 values obtained by assuming olivine silica iron metal osi equilibrium for the primary chromite bearing ureilites the corresponding fo2 were estimated again assuming basaltic melt composition to be iw to iw 1 0 i e several orders of magnitude more oxidizing than the conditions estimated for the chromite free ureilites in terms of fo and cr valence properties ureilites appear to form two groups rather than a single cr valence or fo2 vs fo trend the chromite bearing ureilites show little variation in fo 74 76 but significant variation in cr valence while the non chromite bearing ureilites show significant variation in fo 77 95 and little variation in cr valence these groups are unrelated to petrologic type i e olivine pigeonite olivine orthopyroxene or augite bearing the chromite bearing ureilites also have lower contents of cr in olivine than most non chromite bearing ureilites consistent with predictions based on cr olivine melt partitioning in spinel saturated vs non spinel saturated systems under the assumption that at magmatic temperatures graphite gas equilibria controlled fo2 at all depths on the ureilite parent body we conclude 1 that ureilite precursor materials having the fo and cr valence properties now observed in ureilites are unlikely to have been preserved during planetary processing and 2 that the fo and cr valence properties now observed in ureilites are consistent with having been established by high temperature carbon redox control over a range of depths on a plausible sized ureilite parent body the apparent limit on ureilite fo values around 74 76 suggests that the precursor material s had bulk mg that of ll chondrites

thermal and unroofing history of a thick tilted basin and range crustal section in the tortilla mountains arizona we estimate here a geothermal gradient of only 17 5 c km 1 for the tilted grayback fault block in southeastern arizona when extension began 25 ma this gradient is lower than preextension gradients estimated elsewhere in the basin and range is only about 50 of typical gradients in the basin and range today and needs to be accounted for in models of continental extension the grayback block exposes a 12 km thick crustal section of proterozoic and cretaceous granitoids which was tilted 90 during extension between 25 and 15 ma zircon fission track ages decrease structurally downward westward across the block and were all within a zone of partial track annealing prior to tilting and quenching the zircon age gradient suggests that the 220 240 c isotherm migrated downward 5 6 km during paleogene erosion and regional cooling apatite fission track ages decrease westward from 83 ma in the structurally highest crystalline rocks to 24 ma at 6 km paleodepth and then to 15 ma another 6 km farther west track length analysis confirms that apatites above the break in slope in age at 5 7 km paleodepth resided in a zone of partial annealing prior to tilting and deeper apatites record rapid cooling upon tilting and unroofing beginning 25 ma at that time the 110 10 c isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of 5 7 km and the 220 30 c isotherm as estimated from zircon data resided at a pretilting basement depth of 12 15 km from consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 5 c km 1 from 25 to 15 ma the rotating grayback block cooled rapidly as higher westward moving blocks unroofed it tectonically at a rate of 1 km m y 1

localization and backscattering spectrum of seismic waves in stratified lithology using four sonic well logs from diverse geological environments we analyze the statistics of lithological layers relevant to seismic wave propagation the autocorrelation functions are found to be well approximated by exponentials with correlation lengths generally in the 1 5 to 3 m range we use localization theory to calculate the apparent attenuation caused by random scattering as a function of frequency this attenuation has a peak at 50 to 150 hz with a corresponding localization length of 1 6 to 4 8 km 1 to 3 mi and an apparent q of 120 to 450 at seismic frequencies 20 hz the attenuation is smaller with localization lengths in the 16 to 32 km 10 to 20 mi range and an apparent q of 300 to 700 these values of apparent q are consistent with observations of previous authors who used well log calculations using finite differences we compute synthetic traces for a 20 hz pulse with multiple backscattering from the lithology given by each of the well logs the power spectral density of the trace

almahata sitta asteroid 2008 tc3 and the search for the ureilite parent body abstract this article explores what the recovery of 2008 tc3 in the form of the almahata sitta meteorites may tell us about the source region of ureilites in the main asteroid belt an investigation is made into what is known about asteroids with roughly the same spectroscopic signature as 2008 tc3 a population of low inclination near earth asteroids is identified with spectra similar to 2008 tc3 five asteroid families in the main belt as well as a population of ungrouped asteroids scattered in the inner and central belts are identified as possible source regions for this near earth population and 2008 tc3 three of the families are ruled out on dynamical and spectroscopic grounds new near infrared spectra of 142 polana and 1726 hoffmeister lead objects in the two other families also show a poor match to almahata sitta thus there are no main belt spectral analogs to almahata sitta currently known space weathering effects on ureilitic materials have not been investigated so that it is unclear how the spectrum of the main belt progenitor may look different from the spectra of 2008 tc3 and the almahata sitta meteorites dynamical arguments are discussed as well as ureilite petrogenesis and parent body evolution models but these considerations do not conclusively point to a source region either other than that 2008 tc3 probably originated in the inner asteroid belt

si bearing metal and niningerite in almahata sitta fine grained ureilites and insights into the diversity of metal on the ureilite parent body a detailed mineralogical and chemical study of almahata sitta fine grained ureilites ms 20 ms 165 ms 168 was performed to shed light on the origin of these lithologies and their sulfide and metal the almahata sitta fine grained ureilites silicates 0 5 mn fe 0 5s with some similarities to the mineralogy of enstatite e chondrites overall metal grains show a large compositional variability in ni and si concentrations niningerite grains probably formed as a by product of smelting via sulfidation the large si ni variation in fine grained ureilite metal could be the result of variable degrees of reduction during impact smelting inherited from coarse grained ureilite precursors or a combination of both large si bearing metal grains probably formed via coalescence of existing and newly formed metal during impact smelting bulk and in situ siderophile trace element abundances indicate three distinct populations of 1 metal crystallized from partial melts in ms 20 2 metal resembling bulk chondritic compositions in ms 165 and 3 residual metal in ms 168 almahata sitta fine grained ureilites developed their distinctive mineralogy due to severe reduction during smelting despite the presence of e chondrite and ureilite stones in the almahata sitta fall a mixing relation of e chondrites or their constituents and ureilite material in almahata sitta can be ruled out based on isotopic textural and mineral chemical reasons

experimental constraints on ureilite petrogenesis abstract this experimental study explores the petrogenesis of ureilites by a partial melting smelting process experiments have been performed over temperature 1150 1280 c pressure 5 12 5 mpa and low oxygen fugacity graphite co gas conditions appropriate for a hypothetical ureilite parent body 200 km in size experimental and modeling results indicate that a partial melting smelting model of ureilite petrogenesis can explain many of the unique characteristics displayed by this meteorite group compositional information preserved in the pigeonite olivine ureilites was used to estimate the composition of melts in equilibrium with the ureilites the results of 20 experiments saturated with olivine pyroxene metal and liquid with appropriate ureilite compositions are used to calibrate the phase coefficients and pressure temperature dependence of the smelting reaction the calibrated coefficients are used to model the behavior of a hypothetical residue that is experiencing fractional smelting the residue is initially olivine rich and smelting progressively depletes the olivine content and enriches the pyroxene and metal contents of the residues the modeled residue composition at 1260 c best reproduces the trend of ureilite bulk compositions the model results also indicate that as a ureilite residue undergoes isothermal decompression smelting over a range of temperatures ca al values and cr 2 o 3 contents are enriched at lower temperatures below 1240 c and tend to decrease at higher temperatures therefore fractional smelting can account for the high ca al and cr 2 o 3 wt values observed in ureilites we propose that ureilites were generated from an olivine rich cpx bearing residue smelting began when the residue was partially melted and contained liquid olivine and carbon these residues experienced varying degrees of fractional smelting to produce the compositional variability observed within the pigeonite bearing ureilites variations in mineral composition modal proportions and isotopic signatures are best described by heterogeneous accretion of the ureilite parent body followed by minimal and variable degrees of igneous processing

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

metallic phases and siderophile elements in main group ureilites implications for ureilite petrogenesis abstract metallic phases and siderophile elements are critical to understanding the petrogenesis of the enigmatic ureilite meteorites we obtained petrographic major and minor element and the first in situ trace element data for metallic phases metal sulfides phosphide carbide in 24 main group ureilites of various petrographic types with fo 75 95 the most abundant type of metal 1 3 vol occurs as 10 40 m wide strips along silicate grain boundaries ni contents of this metal range from 0 to 7 3 wt and are correlated with co among all samples ni co 0 64 ci a less abundant type of metal occurs as 5 150 m diameter metallic spherules consisting of cohenite fe 3 c metal phosphide and sulfide enclosed in silicates preferentially low ca pyroxene most samples contain 2 types of sulfide 1 low cr 2 s 4 and troilite abundances of 17 mostly siderophile elements were measured by la icp ms in grain boundary metal spherules graphite sulfides and silicates average compositions of grain boundary metal in 10 samples show decreasing ci normalized abundance with increasing volatility interrupted by depletions in w mo ni and zn and enrichments in au as ga and ge ci normalized os abundances range from 2 to 65 and are correlated with increasing os pt os ni and os pd ratios ci normalized pt os ratios range from 0 3 to 1 bulk cohenite bearing spherules have siderophile element abundances indistinguishable from those of grain boundary metal in the same sample ci normalized patterns of most siderophile elements in the metal are within error identical to those of the bulk rock at 25 40 higher abundances in each sample there are no correlations between siderophile element abundances and fo we infer that at t 1200 c ureilites contained immiscible fe c 3 4 wt c and fe s melts small samples of which were trapped as the spherules within silicates the fe s melt was largely extracted from the rocks and the bulk of the residual fe c melt is now represented by the grain boundary metal assuming that ureilite precursor materials had ci or cv abundances of siderophile elements the large fractionations of hse observed in metal in 7 of the 10 samples require extremely high degrees 98 of batch fe s melt extraction which implies very high x fes wt fes fe fes in the precursors furthermore at such high degrees of fractionation the hse are so strongly concentrated into the residual metal that to match their relatively low absolute abundances in the ureilite metal very high initial metal contents are required together these constraints would imply that ureilite precursors had abundances of fe metal and fes 20 35 wt each far exceeding those of known cc or oc these requirements could be relaxed permitting lower more plausible degrees of melting and lower initial metal and sulfide abundances if ureilite precursors were volatile depleted to a greater extent than bulk cv we suggest that ureilite precursors contained to various degrees an overabundance relative to chondrites of refractory enriched material such as cais excess cais could also account for observed depletions of w and mo otherwise difficult to explain in the ureilite metal and lead to the observed range of siderophile element patterns and abundances among samples such a model can potentially explain the lack of correlation between siderophile element abundances and feo or olivine fo and reconcile the metal and siderophile element data with a redox model for ureilite petrogenesis

a petrologic thermodynamic and experimental study of brachinites partial melt residues of an r chondrite like precursor abstract the primitive achondrites provide a window into the initial melting of asteroids in the early solar system the brachinites are olivine dominated meteorites with a recrystallized texture that we and others interpret as evidence of partial melting and melt removal on the brachinite parent body we present a petrologic thermodynamic and experimental study of the brachinites to evaluate the conditions under which they formed and test our hypothesis that the precursor material to the brachinites was feo rich compared to the precursors of other primitive achondrites petrologic analysis of six brachinites brachina allan hills alh 84025 hughes 026 elephant moraine eet 99402 northwest africa nwa 3151 and nwa 4969 and one brachinite like achondrite nwa 5400 shows that they are meteorites with recrystallized texture that are enriched in olivine 80 vol and depleted in other minerals with respect to a chondritic mineralogy silicates in the brachinites are feo rich fa 32 36 brachinite like achondrite northwest africa 5400 is similar in mineralogy and texture to the brachinites but with a slightly lower feo content fa 30 thermodynamic calculations yield equilibration temperatures above the fe ni fes cotectic temperature 950 c for all meteorites studied here and temperatures above the silicate eutectic 1050 c for all but two brachina formed at an f o 2 of iw and the other brachinites and nwa 5400 formed at iw 1 all the meteorites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineralogies equilibrated mineral compositions and recrystallized textures and having reached temperatures above that required for melt generation in an attempt to simulate the formation of the brachinite meteorites we performed one atmosphere gas mixing partial melting experiments of r4 chondrite lapaz ice field 03639 experiments at 1250 c and an oxygen fugacity of iw 1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites these experiments provide further evidence for the formation of brachinites as a result of partial melting of a chondritic precursor similar in mineralogy and mineral compositions to the r chondrites

mineralogy and petrography of the almahata sitta ureilite abstract we performed a battery of analyses on 17 samples of the almahata sitta meteorite identifying three main lithologies and several minor ones present as clasts the main lithologies are 1 a pyroxene dominated very porous highly reduced lithology 2 a pyroxene dominated compact lithology and 3 an olivine dominated compact lithology although it seems possible that all three lithologies grade smoothly into each other at the kg scale at the g scale this is not apparent the meteorite is a polymict ureilite with some intriguing features including exceptionally variable porosity and pyroxene composition although augite is locally present in almahata sitta it is a minor phase in most but not all samples we have observed low calcium pyroxene 5 mole wollastonite is more abundant than compositionally defined pigeonite however we found that even the low ca pyroxene in almahata sitta has the monoclinic pigeonite crystal structure and thus is properly termed pigeonite as the major pyroxene in almahata sitta is pigeonite and the abundance of pigeonite is generally greater than that of olivine this meteorite might be called a pigeonite olivine ureilite rather than the conventional olivine pigeonite ureilite group the wide variability of lithologies in almahata sitta reveals a complex history including asteroidal igneous crystallization impact disruption reheating and partial vaporization high temperature reduction and carbon burning and re agglomeration

magnetism and mineralogy of almahata sitta polymict ureilite asteroid 2008 tc 3 implications for the ureilite parent body magnetic field the almahata sitta meteorite is the first case of recovered extraterrestrial material originating from an asteroid that was detected in near earth space shortly before entering and exploding in the high atmosphere the aims of our project within the 2008 tc 3 consortium were investigating almahata sitta s as magnetic signature phase composition and mineralogy focussing on the opaque minerals and gaining new insights into the magnetism of the ureilite parent body upb we report on the general magnetic properties and behavior of almahata sitta and try to place the results in context with the existing data set on ureilites and ureilite parent body models the magnetic signature of as is dominated by a set of low ni kamacites with large grain sizes additional contributions come from micron sized kamacites suessite cr troilite and daubreelite mainly found in the olivine grains adjacent to carbon rich veins our results show that the paleomagnetic signal is of extraterrestrial origin as can be seen by comparing with laboratory produced magnetic records irm four types of kamacite i iv have been recognized in the sample the elemental composition of the ureilite vein metal kamacite i particularly co clearly differs from the other kamacites ii iv which are considered to be indigenous element ratios of kamacite i indicate that it was introduced into the upb by an impactor supporting the conclusions of gabriel and pack 2009