the structural control of venusian polygonal impact craters pre impact inhomogeneities of the target material sometimes cause the rim of an impact crater to be composed of several straight segments instead of being circular the venusian surface hosts 121 such polygonal impact craters pics 12 km in diameter their straight rim segments are often parallel to the orientations of the surrounding tectonic structures particularly those in tessera terrain and young rift zones as well as the concentric components of coronae this match is notably more distinct in distances less than two crater diameters between the pics and the tectonic structures than further away surrounding wrinkle ridges lineaments or radial components of volcano tectonic features seem to have very little influence on the orientations of pic rims these results imply that the formation of straight segments of venusian pic rims is controlled by pre existing tectonic structures of the crust but not by the apparently most surficial ones thus pics could be used to provide further constraints on the distribution and orientations of zones of weakness in the venusian crust

minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules we present minor and trace element ree sr y and zr data for pyroxenes and mesostases in four porphyritic chondrules from the semarkona ordinary chondrite apparent partition coefficients for clinoenstatite orthoenstatite pigeonite and augite are compared with experimental and petrologic data from the literature and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated for most elements the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition however for lree apparent partition coefficients are significantly higher than comparable equilibrium data and the ratio of hree lree partition coefficients is lower particularly for ca poor pyroxene we attribute this flattening of ree patterns to the effect of rapid cooling apparent partition coefficients of all ree and y in augite are higher than equilibrium data particularly in one chondrule with a high al 2o3 content we suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge balanced substitutions with al 31 cations

the terrestrial alteration of saharan shergottites dar al gani 476 and 489 a case study of weathering in a hot desert environment abstract rare earth element ree and other selected trace and minor element concentrations were determined in various phases of dar al gani 476 and 489 two basaltic shergottites recently found in the libyan sahara whereas merrillite and feldspathic glass compositions do not seem to have been altered since the fall of these meteorites olivine and pyroxenes commonly show light ree enrichments sometimes accompanied by ce anomalies that are attributed to terrestrial alteration sr and ba enrichments are also observed the calcium carbonate that resides in most cracks is essentially devoid of ree but a fine grained amorphous silicate material that fills some veins and pockets has striking light ree enrichments and elevated p sr and ba it is argued that this material likely to be impact melt as well as the notoriously light ree poor olivine and pyroxenes were preferentially affected by a crustal light ree rich contaminant caution is urged when using whole rock or even mineral separate compositional and isotopic data for hot desert meteorites

ce anomalies in the lew85300 eucrite evidence for ree mobilization during antarctic weathering abstract there is abundant evidence for widespread ree mobilization in antarctic eucrites resulting in ce and eu anomalies and lower whole rock ree abundances relative to typical eucrites using secondary ion mass spectrometry sims we investigated the microdistribution of ce anomalies in the antarctic polymict eucrite lew85300 ce anomalies positive and or negative are present in all three of the minerals plagioclase pyroxene and silica analysed the anomalies occur in grains from all four clasts as well as in mineral and lithic fragments from the surrounding matrix thus there appears to be no lithologic association silica which has been heavily shocked and altered exhibits a lree enriched pattern with negative ce and eu anomalies consistent with a derivation of the ree from ca phosphate dissolution trivalent ree are leached from the phosphates with preferential retention of tetravalent ce and redeposited on silica pyroxene is also widely affected by ree mobilization due to an extensive shock induced microcrack network along cleavage planes a tendency for the largest ce anomalies to occur at low lree concentrations suggests that pyroxene itself experienced ree leaching with concomitant ce retention ce anomalies are rare in plagioclase because this mineral lacks an extensive network along which ree can be mobilized the widespread ree mobilization and evidence for other forms of weathering requires that trace element data be used cautiously especially for antarctic eucrites

petrogenesis of angrites abstract the recent discovery of two new angrites sahara 99555 and d orbigny has revived interest in this small group of achondrites we measured trace element abundances in the individual minerals of these two angrites and compared them with the three antarctic angrites lew 86010 lew 87051 and asuka 881371 trace element variations in four of these meteorites lew 87051 asuka 881371 sahara 99555 and d orbigny indicate rapid crystallization under near closed system conditions consistent with their mineralogical and textural features all four appear to be closely related and crystallized from very similar magmas discrepancies between their bulk ree compositions and melts calculated to be in equilibrium with the major phases may be due in part to kinetic effects of rapid crystallization prior crystallization of olivine and or plagioclase may also account for the elevated parent melt composition of clinopyroxene in some of the angrites such as asuka 881371 lew 86010 also crystallized from a melt and represents a liquid composition but trace element trends in clinopyroxene and olivine differ from those of the other angrites this meteorite seems to have crystallized from a different source magma

assessment of global and regional tectonic models for faulting in the ancient terrains of mars structural landforms within parts of the heavily cratered terrain of mars were studied in order to test various models of global fracturing landforms analyzed were scarps grabens simple ridges and channel wall scarps many of these landforms predate graben formation in tharsis very few structural landforms were found to be either radial to tharsis or normal to principal stress trajectories calculated for tharsis isostasy or flexural loading these results suggest either that the structures are locally derived or if they were produced by tharsis activity that the state of stress in tharsis was different from that which produced the classic graben systems association of narrow valley networks with subdued grabens in one portion of margaritifer sinus suggests that one episode of narrow valley formation coincided with flexural loading in tharsis crustal fracturing in oxia palus that predates outflow channel formation does not correlate with tharsis tectonism and may reflect fracturing by the proposed chryse multiring basin to the north in contrast to previous studies no evidence was found for either an orthogonal martian grid or conjugate sets of shear fractures it is concluded that mars probably did not experience significant tidal despinning calculations of thermal stress within a cooling early lithosphere performed for varying lithospheric thicknesses predict large scale tensile near surface failure many of the large ridges in memnonia that predate classical tharsis centered tectonism may reflect normal faulting generated in part by thermal stress

hidden tectonism on miranda s elsinore corona revealed by polygonal impact craters abstract we mapped the spatial distribution and orientations of polygonal impact craters pics on the classical uranian satellite miranda to gain insight into the complex tectonic history of this moon pic straight rim segments in regions without visible fractures can be used to infer sub regolith and or sub resolution fracture orientations we identified and analyzed 14 pics on miranda nine of which are located on its trailing hemisphere overprinting elsinore corona and the nearby cratered terrain these pics do not follow the e w orientations of the ridges within elsinore s bounding terrain indicating that these ridges are not underlain by deep rooted at least as deep as the crater bowl sub vertical normal or strike slip faults instead the ridges may be contractional features consisting of folds and possibly thrust faults or cryovolcanic features without fault structures furthermore the pics in this region indicate that a previously undetected nw oriented fracture system is present over much of miranda s trailing hemisphere another pic somewhat distal from elsinore on miranda s anti uranus hemisphere might be associated with the nw fractures as well additionally three pics are present proximal to verona rupes on miranda s uranus facing hemisphere these pics exhibit n s orientations consistent with the surrounding fractures that make up miranda s global rift system one other pic is proximal to arden and might be associated with the arden bounding terrain identification of the nw fracture system provides new information on miranda s complex geologic history based on cross cutting relationships with miranda s ancient subdued crater population the nw fracture system is older than elsinore however this fracture system also overprints elsinore suggesting reactivation of the nw fractures after this corona had formed along with discussion of miranda s geologic history we consider the possibility that miranda has undergone true polar wander and we compare its terrains and geologic features to similar features on the saturnian satellite enceladus

polygonal impact craters in the argyre region mars evidence for influence of target structure on the final crater morphology impact craters that in plan view are distinctly polygonal rather than circular or elliptical are common on mars and other planets ohman et al 2005 their actual formation mechanism however is somewhat debatable we studied the polygonal craters of different degradational stages in the region of the argyre impact basin mars the results show that in the same areas heavily degraded moderately degraded and fresh polygonal craters display statistically similar strike distributions of the straight rim segments the fact that the strike distributions are not dependent on lighting conditions was verified by using two data sets viking and moc wa having different illumination geometries but similar resolutions in addition there are no significant differences in the amount of polygonality of craters in different degradational stages these results clearly imply that large scale polygonality is not caused by degradation but originates from the cratering process itself concurring with the findings regarding lunar craters by eppler et al 1983 the straight rims of polygonal craters apparently reflect areal fracture patterns that prevail for a geologically long time

the meteoritical bulletin no 85 2001 september meteoritical bulletin no 85 lists information for 1376 newly classified meteorites comprising 658 from antarctica 409 from africa 265 from asia 262 of which are from oman 31 from north america 7 from south america 3 from australia and 3 from europe information is provided for 11 falls dergaon dunbogan gujba independence itqiy moravka oued el hadjar sayama sologne valera and worden noteworthy non antarctic specimens include 5 martian meteorites dar al gani 876 northwest africa 480 and 817 and sayh al uhaymir 051 and 094 6 lunar meteorites dhofar 081 280 and 287 and northwest africa 479 482 and 773 an ungrouped enstatite rich meteorite itqiy a bencubbin like meteorite gujba 9 iron meteorites and a wide variety of other interesting stony meteorites including ch ck cm co cr cv r enstatite and unequilibrated ordinary chondrites primitive achondrites hed achondrites and ureilites

rare earth elements in angra dos reis and lewis cliff 86010 two meteorites with similar but distinct magma evolutions abstract rare earth element ree and selected trace element abundances were measured with an ion microprobe in individual grains of merrillite fassaite olivine kirschsteinite and plagioclase of lew 86010 the second known angrite and compared with measurements in merrillite and fassaite of angra dos reis ador the rare merrillite is highly enriched in ree the ree patterns show steep decreases from la 3 300 cl to tm 100 cl in lew 86010 and from la 1 500 cl to tm 130 cl in ador ree patterns in fassaite of both meteorites are similar although concentrations are higher in ador whereas ador pyroxenes show a high degree of equilibration the lew 86010 pyroxenes show much variation in composition the abundances of the ree zr al and ti correlate positively with fe fe mg and those of cr sc and v correlate negatively consistent with an igneous origin in a closed system pyroxene and plagioclase cores in lew 86010 formed in equilibrium with a melt whose composition is close to that of the meteorite bulk composition the ador melt had higher ree concentrations but a lower fe mg ratio than that of lew 86010 despite numerous common characteristics the two meteorites were produced in separate magmatic events involving similar but distinct processes and parent melts the unusual angrite composition is not due to purely nebular processes

a global catalogue of ceres impact craters 1 km and preliminary analysis abstract the orbital data products of ceres including global lamo image mosaic and global hamo dtm with a resolution of 35 m pixel and 135 m pixel respectively are utilized in this research to create a global catalogue of impact craters with diameter 1 km and their morphometric parameters are calculated statistics shows 1 there are 29 219 craters in the catalogue and the craters have a various morphologies e g polygonal crater floor fractured crater complex crater with central peak etc 2 the identifiable smallest crater size is extended to 1 km and the crater numbers have been updated when compared with the crater catalogue d 20 km released by the dawn science team 3 the d d ratios for fresh simple craters obviously degraded simple crater and polygonal simple crater are 0 11 0 04 0 05 0 04 and 0 14 0 02 respectively 4 the d d ratios for non polygonal complex crater and polygonal complex crater are 0 08 0 04 and 0 09 0 03 the global crater catalogue created in this work can be further applied to many other scientific researches such as comparing d d with other bodies inferring subsurface properties determining surface age and estimating average erosion rate

the permian to jurassic stratigraphy and structural evolution of the central cheshire basin new seismic reflection data reveal details of the permo triassic basin fill in the central cheshire basin the seismic stratigraphy indicates that the permian and triassic succession is far thicker than previously estimated seismic data and backstripping of the preserved permian to early jurassic sedimentary succession suggest that at least two phases of fault controlled subsidence occurred in early permian and early triassic times an intra triassic unconformity is seen on the seismic data near the top of the sherwood sandstone group this may equate with the hardegsen unconformity it is possible that this unconformity was generated by syn extensional regional uplift accompanying lithospheric thinning the gross morphology and location of the faults on the eastern margin of the cheshire basin are interpreted to be at least locally controlled by extensional re activation of a westwards dipping reverse fault possibly an extension of the pontesford linley lineament

generation of abnormal trace element abundances in antarctic eucrites by weathering processes abstract based on rees antarctic eucrites can be divided into two groups those showing normal trace element characteristics e g similar to juvinas and those showing abnormal trace element abundances many antarctic eucrite polymict eucrite and basaltic clast samples show the abnormal trace element abundances with ree patterns exhibiting positive ce anomalies sometimes negative ce anomalies positive eu anomalies and low abundances of the remainder of the rees with the lrees generally being at lower relative abundances than the hrees most samples of crystalline clasts from the polymict eucrites lew85300 lew85302 and lew85303 show the abnormal patterns while the glassy matrixes of these meteorites show normal patterns exterior samples generally show more abnormal patterns larger anomalies greater depletions than interior samples from the same meteorites comparison of all basaltic eucrite literature data combined with our data shows that positive and negative ce anomalies and positive eu anomalies are found in about 61 of antarctic eucrite analyses and are virtually unknown in non antarctic eucrite analyses further positive ce anomalies and positive eu anomalies are commonly associated with antarctic eucrites having low ree concentrations consideration of mineral melt partition coefficients shows that it is unlikely that ce anomalies are magmatic features from the hed parent body cerium anomalies on earth are generally restricted to the weathering zone where the relative ease of oxidizing ce to the 4 state allows for fractionation of ce from the 3 rees we believe the unusual ree patterns of abnormal antarctic eucrites arise from weathering effects generated in or on the antarctic ice our suggested scenario involves formation of melt water and its equilibration with the atmosphere which promotes dissolution of ree rich phosphates and oxidation of ce tetravalent ce can then be fractionated from the trivalent ree in solution the details of the weathering process are unclear and will require detailed chemical and sem investigations of eucrites for their elucidation we predict that rapidly chilled eucrites with glassy rather than crystalline mesostases will be more likely to survive the antarctic environment without alteration of their ree patterns occasional s se and k enrichments are likely due to weathering in the antarctic environment as well but these enrichments are not well correlated with ce anomalies

global and regional ridge patterns on mars abstract over 16 000 wrinkle ridges on mars have been classified mapped and digitized to provide a large computer accessible data base for analyzing regions subjected to possible compressive stresses the survey has revealed major compressive structures that occur well beyond the tharsis dominated hemisphere the large variety of affected geologic terrains indicate stresses not simply localized in the intercrater plains one major area of inferred compression occurs in the southern cratered highlands near longitude 180 w where major ridges and scarps extend over 3000 km the occurrence and orientation of many ridges are locally controlled by ancient impact basins the chryse basin in particular has an important effect on ridges in northern lunae planum the removal of all basin concentric ridges reveals however a complex global pattern although such patterns may yet be controlled by heavily degraded impact basins major regional trends also emerge that appear to require broader scale global stresses most ridges in the western hemisphere are shown to be orthogonal to three centers corresponding to tharsis 1 n 122 w syria 12 s 104 w and chryse 19 n 47 w ridges not included in these three sets are generally more random and highly localized most but by no means all ridges in the eastern hemisphere are controlled by the hellas and isidis basins a simple global grid is not yet identifiable and may not be recognizable owing to the large number of regional patterns

crystallization kinetics of alkali feldspars in cooling and decompression induced crystallization experiments in trachytic melt cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar the effect of the degree of undercooling t t liquidus t experimental and time on nucleation and crystal growth process this experimental work gives us new data about crystallization kinetics of trachytic melts and it that will be useful to better understand the natural system of campi flegrei volcanoes experiments have been conducted using cold seal pressure vessel apparatus at pressure between 30 and 200 mpa temperature between 750 and 855 c time between 7 200 and 57 600 s and redox condition close to the nno 0 8 buffer these conditions are ideal to reproducing pre and syn eruptive conditions of the campi flegrei volcanoes where the conditions pertain to the complete range of pressures temperatures and time at which the experiments were performed alkali feldspar is the main phase present in this trachyte and its abundance can strongly vary with small changes in pressure temperature and water content in the melt implying appreciable variations in the textures and in the crystallization kinetics the obtained results show that crystallization kinetics are strictly related to t time final pressure superheating t and water content in the melt t is the driving force of the crystallization and it has a strong influence on nucleation and growth processes in fact the growth process dominates crystallization at small t whereas the nucleation dominates crystallization at large t time also is an important variable during crystallization process because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations this is an important aspect to understand magma evolution in the magma chamber and in the conduit which in turn has strong effects on magma rheology

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

the east irish sea basin from caterpillar to butterfly a thirty year metamorphosis in the thirty years since the award of the first licenses the east irish sea basin has emerged as a significant hydrocarbon province this paper first lists some of the occasionally almost arbitrary events that led to the first success in the basin the discovery of the morecambe field in 1974 an attempt is made to review progress over those thirty years in certain topics namely 1 stratigraphy 2 structure 3 sedimentology 4 diagenesis the platy illite problem and others 5 uplift and inversion 6 hydrocarbon sources and types 7 east irish sea basin analogues the paper concludes by summarizing the current state of knowledge 1995 represented the thirtieth anniversary of the first award of licences in the east irish sea basin eisb the twenty sixth anniversary of the drilling of the first wells and the twenty first anniversary of the discovery of the south morecambe field it is perhaps appropriate to look back on some of the crucial events of the last thirty years and on some of the earlier concepts and interpretations with a view to seeing how these last have progressed or been found wanting undoubtedly the biggest spur to exploration was the discovery in 1974 by the british gas corporation subsidiary hgb ltd of the south morecambe field how a fledgling organization came to discover this 5 tcf gas accumulation in its first venture is itself noteworthy key events that led to this discovery might be summarized as follows 1 the wish of the government in power in 1969 to foster a national presence in the exploration of british waters by allowing the then gas council to take 100 interests in eisb blocks 2 the readiness of the gas council to exploit this opportunity 3 the unwillingness of the council s main north sea partner to participate in the proposed application with its offer of a firm well 4 the unwillingness of other majors to join the application 5 the informal gift by gulf in 1973 of the logs of wells 110 8 1 and 2 on the basis of a gentleman s agreement that hgb would likewise make data available from any wells it

the nature and origin of charon s smooth plains charon displays extensive plains that cover the equatorial area and south to the terminator on the sub pluto hemisphere observed by new horizons we hypothesize that these plains are a result of charon s global extension and early subsurface ocean yielding a large cryoflow that completely resurfaced this area leaving the plains and other features that we observe today the cryoflow consisted of ammonia rich material and could have resurfaced this area either by cryovolcanic effusion similar to lunar maria emplacement or a mechanism similar to magmatic stoping where lithospheric blocks foundered geological observations modeling of possible flow rheology and an analysis of rille orientations support these hypotheses

standardizing the nomenclature of martian impact crater ejecta morphologies the mars crater morphology consortium recommends the use of a standardized nomenclature system when discussing martian impact crater ejecta morphologies the system utilizes nongenetic descriptors to identify the various ejecta morphologies seen on mars this system is designed to facilitate communication and collaboration between researchers crater morphology databases will be archived through the u s geological survey in flagstaff where a comprehensive catalog of martian crater morphologic information will be maintained

dynamic crystallization study of barred olivine chondrules abstract chondrules which comprise the bulk of most chondrites formed very early in the solar system and understanding their formation will elucidate early nebular processes barred olivine bo chondrules make up approximately 10 of chondrules and because of their unique dendritic textures represent a restricted set of crystal growth conditions and thus formation conditions bo textures have been replicated using dynamic crystallization techniques on olivine rich chondrule starting compositions in an attempt to determine these formation conditions one composition was modeled after the average bo chondrule composition in l chondrites determined by weisberg 1987 in a survey of bo chondrules chondrule droplets are presumed to form by the melting of crystalline material bo textures developed best if the crystalline precursor was melted slightly above to tens of degrees above the liquidus temperature eliminating all ready nuclei crystalline material embryos of subcritical nuclei size in the melts provide the nuclei when they reach critical nucleus size at degrees of supercooling which stabilize the plate dendritic crystal form of olivine the plate dendrite is the basic crystal unit of the bo texture multiple plate dendrites are the common texture in experimentally produced chondrules the classic single plate dendrite bo chondrule has not been reproduced in the experiments while the preferred bo composition of weisberg does produce bo textures over a broader range of melting temperatures any composition that has olivine on the liquidus for a significant temperature interval will develop bo texture over some range of melting temperatures the range of possible melting temperatures depends on the range of compositions available that have olivine on liquidus the low end of the range will be near 1500 c and the upper end near 1700 c the range of cooling rates that produced bo textures in the compositions studied is 500 to 2300 c h

thermal histories of angrite meteorites trace element partitioning among silicate minerals in angra dos reis lewis cliff 86010 and experimental analogs we measured rare earth element ree abundances in selected silicate phases in the angrites angra dos reis ador and lewis cliff lew 86010 in order to further clarify the thermal history of ador we also carried out a preliminary experimental study designed to examine apparent ree partitioning between silicates fassaite olivine kirschsteinite and melt in synthetic analogs of angrites under disequilibrium conditions at liquidus temperatures silicates in ador are homogeneous with respect to major minor and trace elements which is consistent with the interpretation that ador underwent extensive subsolidus equilibration ree distributions in olivine and kirschsteinite in ador are similar to those in lew 86010 and are consistent with the formation of kirschsteinite by exsolution from olivine during cooling and or annealing there is no evidence for a disequilibrium trace element signature that could have been inherited from rapid cooling at liquidus temperatures this is supported by our petrographic observations of the occurrence of kirschsteinite within olivine aggregates in ador olivine kirschsteinite pairs in ador record closure temperatures around 600 650 c

magnetic lineations in the ancient crust of mars the mars global surveyor spacecraft in a highly elliptical polar orbit obtained vector magnetic field measurements above the surface of mars altitudes 100 kilometers crustal magnetization mainly confined to the most ancient heavily cratered martian highlands is frequently organized in east west trending linear features the longest extending over 2000 kilometers crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude groups of quasi parallel linear features of alternating magnetic polarity were found they are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on earth but with a much larger spatial scale they may be a relic of an era of plate tectonics on mars

the analysis of zoning in magmatic crystals with emphasis on olivine many of the observed features of zoning in magmatic phenocrysts may be due to the orientation of the section rather than inherent properties of the crystals an ideal section for the studying of zoning in magmatic crystals has two characteristics it goes through the center of the crystal and is perpendicular to one or more crystal faces using a model zoned olivine crystal it is possible to construct accurate zoning profiles for different types of section centered symmetrical and skewed the probability of obtaining a random section which passes within x of the center of a crystal is shown to be p 0 0x while the probability that a random section will be within a degrees of perpendicular to a given plane is p sin a a systematic approach to the study of zoned crystals is outlined in particular it is suggested that composition be plotted against distance cubed in order to correct for the volume versus size problem a method of determining if a given section goes through or near the center of a zoned crystal is also presented the reasoning in this work applies to other types of magmatic crystals such as pyroxenes and plagioclase

tectonic implications of mars crustal magnetism mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization presumably by cooling in the presence of an earth like magnetic field thermoremanent magnetization a new map of the magnetic field of mars compiled by using measurements acquired at an 400 km mapping altitude by the mars global surveyor spacecraft is presented here the increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the mars crust variations in the crustal magnetic field appear in association with major faults some previously identified in imagery and topography cerberus rupes and valles marineris two parallel great faults are identified in terra meridiani by offset magnetic field contours they appear similar to transform faults that occur in oceanic crust on earth and support the notion that the mars crust formed during an early era of plate tectonics

hydrocarbon prospectivity in the irish sea area insights from recent exploration of the central irish sea peel and solway basins abstract compared with the prolific success of the triassic play in the east irish sea basin eisb the lack of hydrocarbon discovery in neighbouring permo triassic basins of the irish sea has been an enigma however recent exploration of the peel solway and central irish sea basins has provided new insights into the geology of these basins and the controls upon hydrocarbon prospectivity in the irish sea area regional seismic interpretation suggests that 12 of the 15 exploration wells drilled in the basins adjacent to the eisb tested valid structural closures at top triassic reservoir level re evaluation of the irish sea petroleum system reveals that although effective reservoirs occur in the lower middle triassic ormskirk sandstone formation and evaporites in the middle upper triassic mercia mudstone group provide a regional top seal the major factor controlling hydrocarbon prospectivity is the limited presence of effective source rocks in the underlying carboniferous section a further control upon prospectivity is the timing of hydrocarbon migration from those areas where carboniferous source rocks were deposited and preserved the namurian basinal marine oil and gas prone shales which form the principal source of hydrocarbons for the triassic play in the eisb are restricted to an east west fairway extending from the eisb into the kish bank basin rocks of this age are absent from the peel and solway basins as a result of variscan uplift and erosion however palaeogeographical reconstructions based on well and outcrop data suggest that even if preserved the depositional environment was not conductive to the formation of marine oil and gas prone source rocks well and seismic data suggest that rocks of namurian age were not deposited in the central irish sea area which remained high during much of dinantian and namurian time potential source rock development in the central irish sea area is therefore limited to the westphalian section which is organically lean and dominated by intertinitic kerogens potential hydrocarbon traps in the central irish sea peel and solway basins formed largely as a result of early cretaceous tectonism and were subsequently modified by fault reactivation during tertiary uplift phases trap formation appears to postdate the most likely timing of hydrocarbon charge which this study suggests would have occurred in late triassic and jurassic time hydrocarbon entrapment in the eisb may have been favoured by limited early cretaceous uplift coupled with renewed hydrocarbon generation and re migration during early tertiary time it is concluded that the remaining prospectivity of the triassic play in the irish sea area is likely to be restricted to the proven play fairway within the eisb

degradation of selected terrestrial and martian impact craters degraded craters on mars record the cumulative effects of the complex interplay between erosion transport and deposition by competing intermittent processes fortunately impact craters are instantaneous landforms common to both the earth and mars that are characterized by similar topography and local lithofacies deconvolving the signatures of individual degradation processes around young craters on earth e g drainage scale and density changing wall slope deflation features allows calibrating degradation signatures around craters on mars and places first order limits on controlling processes and perhaps past climates such comparisons are valid despite differences in vegetation cover and time scales over which activity occurs on the two planets analysis of degradation at meteor crater lonar crater and talemzane crater reveals a terrestrial sequence of advancing degradation by fluvial eolian and mass wasting processes at meteor crater fluvial processes currently dominate primary erosion however combined deflation from the ejecta and both alluvium and colluvium make eolian processes most important in overall ejecta degradation lonar and talemzane reveal that advancing fluvial degradation is characterized by larger drainages and decreasing wall slopes at talemzane interior drainages breach the rim crest and pirate headward regions of exterior drainage basins when comparable degradation signatures resolvable at viking resolutions are used in selected areas on mars important differences from the terrestrial sequence are revealed all morphologies of craters in southern ismenius lacus sil are typically incised by small valleys with low drainage densities display rare alluvial deposits possess walls sloped at the angle of repose and are sometimes partially covered by airfall deposit remnants analogy with the terrestrial craters indicates signatures associated with advancing fluvial degradation should be readily detected on mars together with other differences between degraded craters on the two planets the paucity of fluvial signatures in sil implies that mass wasting and eolian activity predominated degradation of martian craters preserved since the hesperian crater statistics for sil indicate most craters were degraded during geologically brief intervals in the noachian and mid hesperian therefore fluvial signatures preserved in sil imply any runoff since the hesperian was short lived locally debouched and unrelated to rainfall

mn fe systematics in major planetary body reservoirs in the solar system and the positioning of the angrite parent body a crystal chemical perspective a revised diagram plotting the fe mn ratio of pyroxene and olivine verses the anorthite content of plagioclase indicates that the angrite parent body originated from a solar system reservoir with a similar mn fe signature to that from which the earth was derived the major difference in terrestrial and angritic basalts is the extreme volatile depletion particularly na and k in the latter considerable research and publication has been focused on angrite meteorites which are among the oldest objects in the solar system 4 56 ga keil 2012 these meteorites include a silica undersaturated mafic mineral assemblage the identity and location of their parent body is still unknown and widely debated our new work on angrites sah 99555 lew 86010 nwa 10463 lew 87051 and angra dos reis focused on olivine and is interpreted in the context of existing pyroxene and plagioclase data sets this paper improves the mn fe in olivine and anorthite content of plagioclase tool for planetary parentage specifically targeted at finding members of the angrite group of meteorites

the cooling history of the lewis cliff 86010 angrite as inferred from kirschsteinite lamellae in olivine olivine in the angritic meteorite lewis cliff lew 86010 contains abundant exsolution lamellae of kirschsteinite compositional gradients adjacent to the interface in both host and lamellae were formed by diffusion of chemical components into and out of the lamellae during cooling and growth we have compared these gradients with compositional profiles calculated from diffusion and heat flow equations to estimate the cooling rate and burial depth of the sample the resulting values for cooling rate and burial depth depend on which values are used for the diffusion rate of ca in olivine and how measured diffusivities are extrapolated to the lower temperatures at which the lamellae grew if the highest diffusion coefficients are used the cooling rates obtained from seven different lamellae range from 30 to 52 c year with an average of 42 c year and burial depths assuming an overburden with a thermal diffusivity typical of solid rock range from 14 to 17 m with an average of 15 m if the lowest reasonable diffusion coefficients are used the cooling rates range from 1 4 to 2 2 c year with an average of 1 7 c year and the depths range from 68 to 83 m with an average of 75 m for the highest ca diffusivities details of the compositional profiles near the olivine kirschsteinite interface suggest that continuous cooling was greatly accelerated at a temperature near 600 700 c the simplest physical explanation for such an acceleration is excavation of the sample from its original burial depth by an impact event if ca diffusivities are lower a two stage cooling history is not required

dynamic crystallization experiments on the angra dos reis achondritic meteorite abstract with the recent discovery of angrite meteorites in the antarctic collection which clearly have a melt origin the hypothesis that angra dos reis ador itself is a cumulate is being reconsidered a recent model suggests that ador is a porphyry not a cumulate dynamic crystallization experiments have been conducted to determine whether a melt of ador composition can crystallize with porphyritic textures at reasonable cooling rates under appropriate heterogeneous nucleation conditions the nucleation conditions were varied by melting the starting material at different degrees of superheat porphyritic textures were produced if nuclei are absent from the melt and embryos are few in number if some degree of supercooling is developed while the embryos are growing to critical size the phenocrysts grow with skeletal shapes fassaitic pyroxene grows over a large temperature interval before the appearance of kirschsteinite spinel and a pyroxferroite like phase alternatively if the nucleation density is high and irregularity distributed at the beginning of cooling a granular texture very similar to ador is produced in this case the large fassaites would grow later and poikilitic enclose the early formed granular fassaite the phases present and their composition depend on the oxygen fugacity and a complex redox history for ador can be inferred olivine is not present in quantities compatible with ador and kirschsteinite appears to have been stabilized kinetically in the experiments while a volcanic or hypabyssal history for ador is possible based on the experiments the presence of olivine particularly as a phenocryst remains unexplained

asymmetric signatures in simple craters as an indicator for an oblique impact direction in oblique impacts with an impact angle under 45 the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector this bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim which remains circular for impact angles down to 10 15 the possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta the overturned flap and the crater rim was explored with the intention of deducing an impact vector a model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim this model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique tooting crater on mars a new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center termed concentric deviation the method was applied to field data from wolfe creek crater western australia bedding in the overturned flap implies an impactor striking from the east which refines earlier publications while bedding from the inner rim shows a correlation with the crater rim morphology

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

an altitude normalized magnetic map of mars and its interpretation techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by mars global surveyor a global distribution of radial br magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude normalized map of br at 200 km the observations are well represented by a potential function of crustal origin consistent with a rema nent origin for the martian magnetic features the correla tion between the 40546 br observations andbr calculated from the magnetization solution at observation locations is 0 978 for a magnetization distribution connedto a 50 km layer calculatedmagnetizationsrangefrom 22to 17a m we see correlations with tectonics that were only hinted at in earlier maps magnetic features appear to be truncated against valles marineris and ganges chasma suggestive of a major change in crustal properties associated with fault ing

the structure and evolution of ancient impact basins on mars the number of martian impact basins once thought deficient owing to a low number of impactors has been significantly increased through the identification of structural imprints that remain even after nearly complete erasure of basin massifs five ancient multiringed impact basins were selected for detailed study and exhibit a systematic pattern of rejuvenation of the multiring plan through endogenic modification concentric zones of unstable terrain are believed to reflect deep seated impact related fractures that have localized regional igneous processes in a manner analogous with the basin controlled distribution of basalts on the moon most martian outflow channels were found to originate along the unstable ring zones hydrothermal melting and temporary confinement of channel source materials at sites analogous to lunar sinuous rille source regions are proposed to play an important role in the distribution of martian outflow channels ancient basins also can be delineated by the concentric pattern of topographically and structurally controlled narrow valley networks that are particularly abundant along the outer rings where basin ejecta deposits occur remnant massifs and scarps commonly exhibit extensive furrowing by narrow valley networks perhaps due to a different paleoclimate or sapping during a period of geothermal melting of trapped ices the systematic patterns of valley networks stable and unstable annuli scarps and massifs provide bases for interpreting impact structure in general and for recognizing multiringed structures elsewhere the chryse basin in particular is proposed to represent a major multiringed structure that controlled the formation and distribution of eastern valles marineris and the chaotic terrains of margaritifer sinus in response to volcanism tectonism related to tharsis we conclude that 1 mars is probably not deficient in large body impactors 2 the multiring patterns of basins represent deep seated fracture zones that can be reexposed through subsequent volcanism hydrothermal activity sapping and differential erosion 3 many martian terrains and features e g chaotic terrains and outflow channels are controlled by ancient basins and 4 the resurrection and destruction of impact basins reveal an important third dimension depth for interpreting impact basin formation

distinct 238 u 235 u ratios and ree patterns in plutonic and volcanic angrites geochronologic implications and evidence for u isotope fractionation during magmatic processes angrites are differentiated meteorites that formed between 4 and 11 myr after solar systemformation when several short lived nuclides e g 26 al 26 mg 53 mn 53 cr 182 hf 182 w were still alive as such angrites are prime anchors to tie the relative chronology inferred from these short lived radionuclides to the absolute pb pb clock the discovery of variable u isotopic composition at the sub permil level calls for a revision of pb pb ages calculated using an assumed constant 238 u 235 u ratio i e pb pb ages published before 2009 2010 in this paper we report high precision u isotope measurement for six angrite samples nwa 4590 nwa 4801 nwa 6291 angra dos reis d orbigny and sahara 99555 using multi collector inductively coupled plasma mass spectrometry and the irmm 3636 u double spike the age corrections range from 0 17 to 1 20 myr depending on the samples after correction concordance between the revised pb pb and hf w and mn cr ages of plutonic and quenched angrites is good and the initial 53 mn 55 mn 0 ratio in the early solar system ess is recalculated as being 7 1 10 6 at the formation of the solar system the error bar incorporates uncertainty in the absolute age of calcium aluminum rich inclusions cais an uncertainty remains as to whether the al mg and pb pb systems agree in large part due to uncertainties in the pb pb age of cais a systematic difference is found in the u isotopic compositions of quenched and plutonic angrites of 0 17 a difference is also found between the rare earth element ree patterns of these two angrite subgroups the 238 u values are consistent with fractionationduring magmatic evolution of the angrite parent melt stable u isotope fractionation due to a change in the coordination environment of u during incorporation into pyroxene could be responsible for such a fractionation in this context pb pb ages derived from pyroxenes fraction should be corrected using the u isotope composition measured in the same pyroxene fraction

dike indicators in the hadriaca patera promethei terra region mars abstract the eastern hellas basin rim has been highly modified by a multitude of both volcanic and fluvial processes it is covered by a large part of the circum hellas volcanic province consisting of the volcanic plains of hesperia planum and western promethei terra as well as the two central volcanoes hadriaca and tyrrhena patera and their associated lava flows large outflow channels of dao niger harmakhis reull and teviot valles dissect the wrinkle ridged plains postdating the major volcanic events the origin of these outflow channels is still ambiguous as the mobilization mechanism for the underground volatile reservoirs has yet to be revealed due to the proximity of the volcanic structures many previous studies have suggested via models that the released ice or ground water was mobilized due to endogenic heating from magmatic activity in the region to find evidence of this we conducted a systematic search for any magmatic intrusions at 50 m pixel resolution this paper provides detailed mapping of several areas where we found features which we interpret to be dikes and shows the overall distribution of such structures in the eastern hellas rim region the hadriaca patera volcano flanks exhibit a concentric pattern of linear ridges and fractures indicative of a vast ring dike system the dikes occupy mostly the area near the summit and the hellas facing flank extending there for over 400 km away from the caldera a hadriaca radial dike swarm pattern is observed extending southwards from the volcano composed of isolated parallel linear ridge group outcrops and cut by the valles canyons an additional isolated dike ridge group exists south of lower reull valles possibly controlled by the tyrrhena patera volcano some 1300 km away all dikes within the study region are roughly late hesperian in age either predating the valles formation or being roughly concurrent with them in total 500 dike segments were identified with a combined length of 2500 km since 90 of the dikes are ridges they suggest that the entire region has undergone large scale erosion of at least a few tens of meters to expose the dikes the dike orientations were most probably controlled by the volcanic centers and possibly partly by a hellas concentric fracture pattern

impact cratering on slopes abstract the majority of impact craters have circular outlines and axially symmetric morphologies deviation from crater circularity is caused by either target heterogeneity a very oblique impact incidence post impact deformation or by topography here we investigate the effect of topography on crater formation and systematically study impact cratering processes on inclined hillsides up to 25 slope utilizing analogue experiments a spring driven air gun mounted in a vertical position shoots into three different types of granular bulk solids two sorts of glass beads quartz sand to emulate impact cratering on slopes in all 170 experiments were conducted the transient crater develops roughly symmetrically perpendicular to the slope plane resulting in higher ejection angles uphill than downhill when measured with respect to a horizontal plane craters become increasingly elliptical with increasing slope angle at slope angles close to angle of repose of the respective bulk solids aspect ratios of the craters reach 1 7 uphill downhill cross sections become increasingly asymmetric the depth diameter ratio of the craters decreases and the deepest point shifts downhill with increasing slope angle mass wasting is initiated both in the uphill and downhill sectors of the crater rim for steep slopes the landslides that emanate from the uphill rim can overshoot the crater cavity and superpose the downhill crater rim in a narrow tongue mass wasting initiated at the downhill sector forms broader and shallower tongues and is triggered by the deposition of ejecta on the inclined slope our experiments help to explain asymmetric crater morphologies observed on asteroids such as ceres vesta lutetia and also on mars

chemical alteration and ree mobilization in meteorites from hot and cold deserts the effects of terrestrial weathering on ree mobilization are evaluated for a variety of uncommon meteorites found in antarctica and in hot deserts the meteorites analyzed include 7 non cumulate eucrites 10 shergottites 3 nakhlites 2 lunar meteorites 4 angrites 10 acapulcoites 1 winonaite and 1 brachinite in situ concentration measurements of lanthanides and selected other minor and trace elements were made on individual grains by secondary ion mass spectrometry sims in antarctic meteorites oxidation converts ce 3 to ce 4 which is less soluble than the trivalent ree resulting in ce anomalies the mineral most affected is low ca pyroxene however not all grains of a given mineral are and distinct analyses of a single grain can even yield ree patterns with and without ce anomalies the effect is most pronounced for antarctic eucrites in which ce anomalies are observed not only in individual minerals but also in whole rock samples although ce anomalies are observed in meteorites from hot deserts as well the most characteristic signs of chemical alteration in this environment are a lree enrichment with a typical crustal signature as well as sr ba and u contaminations these can modify the whole rock ree patterns and disturb the isotope systematics used to date these objects the lree contamination is highly heterogeneous affecting some grains and not others of a given mineral mainly olivine and low ca pyroxene the two minerals with the lowest ree concentrations the major conduit for ree movement is through shock induced cracks and defects and the highest levels of contamination are found in altered material filling such veins and cracks meteorites that experienced low shock levels and those that are highly recrystallized are the least altered copyright 2003 elsevier ltd

rim uplift and crater shape in meteor crater effects of target heterogeneities and trajectory obliquity 1 we have analyzed the rim structure of meteor crater arizona in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters for this purpose we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults we found that rim uplift correlates with the crater shape and increases in the corners of the crater the two main mechanisms of differential uplift are the formation of horizontal interthrust wedges leading to the doubling of strata in the rim and radial corner faults or tear faults that vertically displace bedrock the development of meteor crater s quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets additionally we infer the impact direction with a newly developed technique the two corners model and review the arguments in favor of an oblique trajectory while the data set is ambiguous several indicators suggest an impact direction from the nnw we conclude that oblique impacts should have an effect on early cratering and excavation flow whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

reassessing the origin and chronology of the unique achondrite asuka 881394 implications for distribution of 26al in the early solar system abstract the achondrite asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically chemically and texturally distinct from them previous u pb chronology shows that it is very old forming within the first 0 8 ma of the formation of calcium aluminum rich inclusions cais however the age difference between asuka 881394 and other very old solar system materials cais quenched angrites measured with the 26al 26mg and 53mn 53cr extinct radionuclide chronometers and 207pb 206pb chronometer is not the same this could be interpreted to reflect heterogeneity in the distribution of 26al and 53mn in the early solar system given the significant implications for the early solar system chronology if 26al and 53mn are indeed heterogeneously distributed in this study we further investigate the origin of asuka 881394 and the apparent age discordance between short lived and absolute chronometers by focusing on measurement of its e54cr composition renewed measurements of the absolute pb pb age and new high precision measurements of its 26al 26mg systematics new cr isotope data places additional constraints on the origin of asuka 881394 its e 54cr value of 0 37 0 10e is resolvable outside of uncertainty from hed meteorites 0 72 0 10e reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body unlikely to be 4 vesta new pb pb analyses combined with using a directly measured 238u 235u ratio for age calculation result in a recalculated pb pb age of 4564 95 0 53 ma 1 5 ma younger than the previously reported age with this age asuka 881394 remains one of the oldest known achondrites in our solar system new high precision 26al 26mg data produce an initial 26al 27al ratio of 1 48 0 12 10 6 within error of previous data this ratio corresponds to an al mg age of 4563 69 0 36 ma or 4564 83 0 21 ma relative to cais and the d orbigny angrite respectively thus depending on which age anchor is used the 26al 26mg age is either 1 26 ma or 0 12 ma younger than the new pb pb age the latter being unresolved within analytical uncertainty though a potential age discrepancy between 26al 26mg and pb pb could be a result of heterogeneous distribution of 26al we demonstrate with our new high precision mg isotope data in conjunction with petrographic evidence that the mg isotope system has been disturbed in asuka 881394 we suggest that the 26al 26mg system closed to diffusion after the u pb system either due to slow cooling on the parent body or low grade metamorphic re equilibration of mg thus we can satisfactorily explain the observed age discrepancy between 26al 26mg and u pb systems in asuka 881394 without invoking heterogeneous distribution of 26al 27al ratio in the early solar system comparison of the asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26al 27al ratio because these achondrites have significant differences in their e54cr 17o systematics which could be indicative of location within the early protoplanetary disk it is consistent with homogeneous distribution of 26al in the early solar system angrites remain an outlier either because they evolved from a source with a lower 26al 27al ratio or because their 26al 26mg or pb pb data are problematic in either case this suggests that basaltic angrites are questionable as the age anchor for the entire solar system as a whole and other very old well preserved achondrites should be considered for that role

polygonal impact craters in argyre region mars implications for geology and cratering mechanics impact craters are not always circular sometimes their rims are composed of several straight segments such polygonal impact craters pics are controlled by pre existing target structures mainly faults or other similar planes of weakness in the argyre region mars pics comprise 17 of the total impact crater population 7 km in diameter and pics are relatively more common in older geologic units their formation is mainly controlled by radial fractures induced by the argyre and ladon impact basins and to a lesser extent by the basin concentric fractures also basin induced conjugate shear fractures may play a role unlike the pics ridges and graben in the argyre region are mostly controlled by tharsis induced tectonism with the ridges being concentric and graben radial to tharsis therefore the pics primarily reflect an old impact basin centered tectonic pattern whereas tharsis centered tectonism responsible for the graben and the ridges has only minor influence on the pic rim orientations according to current models of pic formation complex pics should form through a different mechanism than simple pics leading to different orientations of straight rim segments however when simple and complex pics from same areas are studied no statistically significant difference can be observed hence in addition to enhanced excavation parallel to the strike of fractures simple craters and slumping along the fracture planes complex craters we propose a third mechanism involving thrusting along the fracture planes this model is applicable to both simple and small complex craters in targets with some dominating orientations of structural weakness

phase equilibrium constraints on angrite petrogenesis parameterizations of liquidus boundaries and solid solution in the cmas fe system shi 1992 have been employed to depict the liquidus equilibria relevant to the petrogenesis of angrites angrites are basaltic achondrites characterized by highly aluminous augite fassaite intermediate mg fe olivine and late stage cafe olivine kirschsteinite two important features of the equilibria on the olivine liquidus surface relevant to angrite petrogenesis are 1 the presence of a thermal divide on the ol aug plag liq boundary curve which separates the compositions of source materials that produce low silica angritic melts that crystallize highly aluminous augite from those that produce higher silica melts with tholeiitic to eucritic crystallization patterns and 2 the change in the pseudo invariant point on the low silica side of the thermal divide from a plagioclase peritectic involving spinel ol aug plag sp liq at high to intermediate mg mg mg fe to two pseudo eutectics involving kirschsteinite ol aug plag kir liq and ol kir plag sp liq at low mg the fassaitic aluminous augite pyroxene composition in angra dos reis ador the presence of minor green spinel and the absence of primary kirschsteinite prinz et al 1977 indicate that crystallization of the ador parental liquid was governed by the intermediate mg set of equilibria such that following crystallization of ol aug plag the plagioclase reacted completely at the plagioclase peritectic with the interstitial liquid which subsequently crystallized beyond the plagioclase peritectic onto the ol aug sp liquidus boundary curve the ador bulk composition is consistent with trapping 10 of the parental liquid in a cumulate with cotectic proportions of fassaite and olivine lewis cliff lew 86010 crystallized from a liquid with mg similar to that of ador but on the ol plag cotectic closer to the thermal divide such that the first pyroxene to crystallize had much lower al content than that of ador in the late stages of crystallization the 86010 residual liquid and that of lew87051 encountered the low mg set of equilibria involving kirschsteinite these relationships require either a higher degree of melting for the 86010 parent magma or source region different than ador s these relationships are also consistent with compositionally dependent ree partition coefficients between fassaite and the ador liquid being as much as 1 5 2 times higher than those for the 86010 liquid at the onset of pyroxene crystallization the combination of a trapped liquid component higher partition coefficients and smaller degrees of melting help to explain the observation that ador an apparent cumulate has ree concentrations twice as high as those in 86010 mittlefehdlt and lindstrom 1990 an apparent chilled liquid the absence of a strong negative eu anomaly in the ador parent liquid however requires relatively high degrees of partial melting to eliminate plagioclase in the source region resorption of plagioclase at the peritectic eliminates the eu anomaly that develops during crystallization so ultimately different source regions are required progressive iron loss from devolatilized primitive chondrites allende murchison produces source regions capable of producing a wide range of melt compositions with angritic to eucritic crystallization behavior the compositions of carbonaceous and ordinary chondrite provide a similar range of potential source region compositions however primitive chondrite fe source regions that produce angrite like melts have mg that is too low whereas chondrite fe sources that have mg sufficiently high to yield the mg in angrite minerals have too much silica or orthopyroxene component to yield angrite like liquids no single group of meteorites fe simultaneously satisfies the constraints of mg and silica component however mixtures of fe depleted chondrite plus a low silica component similar to ca al rich inclusions cais can satisfy the constraints the absence in angrites of 48ca and 50ti anomalies typical of cais lugmair and galer 1992 suggests that the low silica component was not simply an enrichment of cais but was the result of direct accretion of high temperature condensate grossman 1972 into sizable thermally shielded planetesimals thus angrites cryptically record mixing of planetesimal sized heterogeneities in the early solar system

excess hafnium 176 in meteorites and the early earth zircon record 1 the long lived176lu to 176hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation detrital hadean zircons 3 8 gyr from the jack hills metasedimentary belt of western australia record extremely enriched hf isotope signals suggesting early extraction of a continental crust 4 5 gyr but fail to identify a prevalent complementary depleted mantle reservoir suggesting that crust formation processes in the early earth were fundamentally distinct from today however this conclusion assumes that the hf isotope composition of bulk chondrite meteorites can be used to estimate the composition of earth prior to its differentiation into major silicate reservoirs namely the bulk silicate earth bse we report a176lu 176hf internal mineral isochron age of 4869 34 myr for the pristine sah99555 angrite meteorite this age is 300 myr older than the age of the solar system confirming the existence of an energetic process yielding excess 176hf in affected early formed solar system objects through the production of the 176lu isomer t1 2 3 9 hours thus chondrite meteorites contain excess 176hf and their present day composition cannot be used to infer the lu hf parameters of bse using a revised bse estimate based on the sah99555 isochron we show that earth s oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as 4 3 gyr in earth s history with little evidence for the existence of continental crust prior to 4 4 gyr this new view suggests continuous juvenile crustal growth and recycling throughout the hadean and archean eras perhaps analogous to modern plate tectonics

sources of shape variation in lunar impact craters fourier shape analysis r mode factor analysis of fourier harmonics that describe the shape in plan view of 716 large diameter 15 km nearside lunar craters shows that two factors explain 84 3 of shape variance observed in the sample factor i accounts for 68 2 of the sample variance and describes moderate scale roughness defined by harmonics 7 through 10 shape variation described by these harmonics is related to surficial lunar processes of degradation that modify crater shape in plan dominant among these processes are ejecta scour from large impact events and ongoing aging factor ii accounts for 16 1 of the observed shape variance and describes polygonal shape elements related to harmonics 2 3 4 and 6 variation in these harmonics is tied to variables that distort the spherical symmetry of crater forming processes the dominant contributor among these variables is the nature of geologic structural patterns in impacted material unlike transient features described by factor i polygonal shape elements described by factor ii do not change appreciably with time the permanence of these features and their relation to lunar geologic structure suggest that the shape of old craters carries the imprint of geologic structural relationships present in early lunar crust

sequence and mechanisms of deformation around the hellas and isidis impact basins on mars the distribution and age of early volcanic and tectonic features surrounding the itellas and isidis impact basins are shown to fit a four stage sequence first concentric canyons form outside the basin boundary scarp at or near the time of basin formation followed shortly thereafter by radial troughs extending beyond the boundary scarp after a hiatus concentric graben develop inside the basin within the massif ring finally soon after graben formation widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin three models of basin centered deformation are compared io hese observed deformation events elastic flexure due to loading elastic flexure due to uplift and impact fracture a central load of basin fill can account for concentric fractures by elastic flexure around the load formation of the distant concentric canyons however requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on mars and the moon although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values flexure still fails to account for the large canyon widths observed impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse however is consistent with the three key observational constraints for the distant canyons their age width and distance from the basin as the basin then undergoes isostatic uplift soon after impact elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs later after a long period of basin infilling elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring the localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to mars

reconciliation of the excess 176 hf conundrum in meteorites recent disturbances of the lu hf and sm nd isotope systematics abstract the long lived 176 lu 176 hf and 147 sm 143 nd radioisotope systems are commonly used chronometers but when applied to meteorites they can reveal disturbances specifically lu hf isochrons commonly yield dates up to 300 myr older than the solar system and varying initial 176 hf 177 hf values we investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible minerals scatter away from isochron regressions for both meteorite classes with low hf phases such as plagioclase and olivine typically being most displaced above or left of reference isochrons relatively hf rich pyroxene is less disturbed but still to the point of steepening lu hf errorchrons using our lu hf and sm nd data we tested various hf and lu redistribution scenarios and found that decoupling of lu hf from 176 hf 177 hf must postdate the accumulation of significant radiogenic 176 hf therefore early irradiation or diffusion cannot explain the excess 176 hf instead disturbed meteorite isochrons are more likely caused by terrestrial weathering contamination or common laboratory procedures the partial dissolution of phosphate minerals may predominantly remove rare earth elements including lu leaving relatively immobile and radiogenic hf behind robust lu hf and improved sm nd meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic hf and silicate hosted terrestrial contaminants without disturbing parent daughter ratios

petrology and geochemistry of d orbigny geochemistry of sahara 99555 and the origin of angrites petrology and geochemistry of d orbigny geochemistry of sahara 99555 and the origin of angrites

geochemistry and genesis of the angrites the angrites angra dos reis lew86010 and lew87051 are petrologically and compositionally similar achondrites all angrites have high feomno ratios of 80 94 and very low ci normalized na sm ratios of 0 001 0 003 high abundances of oxidized fe and low abundances of moderately volatile na most likely resulted from parent body processes such as magmatic outgassing rather than nebular processes all angrites have fractionated ca al ratios with angra dos reis exhibiting the most extreme ratio 3 1 ci this ca al fractionation may be due to small amounts of residual spinel in the source region after removal of the angrite melts or to crystallization of spinel from precursor melts for lew86010 and lew87051 the fractionated ca al ratios are not caused by the presence of significant cumulus material for angra dos reis cumulus material may be the cause of the high ca al ratio refractory element ca eu al sc la ce sm tb yb lu ti hf th and ta abundances of lew86010 and lew87051 show similar patterns while angra dos reis has both greater enrichments in these elements and more fractionated patterns compositional and petrologic constraints indicate that lew86010 and lew87051 are related via olivine control the refractory element abundances and mg of lew86010 can be approximated by removal of olivine from lew87051 suggesting that lew86010 may be a residual melt from a lew87051 like precursor alternatively lew87051 may have formed via olivine accumulation from a lew86010 like precursor the origin of the unusual refractory element pattern in angra dos reis remains obscure the differences between the lew86010 lew87051 duo and angra dos reis suggest that either the angrite parent body was heterogeneous or that angra dos reis was formed on a separate parent body based on feomno ratios and normative mineralogies the angrite parent body ies may be similar in bulk composition to one of the carbonaceous chondrite groups particularly ci cm co

clinopyroxene ree distribution coefficients for shergottites the ree content of the shergotty melt abstract ree distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred shergotty intercumulus liquid by in situ analysis with the electron microprobe using samples doped to per cent concentration levels these coefficients exhibit a strong positive correlation with pyroxene wollastonite content recommended values for augites similar in composition to the shergotty cumulus augite cores wo32en48 are d la 0 023 d ce 0 039 d nd 0 10 d sm 0 17 d eu 0 16 d gd 0 20 d yb 0 29 and d lu 0 30 recommended values for pigeonites similar in composition to the shergotty cumulus pigeonite cores wo12en58 are d la 0 002 d ce 0 004 d nd 0 019 d sm 0 031 d yb 0 13 and d lu 0 13 these values resemble distribution coefficients measured for terrestrial basalts of higher mg fe than the shergotty melt rather than those of more evolved siliceous rocks of similar mg fe to the shergotty melt thus it appears that mg fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes ree abundances were computed for the shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions these computed abundances display lree depletion such lree depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the lree enriched patterns required for a two stage sm nd evolution history this result implies that either 1 the melting process was complex e g continuous melting 2 the source region had a more complex history than that of the simple 2 stage model or 3 shergotty does not satisfy the closed system assumption of the calculation and has suffered post magmatic lree enrichment

exsolution lamellae of kirschsteinite in magnesium iron olivine from an angrite meteorite exsolution phenomena of kirschsteinite cafesi04 in olivine have been studied by single crystal x ray diffraction techniques and scanning electron microscopy sem of oriented polished thin sections pts of three single crystals separated from the antarctic angrite lew86010 supplemented by micro area x ray diffraction with the laue method mxl by synchrotron radiation sr for pts of a rock chip of lew8601o the cell dimensions of the host olivine and exsolved kirschsteinite are a 4 79 3 b 10 39 5 and c 6 06 3 a and a 4 87 5 b 11 14 10 and c 6 36 5 a respectively from the precession photos the pts of olivine single crystals oriented parallel to 100 show exsolution lamellae of kirschsteinite up to i 0 m in width the two sets of lamellae are symmetrically related and parallel to 031 and 031 electron microprobe analysis gave si02 33 1 ti02 0 07 al203 0 03 feo 49 4 mno 0 61 mgo 13 4 cao 2 2 cr203 0 02 v203 0 01 nio 0 05 sum 99 4 wt for the host olivine and si02 33 3 ti02 0 03 feo 31 5 mno 0 39 mgo 5 4 cao 28 5 cr203 0 02 nio 0 05 sum 99 2 wt for the exsolved kirschsteinite the results from mxl for the olivine crystals on the rock pts are compatible with the observation on the single crystals that the lamellae are parallel to 031 and 031 the 031 and 031 planes have been known to be twin planes for olivine and the twinning is by reticular pseudomerohedry based on a quadruple lattice although other reported exsolved precipitates in meteoritic olivines exist as inclusions kirschsteinite in lew86010 olivine takes the form of lamellae our explanation is that lew86010 olivine is fe rich and that lamellar precipitates are more easily formed than inclusions because exsolution lamellae along 031 in fe rich olivine maintain lattice coherency