grain size sensitive creep of plagioclase accompanied by solution precipitation and mass transfer under mid crustal conditions abstract we report here on a study of three deformed granitoids two mylonites and an ultramylonite from the inner ductile shear zone of the ryoke metamorphic belt sw japan monophase layers composed of quartz plagioclase or k feldspar are present in all samples the plagioclase rich layers consist of grains 6 10 m in size and sometimes include patchy k feldspar and quartz indicating solution precipitation in the mylonite the fine grained plagioclase is mainly an23 25 and the composition of plagioclase porphyroclast is an21 39 without any significant maximum the an compositions together with textural observations indicate that fine grained plagioclase nucleated from solution with mass transfer during deformation in the ultramylonite fine grained plagioclase is widely changed to be an15 37 indicating that the grain size reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution precipitation process which results in the composition concentrated around an30 in all samples the crystallographic orientations of fine grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts grain size sensitive creep occurred during rock deformation subsequent to the process of solution precipitation that involved mass transfer via fluids

thermal and impact histories of reheated group iva ivb and ungrouped iron meteorites and their parent asteroids the microstructures of six reheated iron meteoritesotwo iva irons maria elena 1935 fuzzy creek one ivb iron ternera and three ungrouped irons hammond babb s mill blake s iron and babb s mill troost s iron owere characterized using scanning and transmission electron microscopy electron probe microanalysis and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids maria elena and hammond were heated below approximately 700 750 c so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite both meteorites retained a record of the original widmanstitten pattern the other four which show no trace of their original microstructure were heated above 600 700 c and recrystallized to form 10 20 lm wide homogeneous taenite grains on cooling kamacite formed on taenite grain boundaries with their close packed planes aligned formation of homogeneous 20 lm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 c or approximately 1 h at 1300 c all six irons contain approximately 5 10 lm wide taenite grains with internal microprecipitates of kamacite and nanometer scale m shaped ni profiles that reach approximately 40 ni indicating cooling over 100 10 000 yr un decomposed high ni martensite a2 in taeniteothe first occurrence in ironsoappears to be a characteristic of strongly reheated irons from our studies and published work we identified four progressive stages of shock and reheating in iva irons using these criteria cloudy taenite m shaped ni profiles in taenite neumann twin lamellae martensite shock hatched kamacite recrystallization microprecipitates of taenite and shock melted troilite maria elena and fuzzy creek represent stages 3 and 4 respectively although not all reheated irons contain evidence for shock it was probably the main cause of reheating cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays if the reheated irons that we studied are representative the iva irons may have been shocked soon after they cooled below 200 c at 4 5 gyr in an impact that created a rubblepile asteroid with fragments from diverse depths the primary cooling rates of the iva irons and the proposed early history are remarkably consistent with the pb pb ages of troilite inclusions in two iva irons including the oldest known differentiated meteorite blichert toft et al 2010

microstructures and c axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites abstract quartzites sheared in low metamorphic grade shear zones that extend along the contact with migmatitic gneissic complexes in the quadrilatere ferifero region southeastern brazil show quartz microstructures indicating deformation by dissolution dissolution occurred adjacent to the foliation planes preferentially in quartz porphyroclasts oriented with their 0001 planes parallel to the foliation dissolution worked as an efficient mechanism to remove porphyroclast edges smoothing foliations and enabling strain accommodation both dissolution and crystal plasticity were competitive crystallographically controlled processes operating in different domains of the evolving microstructure dissolution occurred along the foliation planes while isovolume crystal plasticity and accompanying recrystallization operated essentially in the quartzose domains between folia phyllonites formed in the most strained domains of the shear zones fine grained quartz grains in phyllonites show uncommon c axis fabrics c axis oriented at low angles to the stretching lineation which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution domainal c axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer strengthening c axis maxima at low angles with the stretching lineation at the expense of crystal plastic processes during progressive phyllonitization

development of single crystal σ shaped quartz porphyroclasts by dissolution precipitation creep in a calcite marble shear zone abstract within a greenschist facies calcite marble shear zone isolated quartz grains change shape across the shear zone profile whereas quartz grains have a spherical to elongated shape in the coarse grained marble protolith and protomylonite they are asymmetric shaped porphyroclasts with wedge shaped appendages wings in ultramylonites in all cases quartz grains are single crystals they are sometimes twinned but never recrystallized stress induced dissolution precipitation creep is the favourable shape controlling process during deformation providing a relatively undeformed core truncation of pre existing cathodoluminescence cl patterns and syntaxial precipitation of wings the necessary fluids may have been released from fluid inclusions during dynamic recrystallization of the calcite matrix the development of shape is not related to the crystallographic orientation of the quartz porphyroclasts crystallographic orientation analysis by electron backscatter diffraction ebsd and cl analyses exclude crystal plasticity as a shape controlling mechanism of quartz grains during mylonitisation however in ultramylonites quartz clasts have a strong crystallographic preferred orientation cpo with c axes sub parallel to the shear direction this fabric is uncommon for quartz under greenschist facies conditions it might be explained by a strengthening of a pre existing weak cpo during mylonitisation by rigid particle rotation of elongated quartz grains

thermal history of the h chondrite parent body implications for metamorphic grade and accretionary time scales abstract multiple temperature age constraints for eight h chondrite samples have been used to provide insight into the thermal history of their parent body through combination with numerical models of thermal evolution assuming internal heating by 26al and conductive cooling the effect of spreading accretion out over time is the principal focus of this work a wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo chronological data we conclude that the h chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric onion shell internal structure that body had a radius no larger than 130 km and accretion most probably took place over a time interval on the order of 0 0 0 2 myr approximately 2 myr after cai condensation in any case the time interval of accretion is unlikely to have been more than 0 5 myr supporting evidence in favour of rapid accretion possibly through reassembly of the fragments of an earlier generation of bodies furthermore the h chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism indicating that preservation of the onion shell structure is unlikely the presence of an insulating regolith does not modify this conclusion as appropriate thermal histories for the three h6 samples considered cannot be reproduced at depths near the surface asteroid 6 hebe may be the parent body of the h chondrites but the high bulk density of the latter is difficult to reconcile with a rubble pile structure of pure h chondrite material finally optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types 800 1000 and 1140 k for the h3 4 h4 5 and h5 6 boundaries respectively in detail the type 6 samples studied here are inferred to have experienced a similar peak temperature on the order of 1180 k but spent different times at that temperature while samples of lower metamorphic grade experienced different peak temperature but spent more or less the same time within 10 k of peak temperature on the order of 1 ma these results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

importance of the accretion process in asteroid thermal evolution 6 hebe as an example widespread evidence exists for heating that caused melting thermal metamorphism and aqueous alteration in meteorite parent bodies previous simulations of asteroid heat transfer have assumed that accretion was instantaneous for the first time we present a thermal model that assumes a realistic incremental accretion scenario and takes into account the heat budget produced by decay of 26al during the accretion process by modeling 6 hebe assumed to be the h chondrite parent body we show that in contrast to results from instantaneous accretion models an asteroid may reach its peak temperature during accretion the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface and the volume of high grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core we show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities and thereby highlight the importance of the accretion process in shaping an asteroid s thermal history incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities peak temperatures cooling rates radioisotope closure times in meteorites that were determined by their thermal histories

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

a geochemical study of the winonaites evidence for limited partial melting and constraints on the precursor composition abstract the winonaites are primitive achondrites which are associated with the iab iron meteorites textural evidence implies heating to at least the fe ni fes cotectic but previous geochemical studies are ambiguous about the extent of silicate melting in these samples oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites here we analysed a suite of winonaites for modal mineralogy and bulk major and trace element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite iab parent asteroid modal mineralogy and geochemical data are presented for eight winonaites textural analysis reveals that for our sub set of samples all except the most primitive winonaite northwest africa 1463 reached the fe ni fes cotectic however only one tierra blanca shows geochemical evidence for silicate melting processes tierra blanca is interpreted as a residue of small degree silicate melting our sample of winona shows geochemical evidence for extensive terrestrial weathering all other winonaites studied here fortuna queen alexander range 94535 hammadah al hamra 193 pontlyfni and nwa 1463 have chondritic major element ratios and flat ci normalised bulk rare earth element patterns suggesting that most of the winonaites did not reach the silicate melting temperature the majority of winonaites were therefore heated to a narrow temperature range of between 1220 the fe ni fes cotectic temperature and 1370 k the basaltic partial melting temperature silicate inclusions in the iab irons demonstrate partial melting did occur in some parts of the parent body ruzicka and hutson 2010 thereby implying heterogeneous heat distribution within this asteroid together this indicates that melting was the result of internal heating by short lived radionuclides the brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact which allowed the preservation of the largely unmelted winonaites despite major element similarities to both ordinary and enstatite chondrites trace element analysis suggests the winonaite parent body had a carbonaceous chondrite like precursor composition the parent body of the winonaites was volatile depleted relative to ci but enriched compared to the other carbonaceous classes the closest match are the cm chondrites however the specific precursor is not sampled in current meteorite collections

dissolution precipitation creep of k feldspar in mid crustal granite mylonites abstract the deformation of k feldspar within lower amphibolite facies granite mylonites from the gran paradiso nappe north western alps italy is primarily accomplished by dissolution replacement and precipitation processes with little or no evidence for dislocation glide or creep new elongate grains of k feldspar precipitate in dilational domains within and around k feldspar porphyroclasts as a part of the process of myrmekite formation that progressively consumes the porphyroclasts the new grains grow in epitaxial continuity with the parent porphyroclasts which hinders the development of a bulk crystallographic preferred orientation cpo in the aggregates of new grains when large amounts of relict k feldspar are still present in mylonites the magmatic k feldspar is eventually transformed into 100 300 m thick nearly monomineralic fine grained 20 50 m in size aggregates of new grains showing an oblique shape fabric and a cpo the observed cpo is not consistent with the activity of any slip system in k feldspar instead it is interpreted to result from dissolution precipitation creep with a slow reaction rate parallel to 010 and 001 crystallographic axes and a fast reaction rate parallel the 100 axis consistent with such a dissolution precipitation mechanism boundaries of new k feldspar grains are highly corroded when oriented approximately parallel to the extensional instantaneous stretching axis whereas boundaries approximately orthogonal to the same stretching axis show well developed crystal facets the cpo developed is weak which suggests that the anisotropy in the dissolution growth rate of k feldspar is also weak

the early impact histories of meteorite parent bodies we have developed a statistical framework that uses collisional evolution models shock physics modeling and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies it is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces compacting heating and mixing the outer layers it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers the first 10 20 myr were the most important time for impacts both in terms of the number of impacts and the increase of specific internal energy due to impacts the model has been applied to evaluate the proposed impact histories of several meteorite parent bodies up to 10 parent bodies that were not disrupted in the first 100 myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the cb chondrite parent around 1 5 of bodies that were catastrophically disrupted after 12 myr sustained impacts at times that match the heating events recorded on the iab winonaite parent body more than 75 of 100 km radius parent bodies which survived past 100 myr without being disrupted sustained an impact that excavates to the depth required for mixing in the outer layers of the h chondrite parent body and to protect the magnetic field on the cv chondrite parent body the crust would have had to have been thick approximately 20 km to prevent it being punctured by impacts

lattice preferred orientation of quartz inclusions within garnet porphyroblasts from the imjingang belt south korea abstract quartz inclusions within intertectonic garnet porphyroblasts of metapelites from the southern imjingang belt have a lattice preferred orientation c axis fabrics of the quartz inclusions tend to show a single girdle pattern at a high angle to an earlier foliation together with a submaximum subparallel to an earlier lineation this lattice preferred orientation of the quartz inclusions might result from intracrystalline deformation together with solution creep prior to the garnet growth or from selective entrapment of quartz grains with a specific lattice orientation during the garnet growth

quaternary palaeolakes in the evolution of semidesert basins with special emphasis on lake bonneville and the great basin u s a abstract this paper reviews attributes of quaternary lakes and lake basins which are often important in the environmental prehistory of semideserts basin floor and basin closure morphometry have set limits on palaeolake sizes lake morphometry and basin drainage patterns have influenced lacustrine processes and water and sediment loads have influenced basin neotectonics information regarding inundated runoff producing and extra basin spatial domains is acquired directly from the palaeolake record including the littoral morphostratigraphic record and indirectly by reconstruction increasingly detailed hypotheses regarding lake bonneville the largest late pleistocene palaeolake in the great basin are subjects for further testing and refinement oscillating transgression of lake bonneville began about 28 ka the highest stage occurred about 15 ka and termination occurred abruptly about 13 ka a final resurgence of perennial lakes probably occurred in many subbasins of the great basin between 11 and 10 ka when the highest stage of great salt lake successor to lake bonneville developed the gilbert shoreline and when the russell shoreline formed 500 600 km away in the carson desert of the lake lahontan basin the highest post gilbert stage of great salt lake which has been one of the few permanent lakes in the great basin during holocene time probably occurred between 3 and 2 ka a set of eighteen general observations regarding geologic aspects of the palaeolake record in semidesert basins is helping to guide quaternary studies in the great basin

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

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

the educational value of the history of geomorphology abstract the history of geomorphology can be a valuable tool for educating students of geomorphology the largest and most available record of what geomorphologists thought and did in the past is their original published work also available are some fine papers on the history of geomorphology that review and analyze primary sources and past events in the discipline in order to generalize explain trends and reveal historical associations interest in the history of the discipline varies widely of course among geomorphologists nevertheless several educational benefits in addition to explaining the academic affiliation of geomorphology with two separate disciplines can be derived from incorporating the history of geomorphology into university level geomorphology courses history is a popular subject and presenting some geomorphic concepts using a historical approach can help to maintain or stimulate student interest because of the tendency for older literature to contain more qualitative description and methodological detail than more recent literature undergraduate students may comprehend some concepts better from older papers by reading the original literature students determine for themselves what previous practitioners accomplished rather than relying solely on the interpretations of others reading original literature also helps students realize that older does not mean less intelligent and that like the critical reading of recent geomorphic literature the critical reading of historic geomorphic papers can provide a wealth of new research ideas a thematic set of the historic literature can be used to demonstrate to students the scientific method the origin testing and evolution of hypotheses how explanations develop in this field and that science proceeds through individuals working in a sociological context including history of geomorphology in the curriculum helps to place contemporary research in the perspective of the past as well as the perspective of the future

thermal constraints on the early history of the h chondrite parent body reconsidered abstract reconstructions of the early thermal history of the h chondrite parent body have focused on two competing hypotheses the first posits an undisturbed thermal evolution in which the degree of metamorphism increases with depth yielding an onion shell structure the second posits an early fragmentation reassembly event that interrupted this orderly cooling process here we test these hypotheses by collecting a large number of previously published closure age and cooling rate data and comparing them to a suite of numerical models of thermal evolution in an idealized parent body we find that the onion shell hypothesis when applied to a parent body of radius 75 130 km with a thermally insulating regolith is able to explain 20 of the 21 closure age data and 62 of the 71 cooling rates furthermore six of the eight meteorites for which multiple data at different temperatures are available can be accounted for by onion shell thermal histories we therefore conclude that no catastrophic disruption of the h chondrite parent body occurred during its early thermal history the relatively small number of data not explained by the onion shell hypothesis may indicate the formation of impact craters on the parent body which while large enough to excavate all petrologic types were small enough to leave the parent body largely intact impact events fulfilling these requirements would likely have produced transient crater diameters at least 30 of the parent body diameter

the fate of magmas in planetesimals and the retention of primitive chondritic crusts abstract high abundances of short lived radiogenic isotopes in the early solar system led to interior melting and differentiation on many of the first planetesimals petrologic isotopic and paleomagnetic evidence suggests that some differentiated planetesimals retained primitive chondritic material the preservation of a cold chondritic lid depends on whether deep melts are able to ascend and breach the chondritic crust we evaluate the likelihood of melt ascent on a range of chondritic parent bodies we find that due to the efficient ascent of free volatiles in the gas and supercritical fluid phases at temperatures still below the solidus for silicates and metals mobile silicate melts on planetesimals were likely volatile depleted by calculating the densities of such melts we show that silicate melts likely breached crusts of enstatite chondrite compositions but did not ascend in the cv and cm parent bodies ordinary chondrite melts represent an intermediate case these predictions are consistent with paleomagnetic results from cv and cm chondrites as well as spectral observations of large e type asteroids

deformation induced silica redistribution in banded iron formation hamersley province australia abstract the formation of banded iron formations bif remains controversial despite their potential to provide key information on precambrian atmospheres and hydrospheres it is widely agreed that bif are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases many models address the chemical transformation of such precursor iron oxide phases into bif during compaction and diagenesis however the formation of chert and the influence of physical forces in this process have received less attention microstructural analysis of bif from the hamersley province western australia reveals that significant amounts of silica were redistributed by dissolution precipitation creep during both diagenesis and regional scale deformation this physicochemical process led to silica remobilisation and volume loss by stress induced dissolution of microcrystalline quartz in an aqueous fluid the dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low pressure zones leading to local volume increase these processes were further enhanced by rheological contrasts between different minerals resulting in significant variations of chert band thickness microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations cpo in quartz grains within chert layers the cpo fabrics record strain regimes e g pure and simple shear extension and shortening that modified quartz aggregates by dissolution precipitation creep providing new insights into the metamorphic and deformation history of bif we document microstructures that indicate that non coaxial deformation was active during diagenesis and subsequent deformation of the hamersley province bif further relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution precipitation creep providing a possible explanation for the destruction of primary features we show that different generations of chert alternate on a micro scale suggesting that the role of deformation has been underestimated in many aspects of bif research the discovery of dissolution precipitation creep in low grade metamorphic bif provides a potential link between the physical and chemical processes that occur within these rocks after deposition in the hamersley province dissolution precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events we also suggest that chert layers without cpo record the least overprint by diagenesis and as the most pristine form of bif should be targeted for future paleo environmental research

shock fabrics in fine grained micrometeorites the orientations of dehydration cracks and fracture networks in fine grained unmelted micrometeorites were analyzed using rose diagrams and entropy calculations as cracks exploit pre existing anisotropies analysis of their orientation provides a mechanism with which to study the subtle petrofabrics preserved within fine grained and amorphous materials both uniaxial and biaxial fabrics are discovered often with a relatively wide spread in orientations 40 60 brittle deformation cataclasis and rotated olivine grains are reported from a single micrometeorite this paper provides the first evidence for impact induced shock deformation in fine grained micrometeorites the presence of pervasive low grade shock features in cm chondrites and cm like dust anomalously low density measurements for c type asteroids and impact experiments which suggest cm chondrites are highly prone to disruption all imply that cm parent bodies are unlikely to have remained intact and instead exist as a collection of loosely aggregated rubble pile asteroids composed of primitive shocked clasts

age and paleoclimatic significance of the stansbury shoreline of lake bonneville northeastern great basin abstract the stansbury shoreline one of the conspicuous late pleistocene shorelines of lake bonneville consists of tufa cemented gravel and barrier beaches within a vertical zone of about 45 m the lower limit of which is 70 m above the modern average level of great salt lake stratigraphic evidence at a number of localities including new evidence from crater island on the west side of the great salt lake desert shows that the stansbury shoreline formed during the transgressive phase of late pleistocene lake bonneville sometime between about 22 000 and 20 000 yr b p tufa cemented gravel and barrier beaches were deposited in the stansbury shorezone during one or more fluctuations in water level with a maximum total amplitude of 45 m we refer to the fluctuations as the stansbury oscillation the stansbury oscillation cannot have been caused by basin hypsometric factors such as stabilization of lake level at an external overflow threshold or by expansion into an interior subbasin or by changes in drainage basin size therefore changes in climate must have caused the lake level to reverse its general rise to drop about 45 m in altitude reducing its surface area by about 18 5000 km2 and later to resume its rise if the sizes of great basin lakes are controlled by the mean position of storm tracks and the jetstream which as recently postulated may be controlled by the size of the continental ice sheets the stansbury oscillation may have been caused by a shift in the jetstream during a major interstade of the laurentide ice sheet

fabric transitions in quartz via viscoplastic self consistent modeling part i axial compression and simple shear under constant strain quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures leading to the development of different types of crystallographic preferred orientation cpo patterns in this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self consistent vpsc approach for that we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems we have performed these simulations in axial compression and simple shear regimes under constant von mises equivalent strain of 100 1 73 assuming that the aggregates deformed exclusively by dislocation glide some of the predicted cpos patterns are similar to those observed in naturally and experimentally deformed quartz nevertheless some classical cpo patterns usually interpreted as result from dislocation glide e g y maxima due to prism slip are clearly not developed in the simulated conditions in addition we reported new potential preferred orientation patterns that might happen in high temperature conditions both in axial compression and simple shear we have demonstrated that cpos generated under axial compression are usually stronger that those predicted under simple shear due to the continuous rotation observed in the later simulations the fabric strength depends essentially on the dominant active slip system and normally the stronger cpos result from dominant basal slip in followed by rhomb and prism c slip whereas prism slip does not produce strong fabrics the opening angle of quartz 0001 fabric used as a proxy of temperature seems to be reliable for deformation temperatures of 400 c when the main slip systems have similar behaviors

petrologic evidence for collisional heating of chondritic asteroids abstract the identification of the mechanism s responsible for heating asteroids is among the major problems in planetary science because of difficulties with models of electromagnetic induction and the decay of short lived radionuclides it is worthwhile to evaluate the evidence for collisional heating localized impact heating is responsible for forming shock veins melt pockets metal sulfide mixtures vugs agglutinates and a wide variety of melt rock clasts new evidence for localized impact heating comes from the high proportion of relict type 6 material among impact melt bearing ordinary chondrites oc this relict material was probably metamorphosed by residual heat within large craters olivine aggregates composed of faceted crystals with 120 triple junctions occur within the melted regions of the chico and rose city oc melt rocks the olivine aggregates formed from shocked mosaicized olivine grains that underwent contact metamorphism large scale collisional heating is supported by the correlation in oc between petrologic type and shock stage no other heating mechanism can readily account for this correlation the occurrence of impact melt rock clasts in oc that have been metamorphosed along with their whole rocks indicates that some impact events preceded or accompanied thermal metamorphism such impact events occurring during or shortly after accretion are probably responsible for substantially melting 0 5 of oc these events most have heated a larger percentage of oc to subsolidus temperatures sufficient to have caused significant metamorphism if collisional heating is viable then oc parent asteroids must have been large large oc asteroids in the main belt may include those of the s iv spectral subtype collisional heating is inconsistent with layered onion shell structures in oc asteroids wherein the degree of metamorphism increases with depth but the evidence for such structures is weak it seems likely that collisional heating played an important role in metamorphosing chondritic asteroids

shock compaction heating and collisional processes in the production of type 3 ordinary chondrites lessons from the nearly unique l3 chondrite melt breccia northwest africa 8709

numerical simulations of the differentiation of accreting planetesimals with 26al and 60fe as the heat sources numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26al and 60fe as the heat sources planetesimal accretion was started at chosen times up to 3 ma after ca al rich inclusions cais were formed and was continued for periods of 0 001 1 ma the planetesimals were initially porous unconsolidated bodies at 250 k but became sintered at around 700 k ending up as compact bodies whose final radii were 20 50 100 or 270 km with further heating the planetesimals underwent melting and igneous differentiation two approaches to core segregation were tried in the first labelled a the core grew gradually before silicate began to melt and in the second labelled b the core segregated once the silicate had become 40 molten in a when the silicate had become 20 molten the basaltic melt fraction began migrating upward to the surface carrying 26al with it the 60fe partitioned between core and mantle the results show that the rate and timing of core and crust formation depend mainly on the time after cais when planetesimal accretion started they imply significant melting where accretion was complete before 2 ma and a little melting in the deep interiors of planetesimals that accreted as late as 3 ma the latest melting would have occurred at 10 ma the effect on core and crust formation of the planetesimals final size the duration of accretion and the choice of 60fe 56fe initial were also found to be important particularly where accretion was late the results are consistent with the isotopic ages of differentiated meteorites and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 ma after cais

the portales valley meteorite breccia evidence for impact induced melting and metamorphism of an ordinary chondrite abstract the portales valley h chondrite fall is an annealed impact melt breccia with coarse metal interstitial to angular and subrounded silicate clasts the large metal rich regions exhibit a widmanstatten structure and contain very little troilite we were able to examine a 16 5 kg metal rich specimen of portales valley silicates contain numerous flecks of metallic cu and curvilinear trails of tiny metallic fe ni blebs characteristic of shocked and annealed chondrites one silicate clast appears to have experienced little

post impact thermal evolution of porous planetesimals abstract impacts between planetesimals have largely been ruled out as a heat source in the early solar system by calculations that show them to be an inefficient heat source and unlikely to cause global heating however the long term localized thermal effects of impacts on planetesimals have never been fully quantified here we simulate a range of impact scenarios between planetesimals to determine the post impact thermal histories of the parent bodies and hence the importance of impact heating in the thermal evolution of planetesimals we find on a local scale that heating material to petrologic type 6 is achievable for a range of impact velocities and initial porosities and impact melting is possible in porous material at a velocity of 4 km s burial of heated impactor material beneath the impact crater is common insulating that material and allowing the parent body to retain the heat for extended periods millions of years cooling rates at 773 k are typically 1 1000 k ma matching a wide range of measurements of metallographic cooling rates from chondritic materials while the heating presented here is localized to the impact site multiple impacts over the lifetime of a parent body are likely to have occurred moreover as most meteorite samples are on the centimeter to meter scale the localized effects of impact heating cannot be ignored

earliest rock fabric formed in the solar system preserved in a chondrule rim rock fabrics record the formation compaction and deformation history of that rock high resolution mapping of tiny grains in the allende cv meteorite reveals preservation of a spherical fabric that may have formed in the solar nebula and could be the oldest rock fabric to have formed in the solar system

shock effects in eh6 enstatite chondrites and implications for collisional heating of the eh and el parent asteroids abstract of the six chondrites that were listed as eh6 or eh6 an during the course of this study we confirm the eh classification of y 8404 y 980211 and y 980223 and the eh an classification of y 793225 two chondrites a 882039 and y 980524 are reclassified as el the former contains ferroan alabandite and both contain kamacite with 1 wt si all of the meteorites contain euhedral enstatite grains surrounded by metal sulfide although this texture is rare in y 793225 consistent with enstatite crystallizing from a mixed melt all contain enstatite with 0 5 mg

quartz c axes parallel to stretching directions in very low grade metamorphic rocks abstract quartz preferred orientations with c axes parallel to stretching directions are widespread in slates phyllites and very low grade schists of the south island new zealand in general these rocks lack evidence for plastic deformation of quartz and often preserve the original clastic quartz which naturally tends to be elongate parallel to the c axis the c axis preferred orientation can be explained therefore by mechanical rotation of clastic grains during deformation so that long axes became parallel to the stretching direction plastic deformation does however play a role in some very quartz rich layers which display a preferred orientation due to basal slip with c axes at high angles to the stretching direction solution transfer is important in enhancing the contrasts between pelitic and quartz rich lithologies but its role in producing c axis preferred orientations by competitive anisotropic growth or solution seems important only in slickenfibres and in fibrous quartz veins of shear zones where quartz displays very strong preferred orientations with c axes parallel to the local stretching direction in contrast the non fibrous quartz pressure shadows in some muscovite rich layers display a c axis preferred orientation similar to adjacent quartz rich sand layers and it probably results from nucleation of pressure shadow quartz on pre existing mechanically oriented quartz clasts

new implications for the origin of the iab main group iron meteorites and the isotopic evolution of the noncarbonaceous nc reservoir abstract the origin of the iab main group mg iron meteorites is explored through consideration of 182w isotopic compositions thermal modeling of 26al decay and mass independent nucleosynthetic mo isotopic compositions of planetesimals formed in the noncarbonaceous nc protosolar isotopic reservoir a refined 182w model age for the meteorites campo del cielo canyon diablo and nantan suggests that the iab mg parent body underwent some form of metal silicate segregation as early as 5 3 0 4 myr after calcium aluminum rich inclusion cai formation or as late as 13 8 1 4 myr after cai formation if melting of the iab mg occurred prior to 7 myr after cai formation it was likely driven by 26al decay for a parent body radius 40 km otherwise additional heat from impact is required for melting metal this late in solar system history if melting was partially or wholly the result of internal heating a thermal model of 26al decay heat production constrains the accretion age of the iab mg parent body to 1 7 0 4 myr after cai formation if melting was instead dominantly caused by impact heating thermal modeling suggests the parent body accreted more than 2 myr after cai formation comparison of mo mass independent isotopic compositions of the iab mg to other nc bodies with constrained accretion ages suggests that the mo isotopic composition of the nc reservoir changed with time and that the iab mg parent body accreted between 2 to 3 myr after cai formation thus requiring an origin by impact the relationship between nucleosynthetic mo isotopic compositions and accretion ages of planetesimals from the nc reservoir suggests that isotopic heterogeneity developed from either addition of s process material to or removal of coupled r p process material from the nc reservoir

timing and extent of late quaternary paleolakes in the trans pecos closed basin west texas and south central new mexico abstract the trans pecos closed basin is a hydrographically closed region covering 20 000 km 2 centered on salt basin 160 km east of el paso texas geomorphic and limnetic evidence have been used to identify four major highstands for lake king during the last glacial maximum lgm additional geomorphic features from a second recently identified paleolake lake sacramento have been found in the beargrass subbasin a nested subbasin approximately 75 km northwest of salt basin radiocarbon ages of the organic material in lake king sediments date four abrupt climate changes and rapid lacustrine transgressions during the lgm with a quasi periodicity of 2000 yr geomorphic evidence in the beargrass subbasin identifies lake cycles contemporaneous with those in lake king the dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in new mexico the quasi periodicity of the events approximates that of dansgaard oeschger events identified from greenland ice cores the contemporaneity of the trans pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the lgm

a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body abstract we have studied iab iron meteorites and their silicate bearing inclusions to elucidate the origin of their parent body we have divided iab irons into five categories which best describe the inclusions and other properties of the irons these are 1 sulfide rich irons such as mundrabilla that contain a significant volume fraction of troilite with or without associated silicates 2 irons with non chondritic silicate inclusions such as basaltic inclusions in caddo county 3 irons with rounded troilite and or graphite inclusions sometimes containing embedded angular silicate inclusions 4 irons with angular silicate inclusions similar to those of the winonaites where silicates may comprise up to 40 vol of the rock and 5 irons with phosphate bearing inclusions such as the brianite bearing iab san cristobal similarities in mineralogy mineral and oxygen isotopic compositions suggest that iab irons and winonaites are almost certainly from a common parent body iiicd irons may also have originated on this parent body although evidence supporting this suggestion is less convincing previous models for the origin of iab irons propose that they are either products of selective impact melting in the megaregoliths of a chondritic asteroid or products of partial melting and fractional crystallization during formation of a sulfur rich core the former model suffers from the fact that impact is incapable of causing preferential melting of metal and sulfide on a large scale and pooling of these selective melts into sizable bodies the latter model does not readily explain the occurrence of often fragmentary angular silicate clasts in a metallic fe ni host we propose a new model for the origin of iab irons that incorporates aspects of both previous models we suggest that a chondritic possibly heterogeneous parent body was heated by a non collisional heat source which caused partial but not complete melting of the body after peak temperatures were reached and cooling and crystallization had begun a major impact catastrophically fragmented this body and the debris gravitationally reassembled scrambling the parent body lithologies we suggest that the impact caused extensive mixing of molten metallic fe ni and solid silicate material resulting in the formation of iab irons with silicate inclusions whereas winonaites represent metamorphosed rocks and partial melt residues of the precursor silicate lithologies high temperatures of some of the reassembled debris and deep burial resulted in continued fractionation and formation of the 2

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

the shoreline preservation index as a relative age dating tool for late pleistocene shorelines an example from the bonneville basin u s a this paper evaluates the utility of the shoreline preservation index s p i for estimating the age of late pleistocene palaeolake shorelines the relative influence of various factors on shoreline preservation and whether shoreline preservation varies significantly with shoreline aspect sampled shorelines from the 3300 km2 study area which includes the skull tule and puddle valley portions of the lake bonneville basin range in age from approximately 26 to 12 ka their total s p i values range from 23 to 69 per cent average 46 per cent and do not vary significantly with shoreline aspect the data from skull and tule valleys where studied shorelines are of known age are analysed first in order to determine if there are statistically significant associations between variables representing shoreline preservation age degree of geomorphic development and duration of subaerial exposure pairwise correlation is then repeated using observations from all three valleys in order to determine how data from the puddle valley shorelines whose hypothesized ages are not supported by radiocarbon analyses affect the results results show that s p i is useful as a relative age dating tool that the postulated ages of the puddle valley shorelines are ordinally correct and that geomorphic development is not an important influence on the preservation of these late pleistocene shorelines the relative importance of shoreline age and the relative unimportance of duration of subaerial exposure with respect to shoreline preservation suggest that subaqueous processes play a more substantial role in shoreline obliteration than is generally suggested

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

a global system of furrows on ganymede indicative of their creation in a single impact event abstract furrows are a concentric system of tectonic troughs and are the oldest recognizable surface feature on ganymede we analyzed the distribution of furrows utilizing voyager and galileo images and found that furrows over ganymede s surface are part of a global concentric circular structure if this multi ring structure is impact origin this is the largest impact structure identified so far in the solar system deviations of the shapes of the furrows from the concentricity are small everywhere which implies that the relative location of the blocks of the dark terrains over the entire surface of ganymede has not changed appreciably even during formation of the bright terrains the estimate of the impactor size is difficult but an 150 km radius impactor is consistent with the observed properties of furrows the furrow forming impact should have significant effects on the satellite s geological and internal evolution which are expected to be confirmed by future explorations of jupiter s icy moons such as the juice jupiter icy moon explorer or europa clipper mission

thermal and impact history of the h chondrite parent asteroid during metamorphism constraints from metallic fe ni abstract we have studied cloudy taenite metallographic cooling rates and shock effects in 30 h3 6 chondrites to elucidate the thermal and early impact history of the h chondrite parent body we focused on h chondrites with old ar ar ages 4 4 gyr and unshocked and mildly shocked h chondrites as strongly shocked chondrites with such old ages are very rare cooling rates for most h chondrites at 500 c are 10 50 c myr and do not decrease systematically with increasing petrologic type as predicted by the onion shell model in which types 3 5 are arranged in concentric layers around a type 6 core some type 4 chondrites cooled slower than some type 6 chondrites and type 3 chondrites did not cool faster than other types contrary to the onion shell model cloudy taenite particle sizes which range from 40 to 120 nm are inversely correlated with metallographic cooling rates and show that the latter were not compromised by shock heating the three h4 chondrites that were used to develop the onion shell model ste marguerite beaver creek and forest vale cooled through 500 c at 5000 c myr our thermal modeling shows that these rates are 50 higher than could be achieved in a body that was heated by 26al and cooled without disturbance by impact published ar ar ages do not decrease systematically with increasing petrologic type but do correlate inversely with cloudy taenite particle size suggesting that impact mixing decreased during metamorphism metal and silicate compositions in regolith breccias show that impacts mixed material after metamorphism without causing significant heating impacts during metamorphism created portales valley and two other h6 chondrites with large metallic veins excavated the fast cooled h4 chondrites around 3 4 myr after accretion and mixed petrologic types metallographic data do not require catastrophic disruption by impact during cooling

oxygen isotope and petrological study of silicate inclusions in iie iron meteorites and their relationship with h chondrites abstract the origin of silicate bearing irons especially those in groups iab iiicd and iie is poorly understood as silicate should have separated rapidly from molten metal here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group iie meteorites and a petrological study of silicate inclusions in ten iie irons including those in garhi yasin and tarahumara which have not been described in detail before oxygen isotopes have also been analysed in 20 h chondrites to investigate their possible relationship with the iie irons based on petrographic observations and mineral analysis the silicate bearing iie meteorites have been divided into four types according to the nature of their silicate inclusions 1 primitive chondritic 2 evolved chondritic 3 differentiated with 10 vol orthopyroxene and 4 differentiated with 17 o variation a wide range of values is seen in the dataset as a whole group iie irons with differentiated silicates with the exception of colomera have a range of mean 17 o values that is essentially identical to those of the h4 6 chondrites 0 60 0 77 and 0 61 0 76 respectively colomera inclusions which are differentiated with 17 o value and plot 2 away from the next nearest iie iron however in view of the textural similarities to other iie inclusions a separate source for colomera is deemed unlikely three iie irons with primitive chondritic inclusions garhi yasin netschaevo and techado have relatively low mean 17 o values of 0 56 0 57 as well as relatively reduced silicates with fa 15 17 olivine which have been called hh chondrites given the significant overlap in their oxygen isotope compositions a genetic relationship between iie irons and h chondrites is supported by our new data however derivation of both groups from one parent body seems unlikely instead both groups probably sampled similar precursor materials and accreted at a similar nebular location our data suggest that the iie meteorites formed on an internally heated h hh chondrite like body that experienced the initial stages of differentiation in response to radiogenic heating however prior to full differentiation the iie parent body experienced a major hit and run style collision that resulted in silicate metal mixing the initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates reassembly of the iie parent body produced a large regolith blanket that facilitated subsequent slow cooling the iie parent body has probably experienced numerous subsequent less catastrophic collisions the development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events

ancient porosity preserved in ordinary chondrites examining shock and compaction on young asteroids we use a combination of 2d and 3d petrographic examination and 40ar 39ar analyses to examine the impact histories of a suite of seven ordinary chondrites baszkowka miller nwa 2380 mount tazerzait sahara 98034 tjerebon and mil 99301 that partially preserve their ancient but postaccretionary porosity ranging from 10 to 20 we examine whether materials that seem to be only mildly processed as their large intergranular pore spaces suggest may have more complex shock histories the ages determined for most of the seven ocs studied here indicate closure of the 40ar 39ar system after primary accretion but during baszkowka or shortly after others thermal metamorphism with little subsequent heating exceptions include sahara 98034 and mil 99301 which were heated to some degree at later stages but retain some evidence for the timing of thermal metamorphism in the 40ar 39ar system although each of these chondrites has olivine grains with sharp optical extinction signaling an apparent shock stage of s1 normally indicative of an extremely mild impact history all of the samples contain relict shock indicators given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators initial heating could have resulted from prior collisions the decay of 26al or both processes

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

the use of total lake surface area as an indicator of climatic change examples from the lahontan basin abstract variation in the size of lakes in the lahontan basin is topographically constrained river diversion also has played a major role in regulating lake size in lahontan subbasins the proper gage of lake response to change in the hydrologic balance is neither lake depth level nor lake volume but instead lake surface area normalization of surface area is necessary when comparing surface areas of lakes in basins having different topographies to a first approximation normalization can be accomplished by dividing the paleosurface area of a lake by its mean historical reconstructed surface area

mesoscale and microscale shock effects in the ll6 s4 chondrites saint severin and elbert a tale of two breccias

shock and annealing in the amphibole and mica bearing r chondrites mil 11207 r6 and lap 04840 r6 contain hornblende and phlogopite mil 07440 r6 contains accessory titan phlogopite and no hornblende all three meteorites have been shocked mil 11207 contains extensive sulfide veins pyroxene that formed from dehydrated hornblende and an extensive network of plagioclase glass mil 07440 contains chromite plagioclase assemblages chromite veinlets and blebs pincer shaped plagioclase patches but no sulfide veins lap 04840 contains olivine grains with chromite bleb laden cores and opaque free rims rare grains of pyroxene that formed from dehydrated hornblende and no sulfide veins these meteorites appear to have been heated to maximum temperatures of approximately 700 900 c under conditions of moderately high ph2o perhaps 250 500 bars all three samples underwent postshock annealing during this process olivine crystal lattices healed giving the rocks the appearance of shock stage s1 and diffusion of fe and s from thin sulfide veins to coarse sulfide grains caused the veins to disappear in mil 07440 and lap 04840 this latter process apparently also occurred in most s1 s2 ordinary chondrites of high petrologic type the pressure temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers as suggested in the literature a giant impact or a series of smaller impacts would then have been required to excavate the hornblende and biotite bearing rocks and bring them closer to the surface it was in that latter location where the samples were shocked deposited in a hot ejecta blanket of low thermal diffusivity and annealed

deformation partitioning during folding and transposition of quartz layers abstract banded iron formation bif from the quadrilatero ferrifero southeastern brazil shows a compositional layering with alternating iron rich and quartz rich layers this layering was intensively folded and transposed at a centimeter millimeter scale through a component of bedding parallel shear related to flexural slip at middle to high greenschist facies conditions 400 450 c the microstructure and c axis fabrics of normal limbs inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy sem and digital image analysis in the normal limbs recrystallized quartz grains show undulose extinction relatively dry grain boundaries c axes at high angle to foliation and a pervasive grain shape fabric gsf indicating operation of crystal plastic processes in the inverted limb quartz grains show more serrated and porous wet grain boundaries the gsf is similar to that of the normal limb but c axes are oriented at 90 to those of the normal limb we interpreted these characteristics as reflecting operation of solution precipitation deformation in inverted limbs as a consequence of grains having been rotated to an orientation that was hard to basal a glide but easy to dissolution precipitation creep this deformation partitioning between crystal plasticity and solution transfer during folding transposition of quartz may explain the common occurrence of layered quartz rocks where individual layers show alternating c axis fabrics with opposite asymmetries but a consistent gsf orientation such characteristics may reflect an earlier event of pervasive folding transposition of a preexisting layering

ejecta megaregolith accumulation on planetesimals and large asteroids megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10 i have modeled global average ejecta accumulation as a function of the largest impact size with no explicit modeling of time in conjunction with an assumed cratering size distribution exponent b the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith the largest impactor mass ratio is a major fraction of the catastrophic disruption mass ratio and in general the largest crater s diameter is close to the target s diameter total accumulation is roughly 1 5 of and proportional to the target s radius global accumulations estimated by this approach are higher than in the classic housen et al 1979 study by a factor of roughly 10 this revision is caused mainly by higher typical largest crater size for b 2 the single largest crater typically contributes close to 50 of the total of new nonrecycled ejecta megaregolith can be destroyed by sintering a process whose pressure sensitivity makes it effective at lower temperature on larger bodies planetesimals 100 km in diameter may be surprisingly well suited about as well suited as bodies two to three times larger in diameter for attaining temperatures conducive to widespread melting a water rich composition may be a significant disadvantage in terms of planetesimal heating as the shallow interior may be densified by aqueous metamorphism and will have a low sintering temperature

c axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid rich greenschist conditions the quartz v axis orientations and microstructures have been analyzed in an originally single crystal buckled quartz vein of greenschist metamorphic grade from the sambagawa metamorphic belt southwest japan the basal 0001 plane was inclined by ca 10 20 to the quartz vein surface before deformation and the flexure essentially kink which produced the fold was accomplished by basal 0001 slip alone in the hinges i e flexural slip folding in addition to crystal plasticity dissolution microstructures are ubiquitous in the quartz vein dynamic recrystallization resulted from both subgrain rotation and grain boundary migration rotation recrystallization associated with basal 0001 slip resulted in host controlled v axis orientation distribution along a great circle whereas migration recrystallization perhaps assisted by intergranular fluid resulted in a penetrative development of a weak x elongation maximum v axis fabric nowhere rotated by the folding the migration recrystallization is evidenced by large v axis misorientations across the grain boundaries as well as irregular grain shape based on the weak v axis fabric development and lack of grain shape fabric solution precipitation creep may have dominated in the fluid assisted grain boundary migration regime and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein

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

the ar ar age and petrology of miller range 05029 evidence for a large impact in the very early solar system abstract miller range mil 05029 is a slowly cooled melt rock with metal sulfide depletion and an ar ar age of 4517 11 ma oxygen isotopes and mineral composition indicate that it is an l chondrite impact melt and a well equilibrated igneous rock texture with a lack of clasts favors a melt pool over a melt dike as its probable depositional setting a metallographic cooling rate of approximately 14 c ma 1 indicates that the impact occurred at least approximately 20 ma before the ar ar closure age of 4517 ma possibly even shortly after accretion of its parent body a metal grain with a widmanstatten like pattern further substantiates slow cooling the formation age of mil 05029 is at least as old as the ar ar age of unshocked l and h chondrites indicating that endogenous metamorphism on the parent asteroid was still ongoing at the time of impact its metallographic cooling rate of approximately 14 c ma 1 is similar to that typical for l6 chondrites suggesting a collisional event on the l chondrite asteroid that produced impact melt at a minimum depth of 5 12 km the inferred minimum crater diameter of 25 60 km may have shattered the 100 200 km diameter l chondrite asteroid therefore mil 05029 could record the timing and petrogenetic setting for the observed lack of correlation of cooling rates with metamorphic grades in many l chondrites

original structures and fragmentation and reassembly histories of asteroids evidence from meteorites abstract if chondritic meteorites were internally heated after accretion had ended then the hottest material would have been buried the deepest and should have cooled the slowest if this is correct there ought to be a correlation between cooling rate and petrographic type a measure of the extent to which chondrites were metamorphosed i e heated published and new cooling rates derived from the compositions of metallic iron nickel grains do not display this correlation implying either that chondrite parent asteroids never had onion shell structures or that bodies with onion shell structures were broken up and reassembled prior to cooling to below 500 c the temperature at which cooling rate information is recorded in metallic iron nickel chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids metallic iron nickel grains in h and l chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to 1000 k myr based on thermal calculations these cooling rates suggest that the materials spread out on the surfaces of h and l chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them therefore it appears that at least some asteroids namely the parent bodies of h and l chondrites were disrupted after cooling to below 300 c and then reassembled to create surfaces containing rocks that originated at a wide range of depths these results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles

a combined diffusion and thermal modeling approach to determine peak temperatures of thermal metamorphism experienced by meteorites abstract carbonaceous chondrites are affected to different degrees by thermal and aqueous metamorphism on their parent bodies however the degree of alteration has been categorized mainly by relative scales and achieving quantitative information about metamorphic temperature by conventional mineral thermometry is problematic for low petrologic types we have developed a general approach to estimate the metamorphic peak temperature experienced by type 3 chondrites from diffusion zoning in minerals and have applied this approach to olivine in type i and type ii chondrules of co3 chondrites to obtain metamorphic temperatures from diffusion zoning we have combined diffusion modeling with thermal modeling of the meteorite parent body the integrated diffusion coefficient over time was identified as a useful parameter to quantify the extent of chemical change by diffusion occurring in a mineral during a given thermal history knowing the temperature dependence of the diffusion coefficient values can be calculated for each thermal history and be compared to the values obtained from diffusion modeling for thermal histories realistic for the parent body depends primarily on the metamorphic peak temperature so that values determined from diffusion profiles in meteorite minerals can be directly related to the metamorphic peak temperature this general approach is relatively insensitive to uncertainties in the input parameters for the thermal model we found that chemical zoning in type i and type ii chondrule olivine of the co chondrites kainsaz and lance was largely influenced by solid state diffusion which is evident from the observed correlation of zoning anisotropy with the crystallographic orientation chemical zoning in type ii chondrule olivine is mainly igneous for co chondrites of petrologic types up to at least 3 2 kainsaz and was influenced only minor by diffusion during parent body metamorphism fe mg zoning in type ii chondrule olivine and around sealed cracks in type i chondrule olivine yields similar values indicating a formation of both zoning features during a common thermal history on the parent body in addition values for type ii chondrule olivine correlate with metamorphic grade the application of this approach on fe mg zoning in type ii chondrule olivine of co3 chondrites yields estimates of maximum metamorphic peak temperatures ranging from 653 to 849 k for different petrologic subtypes the fe mg zoning of type i chondrule olivine is not consistent with the peak temperature estimates from type ii chondrule olivine suggesting an additional contribution of solar nebular processes to type i chondrule olivine zoning prior to accretion into the parent body

quartz deformation mechanisms in metasediments from prins karls forland svalbard abstract quartzites and phyllites of presumed precambrian cambrian age in western spitsbergen were deformed and metamorphosed at middle to lower greenschist facies conditions they exhibit a wide variation in quartz content with less variable quartz grain sizes quartz microstructures and lattice orientations in these rocks are used to infer which deformation mechanisms were dominant in the different lithologies three variable factors in the quartzites and phyllites may have influenced the way in which quartz deformed these are 1 1 quartz content 2 2 average quartz grain size 3 3 the distribution of accessory phases particularly the micaceous minerals between the quartz grains generally in a rock with a high quartz content a coarse average quartz grain size and or poorly distributed accessory phases quartz exhibits microstructures and lattice orientations which suggest that intracrystalline mechanisms dominated during deformation on the other hand in a rock with a low quartz content a fine average quartz grain size and or evenly distributed accessory phases quartz exhibits microstructures and lattice orientations which suggest that it deformed by means of intercrystalline mechanisms

late pleistocene to early holocene lake level and paleoclimate insights from stansbury island bonneville basin utah abstract this paper reports on recent multiproxy research conducted to determine the chronology of lake level fluctuations recorded in sediments from a natural exposure at a classic bonneville basin site grain size carbonate percentage magnetic susceptibility amount of charcoal and diatom community composition data were collected from the 16 lacustrine units that compose the 122 cm stratigraphic column in stansbury gulch trends observed in the measured proxies reveal several significant changes in lake level and thereby effective moisture over the approximately 14 500 yr time span represented by the sediments results 1 verify the effectiveness of the multiproxy approach in bonneville basin studies which has been underutilized in this region 2 reaffirm the double nature of lake bonneville s stansbury oscillation 3 suggest a previously undocumented post gilbert highstand of great salt lake and 4 identify possible teleconnections between climate events in the bonneville basin and events in the north atlantic at about 20 500 and 7500 14c yr bp

experimental simulation of oxygen isotopic exchange in olivine and implication for the formation of metamorphosed carbonaceous chondrites we have conducted hydration dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between cm2s and metamorphosed carbonaceous chondrites mccs two experiments were undertaken in the first experiment serpentine was obtained by hydrating terrestrial olivine fo90 9 in the laboratory during this experiment olivine was reacted with isotopically heavy water 18o 21 5 at t 300 c ph2o 300 bar for 100 days the oxygen isotopic composition of the experimental serpentine was enriched in 18o by 10 in 18o due to exchange of oxygen isotopes between olivine and the 18o rich water dehydrated serpentine was then produced during laboratory heating experiment in vacuum at t 930 c for 1 h the oxygen isotopic composition of the dehydrated serpentine was enriched in 18o by a further 7 the net result of the hydration dehydration process was an enrichment of 18o in the final material by approximately 17 the new experimental results suggest that the oxygen isotopic compositions of mccs of the belgica like group including dhofar 225 and dhofar 725 could be derived from those of typical cm2 chondrites via several cycles of hydration dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids

radiocarbon chronology of lake bonneville eastern great basin usa abstract lake bonneville occupied a series of connected topographically closed structural basins in the eastern great basin from about 30 ka to 12 ka the following synthesis of lake bonneville history is based on a critical evaluation of the stratigraphic and geomorphic contexts of 83 radiocarbon ages of a variety of samples including wood charcoal dispersed organic matter mollusk shells and tufa the lake began to rise from levels close to average holocene levels after about 28 ka by 22 ka it had transgressed approximately 100 m between 22 and 20 ka it regressed about 45 m in the stansbury oscillation and the stansbury shoreline was formed transgression after 20 ka proceeded in two phases a rapid phase from 20 to 18 ka and a slower phase from 18 to 15 ka the lake overflowed intermittently at its highest level the bonneville shoreline from about 15 to 14 5 ka then catastrophically dropped 100 m during the bonneville flood to the level of the provo shoreline which it occupied until about 14 ka subsequent closed basin regression was rapid and complete by 12 ka and was followed by a modest transgression to form the gilbert shoreline between 10 9 and 10 3 ka the lake bonneville record is an accurate proxy of the changing water balance in the bonneville basin during the late pleistocene although the nature of the climatic changes during this period are still uncertain

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes

evidence for an impact induced magnetic fabric in allende and exogenous alternatives to the core dynamo theory for allende magnetization we conducted a paleomagnetic study of the matrix of allende cv3 chondritic meteorite isolating the matrix s primary remanent magnetization measuring its magnetic fabric and estimating the ancient magnetic field intensity a strong planar magnetic fabric was identified the remanent magnetization of the matrix was aligned within this plane suggesting a mechanism relating the magnetic fabric and remanence the intensity of the matrix s remanent magnetization was found to be consistent and low 6 t the primary magnetic mineral was found to be pyrrhotite given the thermal history of allende we conclude that the remanent magnetization was formed during or after an impact event recent mesoscale impact modeling where chondrules and matrix are resolved has shown that low velocity collisions can generate significant matrix temperatures as pore space compaction attenuates shock energy and dramatically increases the amount of heating nonporous chondrules are unaffected and act as heat sinks so matrix temperature excursions are brief we extend this work to model allende and show that a 1 km s planar impact generates bulk porosity matrix porosity and fabric in our target that match the observed values bimodal mixtures of a highly porous matrix and nominally zero porosity chondrules make chondrites uniquely capable of recording transient or unstable fields targets that have uniform porosity e g terrestrial impact craters will not record transient or unstable fields rather than a core dynamo it is therefore possible that the origin of the magnetic field in allende was the impact itself or a nebula field recorded during transient impact heating

the initiation and development of metamorphic foliation in the otago schist part 2 evidence from quartz grain shape data shape size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the otago schist new zealand reveal the role of quartz in the progressive development of metamorphic foliation sedimentary compaction and diagenesis contributed little to the formation of a shape preferred orientation spo within the analysed samples metamorphic foliation was initiated at sub greenschist facies conditions as part of a composite s1 bedding structure parallel to the axial planes of tight to isoclinal f1 folds an important component of this foliation is a pronounced quartz spo that formed dominantly by the effect of dissolution precipitation creep on detrital grains in association with f1 strain with increasing grade the following trends are evident from the spo data i a progressive increase in the aspect ratio of grains in sections parallel to lineation and the development of blade shaped grains ii the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation lineation and the intermediate axes of the blades define a partial girdle about the lineation iii a slight thinning and reduction in volume of grains in the one transect and iv an actual increase in thickness and volume in the survivor grains of the second transect the highest grade samples within the chlorite zone of the greenschist facies record segregation into quartz and mica rich layers this segregation resulted largely from f2 crenulation and marks a key change in the distribution deformation and spo of the quartz grains the contribution of quartz spo to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment pressure solution now affects the margins of quartz rich layers rather than individual grains in higher grade samples therefore the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica spo

post impact thermal structure and cooling timescales of occator crater on asteroid 1 ceres abstract occator crater is perhaps the most distinct surface feature observed by nasa s dawn spacecraft on the cerean surface contained within the crater are the highest albedo features on the planet cerealia facula and vinalia faculae and relatively smooth lobate flow deposits we present hydrocode simulations of the formation of occator crater varying the water to rock ratio of our pre impact cerean surface we find that at water to rock mass ratios up to 0 3 sufficient volumes of occator s post impact subsurface would be above the melting point of water to allow for the deposition of faculae like deposits via impact heat driven hydrothermal effusion of brines this reservoir of hydrothermally viable material beneath the crater is composed of a mixture of impactor material and material uplifted from 10 s of kilometers beneath the pre impact surface which could sample a deep subsurface volatile reservoir if present using a conductive cooling model we estimate that the lifetime of hydrothermal activity within such a system depending on choice of material constants is between 0 4 and 4 myr our results suggest that impact heating from the occator forming impact provides a viable mechanism for the creation of the observed faculae with the proviso that the faculae formed within a relatively short time window after the crater itself formed

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

defining the mechanism for compaction of the cv chondrite parent body the allende meteorite a relatively unaltered member of the cv carbonaceous chondrite group contains primitive crystallographic textures that can inform our understanding of early solar system planetary compaction to test between models of porosity reduction on the cv parent body complex microstructures within 0 5 mm diameter chondrules and 10 m long matrix olivine grains were analyzed by electron backscatter diffraction ebsd techniques the large area map presented is one of the most extensive ebsd maps to have been collected in application to extraterrestrial materials chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal plastic deformation crystallographic preferred orientations cpos preserved by matrix olivine grains are strongly coupled to grain shape most pronounced in shortest dimension yet are locally variable in orientation and strength lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold significant local variability in the orientation and intensity of compaction is also inconsistent with a global process detailed microstructures indicative of crystal plastic deformation are consistent with brief heating events that were small in magnitude when combined with a lack of sintered grains and the spatially heterogeneous cpo ubiquitous hot isostatic pressing is unlikely to be responsible furthermore allende is the most metamorphosed cv chondrite so if sintering occurred at all on the cv parent body it would be evident here we conclude that the crystallographic textures observed reflect impact compaction and indicate shock wave directionality we therefore present some of the first significant evidence for shock compaction of the cv parent body

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

efficient early global relaxation of asteroid vesta abstract the asteroid vesta is a differentiated planetesimal from the accretion phase of solar system formation although its present day shape is dominated by a non hydrostatic fossil equatorial bulge and two large mostly unrelaxed impact basins vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating we use a finite element viscoplastic flow model coupled to a 1d conductive cooling model to calculate the expected rate of relaxation throughout vesta s early history we find that given sufficient non hydrostaticity the early elastic lithosphere of vesta experienced extensive brittle failure due to self gravity thereby allowing relaxation to a more hydrostatic figure soon after its accretion vesta reached a closely hydrostatic figure with 40 200 my after formation depending on the assumed depth of megaregolith the veneneia and rheasilvia giant impacts which generated most non hydrostatic topography must have therefore occurred 40 200 my after formation based on crater retention ages topography and relation to known impact generated features we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early vesta the long wavelength figure of this terrain suggests that before the two late giant impacts vesta had a rotation period of 5 02 h 6 3 faster than present while its spin axis was offset by 3 0 from that of the present the evolution of vesta s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non hydrostatic terrains and epochs

p t path development derived from shearband boudin microstructure abstract this work focuses on the development of a regional p t path from the malpica lamego ductile shear zone nw portugal based on the microstructures of shearband boudins evolved during progressive simple shear the combination of microstructural analysis fluid inclusion studies crystallographic preffered orientation and fractal geometry analyses allows to link several stages in the internal evolution of the boudin to regional p t conditions the boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix two main transformations occur simultaneously i change in the external shape with continuous evolution from tabular rigid body to sigmoidal asymmetric morphology shearband boudin and ii localized dynamic recrystallization in the sharp tips of the structure acute edge of shearband boudin and along the boudin s margin and grain boundaries smaller recrystallized grains particularly in the sharp tip domains accommodate most of the external strain and larger relict grains are preserved in the centre dynamic recrystallization under constant strain rates and strain partitioning inside the boudins is indicated by fractal geometry based on grain boundary and grain area analysis progressive deformation leads to the generation of structural and textural heterogeneous domains inside the boudins and is recorded by quartz c axis orientation analysis and fluid inclusion studies the last deformation episode shows the final formation of the blunt tip domain and internal secondary shear planes the regional p t path begins with the crystallization of andalusite after an internal shearband boudin dilation event and ends with quartz dynamic recrystallization on boudin tips the main deformation stage 310 315 ma led to reactivation of internal secondary shear zones with sillimanite crystallization

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

the initiation and development of metamorphic foliation in the otago schist part 1 competitive oriented growth of white mica the 3d shape size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the otago schist new zealand reveal that metamorphic foliation as defined by mica shape preferred orientation spo developed rapidly at sub greenschist facies conditions early in the deformation history the onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition higher aspect ratios and a strong preferred orientation the metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation once initiated foliation progressively intensified by an increase in the aspect ratio size and alignment of grains although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in spo strength despite continued increase in grain size these trends are interpreted in terms of progressive competitive anisotropic growth of blade shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation the competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade from c 1000 relatively small mica grains per square millimetre of thin section at lower grades to c 100 relatively large grains per square millimetre in higher grade samples reversal of spo intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field

first finding of impact melt in the iie netschaevo meteorite about half of the iie nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature the presence of preserved chondrules has been reported for two iie meteorites so far netschaevo and mont dieu which represent the most primitive silicate material within this group in this study silicate inclusions from two samples of netschaevo were examined both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse grained olivine and pyroxene set in a fine grained groundmass that consists of new crystals of olivine and a glassy appearing matrix this texture does not correspond to the description of the previously examined pieces of netschaevo which consist of primitive chondrule bearing angular clasts detailed petrographic observations and geochemical analyses suggest that the investigated samples of netschaevo consist of quenched impact melt this implies that netschaevo is a breccia containing metamorphosed and impact melt rock imr clasts and that collisions played a major role in the formation of the iie group