fire ecology of the carboniferous tropical zone abstract fossil charcoal fusain indicating the occurrence of wildfire is common in the carboniferous rocks of north america and europe during the carboniferous these regions lay on the southern margins of the laurasian continent and within the tropical zone this study examines the facies distribution and botanical identity of charcoal in order to investigate the influence of fire on carboniferous tropical ecology new data from charcoal bearing units in eastern canada poland and the british isles are presented and synthesised with previously published data of charcoal occurrence tournaisian visean charcoal bearing strata were deposited in volcanic lacustrine alluvial plain and coastal marine environments in these deposits charred floras are dominated by pro gymnospermous plants whilst uncharred compressed or permineralised floras are dominated by lycopsids uncharred lycopsid remains were probably derived from wetland communities growing close to the depocentres whilst charred gymnospermous remains may have been sourced from well drained extrabasinal settings based on the vertical distribution of charcoal layers in annually laminated lacustrine deposits it is estimated that fires occurred with high frequency approximately every 3 35 years palaeoclimatic analysis indicates that southern laurasia at this time was subject to a seasonal monsoonal tropical climate such environments today are characterised by tropical savannas with high fire frequencies and early carboniferous fire prone gymnospermous plant communities may have represented a comparable vegetation biome namurian stephanian charcoal bearing strata were deposited in alluvial plain coastal plain and coastal marine environments channelised sandstone units interpreted as river channel deposits contain charcoal derived from cordaitaleans and conifers this assemblage records fires in extrabasinal coniferopsid forests mudstone units containing coals are interpreted as the deposits of floodbasins and peat mires hollow uncharred lepidodendrid stumps occur in this facies and contain lepidodendrid charcoal in the stump bases and lepidodendrid pteridosperm coniferopsid and sphenopsid charcoal outside the stumps this assemblage records fires in wetland lepidodendrid dominated forests lepidodendrid trunks comprising of a spongy parenchymatous core surrounded by dense periderm became hollowed out during fire events an estimate of fire frequency based on the vertical spacing of charcoal layers in coals implies that fires occurred every 105 1085 years in the peat mires palaeoclimatic analysis indicates that southern laurasia was subject to a humid tropical climate during the late carboniferous such environments today are characterised by tropical rainforests where fires occur rarely and late carboniferous communities may have represented a comparable biome

the mineralogy of ordinary chondrites and implications for asteroid spectrophotometry abstract the normative mineralogies of 94 ordinary chondrites have been calculated from high quality bulk chemical analyses relative proportions of olivine orthopyroxene and metal are more variable within the h l and ll classes than has been recognized previously mineralogy within each chondrite class varies systematically with oxidation state these data have important implications for the design of reflectance spectroscopy studies that match asteroid and meteorite spectral features and for quantitative spectral interpretations of asteroids they also provide a tool with which to predict mineral abundances in unsampled asteroids that formed under different redox conditions

topographic inversion and changes in the sediment routing systems in the variscan orogenic belt as revealed by detrital zircon and monazite upb geochronology in post collisional continental basins abstract the basal alluvial and fluvial siliciclastic successions within the late carboniferous intracontinental basins of the west central bohemian massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse we analyze this record using the u pb laser ablation icp ms detrital zircon and monazite geochronology five distinct age groups have been identified across the examined stratigraphic succession i archean paleoproterozoic ii late neoproterozoic iii cambro ordovician iv late devonian and v mid to late carboniferous these age groups indicate mixing of both local and distal extra basinal sources and also point to a two stage post convergent geodynamic evolution of the variscan belt the first stage followed collapse of the central bohemian orogenic plateau at around 346 337 ma and involved rapid cooling at ca 340 320 ma and removal of ca 2 3 km of sedimentary overburden from the top of the plateau no sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands a paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca 330 310 ma in turn these uplifted domains occupied by rapidly unroofed voluminous granite plutons shed detritus into the bohemian sink this was represented by the former plateau collapsed to relatively low elevations but still with significant internal paleorelief dominated by a system of tens of kilometers long nne ssw trending paleovalleys some of the material was further transported in extensive river systems crossing the former plateau to the northerly variscan foreland basin thus the radial sediment routing system changed into axial unidirectional finally we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late orogenic lithospheric mantle delamination events first stage delamination occurred beneath the center of the bohemian massif to cause the plateau collapse denudation and radial sediment transport whereas the second stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport end of deposition and magmatic shutdown at around 305 ma may then define the very cessation of late orogenic vertical movements and the transition to a post orogenic intra plate setting

in situ sims u pb dating of phanerozoic apatite with low u and high common pb abstract apatite is a common accessory mineral in various igneous metamorphic and sedimentary rocks it has strong potential to provide important information for geochronology and petrogenesis however precise u th pb dating of apatite especially young samples is difficult due to low abundances of u and less radiogenic pb posing significant challenges for accurate common pb correction an additional issue is the lack of suitable apatite to serve as a standard for in situ analyses here we present both analytical and data reduction protocols for sims in situ u pb analyses on apatite with low u and high common lead with nw 1 apatite as the u pb age standard which was separated from 1160 ma carbonatite in the prairie lake complex in canada apatites from the kovdor carbonatite in the kola peninsula which contains low u mostly 2 5 ppm and high common pb 20 80 yielded weighted average 207pb corrected u pb ages of 375 13 ma kv 8 and kv 18 and 377 11 ma kv a respectively apatites from the quruqtagh ultramafic intrusion in nw china containing low u 2 3 ppm and high common pb 70 yield 207pb corrected u pb age of 805 21 ma the cenozoic durango apatite was dated at 31 2 ma all these new apatite u pb ages are indistinguishable from independently known age constraints to within 2 4 our results demonstrate that a u pb age can be accurately and precisely measured for apatite with low u 50 by sims with a suitable standard and a careful choice of common pb composition

evolution of the middle westphalian river valley drainage system in central bohemia czech republic and its palaeogeographic implication abstract the basal unit of the kladno rakovnik basin krb in central and western bohemia the radnice member bolsovian is interpreted here as the fill of a combined incised and tectonically formed system of river valleys having a palaeotopography of the depocentre of up to 200 m restoration of the palaeotopography on basement surface is used to reconstruct the palaeo drainage system of this unit consisting of streams of four or five orders the main valleys form conjugated system of narrow straight and more then 15 km long tectonically controlled nw se and nne ssw striking valleys entered by incised valleys these erosional valleys consist of several orders and display a dendritic pattern the fill of this valley system the radnice member consists of two tectonically controlled sequences separated by a basin wide erosional surface with a relief of at least 20 m both units differ in their architecture as a consequence of base level changes expressed by the a s ratio accommodation sediment supply tectonics and sediment supply are considered as the most important controls on deposition whereas palaeotopography controlled the distribution of sediments within a valley system restorated palaeotopography indicates the existence of two orographically separated drainage systems in the krb the eastern part of the kladno rakovnik basin was drained to the nw possibly to the north variscan foredeep whilst its western part was probably drained through a central tectonic valley to the south southwest to the radnice and plze basins and then further ssw perhaps to the naab basin in se bavaria the krb was probably located about 1000 m above the carboniferous sea level

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

a comparative analysis of some late carboniferous basins of variscan europe vegetation diversity and pattern changes and their relation to tectono sedimentary histories are compared between selected euramerican late palaeozoic coalfields to understand better the controls on the dynamics of the pennsylvanian terrestrial ecosystems and to demonstrate the problems with comparing data from various basins the analysis is based on data from the following basins of different geotectonic and palaeogeographical positions the cratonic pennines basin the foreland south wales and upper silesia basins and the fault related intra sudetic and central and western bohemia basins the analysis indicates that complex factors are responsible for changes in plant diversity and vegetation patterns these are related to climate tectonics preservation potential sampling biases and the current state of revision of the flora in each basin plant diversity patterns in the basins differ because of local controls and or the character and detail of the available data maximum diversity varies among the basins within the langsettian and duckmantian substages two apparent step like drops in diversity were detected within coal bearing strata of most basins at the duckmantian bolsovian boundary and at the bolsovian asturian boundary further and more prominent falls are related to transitions from coal bearing to non coal bearing mostly red bed strata or vice versa during stephanian times interpretation of climatic signals recorded in the sedimentary successions indicates that westphalian and middle stephanian times were wet intervals whereas early and late stephanian times were drier

origin of a unique impact melt rock the l chondrite ramsdorf we have studied a unique impact melt rock the ramsdorf l chondrite using optical and scanning microscopy and electron microprobe analysis ramsdorf contains not only clast poor impact melt begemann and wlotzka 1969 but also a chondritic portion 60 g with what appears at low magnification to be a normal well defined chondritic texture however detailed studies at high magnification show that 90 vol of the crystals in the chondritic portion were largely melted by the impact the chondrules lack normal microtextures and are ghosts of the original features the only relics from the precursor chondrules are olivine crystals which have the highest melting temperature 1620 c pyroxene rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine rich melts fine grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts probably because pyroxene rich melts are significantly more viscous complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of ramsdorf formed directly from petrologic type 3 4 material by strong shock we infer that ramsdorf was largely melted by shock pressures of 75 90 gpa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture

the effect of paleotopography tectonics and sediment supply on quality of coal seams in continental basins of central and western bohemia westphalian czech republic abstract the radnice member the most important coal bearing unit of the continental basins in central and western bohemia czech republic is used here to demonstrate the interplay of controls on mire formation and distribution in continental settings its middle westphalian strata are interpreted as tectonically active or erosively incised river valley infills principally controlled by tectonic subsidence and clastic input another important controlling factor was the paleotopography with some hills up to 200 m above the valley floor which was responsible for the distribution of siliciclastics within the dendritic depocenter the interplay of these factors is described here in terms of base level changes expressed in the form of a s ratio accommodation sediment supply ratio martinsen o j ryseth a helland hansen w flesche h torkildsen g idil s 1999 stratigraphic base level and fluvial architecture ericson sandstone campanian rock springs uplift sw wyoming usa sedimentology 46 235 259 vertical changes in a s ratio were related mostly to fluctuation of tectonic subsidence whereas lateral variations of this ratio between valleys or within them were connected with uneven clastic supply of individual valleys from their source areas in valleys where clastic input overreached the accommodation space production a s 1 due to high subsidence rate or low clastic input sediments transported by axial drainage were confined to the upper valley parts the lower and middle parts of the valleys which starved of clastic sedimentation provided ideal conditions for long lasting peat accretion as a result two economically important groups of coals developed in the radnice member in the kladno rakovnik basin the older radnice group and the younger lubna group a high rate of base level rise was typical of the deposition of the radnice group and led to the formation of extensive mires in lower parts of the valleys that had a restricted clastic influx the mires of the lubna group formed during a time when the base level rise was still high but lower than during the deposition of the radnice group mostly small but long lasting mires developed in sedimentary shadows located along the valley margins or in smaller valleys away from the main clastic transport and fluvial dynamics

palynological evidence for pennsylvanian extra basinal vegetation in atlantic canada abstract pollen preserved in latest westphalian and early stephanian clastic deposits of the sydney coalfield cape breton island suggest that there were zones of different vegetational habitats surrounding the wetlands where the coal forming peat was formed on the margins of the wetlands was a narrow band with large cordaitanthacean trees this was surrounded by occasionally dry9 habitats supporting schuetzia plants ruflorioid plants and aethophyllum like herbaceous conifers around this were mesic or seasonally dry habitats dominated by walchian conifers and disaccate producing peltasperms these in turn were surrounded by xeric or seasonally wet habitats dominated by ullmanniacean like conifers cycads and monosaccate producing peltasperms during latest westphalian and early stephanian times the mesic and xeric habitats encroached nearer the wetlands although the vegetation of the wetlands themselves remained essentially unaffected this vegetation change was not a response to climate change but probably to changing drainage patterns and topography in the surrounding hinterland the florinites producing cordaitanthaceans appear not to have been upland trees as previously suggested but occupied mainly coastal habitats or riparian habitats on the margins of the wetlands

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

complexities on the acapulcoite lodranite parent body evidence from trace element distributions in silicate minerals the trace element distributions of individual minerals from seven acapulcoites and lodranites have been studied systematic differences are evident between some members of the two groups specifically pyroxenes from the lodranites macalpine hills mac 88177 and lewis cliff lew 88280 exhibit depletions of the rare earth elements ree and other incompatible trace elements ti zr y relative to acapulcoite acapulco allan hills alh a81261 pyroxenes that are consistent with the formation and removal of 15 or more silicate partial melts from these meteorites phosphate ree patterns in these lodranites also support this scenario however other members of the acapulcoite lodranite clan exhibit more complex trace element variations elephant moraine eet 84302 which has been classified as transitional between the acapulcoites and lodranites generally has trace element characteristics similar to the acapulcoites however its plagioclase ree compositions suggest a somewhat greater degree of metamorphism than that experienced by acapulcoites such as acapulco and alha81261 similar and elevated ree abundances in the silicate phases from acapulcoite alha81187 and lodranite graves nunataks gra 95209 suggest that these two meteorites in fact experienced similar thermal histories this probably included some silicate partial melting although little melt appears to have been lost from the samples the observed variations in the trace element abundances of these samples from the acapulcoite lodranite clan emphasize the complex and varied processes that have acted on their parent body the simple bimodal classification of these meteorites based primarily on petrographic criteria which has been used to date appears to be inadequate to describe this diverse group of samples as they represent a range of degrees of partial melting both with and without accompanying melt migration in some instances secondary processes on the parent body such as cryptic metasomatism have further modified sample compositions

ion microprobe u pb dating of apatite abstract to illustrate in situ sensitive high resolution ion microprobe shrimp analysis of apatites pb isotopes and pb u ratios of five apatite samples with different ages have been measured using shrimp ii ion microprobe at hiroshima university the prap apatite from a carbonatite complex at prairie lake ontario with a 207 pb 206 pb age of 1156 45 ma 2 was used as the standard for 206 pb 238 u 238 u 16 o 238 u calibration apatites from the poohbah lake alkaline complex give 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages of 2726 145 ma 2 and 2701 86 ma 2 in agreement with a conventional tims 207 pb 206 pb age of 2667 10 ma 2 and a k ar isochron age of 2706 46 ma 2 for the remaining three apatite samples the 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages obtained by shrimp are consistent with radiometric and geological ages in the literature

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

disequilibrium partial melting experiments on the leedey l6 chondrite textural controls on melting processes a series of experiments were designed to investigate the textural and compositional changes that take place during disequilibrium partial melting of chondritic material chips of the l6 chondrite leedey were heated at 1200 degreesc and log fo 2 iw 1 for durations of 1 h to 21 days we observed a progression of kinetically controlled textural changes in melt and restite minerals and changes in the liquidus mineralogy in response to factors such as volatile loss during the course of the experiments both olivine and orthopyroxene recrystallized at different times rare relic chondrules could still be identified after 21 days the silicate melts that form are very heterogeneous in terms of both major and trace element chemistry reflecting heterogeneity of the localized mineral assemblage particularly with respect to phosphates and clinopyroxene metal sulfide melts formed in short duration runs are also heterogeneous the experimental data are relevant to aspects of the genesis of primitive achondrites such as the acapulcoites the observed textures are consistent with a model for acapulcoite petrogenesis in which silicate melting was limited to only a few volume percent of the chondritic source rock the experiments are also relevant to the behavior of chondritic material that has been partially melted in an impact environment

ductile normal faulting along the west bohemian shear zone moldanubian tepla barrandian boundary evidence for late variscan extensional collapse in the variscan internides structural and kinematic investigations of the west bohemian shear zone wbs clearly indicate late variscan orogen parallel wsw ene extension within the variscan internides along the wbs the western part of the tepla barrandian tb was downthrown to the east against the adjacent moldanubian according to seismic data the steeply east dipping wbs flattens with depth forming a prominent detachment zone the western part of the tb was tilted along this zone producing the patterns of metamorphic isograds the age of which is probably cadomian cross cutting relationships of wbs mylonites and carboniferous granites as well as the overall cooling ages of hornblende and mica suggest that ductile normal faulting along the wbs was active from about 330 to 310 ma

pennsylvanian fossil forests in growth position t0 assemblages origin taphonomic bias and palaeoecological insights abstract fossil forests buried in growth position in a geological instant t 0 assemblages are far more abundant in pennsylvanian successions than in any other part of the geological record in this review paper we evaluate the fundamental controls on the origin of these phenomena investigate the taphonomic biases that influence their composition and summarize their palaeoecological significance following earlier workers we highlight that high rates of burial and accommodation are essential for the formation and preservation of t 0 assemblages contexts especially favourable for their origin include ashfalls proximal to volcanic centres coastal plains drowned by relative sea level rise and fluvial environments such as channel bars crevasse splays and distributary lobes long term preservation requires high rates of subsidence consequently the vast majority of palaeozoic t 0 assemblages are confined to wetland settings at or close to sea level whereas drylands are poorly represented and uplands rarely sampled if ever however this is not the only major bias in the fossil record taphonomic processes selectively preserve plants dependent on their anatomy and stature and on groundwater chemistry thus although t 0 assemblages offer unrivalled insights into the nature of ancient forests whole plant reconstructions tree density canopy height productivity plant hydraulics cohort dynamics spatial heterogeneity ecological gradients tree sediment interactions and animal plant interactions to name but a few it is naive to believe they provide photographic snapshots9 of palaeoecosystems none the less careful taphonomic analysis of t 0 assemblages offers the potential for a nuanced understanding of these evocative phenomena and much remains to be learned from these important palaeoecological resources

phosphate and feldspar mineralogy of equilibrated l chondrites the record of metasomatism during metamorphism in ordinary chondrite parent bodies in ordinary chondrites ocs phosphates and feldspar are secondary minerals known to be the products of parent body metamorphism both minerals provide evidence that metasomatic fluids played a role during metamorphism we studied the petrology and chemistry of phosphates and feldspar in petrologic type 4 6 l chondrites to examine the role of metasomatic fluids and to compare metamorphic conditions across all three oc groups apatite in l chondrites is cl rich similar to h chondrites whereas apatite in ll chondrites has lower cl f ratios merrillite has similar compositions among the three chondrite groups feldspar in l chondrites shows a similar equilibration trend to ll chondrites from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6 this contrasts with h chondrites which have homogeneous albitic plagioclase in petrologic types 4 6 alkali and halogen rich and likely hydrous metasomatic fluids acted during prograde metamorphism on oc parent bodies resulting in albitization reactions and development of phosphate minerals fluid compositions transitioned to a more anhydrous cl rich composition after the asteroid began to cool differences in secondary minerals between h and l ll chondrites can be explained by differences in fluid abundance duration or timing of fluid release phosphate minerals in the regolith breccia kendleton show lithology dependent apatite compositions bulk cl f ratios for ocs inferred from apatite compositions are higher than measured bulk chondrite values suggesting that bulk f abundances are overestimated and that bulk cl f ratios in ocs are similar to ci

shock metamorphism of planetary silicate rocks and sediments proposal for an updated classification system we reevaluate the systematics and geologic setting of terrestrial lunar martian and asteroidal impactites resulting from single or multiple impacts for impactites derived from silicate rocks and sediments we propose a unified and updated system of progressive shock metamorphism shock metamorphosed rocks occur as lithic clasts or melt particles in proximal impactites at impact craters and rarely in distal impactites they represent a wide range of metamorphism typically ranging from unshocked to shock melted as the degree of shock metamorphism at a given shock pressure depends primarily on the mineralogical composition and the porosity of a rock or sediment sample different shock classification systems are required for different types of planetary rocks and sediments we define shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments 1 crystalline rocks with classes f m a and u 2 chondritic rocks class c and 3 sedimentary rocks and sediments with classes sr se and re the abbreviations stand for felsic f mafic m anorthositic a ultramafic u sedimentary rocks sr unconsolidated sediments se and regoliths re in each class the progressive stages of shock metamorphism are denominated s1 to sx these progressive shock stages are introduced as s1 s7 for f s1 s7 for m s1 s6 for a s1 s7 for u s1 s7 for c s1 s7 for sr s1 s5 for se and s1 s6 for re s1 stands for unshocked and sx variable between s5 and s7 stands for whole rock melting we propose a sequence of symbols characterizing the degree of shock metamorphism of a sample i e f s1 to f s7 with the option to add the tabulated pressure ranges in gpa in parentheses

the methane diurnal variation and microseepage flux at gale crater mars as constrained by the exomars trace gas orbiter and curiosity observations funding for this work was provided by the canadian space agency s mars science laboratory participating scientist program

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

formation of high pressure minerals in shocked l6 chondrite yamato 791384 constraints on shock conditions and parent body size we have studied the high pressure phases observed in yamato 791384 l6 chondrite host meteorite mainly consists of olivine pyroxenes and plagioclase glass mineral fragments observed in the veins and the vein margin region of the host meteorite were partially or totally transformed into high pressure phases ringwoodite majorite akimotoite naalsi3o8 hollandite and jadeite matrix of the shock vein contains majorite pyrope solid solution the spatial distribution indicates that high pressure phases are present in the shock veins and host rocks adjacent to the shock veins investigation of the high pressure phases revealed that in y791384 fragments and adjacent matrix were subjected to pressures around of 18 23 gpa and the vein experienced temperatures around 2300 c during the shock event ringwoodite lamellae were observed in the host olivine adjacent to the vein kinetic investigation for ringwoodite lamellar growth in olivine indicates that the shocked meteorite experienced the pressure for at least 4 s such long duration of the pressure pulse indicates that the size of the parent body was greater than 10 km

hf w thermochronometry ii accretion and thermal history of the acapulcoite lodranite parent body article acapulcoites and lodranites are highly metamorphosed to partially molten meteorites with mineral and bulk compositions similar to those of ordinary chondrites these properties place the acapulcoites and lodranites between the unmelted chondrites and the differentiated meteorites and as such acapulcoites lodranites are of special interest for understanding the initial stages of asteroid differentiation as well as the role of 26 al heating in the thermal history of asteroids to constrain the accretion timescale and thermal history of the acapulcoite lodranite parent body and to compare these results to the thermal histories of other meteorite parent bodies the hf w system was applied to several acapulcoites and lodranites acapulcoites dhofar 125 and nwa 2775 and lodranite nwa 2627 have indistinguishable hf w ages of tcai 5 2 0 9 ma and tcai 5 7 1 0 ma corresponding to absolute ages of 4563 1 0 8 ma and 4562 6 0 9 ma closure temperatures for the hf w system for acapulcoites and lodranites estimated from numerical simulations of w diffusion in high ca pyroxene are 975 50 c and 1025 50 c respectively owing to these high closure temperatures the hf w ages provide information on the earliest high temperature evolution and combined with thermal modeling indicate that the acapulcoite lodranite parent body accreted 1 5 2 ma after cai formation was internally heated by 26

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

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

formation of large scale impact melt dikes a case study of the foy offset dike at the sudbury impact structure canada abstract hypervelocity impacts frequently result in the formation of dikes in the crater floor and central uplift impact melt bearing dikes in large terrestrial impact basins such as the offset dikes at the sudbury impact structure occur at a large scale often tens of meters wide and several kilometers in length the offset dikes host significant ni cu pge deposits which include several well known mines such as totten and copper cliff that have been mined for nearly 100 years the offset dikes typically consist of a clast and sulfide rich core and clast poor margins their formation has been a subject of debate for decades the most widely proposed model is the early emplacement of clast poor impact melt shortly after the impact event followed by the later emplacement of clast and sulfide rich impact melt in the center of the dike an alternative hypothesis is that a single pulse of clast rich impact melt flowed into the fractured target rocks and flow differentiation resulted in a clast rich core and a clast poor margin in this study we examine field and petrographic relationships of the foy offset dike to better understand its emplacement our results show that the characteristics of the foy offset dike namely the gradational nature of the contact between the clast rich and clast poor phases the alignment of clasts sub parallel to this contact the geochemical similarities and the presence of sulfides within both phases of the dike are more consistent with the single injection and flow differentiation hypothesis

petrology and genesis of upper carboniferous seams from the ruhr region west germany abstract the petrology of 21 seam profiles of upper carboniferous age westphalian c and b has been studied in order to determine their depositional environments and diagenetic history the youngest profile was drilled at a depth of 790 m and is mainly overlain by cretaceous sediments the oldest seam was reached at a depth of 1470 m the seam thicknesses vary from 0 4 to 2 5 m the main petrographic compounds are vitrinite intertinite liptinite and minerals the last group occurs as clay bands with illite kaolinite minor chlorite and minor quartz contents or as crack or pore filling calcite fe dolomite siderite pyrite or marcasite or as syngenetic siderite concretions and pyrite crystals the percentages of the different macerals and minerals vary mainly because of different depositional environments diagenetic loss or genesis of compounds is a less important factor in their distribution three types of profiles are distinguished by their different petrologies type i is most abundant and contains much vitrinite many clay bands and syngenetic iron sulfides whereas type iii is rich in inertinite and certain characteristic spores type ii is intermediate but generally contains only low percentages of minerals generally this type is vitrinite rich in the lower and inertinite rich in the central and upper parts of the profiles spores and other liptinites are much better preserved in all the seams than in clay partings or in siltstones and sandstones above and below the seams the seams are interpreted to be former autochthonous peats type i profiles are probably derived from swamps which were sometimes inundated and covered by overbank deposits type ii and iii seams represent former peats which were not inundated by rivers and partly grew under the influence of more oxidizing conditions therefore they contain more inertinite and less sulfide and clay bands they can be interpreted as former raised bogs diagenetic changes are expressed as increases of vitrinite reflectances from 0 65 to 1 0 and of liptinite reflectances a red shift of fluorescence of liptinites was found increasing amounts of exudatinite and micrinite and decreasing amounts of fluorinite and resinite were found minerals seem to be less affected by diagenesis illite crystallinity for example remains poor

a petrologic and isotopic study of lodranites evidence for early formation as partial melt residues from heterogeneous precursors abstract we have conducted petrologic chemical and isotopic studies of lodranites in an attempt to constrain their genesis lodran gibson y 791491 y 791493 y 74357 y 8002 y 75274 mac 88177 lew 88280 eet 84302 fro 90011 and que 93148 are classified as lodranites lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates with lodranites being significantly coarser grained lodranites exhibit a diverse range of petrologic and mineralogic features they range widely in mafic silicate compositions fa3 13 plagioclase 0 11 4 vol fe ni metal 0 5 20 vol and troilite 0 2 5 3 vol contents and shock levels s1 s4 they appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates i e 700 350 c the only dated lodranite gibson cooled to ar closure temperatures at 4 49 0 01 ga lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated heating of their parent body to temperatures between 1050 1200 c resulted in formation of fe ni fes and basaltic partial melts depletions of troilite and or plagioclase in most lodranites testify to the removal of some of these partial melts although melt migration was complex lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures followed by rapid cooling at intermediate temperatures possibly related to breakup of the parent body lodranites were liberated from their parent body during 1 3 impact events with most having cosmic ray exposure ages of 5 5 7 ma the acapulcoites are samples from the same parent body but were heated to lower temperatures and thus experienced lower degrees of partial melting

a review of the middle late pennsylvanian west european regional substages and floral biozones and their correlation to the geological time scale based on new u pb ages abstract lower moscovian duckmantian to lower asselian ash beds in the continental basins of the central and western czech republic have yielded 15 new high precision u pb single crystal zircon ca id tims ages these are used to improve the calibration of the west european regional terrestrial stages and their correlation to the geological time scale they also allow an improved correlation between the macrofloral biozones that are used for biostratigraphy in the european pennsylvanian coalfields the marine biozones on which the global geological time scale was partly established and the fresh water and terrestrial faunal biozones most of the floral biozones have proved to be essentially isochronous although the ranges of some individual but stratigraphically important taxa may differ between basins

shock metamorphism and petrography of the shergotty achondrite the shergotty subsamples 1 and 12 consist of augite and pigeonite 67 5 maskelynite 24 ilmenite and titanomagnetite 2 pyrrhotite 0 4 whitlockite 1 8 apatite 0 1 quartz 0 5 baddeleyite trace fayalite 0 4 mesostasis 3 and shock induced local polymineralic melt products 0 6 the overall modal composition is similar to other shergotty samples except for the rather high whitlockite content the shock effects observed in the mineral constituents include mosaicism deformation bands planar fractures and mechanical twin lamellae in clinopyroxene isotropization of plagioclase with very rare remnants of birefringence planar deformation structures mosaicism and strongly reduced birefringence in quartz mechanical twinning of ilmenite localized in situ melting of neighbouring minerals at the contact of low and high density phases based on the refractive index of maskelynite average 1 5467 with average an content of 49 and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 1 gpa is derived the inferred post shock temperature is 200 20 c no heating event could have exceeded 400 c duke 1968 local stress and temperature concentrations reach 60 80 gpa and 1600 2000 c the observed shock effects can be explained by a single shock event a second weaker shock event as found by others appears to be highly improbable equilibrium shock pressures and post shock temperatures for the other known shergottites are 31 2 gpa and 220 50 c zagami 43 2 gpa and 400 800 c alha 77005 the pressure estimate for eeta 79001 by lambert 1985 is confirmed 34 1 gpa the post shock temperature is 250 50 c the abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence shergotty zagami eeta 79001 alha 77005 undoubtedly the melts could have been formed by the same single shock which produced the equilibrium shock effects e g maskelynitization in these meteorites the shock induced particle velocities inferred from hugoniot data of basalts are in the 1 5 2 0 km s range for the parental rocks of the shergottites ejection velocities are therefore in the order of 3 4 km s special ejection mechanisms are required in order to exceed the escape velocity of a planet like mars without producing higher degrees of shock e g melting than those observed in the shergottites

the pressures and temperatures of meteorite impact evidence from micro raman mapping of mineral phases in the strongly shocked taiban ordinary chondrite taiban is a heavily shocked l6 chondrite showing opaque melt veins raman spectroscopy was used to characterize the high pressure silicate assemblages in a thin section crossed by a shock created 4 mm wide melt vein raman spectra using different excitation wavelengths allowed identification of mineral phases such as olivine wadsleyite ringwoodite high ca clinopyroxene majorite pyrope jadeite maskelynite and lingunite olivine is fe depleted in contact with the ringwoodite which suggests chemical fractionation during a solid state olivine ringwoodite transformation raman imaging revealed a close correlation between the blue ringwoodite color and the peak observed at 877 cm 1 this signal shows strong near resonance raman enhancement when measured with near ir excitation lines 785 and 830 nm close to the optical absorption bands of the ringwoodite we propose that the blue color of the ringwoodite is due to a small amount of iron in fourfold coordination inside the spinel structure and that yields the observed spectral features in differently colored ringwoodite under the formation conditions of the studied silicate pocket all enstatite transformed to a majorite pyrope solid solution whereas the high ca clinopyroxene likely remained unchanged maskelynite grains in the margins of the pocket often contain lingunite or are totally transformed to jadeite based on static high pressure results the mineral assemblages in the pocket suggest peak pressure in the 17 20 gpa range with maximum temperature t max in the range 1850 1900 k as the formation conditions for this taiban chondrite during shock

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

carboniferous permian volcanic evolution in central europe u pb ages of volcanic rocks in saxony germany and northern bohemia czech republic nine shrimp u pb ages on zircon and two pb pb single zircon ages have been determined from late paleozoic volcanic rocks from saxony and northern bohemia samples came from the teplice altenberg volcanic complex the meissen volcanic complex the chemnitz basin the dohlen basin the brandov olbernhau basin and the north saxon volcanic complex the teplice altenberg volcanic complex is subdivided into an early namurian phase mikulov ignimbrite 326 8 4 3 ma thus older than assumed by previous studies and a late caldera forming phase teplice ignimbrite 308 8 4 9 ma the age of the latter however is not well constrained due to a large population of inherited zircon and possible hydrothermal overprint the leutewitz ignimbrite product of an early explosive volcanic episode of the meissen volcanic complex yielded an age of 302 9 2 5 ma stephanian a volcanic rocks intercalated in the brandov olbernhau basin bob 302 2 8 ma chemnitz basin cb 296 6 3 0 ma dohlen basin db 296 3 0 ma and the north saxon volcanic complex nsvc c 300 290 ma yielded well constrained stephanian to sakmarian ages the largest late paleozoic ignimbrite forming eruption in central europe the rochlitz ignimbrite has a well defined middle asselian age of 294 4 1 8 ma ages of palingenic zircon revealed that the namurian westphalian magmatism assimilated larger amounts of crystalline basement that formed during previous paleozoic geodynamic phases the precambrian inherited ages support the chronostratigraphic structure assumed for the saxo thuringian zone of the variscan orogen the present results help to improve the chronostratigraphic allocation of the late paleozoic volcanic zones in central europe at the same time the radiometric ages have implications for the interbasinal correlation and for the geodynamic evolution of the variscan orogeny

a petrologic chemical and isotopic study of monument draw and comparison with other acapulcoites evidence for formation by incipient partial melting abstract we have conducted petrologic chemical and isotopic studies of acapulcoites acapulco monument draw yamato 74063 alh a77081 alh a81261 alh a81315 alh 78230 alh a81187 and alh 84190 in an attempt to constrain their genesis acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in iab and iiicd irons winonaites and ureilites and thus formed on a different parent body oxygen isotopic compositions which are slightly heterogeneous within the group overlap with lodranites indicating a likely origin on a common parent body these groups can be distinguished on the basis of mafic silicate grain size all acapulcoites have mafic silicate compositions intermediate between e and h chondrites roughly chondritic mineralogies achondritic equigranular textures micrometer to centimeter sized veins of fe ni fes which cross cut silicate phases rapid metallographic cooling rates at 600 400 c 103 105 c myr and trapped noble gas abundances comparable to type 3 4 ordinary chondrites they exhibit variable mafic silicate zoning abundance of fe ni fes veins ree abundances and patterns and possibly cosmic ray exposure ages 5 7 ma momument draw and yamato 74063 retain rare relict chondrules phosphates are associated with fe ni fes veins or form separate veins in monument draw and acapulco heating and cooling of acapulcoites occurred very early in the history of the solar system as evidenced by the 39ar 40ar ages of 4 51 ga these ages appear distinctly younger than the likely formation time for acapulco of 4 557 ga but are older than analogous 39ar 40ar ages for most chondrites acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions heating to 950 1000 c resulted in melting at the fe ni fes cotectic but silicates did not melt silicate textures resulted from extensive solid state recrystallization heating was by noncollisional sources e g 26al electromagnetic induction despite uncertainties owing to a lack of data acapulcoites may have experienced a three stage thermal history of slow cooling at high temperature rapid cooling at intermediate temperatures and slow cooling at low temperatures possibly resulting from breakup and gravitational reassembly of the parent body the complex thermal history is also reflected in disequilibrium ree abundances one or at most two impact events 7 ma and possibly 5 ma ago are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist

climatic and biotic changes around the carboniferous permian boundary recorded in the continental basins of the czech republic abstract the paper provides an overview of a several decades long study of transitional carboniferous permian stephanian c autunian sedimentary successions in continental basins of the czech part of the bohemian massif these predominantly monotonous fluvial red beds intercalate with laterally widespread grey to variegated sediments of dominantly lacustrine origin both fossil and climatic records show that apart from a generally known long term climatic shift to drier conditions in early permian the climate oscillated on several time scales throughout the study interval climatic indicators in the red beds part of the succession include palaeosols ranging between red vertisols and vertic calcisols suggesting strongly seasonal dry sub humid climate this is in agreement with the rarity of plant remains which were mostly completely oxidised and only rarely preserved as plant impression in red mudstones or as silicified mostly gymnosperm woods in sandy channel fills silicification instead of coalification was the dominant fossilisation process during red beds deposition even drier possibly semi arid climate may be indicated by spatially and temporarily restricted bimodal sandstones dominated by well rounded quartz grains and interpreted as eolian in origin periods of moist sub humid or even humid climate were accompanied by formation of perennial lakes containing grey laminated mudstones dark grey bituminous mudstones or limestones muddy limestones chert layers or even spatially restricted coals some of them however of economic importance shorter climatic oscillations operating on a scale of tens to possibly hundreds of thousands of years are represented by transgressive regressive lacustrine cycles followed by significant changes in lake water salinity reflected by boron content the fossil record indicates the presence of dryland and wetland biomes in basinal lowlands although their proportions varied significantly as the climate changed during deposition of red beds the alluvial plain was vegetated dominantly by dryland biome assemblages the composition of these assemblages is indicated by fairly common silicified gymnosperm cordaitalean and coniferous woods in sandstone conglomerate fluvial channel bedforms and by poorly preserved impressions of walchian conifer shoots and cordaitalean leaves in associated mudstone intercalations this is in agreement with sub vertical root rhizolites and haloes in calcic vertisols occurrence of wet spots colonised by wetland assemblages is indicated by rather exceptional findings of silicified calamite stems in fluvial red beds associated with gymnospermous woods during the humid intervals parts of the basinal lowlands were occupied by lakes surrounded by broad belts of wetland biome floras during the stephanian c most of these floras were dominated by tree ferns calamites and sub dominant pteridosperms local peat swamps were colonised by lycopsids including sigillaria brardii asolanus camptotaenia and even some lepidodendrid lycopsids in contrast the fossil record of stephanian c dryland floras is rarely preserved in lacustrine sediments the fossil record of autunian lakes however suggests increasing proportions of dryland elements including conifers and peltasperms the response of lacustrine faunas to climatic oscillations around the carboniferous permian transition is less prominent than that of plants the origin of the transition between the local elonichthys sphaerolepis and acanthodes gracilis bio ecozones around the carboniferous permian boundary is impossible to deduce from the existing fossil record

middle moscovian climate of eastern equatorial pangea recorded in paleosols and fluvial architecture abstract fluvial architecture in conjunction with the morphology petrographical mineralogical and geochemical composition of intercalated paleosols of the middle moscovian upper radnice member of the continental kladno rakovnik basin czech republic were studied in order to decipher climatic signals in the eastern pangea region during the late paleozoic ice age analysis of architectural elements reveals vertical changes from strongly amalgamated channel fills of bedload dominated braided streams to isolated ribbon like meandering channel fills in mud dominated floodplain strata the resulting large scale fining upward cycles separated by laterally widespread bounding surfaces are interpreted as having been formed in response to allocyclic processes driven by variable intensities of precipitation which affected the density of vegetation cover across the landscape and in turn the amount and character of sediment supply bedload dominated parts of the cycles were deposited during periods of low water tables and sparse vegetation whereas periods of maximum precipitation resulted in higher water tables and a greater density of vegetation cover which in turn stabilized the floodplains and fixed streams to narrow meandering anastomosing channels in mud dominated floodplain strata associated paleosols resemble modern vertisols which form under seasonal precipitation in modern settings however coeval formation of inertinite rich peat represented by histosols and gleyed low chroma vertisols in the stratigraphically lowest paleosol succession occurred under overlapping marginal conditions for the climatically different types of soils i e under a seasonal climate where precipitation exceeds evapotranspiration for 6 to 8 months out of the year mean annual precipitation calculated from the geochemistry of b horizons of vertisols calmag proxy varies between 1815 and 1730 mm yr

palaeofloristics of middle pennsylvanian medullosaleans in variscan euramerica abstract medullosaleans were an abundant and widely distributed group of seed plants in the moscovian palaeotropical wetland forests coal forests using mainly a combination of cluster analysis and detrended correspondence analysis the distribution of medullosalean species has revealed five distinct palaeofloristic units within variscan euramerica two palaeofloristic units characterise lowland paralic areas the rhine palaeoprovince found in parts of the variscan foreland and the donets basin and the iberia palaeoprovince found in northern spain and portugal the silesia palaeoprovince represents somewhat lower diversity medullosalean macrofloras mainly associated with more marginal areas of the variscan foreland intramontane basins are characterised by two palaeofloristic units that are quite distinct from those of the more lowland areas and to a lesser extent different from each other the saar lorraine and central bohemia palaeoprovinces the latter found in the intrasudetic basin and zwickau as well as central bohemia except for the appearance of the iberia palaeoprovince in late moscovian times the basic palaeofloristic structure of variscan euramerica remained essentially unchanged through the moscovian age those palaeofloristic changes that did take place mainly reflect localised increased influxes of sediment or changes in water table the palaeofloristic data provides no evidence of regional scale environmental changes taking place such as marked climate change regional uplift or major palaeolatitudinal change

brachina a new type of meteorite not a chassignite the brachina meteorite previously classified as a chassignite is found to be a nearly primitive and unique meteorite it is a fine grained unshocked equigranular igneous rock containing small melt inclusions it contains 80 olivine fo70 5 5 clinopyroxene wo38en49fs13 10 plagioclase an22 0 5 chromite 0 5 chlorapatite 3 fe sulfide 0 3 pentlandite and a trace of ni rich metal orthopyroxene en70 and anorthoclase rich glass are found only in melt inclusions siderophiles are high in the sulfides and the oxidation state is similar to that of ll chondrites major element and mineralogic data of brachina are similar to chassigny but trace element data are dramatically different the major differences are higher feo mno ga al and k la ratios w depletion high siderophile content near horizontal ree pattern at about 1 chondrite level high refractory nonrefractory metal ratios e g ir au and chalcophile element depletion compositionally brachina is close to c2 chemistry but has undergone some segregation of fe ni s melt these data in addition to its near 4 5 b y age and oxygen isotopic signature within the eucritic field suggest that brachina is not related to chassigny or snc meteorites it may be related to primitive meteorites such as silicate inclusions in iab irons and winonaites but is more oxidized reduction would make it very similar to winonaites the assemblage may have experienced the beginning of melting in its parent body since it has characteristics that differ from any other meteorite we suggest it be called a brachinite

the impact of climate change tectonism and hydrology on the formation of carboniferous tropical intermontane mires the springhill coalfield cumberland basin nova scotia abstract both climate and tectonism are influential in determining basin and mire hydrology in the rapidly subsiding intermontane basins of euramerica which formed in response to the variscan hercynian alleghenian orogeny however the record of climate change has been muted and may therefore go unrecognized the complex interplay of climate and tectonism in a tropical continental setting is recorded in the 4 km thick westphalian a b fill of the cumberland basin thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4 9 km wide flourished between basin axis rivers and coalesced alluvial fans derived from the cobequid highlands as a result of their geomorphic setting the thicker coals 4 3 m exhibit distinct areal zones piedmont inner mire and riverine evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin margin conglomerates interfingering of distal fan sheetflood and mire deposits petrographic evidence of elevated ph partial gelification of tissues and predominance of the arboreous lycopsids lepidodendron and paralycopodites whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods a subhumid climate is implied coals at joggins further removed from groundwater discharge at the basin margin are less well developed a 600 m thick coal window within the fining and reddening upward basin fill sequence at springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence the ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the cobequid massif was denuded and the basin filled the coal window ca 6 9 10 5 yr is punctuated by non marine cyclothems of coal grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession ca 1 4 10 4 yr the stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence the late quaternary history of certain african mires illustrates the dramatic impact of climate change especially temperature and seasonality and orographic effects on tropical continental peat accumulation aspects of the groundwater recharge model may be applicable to the westphalian stephanian intermontane mires of spain and mesozoic coal basins of northern china

noble gases in fossil micrometeorites and meteorites from 470 myr old sediments from southern sweden and new evidence for the l chondrite parent body breakup event we present noble gas analyses of sediment dispersed extraterrestrial chromite grains recovered from similar to 470 myr old sediments from two quarries hallekis and thorsberg and of relict chromites in a coeval fossil meteorite from the gullhogen quarry all located ill southern sweden both the sediment dispersed grains and the meteorite gullhogen 001 were generated in the l chondrite parent body breakup about 470 myr ago which was also the event responsible for the abundant fossil meteorites previously found in the thorsberg quarry trapped solar noble gases in the sediment dispersed chromite grains have partly been retained during similar to 470 myr of terrestrial residence and despite harsh chemical treatment in the laboratory this shows that chromite is highly retentive for solar noble gases the solar noble gases imply that a sizeable fraction of the sediment dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites the grains in the oldest sediment beds were rapidly delivered to earth likely by direct injection into all orbital resonance in the inner asteroid belt whereas grains in younger sediments arrived by orbital decay due to poynting robertson p r drag the fossil meteorite gullhogen 001 has a low cosmic ray exposure age of similar to 0 9 myr based on new he and ne production rates in chromite determined experimentally this age is comparable to the ages of the fossil meteorites from thorsberg providing additional evidence for very rapid transfer times of material after the l chondrite parent body breakup

the formation and chronology of the pat 91501 impact melt l chondrite with vesicle metal sulfide assemblages the l chondrite patuxent range pat 91501 is an 8 5 kg unshocked homogeneous igneous textured impact melt that cooled slowly compared to other meteoritic impact melts in a crater floor melt sheet or sub crater dike mittlefehldt d w and lindstrom m m 2001 petrology and geochemistry of patuxent range 91501 and lewis cliff 88663 meteoritics planet sci 36 439 457 we conducted mineralogical and tomographic studies of previously unstudied mm to cm sized metal sulfide vesicle assemblages and chronologic studies of the silicate host metal sulfide clasts constitute about 1 vol comprise zoned taenite troilite and pentlandite and exhibit a consistent orientation between metal and sulfide and of metal sulfide contacts vesicles make up 2 vol and exhibit a similar orientation of long axes 39ar 40ar measurements probably date the time of impact at 4 461 0 008 gyr b p cosmogenic noble gases and 10be and 26al activities suggest a pre atmospheric radius of 40 60 cm and a cosmic ray exposure age of 25 29 myr similar to ages of a cluster of l chondrites pat 91501 dates the oldest known impact on the l chondrite parent body the dominant vesicle forming gas was s2 15 20 ppm which formed in equilibrium with impact melted sulfides the meteorite formed in an impact melt dike beneath a crater as did other impact melted l chondrites such as chico cooling and solidification occurred over 2 h during this time 90 of metal and sulfide segregated from the local melt remaining metal and sulfide grains oriented themselves in the local gravitational field a feature nearly unique among meteorites many of these metal sulfide grains adhered to vesicles to form aggregates that may have been close to neutrally buoyant these aggregates would have been carried upward with the residual melt inhibiting further buoyancy driven segregation although similar processes operated individually in other chondritic impact melts their interaction produced the unique assemblage observed in pat 91501

partial melting and melt migration in the acapulcoite lodranite parent body we review petrologic evidence that the acapulcoites and lodranites formed by 1 vol to probably 20 vol whole rock partial melting of a chondritic precursor material at low degrees of partial melting only fe ni fes cotectic melting occurred migration distances for partial melts were short resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents but achondritic textures at high degrees of partial melting both fe ni fes and basaltic plagioclase pyroxene partial melts formed and the melts may have migrated out of the source rock the partial melt residues which are more or less depleted in fe ni fes and plagioclase are the lodranites melt migration was complex most acapulcoites which experienced relatively low degrees of partial melting lost little if any of the partial melt one acapulcoite lew 86220 represents a unique case in which fe ni fes and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region where they were trapped in cases of the relatively high degrees of partial melting experienced by lodranites melts may have been partly selectively or totally removed from the rocks and fe ni fes and or basaltic partial melts may have been removed to different degrees and may in fact have been trapped on occasion in greater than chondritic proportions we model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting our calculations are broadly consistent with observations indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting although the size of the acapulcoite lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained the lack of basaltic rocks in the world s meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by wilson and keil 1991 and ejected into space the diversity of rocks from the acapulcoite lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the s type asteroids

evidence for high temperature diffusional creep preserved by rapid cooling of lower crust north bohemian shear zone czech republic diffusional creep structures in orthogneisses and granulites of the north bohemia shear zone eastern part of the erzgebirge crystalline complex imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling u pb dating of monazite and ar ar dating of muscovite of kadan orthogneisses result in a high cooling rate this is consistent with a rapid tectonic movements resulting in the juxtaposition of upper crustal rocks of the tepla barrandian unit against lower crustal rocks of the erzgebirge crystalline complex

uplands lowlands and climate taphonomic megabiases and the apparent rise of a xeromorphic drought tolerant flora during the pennsylvanian permian transition abstract the late mississippian and pennsylvanian have been referred to as the coal age due to enormous paleotropical peat accumulations coal beds numerous fossil floras have been collected from these coals and their associated seat earth paleosols and roof shales over more than two centuries leading to the inference of vast swampy wetlands covering the pangean tropics during the pennsylvanian in contrast the permian tropics are characterized as more arid with sparser and more heterogeneous vegetation than inferred for the pennsylvanian in the tropics the pennsylvanian to permian transition has been described as a changeover from a pteridophyte dominated paleophytic flora to a seed plant dominated mesophytic flora this view notwithstanding floras dominated by xeromorphic seed plants also are well known from the pennsylvanian tropics some authors have characterized these plants as being occupants of uplands subsequently transported into basinal lowland preservational environments in this model uplands are well drained causing areas of drought under otherwise everwet climates in this paper we present an alternative interpretation that the apparent transition in pennsylvanian permian tropical vegetation reflects two types of taphonomic megabias first is a preservational megabias strongly favoring the vegetation of humid climates over that of seasonally dry climates accordingly tropical plant preservational potential fluctuated in concert with late paleozoic ice age glacial interglacial oscillations and contemporaneous sea level and climatic changes second is an analytical megabias strongly favoring the discovery and collection of the wetland biome from pennsylvanian strata overlooking the less frequently and more poorly preserved drought tolerant biome by permian times vast wetlands and their fossil record had largely disappeared from central pangea although continuing in cathaysia making drought tolerant vegetation more visible to searchers without changing its preservational circumstances we demonstrate that the upland model is untenable being inconsistent with the principles of plant biogeography and with geological aspects of the fossil record

late palaeozoic palaeomagnetic and tectonic constraints for amalgamation of pangea supercontinent in the european variscan belt abstract this paper provides a review of the evolution of palaeomagnetic directions and sequence of tectonic events in the european variscan belt during late palaeozoic times these data are correlated with palaeomagnetic records from gondwana and baltica in order to provide a large scale geodynamic framework during the carboniferous to permian the early carboniferous palaeomagnetic records reported mainly from the rhenohercynian and saxothuringian magmatic arcs indicate a 70 anticlockwise rotation while late carboniferous to permian magnetic directions from various rocks of western and central europe are consistent with 120 clockwise rotation of the eastern and central parts of the variscan belt our review of the chronology of tectonic events is based on a robust database of geochronologically constrained deformation metamorphic and magmatic events which allow discretizing the 80 ma long orogenic evolution into five principal events in this scenario the variscan belt can be regarded as a linear sub plate isolated from gondwana and laurussia by the rhenohercynian and palaeotethysian oceans during devonian times this linear composite belt was segmented by transform faults and boundaries in the late devonian to early carboniferous times 360 335 ma during progressive e w closure of rhenohercynian ocean synchronously with collision between saxothuringian and moldanubian blocks subsequent relocation of subduction to the northern boundary palaeotethysian ocean was responsible for n s shortening almost orthogonal to the ancient sub plate n s elongation at around 335 325 ma this deformation resulted in dextral reactivation of transform boundaries associated with anticlockwise rotation of intermittent blocks at the end of this rotation the faults were parallelized to the teysseire tornquist zone the southern margin of baltica while the lozenge shaped blocks of the former variscan sub plate were further shortened during continuous contraction this evolution can fully explain the 70 anticlockwise rotation from cn3 to cn2 palaeomagnetic directions which are regarded as successive carboniferous magnetizations subsequently the variscan belt suffered a giant transtensional event from 325 to 310 ma that was related to the development of extensional syn magmatic core complexes over the whole belt and significant dextral reactivation of earlier nw se trending transform faults this extensional event was associated with important tilts recorded by cp magnetic overprint resulting from a major thermally induced remagnetization during this event new sets of sinistral transfer nne ssw trending faults originated that partly reactivated boundaries of the principal tectonic zones blocks delimited by second order sinistral and first order dextral faults then rotated in a clockwise manner by 80 the whole system subsequently suffered a period of nne ssw shortening that affected the variscan belt namely along the former laurussian and former variscan sub plate contact in the north and in the south where the giant cantabrian orocline developed at around 310 300 ma due to hard collision with gondwana this deformation is associated with the clockwise rotation of laurussia together with the accreted northern sector of the variscan belt and the anticlockwise of gondwana this clockwise 30 rotation is achieved by a1 remagnetizations while the southern part of iberia suffered anticlockwise rotation the final stage of rapid clockwise rotation affecting the northern limb of the iberian arc including corsica and sardinia is attributed to giant n s extension affecting the whole variscan belt at the onset of permo triassic opening of the tethys ocean this complex evolution is regarded as a result of the readjustment of inhomogeneous and thermally and mechanically instable mobile space in between reorganizing the gondwana and laurussia megaplates and the opening of the palaeotethys ocean during the final stages of formation of the pangea supercontinent finally the palaeomagnetically constrained rotations and tectonic evolution of the variscan belt are explained using a pinned model of internal and external rotations of blocks driven by activation of dextral shear zones by n s compression and e w transtension in particular

the northwest africa 1500 meteorite not a ureilite maybe a brachinite the northwest africa nwa 1500 meteorite is an olivine rich achondrite containing approximately 2 3 vol augite 1 2 vol plagioclase 1 vol chromite and minor orthopyroxene cl apatite metal and sulfide it was originally classified as a ureilite but is currently ungrouped we re examined the oxygen three isotope composition of nwa 1500 results of ultra high precision 0 03 for d 17 o laser fluorination analyses of two bulk chips and high precision 0 3 secondary ion mass spectrometry sims analyses of olivine and plagioclase in a thin section show that the oxygen isotope composition of nwa 1500 d 17 o 0 22 from bulk samples and 0 18 0 06 from 16 mineral analyses is within the range of brachinites we compare petrologic and geochemical characteristics of nwa 1500 with those of brachinites and other olivine rich primitive achondrites including new petrographic mineral compositional and bulk compositional data for brachinites hughes 026 reid 013 nwa 5191 nwa 595 and brachina modal mineral abundances texture olivine and pyroxene major and minor element compositions plagioclase major element compositions rare earth element abundances and siderophile element abundances of nwa 1500 are within the range of those in brachinites and in most cases well distinguished from those of winonaites iab silicates acapulcoites lodranites ureilites and divnoe nwa 1500 shows evidence of internal reduction in the form of reversely zoned olivine fo 65 73 core to rim and fine grained intergrowths of orthopyroxene metal along olivine grain margins the latter also occur in reid 013 hughes 026 nwa 5191 and nwa 595 we argue that reduction olivine fi enstatite fe 0 o 2 is the best hypothesis for their origin in these samples as well we suggest that nwa 1500 should be classified as a brachinite which has implications for the petrogenesis of brachinites fe mn mg compositions of brachinite olivine provide evidence of redox processes among bulk samples nwa 1500 provides evidence for redox processes on a smaller scale as well which supports the interpretation that these processes occurred in a parent body setting sims data for 26 al 26 mg isotopes in plagioclase in nwa 1500 show no 26 mg excesses beyond analytical uncertainties 1 2 the calculated upper limit for the initial 26 al 27 al ratio of the plagioclase corresponds to an age younger than 7 ma after cai compared to 53 mn 53 cr data for brachina wadhwa et al 1998b this implies either a much younger formation age or a more protracted cooling history however brachina is atypical and this comparison may not extend to other brachinites

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

the nature of the l chondrite parent body s disruption as deduced from high pressure phases in the sixiangkou l6 chondrite the disruption of the l chondrite parent body lcpb at similar to 470ma is currently the best documented catastrophic celestial impact event based on the large number of l chondritic materials associated with this event uranium lead u pb dating of apatite and its high pressure decomposition product tuite in the sixiangkou l6 chondrite provides a temporal link to this event the u pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the u pb system correlates closely with the extent of apatite decomposition this suggests that the u pb system of apatite could be substantially disturbed by high temperature pulse during shock compression from natural impacts at least on the scale of mineral grains although many l chondrites can be temporally related to the catastrophic lcpb impact event the shock conditions experienced by each individual meteorite vary this could be due to the different geologic settings of these meteorites on their parent body the shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions although they could provide lower limits to the impact conditions the sixiangkou shock duration similar to 4s estimated from high pressure transformation kinetics provides a lower limit to the high pressure pulse of the lcpb disruption impact combined with available literature data of l chondrites associated with this impact event our results suggest that the lcpb suffered a catastrophic collision with a large projectile with a diameter of at least 18 22km at a low impact velocity 5 6kms 1 this is consistent with astronomical estimates based on the dynamical evolution of l chondritic asteroids

petrogenesis of acapulcoites and lodranites a shock melting model abstract acapulcoites are modeled as having formed by shock melting cr like carbonaceous chondrite precursors the degree of melting of some acapulcoites was low enough to allow the preservation of 3 6 vol relict chondrules shock effects in acapulcoites include veins of metallic fe ni and troilite polycrystalline kamacite fine grained metal troilite assemblages metallic cu and irregularly shaped troilite grains within metallic fe ni while at elevated temperatures acapulcoites experienced appreciable reduction because graphite is present in some acapulcoites and lodranites it seems likely that carbon was the principal reducing agent reduction is responsible for the low contents of olivine fa 4 14 mol and low ca pyroxene fs 3 13 mol in the acapulcoites the observation that in more than two thirds of the acapulcoites the fa value is lower than the fs value in contrast to the case for equilibrated ordinary chondrites the low feo mno ratios in acapulcoite olivine 16 18 compared to 32 38 in equilibrated h chondrites the relatively high modal orthopyroxene olivine ratios e g 1 7 in monument draw compared to 0 74 in h chondrites and reverse zoning in some mafic silicate grains lodranites formed in a similar manner to acapulcoites but suffered more extensive heating loss of plagioclase and loss of an fe ni s melt acapulcoites and lodranites experienced moderate post shock annealing presumably resulting from burial beneath material of low thermal diffusivity the annealing process repaired damaged olivine crystal lattices lending acapulcoites and lodranites the appearance of unshocked i e shock stage s1 rocks any high pressure phases that may have formed during initial shock reverted to their low pressure polymorphs during annealing some samples were subsequently shocked again several acapulcoites reached shock stage s2 levels alh 84190 reached s3 and the lodranite mac 88177 reached s5

conjugate shear zones in the southern bohemian massif austria implications for variscan and alpine tectonothermal activity abstract mylonitic fabrics developed within conjugate wrench ductile shear and fault systems in the southern bohemian massif display both dextral nw se trending systems and sinistral ne sw trending systems shear senses contrasting temperature conditions of deformation can be observed in the different shear zones temperatures above 650 c were reached in the pfahl shear zone shearing under greenschist facies conditions took place in the rodl and danube shear zones brittle deformation dominated in the vitis and diendorf shear zones 40 ar 39 ar dating of various size fractions of muscovite formed and or deformed during mylonitization yield ages of ca 287 ma the nw se trending system and ca 288 281 ma the ne sw trending system the shear zones are interpreted as a late variscan conjugate system 40 ar 39 ar age spectra of fine grained newly grown sericite or rejuvenation of ages give evidence for post variscan reactivation of the shear zones brittle deformation within the shear zones was probably associated with maintenance of very high fluid pressure during variscan deformation foreland deformation during the alpine orogeny also played a significant role leading not only to the development of the ceske budejovice graben but also to local reactivation of the shear zones at higher crustal levels

pre sedimentary palaeo relief and compaction controls on peat deposition and clastic sedimentation in the radnice member kladno basin bohemia

host rock solid state transformation in a shock induced melt vein of tenham l6 chondrite abstract the host rock fragments entrained in a 580 m wide melt vein of the tenham l6 chondrite were investigated using field emission scanning electron microscopy fesem and transmission electron microscopy tem to better understand the solid state transformation mechanisms and the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide that crystallized from immiscible melts and sub rounded host rock fragments that have been entrained in the melt and transformed to polycrystalline high pressure silicates by solid state transformation mechanisms these high pressure phases include ringwoodite low ca majorite clinoenstatite hollandite structured plagioclase and ca rich majorite the ca rich majorite occurs as a symplectitic intergrowth with a ca poor amorphous silicate phase in a 200 m diameter chondrule in the vein this intergrowth seems to be the result of a disproportionate breakdown of a ca rich clinopyroxene precursor into ca rich majorite and femg sio 3 perovskite which subsequently vitrified upon pressure release the tem observations suggest that most solid state transformations in the tenham are reconstructive the transformation of olivine to polycrystalline ringwoodite appears to involve incoherent intracrystalline nucleation and interface controlled growth lamellae in partially transformed olivine are not continuous coherent lamellae but rather lamellae of polycrystalline ringwoodite which is inconsistent with a coherent lamellar transformation mechanism growth rate calculations based on published kinetic data suggest that the time required to grow 1 m ringwoodite crystal is 100 ms at 1600 k suggesting that the minimum shock pulse of approximately 100 ms

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

records of the moon forming impact and the 470 ma disruption of the l chondrite parent body in the asteroid belt from u pb apatite ages of novato l6 novato a newly observed fall in the san francisco bay area is a shocked and brecciated l6 ordinary chondrite containing dark and light lithologies we have investigated the u pb isotope systematics of coarse cl apatite grains of metamorphic origin in novato with a large geometry ion microprobe the u pb systematics of novato apatite reveals an upper intercept age of 4472 31 ma and lower intercept age of 473 38 ma the upper intercept age is within error identical to the u pb apatite age of 4452 21 ma measured in the chelyabinsk ll5 chondrite this age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the moon forming giant impact on earth the lower intercept age is consistent with the most precisely dated ar ar ages of 470 6 ma of shocked l chondrites and the fossil meteorites and extraterrestrial chromite relicts found in ordovician limestones with an age of 467 3 1 6 ma in sweden and china the lower intercept age reflects a major disturbance related to the catastrophic disruption of the l chondrite parent body most likely associated with the gefion asteroid family which produced an initially intense meteorite bombardment of the earth in ordovician period and reset and degassed at least approximately 35 of the l chondrite falls today we predict that the 470 ma impact event is likely to be found on the moon and mars if not mercury

10 km minimum throw along the west bohemian shear zone evidence for dramatic crustal thickening and high topography in the bohemian massif european variscides the west bohemian shear zone wbsz forms a steep collapse structure along which east side down normal movements led to the juxtaposition of the relatively cold cadomian basement of the tepla barrandian unit against high grade moldanubian rocks synkinematic plutons straddle the wbsz the mut nin pluton intruded into moldanubian crust at a depth of 23 4 km as derived by using al in hornblende barometry the tepla barrandian babylon pluton intruded at 12 km depth as indicated by phengite barometry and petrogenetic considerations both emplacement depths together with mineral cooling ages result in a minimum vertical displacement of 10 km between 340 and 320 ma this large throw could be explained by over thickened crust that was weakened from below the alkaline signature of the mut nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 ma the cold tepla barrandian upper crust sank into its weak partly molten moldanubian substratum resulting in elevator style movements not only along the wbsz but also along the hoher bogen and central bohemian shear zone all these ductile normal shear zones were active simultaneously during the lower carboniferous and dip steeply towards the tepla barrandian unit that probably formed a highly elevated plateau at this time

plagioclase breakdown as an indicator for shock conditions of meteorites the collision history of asteroids in the early solar system is difficult to reconstruct a study of plagioclase breakdown using x ray diffraction measurements under increasing pressures and temperatures suggests that peak pressures in collisions that formed shocked meteorites have been overestimated shocked meteorites that were formed when their parent body underwent shock metamorphism often contain the mineral plagioclase either in an amorphous form or in its high pressure phase the peak pressures in shock metamorphism can be constrained by shock recovery experiments that determine the amorphization pressures of plagioclase1 2 3 however in these experiments temperatures have been unrealistically low and timescales much shorter than those in natural events here we present in situ x ray diffraction measurements of two kinds of plagioclase feldspar in conditions of increasing pressures and temperatures we find that the amorphization pressure of plagioclase decreases with increasing temperature suggesting that previous studies overestimated this parameter1 2 3 we also found that the mineral jadeite forms first from amorphous plagioclase whereas the nucleation of other minerals such as stishovite or garnet is significantly delayed the occurrence of jadeite in shocked meteorites that do not contain stishovite4 5 6 7 8 can therefore be explained as a result of the crystallization kinetics we conclude that the study of plagioclase breakdown can constrain the pressure temperature time history of shock events and thus help to reconstruct the collisional history of asteroids in the early solar system

ordinary chondrite metallography part 2 formation of zoned and unzoned metal particles in relatively unshocked h l and ll chondrites we studied the metallography of fe ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of p free fe ni alloy cooling experiments described in reisener and goldstein 2003 two types of fe ni metal particles were observed in the chondrites zoned taenite kamacite particles and zoneless plessite particles which lack systematic ni zoning and consist of tetrataenite in a kamacite matrix both types of metal particles formed during metamorphism in a parent body from homogeneous p poor taenite grains the phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles polycrystalline taenite particles transformed to zoned taenite kamacite particles by kamacite nucleation at taenite taenite grain boundaries during cooling monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process the varying proportions of zoned taenite kamacite particles and zoneless plessite particles in types 46 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles the distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of fe ni alloy phase transformations during cooling the distribution cannot be explained by hot accretion autometamorphism post metamorphic brecciation or shock processing

dating phosphates of the strongly shocked suizhou chondrite impacts on undifferentiated asteroidal bodies provide invaluable information for understanding the solar system evolution nevertheless dating early and small scale impact events is technically challenging uranium lead u pb systematics of u bearing phases within shock induced melt veins smvs of chondrites may be significantly disturbed by localized heating thus providing opportunities to date these impact events as one of the major u hosts in chondrites apatite in the suizhou l6 chondrite has been compositionally and structurally modified to varying degrees by shock metamorphism apatite grains in the host remained largely intact during the impact and have recorded the initial thermal cooling time similar to 4550 ma on their parent body apatite grains in regions less than 100 mu m bordering the smvs or in relatively cold regions within the smvs were partly decomposed to tuite driven by the localized transient heating within the smvs their u pb systematics were disrupted to varying extents apatite in regions close to the center of the smvs has been completely transformed to tuite which yields an age of 4481 30 ma 2 sigma providing an upper limit to the impact event this study clearly demonstrates that by integrating in situ u pb isotope analysis with detailed microstructural and compositional analysis of phosphates it is possible to deduce the timing of early and small scale celestial impact events and hence create a more comprehensive understanding of the impact history of the solar system

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

hercynian high altitude phenomena in the french massif central tectonic implications abstract stratified slope deposits diamictite and sedimentary structures such as water escape craters ghosts of ice crystals and solifluction flowage in the stephanian autunian of the french massif central are related to high altitude periglacial effects the clay mineral assemblage dominated by the association illite chlorite irregular interlayed clays and the slight weathering of feldspathic clasts and granite gneiss pebbles favour the prevalence of a cold to temperate climate at this time rather than a hydrolyzing climate of humid equatorial or tropical type the flora and entomofauna also indicate a seasonally varying climate with cold winters favouring the development of periglacial phenomena at high altitude in view of the equatorial paleogeographic position of the hercynian belt this type of climate could exist only at altitudes higher than 4500 m the tectonic implications of these altitudes are examined through a comparison with the present day himalayas and tibetan plateau

acapulco and lodran like achondrites petrology geochemistry chronology and origin we have performed petrologic and geochemical studies of five primitive achondrites alha81187 and alha81261 acapulco like eet 84302 transitional but acapulco like and lew 88280 and mac 88177 lodran like we have also performed 39ar 40ar chronology on alha81187 alha81261 and eet 84302 lew 88280 and mac 88177 contain more ferroan olivines orthopyroxenes clinopyroxenes and chromites than do the acapulco like achondrites plagioclase is present in only alha81187 alha81261 and eet 84302 its composition does not track that of the mafic silicates eet 84302 with the most calcic plagioclase contains olivine and pyroxene with an mg intermediate between alha81187 and alha81261 similarly spinels in eet 84302 have the same mg as those in alha81187 while pyroxenes in the former are more ferroan than in the latter acapulco like achondrites have sm sc ratios between 0 8 1 5 times h chondrites and na sc ratios are between 0 81 0 times h chondrites indicating that silicate partial melts have not been lost from these rocks siderophile and chalcophile elements are fractionated and show a trend of increasing ir ni ratios with decreasing se co ratios this variation is consistent with fractionation by partial melting in the fe ni s system eet 84302 is very depleted in se and troilite is a accessory phase indicating essentially total loss of the low temperature fe ni s melt in contrast the lodran like achondrites are depleted in the highly incompatible lithophile elements relative to more compatible elements sm sc ratios are 0 2 0 7 times and na sc ratios are 0 04 0 18 times h chondrites these data are consistent with a model for formation of the lodran like achondrites as partial melting residues however the low ir ni and high se co ratios in two lodran like achondrites suggest the metal troilite in these rocks is dominated by the low melting fraction in the fe ni s system this metal troilite was added to the silicates after partial melting depleted them in a basaltic fraction the ar ar release spectra yield good plateau ages of 4 507 0 024 ga alha81187 4 511 0 007 ga alha81261 and 4 519 0 017 ga eet 84302 in good agreement with previously determined ar ar ages for other acapulco like achondrites together the ar ar ages demonstrate that metamorphism on the acapulco lodran parent body occurred 4 51 0 02 ga ago roughly 50 ma after its formation the geochemical and petrologic evidence on the acapulco and lodran like achondrites suggest that heating on the parent body was localized and heterogeneous in space and time the parent body was probably not subjected to parent body wide magmatism in contrast to the case for 4 vesta and the hed meteorites

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

trace element inventory of meteoritic ca phosphates most extraterrestrial samples feature the two accessory ca phosphates apatite group minerals and merrillite which are important carrier phases of the rare earth elements ree the trace element concentrations ree sc ti v cr mn co as rb sr y zr nb ba hf ta pb th and u of selected grains were analyzed by la icp ms and or sims ree only this systematic investigation includes 99 apatite and 149 merrillite analyses from meteorites deriving from various asteroidal bodies including 1 carbonaceous chondrite 8 ordinary chondrites 3 acapulcoites 1 winonaite 2 eucrites 5 shergottites 1 ureilitic trachyandesite 2 mesosiderites and 1 silicate bearing iab iron meteorite although ca phosphates predominantly form in metamorphic and or metasomatic reactions some are of igneous origin as late stage phases that often incorporate the vast majority of their host s bulk ree budget the investigated ca phosphates have ree enrichments of up to two orders of magnitude compared to the host rock s bulk concentrations within a single sample each phosphate species displays a uniform ree pattern and variations are mainly restricted to their enrichment therefore indicating similar formation conditions exceptions are brecciated samples i e the adzhi bogdo ll3 6 ordinary chondrite despite this uniformity within single samples distinct meteorite groups do not necessarily have unique ree patterns four basic shapes dominate the ree patterns of meteoritic ca phosphates 1 flat patterns smoothly decreasing from la lu with prominent negative eu anomalies acapulcoites eucrites apatite from the winonaite and the ureilitic trachyandesite merrillite from ordinary chondrites 2 unfractionated patterns with only minor or no anomalies mesosiderites enriched shergottites iab iron meteorite 3 lree enriched patterns with either positive or slightly negative eu anomalies chondritic apatite and 4 strongly lree depleted patterns with negative eu anomalies depleted shergottites the patterns do not correlate with the grade of metamorphism petrologic type specific adjacent mineral assemblages or with ca phosphate grain size neither the proportions of different ree nor particular ree patterns themselves are universally correlated to a specific formation mechanism yet eu i e magnitude of the eu anomaly is a sensitive indicator to evaluate the timing of plagioclase and phosphate crystallization based on our data u and th abundances in apatite increase almost linearly with the grade of metamorphism as well as with the differentiation of their host rock

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

chondrites as samples of differentiated planetesimals abstract chondritic meteorites are unmelted and variably metamorphosed aggregates of the earliest solids of the solar system the variety of metamorphic textures in chondrites motivated the onion shell model in which chondrites originated at varying depths within a parent body heated primarily by the short lived radioisotope 26 al with the highest metamorphic grade originating nearest the center allende and a few other chondrites possess a unidirectional magnetization that can be best explained by a core dynamo on their parent body indicating internal melting and differentiation here we show that a parent body that accreted to 200 km in radius by 1 5 ma after the formation of calcium aluminum rich inclusions cais would have a differentiated interior and ongoing accretion would add a solid undifferentiated crust overlying a differentiated interior consistent with formational and evolutionary constraints inferred for the cv parent body this body could have produced a magnetic field lasting more than 10 ma this hypothesis represents a new model for the origin of some chondrites presenting them as the unprocessed crusts of internally differentiated early planetesimals such bodies may exist in the asteroid belt today the shapes and masses of the two largest asteroids 1 ceres and 2 pallas can be consistent with differentiated interiors conceivably with small iron cores with hydrated silicate or ice silicate mantles covered with undifferentiated crusts

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

sulfide rich metallic impact melts from chondritic parent bodies sacramento wash 005 saw 005 meteorite hills 00428 met 00428 and mount howe 88403 how 88403 are s rich fe ni rich metal meteorites with fine metal structures and homogeneous troilite we compare them with the h metal meteorite lewis cliff 88432 phase diagram analyses suggest that saw 005 met 00428 and how 88403 were liquids at temperatures above 1350 c tridymite in how 88403 constrains formation to a high temperature and low pressure environment the morphology of their metal troilite structures may suggest that met 00428 cooled the slowest saw 005 cooled faster and how 88403 cooled the quickest saw 005 and met 00428 contain h chondrite like silicates and saw 005 contains a chondrule bearing inclusion that is texturally and compositionally similar to h4 chondrites the compositional and morphological similarities of saw 005 and met 00428 suggest that they are likely the result of impact processing on the h chondrite parent body saw 005 and met 00428 are the first recognized iron and sulfide rich meteorites which formed by impact on the h chondrite parent body which are distinct from the iie iron meteorite group the morphological and chemical differences of how 88403 suggest that it is not from the h chondrite body although it likely formed during an impact on a chondritic parent body

contrasting styles of deformation during progressive nappe stacking at the southeastern margin of the bohemian massif thaya dome abstract variscan nappe stacking at the southeastern margin of the bohemian massif is a polyphase process implying a progressive modification of the deformation regime from thrusting to wrenching associated change of finite strain axes orientations and progressive downwards migration of thrust or wrench planes a first high temperature deformation phase d 1 is related to nappe stacking of moldanubian units in a nw se direction it is recorded only in these units d 2 corresponds to the piling of the moldanubian units over the moravian ones producing internal nappe stacking of the latter and a first medium temperature mylonitization of thaya basement rocks the main feature of this stage is a switch in finite strain axes orientation and northeastward transpressional shearing strain partitioning is important within the moravian nappes and implies alternation of plane strain oblate and constrictional fabrics d 3 is also a phase connected with strong non coaxial ne directed shearing it is concentrated in zones of weakness and causes reactivation of older structures the structural pattern strongly depends on the position in the nappe pile strike slip tectonics dominate near the resistant eastern cadomian basement border foliation parallel extensional reactivation producing large scale folds is typical in moravian nappes while extensional tectonics prevail in the uppermost gfohl nappe large scale strain partitioning during d 2 stacking of moravian nappes and the extension of the upper part of the nappe pile during d 3 reactivation are mechanically controlled by a lateral ramp basement geometry along the bruno vistulian cadomian foreland

heterogeneous distribution of h2o in the martian interior implications for the abundance of h2o in depleted and enriched mantle sources we conducted a petrologic study of apatite within 12 martian meteorites including 11 shergottites and one basaltic regolith breccia these data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the martian interior apatites in individual martian meteorites span a wide range of compositions indicating they did not form by equilibrium crystallization in fact the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a cl rich crustal component we determined that most martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the martian mantle using the subset of samples that did not exhibit crustal contamination we determined that the enriched shergottite source has 36 73 ppm h2o and the depleted source has 14 23 ppm h2o this result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the martian mantle we also estimated the h2o cl and f content of the martian crust using known crust mantle distributions for incompatible lithophile elements we determined that the bulk martian crust has 1410 ppm h2o 450 ppm cl and 106 ppm f and cl and h2o are preferentially distributed toward the martian surface the estimate of crustal h2o results in a global equivalent surface layer gel of 229 m which can account for at least some of the surface features on mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on mars surface

petrology and geochemistry of patuxent range 91501 a clast poor impact melt from the l chondrite parent body and lewis cliff 88663 an l7 chondrite we have performed petrologic and geochemical studies of patuxent range pat 91501 and lewis cliff lew 88663 pat 91501 originally classified as an l7 chondrite is rather a unique near total impact melt from the l chondrite parent body lewis cliff 88663 was originally classified as an achondrite but we find that it is a very weakly shocked l7 chondrite pat 91501 is an unshocked homogeneous igneous textured ultramafic rock composed of euhedral to subhedral olivine low ca pyroxene augite and chrome rich spinels with interstitial albitic plagioclase and minor silica alumina alkali rich glass only 10 relic chondritic material is present olivine grains are homogeneous fa25 2 26 8 low ca pyroxene wo1 9 7 2en71 9 78 2fs19 9 20 9 and augite wo29 8 39 0en49 2 55 3fs11 8 14 9 display a strong linear tio2 al2o3 correlation resulting from igneous fractionation plagioclase is variable in composition or3 0 7 7ab79 8 84 1an8 2 17 2 chrome rich spinels are variable in composition and zoned from cr rich cores to ti al rich rims some have evolved compositions with up to 7 9 wt tio2 pat 91501 bulk silicate has an l chondrite lithophile element composition except for depletions in zn and br siderophile and chalcophile elements are highly depleted due to sequestration in centimeter size metal troilite nodules the minerals in lew 88663 are more uniform in composition than those in pat 91501 olivine grains have low cao and cr2o3 contents similar to those in l5 6 chondrites pyroxenes have high tio2 contents with only a diffuse tio2 al2o3 correlation low ca pyroxenes are less calcic wo1 6 3 1en76 5 77 0fs20 4 21 4 while augites wo39 5 45 6en46 8 51 1fs7 6 9 4 and plagioclases or2 6 5 7ab74 1 83 1an11 2 23 3 are more calcic spinels are homogeneous and compositionally similar to those in l6 chondrites lew 88663 has an l chondrite bulk composition for lithophile elements and only slight depletions in siderophile and chalcophile elements that are plausibly due to weathering and or sample heterogeneity