chondrule precursor minerals as anhydrous phases abstract chondrules might be generated from hydrous or anhydrous precursor minerals but vesicles due to dehydration during melting are absent in natural chondrules the occurrence of vesicles in experimental charges made from serpentine rich starting material that was a flash heated and remelted several times b melted for long durations and c melted in a vacuum has been studied in order to test whether vesicles could be eliminated in chondrules made from hydrous precursors high percentages of vesicles in all experiments indicate that chondrules whether melted once multiple times for a brief or extended period at solar nebula or higher pressure were made from anhydrous phases in t r 0 duct i0 n chondrules are millimeter sized igneous textured spheres of mostly silicate composition grossman et al 1988 that were melted in the solar nebula and are found in meteorites the pervasive chondrule forming event has to a large extent obscured the meteor itic record of earlier nebular events such

dynamic crystallization of chondrule melts of porphyritic and radial pyroxene composition abstract dynamic crystallization experiments in which heterogeneous nucleation is an important variable have been completed on four melts of chondrule composition compositions were chosen to best represent chondrules with porphyritic pyroxene and radial pyroxene textures experimental results show that heterogeneous nucleation is essential for the formation of porphyritic textures without preexisting nuclei too much supercooling is established before crystallization is initiated and the textures are more likely to be dendritic or radial in the near total absence of nuclei radial textures can form at cooling rates as slow as 5 c hr in this study by varying the heterogeneous nucleation conditions and having a melt in which the appropriate phases are stable or metastable analogs to most of the recognized chondrule textures can be produced in a single melt composition olivine inclusions in pyroxene can form readily during an experiment from a starting material which did not initially contain olivine crystals thus care must be taken in the assumption that olivine inclusions in pyroxene represent preexisting crystals

chondrules in h3 chondrites textures compositions and origins major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated type 3 h group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types barred olivine porphyritic olivine porphyritic pyroxene barred pyroxene radiating pyroxene fine grained although analyses of individual chondrules scatter widely the mean composition of each textural type except barred pyroxene is rather distinct as verified by discriminant function analysis al2o3 tio2 and na3o are correlated in chondrules but al2o3 and cao do not correlate compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition the data suggest that composition played a conspicuous role in producing the observed textures of chondrules though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important if compound chondrules formed and joined when they were still molten or plastic then the data suggest that chondrules of each textural type could have formed together in space or time the correlation of al2o3 and tio2 with na2o and not with cao appears to rule out formation of chondrules by direct equilibrium condensation from the nebula we conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high intermediate and low temperature nebular condensates that underwent melting in space a small percentage of chondrules might have formed by impacts in meteorite parent body regoliths

microchondrules in three unequilibrated ordinary chondrites we report on a suite of microchondrules from three unequilibrated ordinary chondrites uocs microchondrules a subset of chondrules that are ubiquitous components of uocs commonly occur in fine grained chondrule rims although may also occur within matrix microchondrules have a variety of textures cryptocrystalline microporphyritic radial glassy in some cases their textures and in many cases their compositions are similar to their larger host chondrules bulk compositions for both chondrule populations frequently overlap the primary material that composes many of the microchondrules has compositions that are pyroxene normative and is similar to low ca pyroxene phenocrysts from host chondrules primary material rarely resembles olivine or plagioclase some microchondrules are composed of feo rich material that has compositions similar to the bulk submicron fine grained rim material these microchondrules however are not a common compositional type and probably represent secondary feo enrichment microchondrules may also be porous suggestive of degasing to form vesicles our work shows that the occurrence of microchondrules in chondrule rims is an important constraint that needs to be considered when evaluating chondrule forming mechanisms we propose that microchondrules represent melted portions of the chondrule surfaces and or the melt products of coagulated dust in the immediate vicinity of the larger chondrules we suggest that through recycling events the outer surfaces of chondrules were heated enough to allow microchondrules to bud off as protuberances and become entrained in the surrounding dusty environment as chondrules were accreting fine grained rims microchondrules are thus byproducts of cyclic processing of chondrules in localized environments their occurrence in fine grained rims represents a snapshot of the chondrule forming environment we evaluate mechanisms for microchondrule formation and hypothesize a potential link between the emergence of type ii chondrules in the early solar system and the microchondrule bearing fine grained rims surrounding type i chondrules

models of passive margin inversion implications for the rhenohercynian fold and thrust belt belgium and germany abstract positive tectonic inversion is related to the transmission of compressional stresses along a decollement into the foreland of an orogenic zone this stress and strain concentration in regions remote from the main orogenic front is commonly related to the presence of pre existing rheological heterogeneities such as normal syn depositional faults during inversion these pre existing normal faults are reactivated as reverse faults tectonic inversion in the rhenohercynian fold and thrust belt during the variscan orogeny shows that inversion is likely synchronous with the onset of collision in the hinterland here we present the results of a simplified thermo mechanical model stm which allows one to study strain partitioning between two orogenic zones we show that if the two orogenic zones have the same mechanical properties the viscosity of the decollement which links them controls the initial strain partitioning during subsequent finite shortening erosional processes determine the partitioning of strain rate the presence of a weak structure in the inverted zone and of a low viscosity decollement leads to initial strain concentration in the inverted track rather than in the collision zone and a progressive decrease in strain partitioning between the two orogenic zones the stm results are in good agreement with results of a 2d finite element model we conclude that in the western part of the rhenohercynian massif simultaneous uplift and deformation within the mid german crystalline rise the main collision zone and the ardenne anticlinorium the inverted zone lead to interpreting this orogenic event as a case of vice tectonic rather than the propagation of a wave of folding towards the variscan front as suggested by previous authors

continental collision including a weak zone the vise model and its application to the newfoundland appalachians continental collision generally occurs after a protracted history of rifting sedimentation and subduction inherited material heterogeneities and thermal effects from terrane accretion and amalgamation may control subsequent collisional deformation such strength contrasts may have played an important role in the silurian evolution of the newfoundland appalachians a cross sectional plane strain numerical model is used to investigate the effect of a weak zone embedded between stronger model crust the vise on the deformation style of orogens in contrast to collision of uniform strong crust in which deformation is directly related to the underlying subduction of mantle lithosphere deformation in vise models can propagate out to the ends of the vise in a diffuse manner this distributed tectonic style depends on the relative strength of the embedded weak zone the degree of coupling of this zone to underlying layers and the effect of gravity acting on thickened crust for weak coupling at the base o

rapid burial and exhumation during orogeny thickening and synconvergent exhumation of thermally weakened and thinned crust variscan orogen in western europe we present a model demonstrating that the presence of a thinned and thermally weakened crustal domain is a prerequisite for the formation of an orogenic root in mountain belts furthermore this mechanism permits burial to 50 to 70 km in time periods of less than 20 myr and exhumation from these depths in less than 10 myr our example from the variscan belt is based on field geology petrology of metamorphic rocks and detailed geochronology it describes an evolution from i sedimentation and magmatism during back arc extension ii compression leading to burial thickening of the lithosphere and granulite facies metamorphism followed by iii exhumation formation of dehydration melts migmatites and late intrusive granites the existing u pb ar ar data constrain this evolution to no more than 20 to 25 myr thermorheological modelling starts from a hot thinned crust forming a narrow 50 km wide orogenic root within 15 myr at a maximum of 50 km depth and subsequent exhumation by extrusion within 7 myr the model explains exhumation rates of 3 to 4 mm year by compressional extrusion and shows that extension cannot do the same job within the same time extrusion transported the internal thermal structure into the upper crust within a short time followed by rapid cooling whereas exhumation through extension would cause mantle upwelling and the formation of a long lived thermal anomaly the latter would contradict the geochronological and petrological database the model explains orogeny as a thermorheological process without needing to involve an external heat source from the mantle and presents a new explanation of the high temperature and low pressure conditions very typical of the variscan orogenic crust

extensional inheritance and surface processes as controlling factors of mountain belt structure surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts the first order effects of these parameters have been studied extensively throughout the last decades but their relative importance remains notoriously difficult to assess and document we use lithospheric scale plane strain thermomechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion surface processes are modeled through the combination of a simple sedimentation algorithm where all negative topography is filled up to a prescribed reference level and an elevation dependent erosion model our results show that 1 extensional inheritance facilitates the propagation of basement deformation in the retro wedge and 2 increases the width of the orogen 3 sedimentation increases the length scale of both thin skinned and thick skinned thrust sheets and 4 results in a wider orogen 5 erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro wedge a comparison of the modeled behaviors to the high atlas the pyrenees and the central alps three extensively studied natural examples characterized by different degrees of inversion is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

seismic refraction profiles in the gulf of saint lawrence and implications for extent of continuous grenville lower crust near the exposed grenville craton a two layer crust with velocities of 6 2 km s 1 and 6 65 km s 1 and low gradient is measured the 6 65 km s 1 velocity is observed at the shallow depth of 15 km and the moho at greater than 40 km depth towards the appalachian orogen a southwards dipping ramp is developed in the mid crust on the top of the 6 65 km s 1 layer a collinear reflection profile confirms the southwards dipping ramp the shallowing moho and the lateral changes in the lower crust a published refraction profile that crosses this reflection profile near the southern end confirms the termination of the 6 65 km s 1 layer and its replacement by a slower velocity with the publishing of wide angle reflection refraction war profile 88 2 two pairs of lines are now available in the gulf that cross the craton into the orogen with both war and deep seismic reflection coverage the combined data sets are used to distinguish the grenville lower crustal block based on patterns of reflectivity and associat

testing thin skinned inversion of a prerift salt bearing passive margin eastern prebetic zone se iberia abstract detailed geologic mapping combined with well and seismic data from the eastern prebetic zone se iberia reveal extensional and contractional structures that permit characterization of passive margin development and its incorporation into a thin skinned fold and thrust belt the study area is represented by nw directed ene trending folds and thrusts faults locally disrupted by the nw trending matamoros basin and the active jumilla and la rosa diapirs these structures resulted from the thin skinned inversion of the proximal part of the eastern south iberian passive margin containing prerift salt here upper jurassic to santonian thick skinned extension controlled the accumulation of sediment over mobile prerift salt this in turn defined the style of salt tectonics characterized by monoclinal drape folds suprasalt extensional faults and diapirs the structural and sedimentological analysis suggests that during extension salt localizes strain thus decoupling sub and suprasalt deformation thick skinned extension controls suprasalt deformation as well as its location and distribution which changes over time salt also localizes strain during inversion the preexisting salt structures weaker than adjacent areas preferentially absorb the contractional deformation in addition the stepped subsalt geometry that results from thick skinned extension also controls the shortening propagation therefore the degree of strain localization depends on the thickness of the suprasalt cover and on the dip of subsalt faults relative to the thin skinned transport direction

relict and other anomalous grains in chondrules implications for chondrule formation we have identified relict olivine and pyroxene grains in chondrules from ordinary and carbonaceous chondrites that probably did not crystallize in situ some of these olivines are clear but others contain fine grained fe ni dusty olivines and resemble previously described occurrences in ordinary chondrites there are also chondrules in which all olivine is dusty we conclude 1 not all relict olivines are dusty 2 not all dusty olivines crystallized outside the chondrule in which they occur and 3 some dusty olivines were produced during chondrule formation by a reduction process that affected the whole chondrule the occurrence of dusty olivines and relict pyroxenes and olivines in chondrules from carbonaceous as well as ordinary chondrites supports the argument that chondrules from all chondrites had similar origins and histories we propose that chondrules and mineral fragments were transported across f o2 gradients in the solar nebula while they were hot or were reheated in an environment different from the one in which they formed partially molten chondrules sometimes incorporated mineral grains or chondrules with different redox states producing compound chondrules and chondrules containing anomalous grains dusty olivines may also have formed when hot chondrules were transported to regions of lower oxygen fugacity

tectonic evolution of the lachlan fold belt southeastern australia constraints from coupled numerical models of crustal deformation and surface erosion driven by subduction of the underlying mantle abstract we have used a coupled thermo mechanical finite element fe model of crustal deformation driven by mantle oceanic subduction to demonstrate that the tectonic evolution of the lachlan fold belt lfb during the mid palaeozoic late ordovician to early carboniferous can be linked to continuous subduction along a single subduction zone this contrasts with most models proposed to date which assume that separate subduction zones were active beneath the western central and eastern sections of the lachlan orogen we demonstrate how the existing data on the structural volcanic and erosional evolution of the lachlan fold belt can be accounted for by our model we focus particularly on the timing of fault movement in the various sectors of the orogen we demonstrate that the presence of the weak basal decollement on which most of the lachlan fold belt is constructed effectively decouples crustal structures from those in the underlying mantle the patterns of faulting in the upper crust appears therefore to be controlled by lateral strength contrasts inherited from previous orogenic events rather than the location of one or several subduction zones the model also predicts that the uplift and deep exhumation of the wagga omeo metamorphic belt womb is associated with the advection of this terrane above the subduction point and is the only tectonic event that gives us direct constraints on the location of the subduction zone we also discuss the implications of our model for the nature of the basement underlying the present day orogen

thermo mechanical modeling of subduction of continental lithosphere abstract we consider two dimensional thermo chemical mantle convection models to investigate the deformation of the continental lithosphere that follows the oceanic lithosphere into the subduction zone the models account for the compositional buoyancy forces by considering lithospheric plates with distinct crustal layers continental convergence results in crust mantle detachment in the subducting plate and crustal thickening in both subducting and overriding plates the depth of detachment ranges from 90 to 200 km depending on the strength of the lithosphere and the density of the continental crust as the crust thickens the convergence velocity in the collision zone decreases and the locus of subduction gradually shifts toward the interior of the subducting plate in models with greater viscosity the subducting mantle lithosphere maintains its integrity and does not break up in models with weaker rheology the positive buoyancy of the thickened crust can overcome the strength of the subducting lithosphere and causes the oceanic slab to break off and sink into the mantle the breakoff occurs over a time interval of 10 20 million years which is roughly the time needed for the crust to reach its maximum thickness crustal detachment takes place over a wide range of lithospheric strength suggesting that crustal buoyancy has an important role in the dynamics of continental subduction

26al records in chondrules from unequilibrated ordinary chondrites ii duration of chondrule formation and parent body thermal metamorphism abstract fossil records of the now extinct short lived nuclide 26al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites uocs semarkona 3 0 bishunpur and y 791324 both 3 1 and alha76004 3 3 belonging to the ll group to infer their time of formation the present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of uoc chondrule formation nebular processes responsible for chondrule formation started 1 ma after formation of cais and effectively stopped within another one and a half million years in the inner region of the asteroidal belt modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of uoc chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in uoc parent bodies the duration of formation of uoc chondrules is significantly shorter than that for carbonaceous chondrite chondrules

the influence of bulk composition and dynamic melting conditions on olivine chondrule textures abstract variation of olivine chondrule textures has been produced by varying the feo feo mgo ratio between average type ia and type ii chondrule compositions for a constant initial melting temperature and heating time earlier experiments produced the same variation of olivine chondrule textures by varying the initial heating temperature for a constant feo feo mgo ratio if chondrule textures are produced by heterogeneous nucleation with no external seeding then the degree of melting directly affects the number of growth sites nuclei and relic crystals remaining after the initial chondrule forming event the degree of melting can be controlled either by changing liquidus temperature i e feo feo mgo ratio or initial melting temperature a range of heating times and masses of precursor spheres causes variation in the degree of melting and produces variation in chondrule textures such effects could be minor in nature compared to initial heating temperatures or bulk composition chondrule textures are distributed on a graph of initial temperature vs feo feo mgo ratios as bands parallel to the olivine disappearance curve using this graph chondrule textures can be predicted for feo feo mgo ratios at specific initial melting temperatures therefore whether chondrules formed within a restricted temperature range or not bulk composition was an important variable affecting their textures

coarse grained chondrule rims in type 3 chondrites abstract relatively coarse grained rims occur around all types of chondrules in type 3 carbonaceous and ordinary chondrites those in h l ll3 chondrites are composed primarily of olivine and low ca pyroxene those in cv3 chondrites contain much less low ca pyroxene average grain sizes range from 4 m in h l ll3 chondrites to 10 m in cv3 chondrites such rims surround 50 10 and 1 of chondrules in cv3 h l ll3 and co3 chondrites respectively but are rare 1 around cv3 ca al rich inclusions rim thicknesses average 150 m in h l ll3 chondrites and 400 m in cv3 chondrites the rims in h l ll3 chondrites are composed of material very similar to that which comprises darkzoned chondrules and recrysiallized matrix dark zoned chondrules and coarse grained rims probably formed in the solar nebula from clumps of opaque matrix material heated to sub solidus to sub liquidus temperatures during chondrule formation mechanisms capable of completely melting some material while only sintering other material require steep thermal gradients suitable processes are lightning reconnecting magnetic field lines and possibly aerodynamic drag heating cv chondrites may have formed in a region where the chondrule formation mechanism was less efficient probably at greater solar distances than the ordinary chondrites the lesser efficiency of heating could be responsible for the greater abundance of coarse grained rims around cv chondrules alternatively cv chondrules may have suffered fewer particle collisions prior to agglomeration

chondrule cooling rates inferred from diffusive profiles in metal lumps from the acfer 097 cr2 chondrite abstract cr chondrites contain metal lumps 300 m either attached to chondrule silicates or apparently isolated in the matrix here laser ablation microanalysis of six metal lumps from a polished thin section of the acfer 097 cr2 chondrite at 15 m spatial resolution revealed zoning profiles for the volatile elements cu and ga the mutual diffusivities of cu and ga were used to infer t 1473 100 k from the correlation of cu versus ga the cooling rates of the metal lumps were calculated to be 0 5 50 k h 1 for tp 1473 100 k with a maximum possible range of 0 1 400 k h 1 for tp 1200 1800 k overlapping the range of cooling rates inferred from petrological studies of type i chondrules 10 1000 k h 1 chondrule textures were established near the peak heating temperatures of chondrules approximately 1900 2000 k while the cu and ga diffusive profiles were established after solidification t 1500 k consistent with nonlinear cooling furthermore one chondrule n2 has a more complex zoning profile that is modeled as a three stage cooling history involving initial cooling at approximately 1 k h 1 followed by mild re heating t 1700 k that re evaporated cu and ga from the outer approximately 100 m of the metal lump and then cooled again at approximately 5 k h 1 the thermal effects of parent body and other preaccretionary heating events on the cu and ga zoning profiles are examined although cr parent bodies have experienced aqueous alteration the thermal effects of this process can neither produce nor erase the cu and ga diffusive profiles thus metal lumps in cr chondrites record the solid state thermal history of chondrules as they travelled away from the chondrule forming region

the origin and nebular history of the metal phase of ordinary chondrites abstract we present new inaa results for bulk metal from h and ll chondrites both have characteristic elemental patterns similar to l chondrite metal in particular increasing abundances of w and ga from unequilibrated to equilibrated chondrites but a reverse variation of v and cr abundances these characteristics indicate that metal in ordinary chondrites formed by melting and reduction of highly oxidized material the similarities of melting features and the complementary nature of compositions between metal and chondrules suggest that these two components were derived from a common precursor similar to ci or cm material in redox state and compositionally related to the matrix of highly unequilibrated ordinary chondrites co ni abundance ratios are similarly low for bulk metal in the least metamorphosed ordinary chondrites of all three chemical groups this suggests that metal in ordinary chondrites initially had the same composition and formed under the same melting conditions the chondrites that accreted earliest have preserved their melting characteristics while those that accreted later reequilibrated with the ambient gas at different temperatures the differences in redox state between equilibrated and unequilibrated chondrites show that formation of chondritic metal and chondrules by melting occurred during the accretion of ordinary chondrite parent bodies the initial metal composition established during the melting stage 13 wt ni co ni 0 031 is inferred from the metal in highly unequilibrated chondrites the accretion temperatures of about 600 k for ordinary chondrites are calculated from the reequilibration reaction h chondrites were accreted at the highest temperature and are most reduced among the three ordinary chondrite groups ll chondrites were accreted at the lowest temperature and are most oxidized compositionally the metal component that would account for the fe si fractionation among h l and ll chondrites is different from the metal formed by melting indicating that the different fe s ratios of h l and ll chondrites were established before their accretion

matrix material in type 3 chondrites occurrence heterogeneity and relationship with chondrules abstract matrix material in type 3 chondrites forms rims on chondrules metal sulfide aggregates ca al rich inclusions and chondritic clasts it also forms lumps up to a millimeter in size which may contain coarser silicates chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized mean concentrations of mg na al and ca in matrix occurrences show up to fivefold variations in a single chondrite variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between h l and ll matrices such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates compositions of chondrules and their matrix rims are normally unrelated although rim compositions are correlated with those of matrix lumps inside chondrules a single chondrule was found with a composition nearly identical to that of its internal matrix lump suggesting that some chondrules may have formed from matrix material matrix lumps are as heterogeneous as chondrules but mean chondrule and matrix compositions differ even allowing for possible loss of metallic fe ni during chondrule formation since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for fe mg exchange our matrix samples cannot represent typical chondrule precursor materials

elemental redistribution in tieschitz and the origin of white matrix two types of mesostasis coexist within some porphyritic chondrules in tieschitz one type is smooth the other confined to chondrule margins is blocky on a 5 10 m scale mesostases in one porphyritic olivine pyroxene pop chondrule and one porphyritic olivine po chondrule were analysed by scanning electron microscopy sem and energy dispersive x ray spectrometry eds as was white matrix nearby mesostases in the po chondrule and in four others were analysed by ion probe pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change relative to smooth mesostasis blocky mesostasis is enriched in al alkalis ba f and cl but depleted in si fe and ca white matrix fills channels between the chondrules it is physically and chemically similar to blocky mesostasis but three ion probe analyses indicate that unlike the mesostases it is poor in sc and has variable and fractionated rare earth elements rees smooth mesostasis is interpreted as solidified primary chondrule liquid whereas blocky mesostasis is its alteration product or less likely a precipitate replacing smooth mesostasis leached out by aqueous fluid white matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion or as a precipitate filling interchondrule voids iron may have been lost from the bulk meteorite but most other elements merely underwent internal redistribution disturbed isotopic systems indicate that aqueous fluid may have been active on the tieschitz parent body only 2 ga ago if correct this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4 5 ga ago

granoblastic olivine aggregates in magnesian chondrules planetesimal fragments or thermally annealed solar nebula condensates abstract granoblastic olivine aggregates goa have been discovered in some type i magnesian chondrules within carbonaceous chondrites by libourel and krot libourel g krot a n 2007 evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin earth planet sci lett 254 1 8 who proposed an origin from pre existing planetesimals amoeboid olivine aggregates aoa generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites display similar equilibrium texture though on a finer scale for these reasons we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating pressed 88 4 was isothermally heated at temperatures ranging from 1350 1550 c for 1 100 h the 100 h runs yield olivine aggregates with well developed granoblastic texture at all temperatures manifest as a network of randomly oriented and sutured olivine grains with 120 triple junctions individual olivine grains are 4 6 sided and polygonal by 1450 c and equigranular texture is developed at high temperature 1500 1550 c melting of olivine commences at 1450 1500 c and aids in ripening and suturing grain coarsening and grain boundary migration textural equilibrium is clearly met at 1550 c a planetesimal origin cannot be ruled out however the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects goa may have experienced higher degrees of thermal processing than the finer grained aoa if the precursors were the same grain coarsening would have to be accompanied by modification to bulk and isotopic compositions however the precursors could have been olivine condensates formed later than aoa annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of goa prior to their incorporation into chondrules

indentation as an extrusion mechanism of lower crustal rocks insight from analogue and numerical modelling application to the eastern bohemian massif abstract recent petrological structural and geochronological studies of the eastern margin of the bohemian massif czech republic suggest a conceptual geodynamical model to explain exhumation of lower crustal 20 kbar 800 c felsic rocks the model involves indentation of a weak orogenic lower crust by an adjacent rigid mantle lithosphere resulting in crustal scale buckling of the weak orogenic lower middle crust interface followed by extrusion of a ductile nappe over the rigid promontory the hypothesis has been investigated using both analogue and numerical models analogue experiments using a three layer sand silicone setup were carried out in rennes laboratory france results show that the most important features of the conceptual model can be reproduced extrusion of lowermost silicone over the indenter and flow of horizontal viscous channel underneath a rigid lid above the actively progressing promontory furthermore experimental results show that a plateau develops above the channelling lower crust two sets of sandbox scale numerical simulations were performed the first set of experiments is designed to study the influence of viscosity stratification within the crust on the extrusion process a second set of experiments were performed in order to quantify the influence of the viscosity and the geometry of the indentor non dimensional scaling laws were derived to predict the maximum extrusion rates associated with the indentation mechanism such laws enable the computation vertical extrusion rates that are in good agreement with natural exhumation rates inferred from petrological data finally we discuss the potential positive feedback of rayleigh taylor instability on vertical extrusion for the case of eastern bohemian massif

quantifying the error of 2d bulk chondrule analyses using a computer model to simulate chondrules simchon bulk chondrule compositions are important to many questions in cosmochemistry however the number of available bulk chondrule data sets is still small a main reason for this is the difficulties of determining bulk chondrule compositions a commonly used technique is to obtain 2d bulk chondrule compositions from meteorite sections this technique has an error that we quantify here for the first time using a mathematical model of a chondrule called simchon the theoretically calculated errors are compared to errors that we determined from serial sectioning of eight efremovka chondrules the errors obtained from both approaches are in excellent agreement proving that our mathematical model produces reliable errors that can be assigned to 2d bulk chondrule compositions these errors allow a much better interpretation of 2d bulk chondrule data we provide a table that contains typical errors for 2d bulk compositions of porphyritic chondrules the errors are in the range of 1 30 relative this should be acceptable for many problems in cosmochemistry the effect of a chemical layering inside chondrules and the occurrence of a rim around them as well as the occurrence of opaque and other accessory phases have been studied a spreadsheet is provided that enables the calculation of errors for any desired chondrule mineral composition bo chondrules have a negligible error but it is impossible to provide reasonable error estimates for bo chondrules with an igneous rim radial pyroxene chondrules have negligible errors

high carbon concentrations in meteoritic chondrules a record of metal silicate differentiation abstract both reduced type i and oxidized type ii chondrules from the primitive chondrites allende semarkona and bishunpur were analysed with ion microprobe for the carbon and hydrogen concentrations of their silicates the high and variable h and c concentrations found i e from 100 to 1500 ppm h 2 o and from 100 to 3900 ppm c show that these volatile elements were present in chondrule precursors as solid grains mean c concentrations are 3 times higher in type i chondrules than in type ii chondrules therefore it seems likely that metal silicate oxydo reduction reactions were promoted by c rich grains whose heterogeneous distribution in the protosolar nebula caused the variation in the redox states of chondrules based on the present results an in situ reduction of the silicates during chondrule melting seems plausible although reduction in the gas phase before condensation cannot be ruled out calculations of the devolatilisation rate of co from hot chondrules account for the presently measured high c concentrations of type i chondrules if 2 6 wt c grains were originally present among chondrule precursors such a result is in agreement with the c concentration in c1 chondrites and with laboratory simulations which have demonstrated that up to 4 wt c grains were necessary to yield type i chondrules by in situ reduction connolly et al 1994 assuming a solar c o ratio in the chondrule forming regions of the protosolar nebula it is likely that the high density of c grains in the gas resulted from presolar chemistry this interpretation is supported by the corresponding c grain atom gas ratio 2 7 10 13 which is 10 times below the ratio measured in the prestellar clouds

the formation of chondrules petrologic tests of the shock wave model chondrules are millimeter sized rounded igneous rocks within chondritic meteorites their textures and fractionated mineral chemistries suggest that they formed by repeated localized brief minutes to hours melting of cold aggregates of mineral dust in the protoplanetary nebula astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites however a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed it also explains the volatile rich nature of chondrule rims and the chondrite matrix

physical properties of chondrules in different chondrite groups implications for multiple melting events in dusty environments chondrite groups cv ck cr with large average chondrule sizes have low proportions of rp plus c chondrules high proportions of enveloping compound chondrules high proportions of chondrules with thick igneous rims and relatively low proportions of type i chondrules containing sulfide in contrast chondrite groups cm co oc r eh el with smaller average chondrule sizes have the opposite properties equilibrated ck chondrites have plagioclase with relatively low na equilibrated oc r eh and el chondrites have more sodic plagioclase enveloping compound chondrules and chondrules with igneous rims formed during a remelting event after the primary chondrule was incorporated into a dustball repeated episodes of remelting after chondrules were surrounded by dust would tend to produce large chondrules rp and c chondrules formed by complete melting of their precursor assemblages remelting of rp and c chondrules surrounded by dust would tend to produce porphyritic chondrules as small dust particles mixed with the melt providing nuclei for crystallizing phenocrysts this process would tend to diminish the numbers of rp and c chondrules correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust rich mantles that formed in locally dusty nebular environments chondrules that were surrounded by thick dust mantles tended to cool more slowly because heat could not quickly radiate away slow cooling led to enhanced migration of sulfide to chondrule surfaces and more extensive sulfide evaporation these chondrules also lost na the plagioclase that formed from equilibrated ck chondrites was thus depleted in na

chondrule collisions in shock waves detailed numerical models have shown that solar nebula shock waves would be able to thermally process chondrules in a way that is consistent with experimental constraints however it has recently been argued that the high relative velocities that would be generated between chondrules of different sizes immediately behind the shock front would lead to energetic collisions that would destroy the chondrules as they were processed rather than preserving them for incorporation into meteorite parent bodies here the outcome of these collisions is quantitatively explored using a simple analytic expression for the viscous dissipation of collisional energy in a liquid layer it is shown that molten chondrules can survive collisions at velocities as high as a few hundred meters per second it is also shown that the thermal evolution of chondrules in a given shock wave varies with chondrule size which may allow chondrules of different textures to form in a given shock wave while experiments are needed to further constrain the parameters used in this work these calculations show that the expected outcomes from collisions behind shock waves are consistent with what is observed in meteorites

correlating multiple neoarchean paleoproterozoic impact spherule layers between south africa and western australia abstract well studied successions in the griqualand west basin south africa and the hamersley basin western australia that both straddle the archean proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities each succession hosts a minimum of three impact spherule layers implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years the occurrences of the youngest spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact close similarities in stratigraphic setting age and petrographic characteristics of the spherules as well as the restriction of distinctive irregular melt particles to only these layers suggest the oldest layers in the two basins were likewise formed by a single impact in contrast the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the spherules suggesting they are products of different impact events these results suggest that using impact spherule layers to establish a global network of high resolution stratigraphic markers for early precambrian successions may be an achievable goal but careful comparisons between potentially correlative layers will be necessary to achieve it

the cr chondrite clan implications for early solar system processes in this paper we review the mineralogy and chemistry of calcium aluminum rich inclusions cais chondrules feni metal and fine grained materials of the cr chondrite clan including cr ch and the metal rich cb chondrites queen alexandra range 94411 hammadah al hamra 237 bencubbin gujba and weatherford the members of the cr chondrite clan are among the most pristine early solar system materials which largely escaped thermal processing in an asteroidal setting bencubbin weatherford and gujba may be exceptions and provide important constraints on the solar nebula models these constraints include 1 multiplicity of cai formation 2 formation of cals and chondrules in spatially separated nebular regions 3 formation of cais in gaseous reservoir s having o 16 rich isotopic compositions chondrules appear to have formed in the presence of o 16 poor nebular gas 4 isolation of cais and chondrules from nebular gas at various ambient temperatures 5 heterogeneous distribution of al 26 in the solar nebula and 6 absence of matrix material in the regions of cai and chondrule formation

compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body abstract chemical compositions of eleven enstatite chondrites 1 eh5 4 eh3 3 el3 i el5 and 2 el6 and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported the abundances of nonvolatile lithophile elements such as al sc and mg increase from eh to el while those of siderophile and moderately volatile elements decrease in the sequence of eh5 to eh3 el3 and el5 6 the continuity in compositions of enstatite chondrites and in particular the inverse variations of moderately volatile element abundances with petrographic type between eh and el groups demonstrate that enstatite chondrites have been derived from a common parent body the enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region metal was effectively accreted into the core where eh5 chondrites were derived and the abundances of metal decreased as accretion proceeded in complement silicate abundances gradually increased in the layers that accreted later the lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes furthermore it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites it is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating the chondrule formation process formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period the mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism the metamorphic trend from eh3 to eh5 resulted from internal heating whereas the trend from el3 to el6 was due to an external heat source it seems very likely that the eh chondrites were metamorphosed during accretion thus cooled rather rapidly activities of the early sun could serve as an energy source for the external heating

isotopically uniform 16o depleted calcium aluminum rich inclusions in ch and cb carbonaceous chondrites abstract in situ oxygen isotope measurements of calcium aluminum rich inclusions cais from the metal rich carbonaceous chondrites isheyevo ch cb like acfer 214 paired with acfer 182 ch que 94411 paired with que 94627 cbb and hammadah al hamra 237 cbb revealed the presence of a common population of igneous isotopically uniform 16o depleted inclusions 17o average 2 standard deviations 7 4 6 5 and 8 3 respectively all cais from cbs and a significant fraction of those from chs and isheyevo are 16o depleted most of the 16o depleted cais consist of ti poor al diopside spinel melilite and forsterite and surrounded by a single and double layered rim of forsterite diopside the 16o depleted cais composed of hibonite grossite melilite and spinel and surrounded by the multilayered melilite diopside forsterite rims are less common some of the 16o depleted refractory igneous inclusions composed of al diopside forsterite and spinel have chondrule like textures skeletal or barred they are mineralogically most similar to al diopside rich chondrules found in metal rich carbonaceous chondrites and composed of al diopside forsterite al rich low ca pyroxene glassy mesostasis and spinel suggesting there is a continuum between these objects we suggest that i most of the isotopically uniform and 16o depleted cais resulted from remelting of pre existing possibly 16o rich refractory inclusions the remelting may have occurred during formation of the magnesian non porphyritic cryptocrystalline and skeletal chondrules in chs cbs and isheyevo either by an unspecified late single stage highly energetic event or in an impact generated plume previously hypothesized for their origin both mechanisms probably occurred in the solar nebula i e in the presence of the nebula gas the forsterite pyroxene rims around 16o depleted cais may have resulted from evaporation recondensation of silicon and magnesium during this event some of the al diopside rich cais may have formed by evaporation of the al rich chondrule melts ii in addition to these components the chs and isheyevo contain a high abundance of chondrules and refractory inclusions formed by the commonly inferred nebular processes evaporation condensation and incomplete melting of dust aggregates during multiple transient heating events these include 16o rich cais amoeboid olivine aggregates and ferromagnesian and aluminum rich chondrules with porphyritic textures such components are also present in cb chondrites but they are exceptionally rare these observations indicate that there are multiple generations of cais in metal rich carbonaceous chondrites iii because the isotopically uniform 16o depleted igneous cais and the magnesian cryptocrystalline and skeletal olivine pyroxene chondrules are found almost exclusively in metal rich carbonaceous chondrites the hypothesized impact plume mechanism of chondrule formation and recycling of cais are not common processes

dynamic cyrstallization of chondrule melts of porphyritic olivine composition textures experimental and natural abstract melts of the same composition as has been defined for porphyritic olivine chondrules have been crystallized under controlled cooling conditions dynamic crystallization the full range of textures observed in such chondrules has been duplicated in the experimentally produced analogs the primary controlling factor in producing this range of textuures is the heterogeneous nucleation behavior in general if nuclei are present when cooling begins growth starts immediately at low degrees of supercooling and porphyritic textures result the shapes of the phenocyrsts depend on the growth rate which in turn depends on the cooling rate with high nucleation densities and modest to low cooling rates the textures are microporphyritic with generally equant euhedral phenocrysts as the nucleatio density decreases the phenocrysts become larger and as the cooling rate increases they become increasingly skeletal if nuclei are not present when cooling begins growth is delayed until subcritical sized embryos graduate to supercritical nuclei growth usually begins at some significant degree of supercooling depending on the cooling rate and how long it takes the embryos to become nuclei the shapes of the crystals depend on the degree of supercooling when growth is initiated and on the subsequent cooling rate the resulting textures vary from plate dendrites barred olivine chondrules to randomly oriented elongated skeletal crystals as the degree of supercooling and cooling rates decreases the results of a matrix of experiments set limits on the maximum melting temperatures of chondrule forming materials and the subsequent cooling rates based primarily on crystal shape the porphyritic textures develop best for this composition at melting temperatures less than 1590 c and cooling rates near 100 c hr and definitely less than 1000 c hr at cooling rates significantly less than 100 c hr the phenocrysts are generally larger than observed in chondrules and as cooling rates approach 1000 c hr they become more skeletal barred olivine chondrule textures form best from materials melted at higher temperatures and cooling rates in the range 1000 to 3000 c hr

the origin of chondrules and chondrites debris from low velocity impacts between molten planetesimals we investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin shelled largely molten planetesimals collided at low speed during accretion this scenario here dubbed splashing stems from evidence that such planetesimals intensely heated by 26al were abundant in the protoplanetary disk when chondrules were being formed approximately 2 myr after calcium aluminum rich inclusions cais and that chondrites far from sampling the earliest planetesimals are made from material that accreted later when 26al could no longer induce melting we show how splashing is reconcilable with many features of chondrules including their ages chemistry peak temperatures abundances sizes cooling rates indented shapes relict grains igneous rims and metal blebs and is also reconcilable with features that challenge the conventional view that chondrules are flash melted dust clumps particularly the high concentrations of na and feo in chondrules but also including chondrule diversity large phenocrysts macrochondrules scarcity of dust clumps and heating we speculate that type i feo poor chondrules come from planetesimals that accreted early in the reduced partially condensed hot inner nebula and that type ii feo rich chondrules come from planetesimals that accreted in a later or more distal cool nebular setting where incorporation of water ice with high 17o aided oxidation during heating we propose that multiple collisions and repeated re accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties and led to so called complementarity and metal depletion in chondrites we suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

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

petrography and geochemistry of distal spherules from the k pg boundary in the umbria marche region italy and their origin as fractional condensates and melts in the chicxulub impact plume the impact spherules from the distal k pg boundary sections are considered to represent silicate droplets condensed and solidified from a laterally expanding cooling vapor plume formed upon hypervelocity impact in the present day cretaceous paleogene boundary k pg spherule population of the umbria marche region in italy three texturally and compositionally distinct types of impact spherules can be identified that are dominantly composed of 1 goethite 2 k feldspar or 3 glauconite although these phases represent the products of diagenetic alteration the remnant textural characteristics of the spherules and the type of alteration product are indicative of the spherules original compositions which are important to constrain the physicochemical conditions prevalent throughout the impact vapor plume the presence of relict ghost crystals and the identification of iddingsite indicate that goethite likely represents pseudomorphic replacement after olivine goethite spherules contain numerous dendritic euhedral and skeletal spinel crystals variably dispersed in the groundmass in terms of textures five types of goethite spherules can be distinguished showing striking similarities to chondrules i skeletal ii barred iii radial barred iv porphyritic and v relict granular the morphology of both spinel and olivine pseudomorphs is consistent with established formation conditions peak temperature tmax degree of supercooling t cooling rate presence of nucleation sites for different chondrule textural types as goethite spherules are anomalously enriched in moderately to highly refractory lithophile sc v y zr nb ree hf ta th and siderophile cr co ni w ir pt elements they are interpreted to represent diagenetically altered refractory high t condensation products from a well homogenized plume consisting of both vaporized target and projectile matter different from goethite spherules k feldspar spherules exhibit pseudomorphic textures after lower liquidus silicates such as ca rich pyroxene and plagioclase furthermore the k feldspar spherules yield systematically lower abundances of the most refractory trace elements this suggests that the pre altered k feldspar spherules are part of the same fractional condensation sequence of the target impactor vapor plume cooled during lateral expansion glauconite spherules are cryptocrystalline exhibiting hemispherical lamellae that resemble the palagonite smectite alteration layers of basaltic glasses and the k pg spherules microtektites found at proximal sites around the gulf of mexico region their trace element contents and ree patterns are strikingly similarity to those of altered k pg microtektites suggesting that glauconite spherules represent former glass spherules without crystallites microlites glauconite spherules are interpreted not to be part of the fractional condensation sequence of the impact vapor plume that led to the formation of the replaced goethite and k felspar spherules they were likely formed by entrainment of melt expelled at the steepest angles and highest ejection velocities from the central melt sheet within the edges of the vapor plume this work is the first to create a geochemical foundation for vapor plume models both at the major and trace element scale in addition it highlights the unique characteristics of the chicxulub impact event and emphasizes the importance of its unusual target lithologies the heterogeneous layered and volatile rich target contributed significantly to the formation of a dust rich environment high oxygen fugacities and the preservation of some degree of heterogeneity in the vapor plume in addition striking similarities between distal goethite type k pg spherules and chondrules may argue for a reevaluation of the impact model as a possible origin for the formation of certain types of cosmic chondrules

convergent margin tectonics central wabigoon subprovince superior province canada abstract best known for its neoarchean greenstone granite terranes in the east and west the wabigoon subprovince of the superior province features a central transverse corridor of meso and neoarchean gneissic granitoid and minor supracrustal rocks resembling an eroded intermontane plateau structural analysis and u pb geochronology in the north central wabigoon subprovince reveal a complex history of plutonism supracrustal rock deposition polyphase deformation and amphibolite facies metamorphism between 3075 and 2670 ma the geometric and kinematic character of d 1 and d 2 deformational events recorded in 2890 2718 ma tonalitic gneisses remain cryptic as a result of magmatic and structural d 3 5 overprinting but the early structures are bracketed in age between 2725 and 2713 ma by a polydeformed gneiss and tonalite dyke these events occurred deep within an active continental magmatic arc and correspond in age to a collision between the juvenile wabigoon and continental winnipeg river subprovinces recognized to the west clastic sediments deposited after 2701 ma are the youngest rocks involved in subsequent regional deformation a d 3 event is responsible for rapid burial of supracrustal units to 15 km within warm arc crust and imposition of a regional penetrative s 3 foliation that accommodated some relative uplift prior to 2697 ma the age of a discordant granodiorite dyke this event corresponds to variably preserved steep north trending d 1 structures recognized in adjacent greenstone belts subsequent d 4 ductile deformation defines the map pattern in the deep crustal rocks of the central wabigoon subprovince upright east trending f 4 folds developed synchronously with d 2 dextral transpressive structures that formed throughout the wabigoon subprovince prior to 2698 ma the final d 5 sinistral shearing event occurred during emplacement of mantle derived granodiorites of sanukitoid affinity 2688 ma and crust derived granites 2685 ma this event appears to have triggered rapid cooling reflected in 2680 2670 ma titanite dates and late brittle increments of displacement

clues to chondrule precursors an investigation of vesicle formation in experimental chondrules abstract the absence of vesicles in chondrules and their presence in synthetic analogs yields information about the origin of chondrules a variety of melting crystallization experiments demonstrate the cause of vesicles in synthetic chondrules experiments involving the use of binding agents in sample preparation samples with residual adsorbed moisture incompletely melted samples and the use of fine grained sizesorted starting powder all generated more vesicles than experiments on control samples volatiles such as na were not responsible for vesicles in our experiments because na was not lost under our flashheating conditions because w dowiak 1983 assumed chondrule precursors contained volatiles and his electrical discharge melting generated vesicles he suggested chondrules were not formed by flashmelting events however vesicle free chondrules are to be expected with flash melting provided that the precursors were poor in highly volatile material flash melting experiments with serpentine in the precursor powder developed extremely porous popcorn spherules as in some meteorite ablation spherules chondrule precursors must have consisted of anhydrous phases assembled at low ambient gas pressure above the condensation temperature of ice the absence of vesicles in all chondrules including those unlikely to have been heated multiple times e g 16o rich and granular chondrules demonstrates that their original precursors whether interstellar dust or nebular condensates cannot have consisted of hydrous silicates

late cretaceous intraplate thrusting in central europe effect of africa iberia europe convergence not alpine collision a well established event of intraplate basin inversion and basement thrusting affected central europe in late cretaceous time it is widely accepted to have resulted from the collision of the alpine orogen with europe s margin at that time an early alpine orogen located on the leading edge of the adria microplate still lay far southeast of its present day position and had entered a phase of extension after a first orogenic event characterized by w to nw directed thrusting this configuration is not likely to have induced ssw nne directed thrusting and folding in the future european foreland that was still separated from the alpine wedge by a strip of oceanic lithosphere by contrast the onset of intraplate contraction coincides with an important change in relative motion between the european and african plates at ca 90 ma africa s sse directed sinistral transform motion relative to europe changed to ne directed convergence this agrees well with the timing and kinematics of intraplate thrusting in central europe structures of similar age and kinematics occurring in southern france spain and north africa suggest that the late cretaceous pulse of contraction was caused by pinching west central europe s thin lithosphere between baltica and africa only since the onset of n directed thrusting in the alps in paleocene or eocene time are the kinematics of the alps and their european foreland compatible indicating that mechanical coupling between africa europe and the adria microplate had been achieved

subduction zone retreat and recent tectonics of the south island of new zealand the central south island of new zealand is a zone of recent oblique continent continent convergence between the australian and pacific plates convergence is localized on the alpine fault and its broader expression has created the southern alps we propose that many of the strike variations in orogenic style of this zone e g physical extent and style of crustal shortening argon release patterns and fission track observations can be explained by differential amounts of subduction zone retreat beneath the orogen the observed patterns suggest greater eastward retreat of the subducting pacific plate in the south than in the north to support this hypothesis we compare geodynamic models having varying amounts of subduction zone retreat with geological and geophysical observations incorporating subduction zone retreat not only creates tectonic variations similar to those observed but also provides a geodynamically consistent explanation for the size shape and location of the broad negative bouguer gravity anomaly associated with the orogen in addition these geodynamic models suggest that subduction zone retreat may result in local topographic lows which may serve as preferential locations for the formation of intermontane basins and associated lakes possibly explaining the chain of south island lakes our preferred interpretation of the geologic history of the central south island convergence has an initial phase of pacific plate lower crust and mantle subduction underthrusting with no retreat followed by a second phase of subduction zone retreat in the southern part of the zone this retreat may be a consequence of oblique convergence which creates interference beneath fiordland between the south east dipping australian plate and the underthrust west dipping pacific plate to the north and forces retreat

indosinian high strain deformation for the yunkaidashan tectonic belt south china kinematics and 40ar 39ar geochronological constraints 1 structural and 40ar 39ar data from the yunkaidashan belt document kinematic and tectonothermal characteristics of early mesozoic indosinian orogenesis in the southern part of the south china block the yunkaidashan belt is tectonically divided from east to west into the wuchuang sihui shear zone xinyi gaozhou block and the fengshan qinxi shear zone indosinian structural elements ascribed to the indosinian orogeny include d2 and d3 deformation the early d2 phase is characterized by folding and thrusting with associated foliation and lineation development related to nw se transpression under amphibolite to greenschist facies conditions this event is heterogeneously overprinted by d3 deformation characterized by a gentle dipping s3 foliation subhorizontally to shallowly plunging l3 lineation some reactived d2 folds and low angle normal faults the d3 fabrics suggest a sinistral transtensional regime under greenschist facies metamorphism the timing of the d2 and d3 events have been constrained to the early to middle triassic 248 220 ma and late triassic 220 200 ma respectively on the basis of 40ar 39ar geochronology and regional geological relations the change from oblique thrusting d2 to sinistral transtension d3 may reflect oblique convergence and crustal thickening followed by relaxation of the overthickened crust in combination with the regional relations from xuefengshan to yunkaidashan and on to wuyishan the early phase of the indosinian orogeny constituted a large scale positive flower structure and is related to the intracontinental convergence during the assembly of pangea in which the less competent south china orogen was squeezed between the more competent north china and indosinian blocks

a doubly vergent fan structure in the peninsular ranges batholith transpression or local complex flow around a continental margin buttress 1 the jura cretaceous peninsular ranges batholith prb of southern and baja california contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs jura cretaceous deformation in the prb is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith the western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures from 2 to 6 kbar and temperatures from 650 c elsewhere in the north american cordillera similar lithospheric boundaries are thought to accommodate various amounts of orogen parallel displacements of outboard tectonic fragments to test this proposal in the prb we examined an unusually well preserved part of the transition zone in the southern sierra san pedro martir sspm here a spectacular 20 km wide doubly vergent fan structure which developed over a period 40 m y during both jurassic and cretaceous magmatism occurs along the transition zone this structure consists of moderately inward dipping outward vergent fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic vertically sheeted plutons mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center a number of constraints on possible displacements rotation of units and duration of fabric formation limit the amount of orogen parallel displacement to a few kilometers across the fan structure in the sspm instead we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid pre mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex focused deformation along the lithospheric discontinuity and subsequent strain softening we suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression emphasizing the need for caution in interpreting such features as zones of significant orogen parallel displacement

chondrules as products of dust collisions with totally molten droplets within a dust rich nebular environment an experimental investigation abstract chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting if barred olivine bo chondrules formed by nearly total melting at their respective liquidi i e over a wide temperature range it is surprising that there are so few totally glassy chondrules the occurrence of fine grained accretionary rims on chondrules suggests mineral dust in the chondrule forming environment which might have collided with totally molten droplets to produce chondrule textures we have therefore conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500 c h and cause nucleation our experiments reproduce the majority of common chondrule textures the production of textures in our experiments is a function of the temperature at which dust is encountered the number of dust grains encountered and to a lesser extent the mineralogy of the encountered mineral grains crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters we have produced some less commonly discussed types of chondrule textures e g barred olivine porphyritic olivine pyroxene bo pop by dust seeding that have not been reproduced by the incomplete melting of starting materials the reproduction of bo pop and excentroradial textures by dust collisions and the scarcity of natural totally glassy chondrules suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures e g feo rich chondrules from these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs however we have reproduced the very fine grained microporphyritic textures characteristic of type ia chondrules only by incomplete melting such chondrules could not have reached average internal temperatures above their liquidus temperatures the conclusion that at least some chondrules formed from total melts that collided with mineral dust during cooling indicates that chondrule formation occurred within a region of the nebula rich in mineral dust

granoblastic olivine aggregates as precursors of type i chondrules an experimental test abstract chondrule formation models involving precursors of granoblastic olivine aggregates goa of either planetesimal or nebular origin have recently been proposed we have therefore conducted chondrule simulation experiments using mixtures of 100 h thermally annealed goa and an en to test the viability of goa as predecessors of porphyritic olivine po chondrules isothermal runs of less than 5 min at 1350 1550 c result in goa disaggregation and fe mg exchange runs of 0 5 4 h show textures superficially similar to granular and po chondrules but with reversely zoned olivine charges isothermally heated at 1550 c for 1 and 4 h before being cooled at 10 and 100 c h undergo olivine crystallization and yield classical po textures although most evidence of origin from goa is erased the cores of normally zoned euhedral crystals are relict as phenocrysts in type i chondrules can be relict such chondrules could have experienced similar peak temperatures to those of type ii chondrules chondrules containing goa with olivine triple junctions resemble experimental charges heated for minutes at temperatures between 1350 and 1450 c and type i chondrules with subhedral to anhedral olivine plus goa relicts resemble charges heated at the same temperatures but for longer duration type i chondrules with a mass of granular olivine or irregular anhedral olivine grains in the center and much glass nearer the margin on the other hand require limited heating at high temperature 1550 c while type i chondrules with euhedral olivines resemble charges heated at 1550 c for 4 h the majority of type i chondrules in cv chondrites display evidence of derivation from goa many finer grained chondrules in cr and uoc on the other hand could not have been derived from such coarse grained precursors but could have formed from fine grained dustballs as stipulated in the standard paradigm thus both goa and dustballs represent viable chondrule precursors of coarser and finer grained type i po chondrules respectively

variations in the o isotope composition of gas during the formation of chondrules from the cr chondrites to better understand the environment of chondrule formation and constrain the o isotope composition of the ambient gas in the renazzo like carbonaceous cr chondrite chondrule forming region we studied the mineralogy petrology and in situ o isotope compositions of olivine in 11 barred olivine bo chondrules and pyroxene and silica in three type i porphyritic chondrules from the cr chondrites gao guenie b graves nunataks gra 95229 pecora escarpment pca 91082 and shisr 033 bo chondrules experienced a higher degree of melting than porphyritic chondrules and therefore it has been hypothesized that they more accurately recorded the o isotope composition of the gas in chondrule forming regions we studied the o isotope composition of silica as it has been hypothesized to have formed via direct condensation from the gas bo chondrules constitute 4 of the total cr chondrule population by volume on a three isotope oxygen diagram 17o vs 18o olivine phenocrysts in type i and type ii bo chondrules plot along slope 1 line with the exception of a type ii bo chondrule that plots along slope 0 5 line olivine phenocrysts in type i and type ii bo chondrules have similar but more restricted ranges of 17o values 3 8 to 1 3 and 0 8 to 1 4 respectively than those in type i and type ii porphyritic chondrules 4 6 to 0 3 and 1 8 to 0 9 respectively the observation that olivine grains in type i bo chondrules have similar chemical and o isotope compositions to those of olivine in their porphyritic counterparts argues against the hypothesis that olivine grains in type i porphyritic chondrules are xenocrysts and represent relict fragments of early formed planetesimals the compositional and o isotope data suggest that bo chondrules experienced more extensive but incomplete exchange with the ambient gas than porphyritic chondrules we suggest that cr chondrules formed from relatively 16o enriched solids in the presence of relatively 16o depleted gaseous h2o the o isotope compositions of chondrule olivine likely result from differences in the o isotope composition of both the chondrule precursors and the ambient gas during chondrule formation the inferred o isotope composition of this gas 17o ranges from 3 to 3 is inconsistent with a high abundance of water from the outer solar system which has been predicted to be isotopically heavy

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

late eocene neogene sedimentary geology of the girne kyrenia range northern cyprus a case history of sedimentation related to progressive and diachronous continental collision abstract sedimentary geology can shed light on the processes of continental collision the easternmost mediterranean provides an exceptional opportunity to study sediments deposited during progressive and diachronous continental collision with implications for comparable settings elsewhere here we focus on sediment sources pathways and deposition of mostly deep sea clastic sediments exposed in northern cyprus the sedimentary basin forms part of the northern continental margin of the southern neotethys ocean and is represented by a ca 200 km long e w trending arcuate lineament encompassing northern cyprus and its submarine link with turkey the upper eocene upper miocene basin was divided into northerly and southerly sub basins during mid to late miocene time by an e w trending inferred syn depositional fault lineament the sequence begins with non marine to shallow marine basal conglomerates late eocene that abruptly fine upwards into siliciclastic turbidites oligocene followed by biogenic calciturbidites and marls aquitanian langhian during mid late miocene the northerly sub basin was characterised by thin to medium bedded calciturbidites and marls serravallian overlain by thick bedded medium grained lithic sandstones tortonian in contrast the southern sub basin is dominated by siliciclastic turbidites with abundant sole structures burdigalian tortonian palaeocurrent evidence indicates dominantly westward sediment transport especially during the late miocene the serravallian tortonian sandstones of the northern sub basin are relatively rich in carbonates while the southern sub basin contains more siliciclastic and ophiolite derived detritus indicating differences in sediment supply in general the upper eocene basal conglomerates were mainly derived from the north from within the girne kyrenia range and its inferred northward extension in response to rapid subsidence reflecting possible southward subduction zone retreat clastic sediment sources to the north then submerged while clastic sediment was instead mainly supplied from the south neotethyan suture zone to the east and northeast in southern turkey reflecting diachronous collision deep water deposition persisted in the easternmost mediterranean basin further west until this was terminated in northern cyprus by thrusting folding during latest miocene earliest pliocene quaternary collision related uplift ensued similar sedimentary development is to be expected in collisional settings elsewhere e g tethyan closure in the himalayas iapetus closure in the appalachians

the formation and alteration of the renazzo like carbonaceous chondrites iii toward understanding the genesis of ferromagnesian chondrules to better understand the formation conditions of ferromagnesian chondrules from the renazzo like carbonaceous cr chondrites a systematic study of 210 chondrules from 15 cr chondrites was conducted the texture and composition of silicate and opaque minerals from each observed feo rich type ii chondrule and a representative number of feo poor type i chondrules were studied to build a substantial and self consistent data set the average abundances and standard deviations of cr2o3 in feo rich olivine phenocrysts are consistent with previous work that the cr chondrites are among the least thermally altered samples from the early solar system type ii chondrules from the cr chondrites formed under highly variable conditions e g precursor composition redox conditions cooling rate with each chondrule recording a distinct igneous history the opaque minerals within type ii chondrules are consistent with formation during chondrule melting and cooling starting as s and ni rich liquids at 988 1350 c then cooling to form monosulfide solid solution mss that crystallized around olivine pyroxene phenocrysts during cooling fe ni metal crystallized from the s and ni rich liquid and upon further cooling mss decomposed into pentlandite and pyrrhotite with pentlandite exsolving from mss at 400 600 c the composition texture and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration however some opaque minerals fe ni metal versus magnetite and panethite present in type ii chondrules are a proxy for the degree of whole rock aqueous alteration the texture and composition of sulfide bearing opaque minerals in graves nunataks 06100 and grosvenor mountains 03116 suggest that they are the most thermally altered cr chondrites

the importance of experiments constraints on chondrule formation models abstract we review a number of constraints that have been placed on the formation of chondrules and show how these can be used to test chondrule formation models four models in particular are examined the x wind model sudden exposure to sunlight 0 1 au from the proto sun with subsequent launching in a magnetocentrifugal outflow solar nebula lightning nebular shocks driven by eccentric planetesimals and nebular shocks driven by diskwide gravitational instabilities we show that constraints on the thermal histories of chondrules during their melting and crystallization are the most powerful constraints and provide the least ambiguous tests of the chondrule formation models such constraints strongly favor melting of chondrules in nebular shocks shocks driven by gravitational instabilities are somewhat favored over planetesimal bow shocks

the ordovician sierras pampeanas puna basin connection basement thinning and basin formation in the proto andean back arc article i nfo the ordovician sierras pampeanas located in a continental back arc position at the proto andean margin of southwest gondwana experienced substantial mantle heat transfer during the ordovician famatina orogeny converting neoproterozoic and early cambrian metasediments to migmatites and granites the high grade metamorphic basement underwent intense extensional shearing during the early and middle ordovician contemporaneously up to 7000 m marine sediments were deposited in extensional back arc basins covering the pre ordovician basement extensional ordovician tectonics were more effective in mid and lower crustal migmatites than in higher levels of the crust at a depth of about 13 km the separating boundary between low strain solid upper and high strain lower migmatitic crust evolved to an intra crustal detachment the detachment zone varies in thickness but does not exceed about 500 m the formation of anatectic melt at the metamorphic peak and the resulting drop in shear strength initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies p t conditions p t d t data in combination with field evidence suggest significant ca 52 crustal thinning below the detachment corresponding to a thinning factor of 2 1 ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25 and 44 constituting total crustal thinning factors of 1 7 2 0 while the migmatites experienced retrograde decompression during the ordovician rocks along and above the detachment show isobaric cooling this suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface upper crustal extension and thinning is compensated by newly deposited sediments maintaining constant pressure at detachment level thinning of the migmatitic lower crust is compensated by elevation of the crust mantle boundary the degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension basin formation and sediment thickness in the back arc basin the initiation of crustal extension in the back arc however crucially depends on the presence of anatectic melt in the middle and lower crust consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension

decoupling during continental collision and intra plate deformation abstract fundamental aspects of mountain building such as the dominant deformation mechanism strain localisation and topography development in response to the presence of decoupling zones either along boundaries of weak and strong lithospheres or within weak domains of continental lithosphere are studied by analogue lithospheric models scenarios are investigated where a weak part of the lithosphere is bordered on either side by stronger lithospheres geometries of the weak zone vary from box to wedge to graben shaped representing lateral strength variations as a consequence of earlier deformation phases the rheologic and geometric configuration at the onset of shortening is therefore considered as representative for continental collision and intra plate settings the modelling results show that the geometric and topographic evolution of mountain belts through time is sensitive to the presence of decoupling horizons along plate or tectonic boundaries this is because the decoupled boundaries localise strain control the dominant deformation mechanism and hence the locus of uplift and subsidence accordingly decoupled boundaries favour deformation by thrusting and particularly in the case of an initially wedge shaped weak zone the development of mountain belts with overall triangular shape in contrast lithospheres with coupled boundaries dominantly deform by buckling irrespective of the initial geometry of the weak zone as this deformation mechanism affects all model domains it produces a distinguishable pattern of uplift and subsidence controlled by buckling strong decoupling at mid crustal levels within the weak zone favours vertical strain partitioning and the formation of plateau shaped mountains the modelling results suggest that natural collisional systems should respond to an increase in plate coupling through time by widespread uplift of the mountain belt including its foreland basins which were formerly created under decoupled conditions due to the absence of decoupling zones like plate contacts weak domains in intra plate settings are predicted to deform dominantly by buckling

seismic refraction data in the gulf of saint lawrence implications for the lower crustal blocks a refined model for the wide angle reflection refraction profile 88 3 that crosses the foreland basin of the appalachian orogen is presented the two major layers of the cratonic crust have velocities of 6 2 and 6 7 km s 1 with low gradients near the exposed north american craton the 6 7 km s 1 velocity appears at the shallow depth of 13 km and the m discontinuity at greater than 40 km depth towards the orogen a southeastward dipping ramp is developed at mid crustal levels at the top of the 6 7 km s 1 layer near the appalachian front lower crustal velocities typical of the grenville are replaced with a velocity of 7 2 km s 1 the collinear reflection profile confirms the southward dipping ramp the shallowing m discontinuity and the lateral changes in the lower crust refraction profiles combined with reflection and gravity data are used to distinguish the grenville lower crustal block the grenville lower crustal block is defined as a pattern of lower crustal and upper mantle reflectivity with associated velocities and a more negative gravity anomaly than lower crustal blocks of the appalachian terranes the grenville block terminates northwest of the coast of newfoundland near the appalachian front thus this lower crustal province does not underlie the humber tectono stratigraphic zone this is different than the relationship of the edge of the grenville lower crustal block to surface zones in new england a thick skinned tectonic style with basement involved deformation appears to be typical of the newfoundland appalachians in contrast a thin skinned structural style is observed in new england the change in structural styles occurs in the bend of the appalachian orogen across the gulf of st lawrence

tectonic and economic implications of trace element 40ar 39ar and sm nd data from mafic dykes associated with orogenic gold minerals in central victoria australia comment

experimental reproduction of classic barred olivine chondrules open system behavior of chondrule formation barred olivine bo chondrule texture with olivine rim was successfully reproduced by cooling experiments under evacuated conditions for an feo rich alkali free low liquidus 1215 c analog chondrule composition the rim formation requires a heating temperature about 100 c or more above the liquidus to promote heterogeneous nucleation of olivine on the surface of a melt droplet partial evaporation mainly feo enhances the selective nucleation too because the surface should be cooler than the interior due to the latent heat of the evaporation the evaporation also causes subsequent growth along the cooler surface this rim formation process suggests chondrule formation as open systems if evaporation occurred during chondrule formation evaporation of na and sio2 should enhance the selective nucleation and growth to form olivine rims more easily textures close to classic bo which consists of a single set of parallel olivine platelets with olivine rim were also reproduced by using small samples 10 mg and 1 mm in diameter where only a limited number of nucleation events is possible in a small surface area three dimensional structures of reproduced bo chondrules were observed by x ray microtomography using synchrotron radiation olivine bars in a two dimensional section are really platelets and the platelets grow from the surface of a melt spherule inwards the crystallographic orientations of the olivine platelets which are normal to the b axis are also consistent with those of natural bo chondrules bo textures close to natural chondrules were formed by heating at super liquidus temperatures of about 100 200 c for 10 min and cooled at 1000 c h however detailed morphologies of olivine rims and platelets glass inclusions and dendritic side arms and olivine zoning are inconsistent with those of natural olivine irrespective of the experimental conditions these discrepancies can be explained by annealing diffusion calculations for cooling and annealing of chondrules suggest that annealing at low temperatures 1000 c such as by thermal metamorphism cannot explain mg fe and ca zoning in bo the annealing was possibly due to multiple heating in connection with chondrule recycling process

chondrule formation by the ablation of small planetesimals numerous models have been proposed to explain the formation of chondrules but none can be reconciled with the highly diverse properties of these objects here the formation of chondrules by the surface melting and ablation of small planetesimals in nebula shock waves is investigated using a numerical model it is shown that bodies between 1 mm and 500 m in diameter would have produced molten droplets by ablation during gas drag in nebula shocks stronger than 2 0 mach the properties of chondrules produced by ablation are estimated by comparison with meteorite fusion crusts and through consideration of the environment within the bow shock envelope of ablating planetesimals it is suggested that most ablation chondrules will have broadly chondritic compositions with depletions in siderophile and chalcophile elements and relatively high volatile contents and textures that are mainly porphyritic the formation of chondrules by ablation of planetesimals in shock waves was probably most important at a late stage in nebula history and occurred at the same time as chondrules formed by the melting of dust particles the high abundance of dust particles relative to larger bodies at all stages of accretion implies that only a proportion of chondrules may have been formed by ablation and that genetic groups of chondrules with very different origins may coexist in meteorites

formation conditions of pyroxene olivine and magnesian olivine chondrules abstract dynamic crystallization experiments performed on a type i and an intermediate type ii iii composition demonstrate chondrule texture is controlled by the abundance of heterogeneous nuclei this factor is controlled by the maximum initial temperature relative to the olivine disappearance temperature for the given heating time glassy and excentroradial textures form from superheated melts with no nuclei and barred textures form with initial temperatures near the olivine disappearance temperature porphyritic and granular textures form with initial temperatures below the liquidus with abundant nuclei comparison of zoning characteristics of experimental olivines with those in natural porphyritic olivine chondrules indicates cooling rates of 100 to 1000 c h whereas olivine morphology indicates barred olivines cooled at rates of 250 to 1000 c h texture composition relationships suggest chondrules experienced initial temperatures below 1750 c these observations are consistent with the idea that chondrules formed by transient heating in dust rich regions i e in the midplane of the solar nebula

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

the fountain hills unique cb chondrite insights into thermal processes on the cb parent body we report the results of an extensive study of the fountain hills chondritic meteorite this meteorite is closely related to the cba class mineral compositions and o isotopic ratios are indistinguishable from other members of this group however many features of fountain hills are distinct from the other cb chondrites fountain hills contains 23 volume percent metal significantly lower than other members of this class in addition fountain hills contains porphyritic chondrules which are extremely rare in other cba chondrites fountain hills does not appear to have experienced the extensive shock seen in other cb chondrites the chondrule textures and lack of fine grained matrix suggests that fountain hills formed in a dust poor region of the early solar system by melting of solid precursors refractory siderophiles and lithophile elements are present in near ci abundances within a factor of two related to the enhancement of metal moderately volatile and highly volatile elements are significantly depleted in fountain hills the abundances of refractory siderophile trace elements in metal grains are consistent with condensation from a gas that is reduced relative to solar composition and at relatively high pressures 10 3 bars fountain hills experienced significant thermal metamorphism on its parent asteroid combining results from the chemical gradients in an isolated spinel grain with olivine spinel geothermometry suggests a peak temperature of metamorphism between 535 c and 878 c similar to type 4 ordinary chondrites

petrology and mineralogy of type ii feo rich chondrules in semarkona ll3 0 origin by closed system fractional crystallization with evidence for supercooling abstract the petrology of type ii porphyritic olivine chondrules in semarkona ll3 0 has been studied in detail olivines in these chondrules are euhedral fe richand are strongly zoned from cores to rims of grains in feo fa10 30 cr2o3 0 2 0 6 wt mno 0 2 0 7 wt and cao 0 1 0 4 wt interstitial mesostasis is rich in si aland ca and is glassy with abundant microcrystallites minor minerals include troilite fe ni metaland chromite some olivine grains contain euhedral fayalite rich cores that are probably produced during initial supercooling of the chondrule melt rare relict grains of forsteritic olivine have compositions very similar to olivines in type ia chondrules in semarkona and may result from disaggregation of such chondrules apart from these relics all properties of type ii chondrules can be described by closed system fractional crystallization of droplets which were essentially entirely molten the chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks cooling rates of the order of 1000 c h are indicated from peak temperatures of around 1600 c the chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization there is no evidence for chondrule nebula exchange either during or after crystallization or any secondary processes such as aqueous alteration or thermal metamorphism bulk composition data suggest that precursor material included a refractory ca al ti rich component similar to one of the precursor components of type ia chondrulesand a non refractory fe and si rich component type ia chondrules may have formed from type ii chondrules by loss of fe and volatiles alternatively the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different fe mg ratios

the transition from thick skinned to thin skinned tectonics in the basque cantabrian pyrenees the burgalesa platform and surroundings interpretation of seismic data in the margins of the burgalesa platform in the basque cantabrian pyrenees has allowed proposition of a new structural model that combines different modes of deformation during oblique tectonic inversion conditioned by the distribution of triassic salts deformation was decoupled by the presence of the salt horizon between basement involved thrusts inverting formerly triassic and late jurassic early cretaceous extensional faults and a detached thrust system involving the upper triassic to neogene sedimentary package structural units experiencing different styles of deformation are not only stacked vertically above and below the salt but most importantly they change from one to the other along strike across the transversal edges of the triassic salts the burgalesa platform detached thrust system was confined between the basement involved structures of the cantabrian mountains westward and the nw tip of the iberian basement involved structures san pedro southward this together with the obliquity between the pyrenean shortening direction and the strike of the previous extensional faults mostly during the late stages of deformation determined the strike slip reactivation of the basement involved inverted faults and the lateral extrusion of the burgalesa platform detached mesozoic successions above the salt towards the se to form a prominent thrust salient oblique to the main pyrenean trend the proposed model combines thick skinned with thin skinned structural styles during oblique tectonic inversion and is consistent with the surface data including the fracture system the available subsurface data and the mechanical stratigraphy

feo rich porphyritic pyroxene chondrules in unequilibrated ordinary chondrites abstract a suite of feo rich type ii porphyritic olivine pyroxene pop chondrules has been studied in detail data for ten chondrules from semarkona ll3 0 are emphasized and one chondrule from chainpur ll3 4 and two from parnallee ll3 6 are included as further examples of certain properties the chondrules contain phenocrysts of pyroxene and olivine in varying proportions and have fe fe mg 10 mol in the silicate minerals four pyroxene minerals are present clinoenstatite orthorhombic enstatite pigeonite and augite pyroxene phenocrysts may consist of all four minerals with progressive overgrowths in order of increasing wo content enstatite may occur independently of clinoenstatite but ca rich pyroxenes pigeonite and augite always nucleate on low ca pyroxene phenocrysts pyroxene and olivine phenocrysts may be independent of each other or may be intergrown and commonly show hopper morphologies indicative of rapid growth pyroxenes and olivines are commonly strongly zoned as a result of fractional crystallization of the chondrules in addition low ca pyroxene may contain ghost regions of slightly more feo and minor element rich material in the cores of grains these regions are partially resorbed and appear to be derived from previous crystallization of essentially the same chondrule oscillatory zoning is observed in enstatite in two chondrules and is believed to be the result of disquilibrium kinetic effects relict grains including a forsterite relict and a partially reduced feo rich relict in the same chondrule and clinoenstatite cores of pyroxene phenocrysts in another chondrule are evidence for recycling and mixing of chondrule material all of the chondrules are highly disequilibrium assemblages they appear to have cooled from essentially molten droplets with little volatile loss or recondensation occurring during the heating event peak temperatures were probably close to liquidus temperatures 1500 1700 c and cooling rates were probably of the order of hundreds of degrees per hour this description will serve as a basis for future interpretation of the effects of metamorphism on porphyritic pyroxene rich chondrules

petrologic geochemical and experimental constraints on models of chondrule formation abstract the petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule formation models nebular formation is indicated by the non mass fractionated oxygen isotopic compositions of bulk chondrules chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral phenocrysts and glassy mesostases incomplete melting is indicated by the abundance of porphyritic chondrules which experiments demonstrate require relict nuclei and coarse relict grains the length of time that chondrules were hot is constrained by their retention of relict grains and moderately volatile elements rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts isotopic anomalies and dynamic crystallization experiments it is clear from the presence of relict grains enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling the heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above 900 to below 600 k the chondrule formation mechanism provided a repeatable source of energy capable of highly localized melting characteristic of flash heating the occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high temperature regions near the sun as required in bipolar outflow models mechanisms for forming chondrules that are consistent with the constraints include various flash heating models nebular lightning magnetic reconnection flares gas dynamic shock waves and radiative heating

textural constraints on the formation of impact spherules a case study from the dales gorge bif paleoproterozoic hamersley group of western australia impact ejecta about 2 5 gyr old in the ds4 layer of the dales gorge bif hamersley group western australia are so well preserved that many original textures such as vesicles and microlites are faithfully preserved about 65 of the particles in the layer originated as impact ejecta of which 81 are splash forms the remaining 19 are angular but the splash forms and angular particles have the same composition mainly diagenetic stilpnomelane and k feldspar and share a common suite of internal textures some particles contain randomly oriented microlites texturally identical to plagioclase in basalts most splash forms have rims of inward growing crystals that may have formed from the melt perhaps nucleated by impinging dust or via thermal devitrification the rims clearly formed in flight because in broken particles which make up about 13 of the splash forms they are generally not present on broken surfaces the origin of the angular particles is uncertain but they may represent solid ejecta given the large sizes and variable shapes of the splash forms they are probably droplets of impact melt emplaced ballistically this is largely by analogy to the k t boundary layer but ds4 splash forms differ from k t spherules in important ways suggesting the k t model is not universal the occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust the diverse textures formed via in flight crystallization suggest particle paths in the plume are more complex than is generally appreciated

crustal velocity structure from sarex the southern alberta refraction experiment lithoprobe s southern alberta refraction experiment sarex extends 800 km from east central alberta to central montana it was designed to investigate crustal velocity structure of the archean domains underlying the western canada sedimentary basin from north to south sarex crosses the loverna domain of the hearne province the vulcan structure the medicine hat block previously considered part of the hearne province the great falls tectonic zone and the northern wyoming province ten shot points along the profile in canada were recorded on 521 seismographs deployed at 1 km intervals to extend the line an additional 140 seismographs were deployed at intervals of 1 25 2 50 km in montana data interpretation used an iterative application of damped least squares inversion of traveltime picks and forward modeling results show different velocity structures for the major blocks loverna medicine hat and wyoming indicating that each is distinct wavy undulations in the velocity structure of the lov

models of convergent boundary tectonics implications for the interpretation of lithoprobe data physical models of convergent boundary processes can provide insights into compressional orogens such as those studied by lithoprobe we summarize the qualitative tectonic style of a series of mode

first order control of syntectonic sedimentation on crustal scale structure of mountain belts the first order characteristics of collisional mountain belts and the potential feedback with surface processes are predicted by critical taper theory while the feedback between erosion and mountain belt structure has been fairly extensively studied less attention has been given to the potential role of synorogenic deposition for thin skinned fold and thrust belts recent studies indicate a strong control of syntectonic deposition on structure as sedimentation tends to stabilize the thin skinned wedge however the factors controlling basement deformation below fold and thrust belts as evident for example in the zagros mountains or in the swiss alps remain largely unknown previous work has suggested that such variations in orogenic structure may be explained by the thermotectonic age of the deforming lithosphere and hence its rheology here we demonstrate that sediment loading of the foreland basin area provides an additional control and may explain the variable basement involvement in orogenic belts when examining the role of sedimentation we identify two end members 1 sediment starved orogenic systems with thick skinned basement deformation in an axial orogenic core and thin skinned deformation in the bordering forelands and 2 sediment loaded orogens with thick packages of synorogenic deposits derived from the axial basement zone deposited on the surrounding foreland fold and thrust belts and characterized by basement deformation below the foreland using high resolution thermomechanical models we demonstrate a strong feedback between deposition and crustal scale thick skinned deformation our results show that the loading effects of syntectonic sediments lead to long crustal scale thrust sheets beneath the orogenic foreland and explain the contrasting characteristics of sediment starved and sediment loaded orogens showing for the first time how both thin and thick skinned crustal deformations are linked to sediment deposition in these orogenic systems we show that the observed model behavior is consistent with observations from a number of natural orogenic systems

correlation between relative ages inferred from 26al and bulk compositions of ferromagnesian chondrules in least equilibrated ordinary chondrites we have studied the relationship between bulk chemical compositions and relative formation ages inferred from the initial 26al 27al ratios for sixteen ferromagnesian chondrules in least equilibrated ordinary chondrites semarkona ll3 0 and bishunpur ll3 1 the initial 26al 27al ratios of these chondrules were obtained by kita et al 2000 and mostefaoui et al 2002 corresponding to relative ages from 0 7 0 2 to 2 4 0 4 0 7 myr after calcium aluminum rich inclusions cais by assuming a homogeneous distribution of 26al in the early solar system the measured bulk compositions of the chondrules cover the compositional range of ferromagnesian chondrules reported in the literature and thus the chondrules in this study are regarded as representatives of ferromagnesian chondrules the relative ages of the chondrules appear to correlate with bulk abundances of si and the volatile elements na k mn and cr but there seems to exist no correlation of relative ages neither with fe nor with refractory elements younger chondrules tend to be richer in si and volatile elements our result supports the result of mostefaoui et al 2002 who suggested that pyroxene rich chondrules are younger than olivine rich ones the correlation provides an important constraint on chondrule formation in the early solar system it is explained by chondrule formation in an open system where silicon and volatile elements evaporated from chondrule melts during chondrule formation and recondensed as chondrule precursors of the next generation

models of large hot orogens containing a collage of reworked and accreted terranes we describe a classification scheme for orogens using temperature magnitude t m diagrams and use this framework for modelling large hot orogens that evolve in continents comprising cratonic nuclei bordered by a series of juvenile accreted reworked and metamorphosed terranes modelling the complete evolution of an orogen is difficult particularly large orogens with multiple orogenic phases early phases during which a continent is assembled produce a tectonic and metamorphic fabric that needs to be taken into account when modelling the main collisional orogeny this inherited fabric is represented in a simple way in models described here by a series of lower crustal blocks that are arranged to be systematically stronger toward the cratonic continental interiors we investigate how this fabric influences the development of the model orogen during the main collisional phase using upper mantle scale ums and crustal scale cs finite element models the models exhibit a diachronous three phase evolutio

relict subduction zones in canada 1 relict subduction zones that are interpreted on 10 lithoprobe deep seismic reflection profiles in canada provide key observational evidence that lithosphere plate interactions have operated with essentially the same resultant geometry for at least 2 7 gyr subduction zone reflections are generally found beneath known collisional belts and their orientations are often consistent with the locations of exposed arcs sutures and subduction complexes they project from the lower crust into the upper mantle and provide key geometric relationships between the crust moho and upper mantle the interpretation of subduction zone reflections as being syntectonic establishes a minimum age for the overlying lower crust and mantle wedge and precludes inferred formation of the moho and upper mantle by postorogenic delamination in these areas the orientation of subduction zone reflections may be synthetic or antithetic to exposed sutures indicating juxtaposition by subcretion or by delamination respectively the mantle wedge above the subduction zone reflections may exhibit sparse reflectivity subduction zone reflections commonly dip toward the craton indicating a preferred dip of subduction during the last stages of convergence within an orogen or else a higher likelihood of preservation beneath older domains interpreted ages of the canadian examples range from recent to neoarchean 2 7 ga providing observational evidence that plate tectonics or similar processes have been active since at least the neoarchean the locations and geometries of these relict subduction zones provide a snapshot of accretion boundaries in the mantle and establish a framework for the neoarchean recent accretionary architecture of the north american lithosphere