three dimensional imaging of ordinary chondrite microporosity at 2 6 μm resolution abstract we show that high resolution 2 6 m voxel synchrotron x ray microtomography ct scans are able to observe the majority of microporosity in ordinary chondrites this porosity is present in the form of microcracks and voids among and between mineral grains we examined in total seven small 6 12 mm 3 chips of the ordinary chondrites baszkowka l5 s1 alh a77258 h5 s2 moorleah l6 s3 and kyushu l6 s5 these four samples were chosen because of their variable impact histories using ct and various digital isolation and visualization techniques we found that the structure of microporosity varies with the degree of compaction and shock loading experienced by the materials the microporosity in the compacted and more strongly shock samples moorleah and kyushu is visible as sheet like fractures within brittle silicates as well as in discontinuous distributed voids sometimes associated with chondrule rims and grain boundaries in samples that have experienced less shock and only mild compaction we found few if any microcracks in the silicate grains of the materials microporosity in the baszkowka and alh a77258 samples is largely represented by intergranular voids with occasional intragranular voids being present regardless of degree of shock loading ductile fe ni or fes grains give no evidence of shock related internal fractures contributing to porosity

reduction during metamorphism of four ordinary chondrites abstract three type 3 chondrite finds willaroy suwahib buwah and moorabie and one type 4 find cerro los calvos are ordinary chondrites oc in all respects except redox state a they have oc abundances of refractory lithophiles cerro los calvos data are ambiguous zn and other taxonomic elements b they have oc matrix chondrule abundance ratios and their chondrule sizes are consistent with bulk siderophile indicators of group membership h for willaroy and cerro los calvos l for suwahib buwah and moorabie and c their oxygen isotope compositions are far closer to those of oc than to those of any other chondrite group in each of the four chondrites the range of olivine compositions is relatively narrow fayalite modes and means are distinctly below those of other members of their respective groups having similar or greater degrees of equilibration the most plausible explanation of these properties is that these four oc experienced reduction during metamorphism this is consistent with reverse zoning in olivine from one type ii chondrule in willaroy the reducing agents may have been foreign clasts incorporated during an earlier accretional history alternatively these chondrites could themselves have been foreign materials incorporated into a reduced regolith examples of reducing clasts are the c rich aggregates in sharps examples of reducing host regoliths are the host materials in bencubbin and cumberland falls confirmation of this interpretation will require identification of the reduced foreign material or their relicts in the four reduced oc initial searches have been unsuccessful

high pressure phases in shock induced melt veins of the umbarger l6 chondrite constraints of shock pressure we report a previously undocumented set of high pressure minerals in shock induced melt veins of the umbarger l6 chondrite high pressure minerals were identified with transmission electron microscopy tem using selected area electron diffraction and energy dispersive x ray spectroscopy ringwoodite fa30 akimotoite en11fs89 and augite en42wo33fs25 were found in the silicate matrix of the melt vein representing the crystallization from a silicate melt during the shock pulse ringwoodite fa27 and hollandite structured plagioclase were also found as polycrystalline aggregates in the melt vein representing solid state transformation or melting with subsequent crystallization of entrained host rock fragments in the vein in addition fe2sio4 spinel fa66 fa99 and stishovite crystallized from a feo sio2 rich zone in the melt vein which formed by shock melting of feo sio2 rich material that had been altered and metasomatized before shock based on the pressure stabilities of the high pressure minerals ringwoodite akimotoite and ca clinopyroxene the melt vein crystallized at approximately 18 gpa the fe2sio4 spinel stishovite assemblage in the feo sio2 rich melts is consistent with crystallization of the melt vein matrix at the pressure up to 18 gpa the crystallization pressure of 18 gpa is much lower than the 4590 gpa pressure one would conclude from the s6 shock effects in melt veins stffler et al 1991 and somewhat less than the 25 30 gpa inferred from s5 shock effects schmitt 2000 found in the bulk rock

rapid oscillations in vostok and grip ice cores we investigate the spectral properties of climatic times series derived from two recent ice cores in greenland and east antarctica we find that the signals behave in a similar way in the high frequency part of their spectra the rapid oscillations found in the grip ice core were closely correlated to heinrich events a comparable spectral feature is detected in the vostok ice core we discuss the possibilities of connections between the two hemispheres and proper ice sheet oscillations in the light of simple oscillating climate models

incompletely compacted equilibrated ordinary chondrites we document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high resolution synchrotron x ray microtomography ct and helium pycnometry we found total porosities ranging from 10 to 20 within these chondrites and with ct we show that up to 64 of the void space is located within intergranular voids within the rock given the low s1 s2 shock stages of the samples and the large voids between mineral grains we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history with fe metal and fes metal abundances and grain size distributions we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism these materials were not fluffed on their parent body by impact related regolith gardening or events caused by seismic vibrations samples of all three chemical types of ordinary chondrites ll l h are represented in this study and we conclude that incomplete compaction is common within the asteroid belt

natural variability of atmospheric temperatures and geomagnetic intensity over a wide range of time scales the majority of numerical models in climatology and geomagnetism rely on deterministic finite difference techniques and attempt to include as many empirical constraints on the many processes and boundary conditions applicable to their very complex systems despite their sophistication many of these models are unable to reproduce basic aspects of climatic or geomagnetic dynamics we show that a simple stochastic model which treats the flux of heat energy in the atmosphere by convective instabilities with random advection and diffusive mixing does a remarkable job at matching the observed power spectrum of historical and proxy records for atmospheric temperatures from time scales of one day to one million years myr with this approach distinct changes in the power spectral form can be associated with characteristic time scales of ocean mixing and radiative damping similarly a simple model of the diffusion of magnetic intensity in earth s core coupled with amplification and destruction of the local intensity can reproduce the observed 1 f noise behavior of earth s geomagnetic intensity from time scales of 1 myr to 100 yr in addition the statistics of the fluctuations in the polarity reversal rate from time scales of 1 myr to 100 myr are consistent with the hypothesis that reversals are the result of variations in 1 f noise geomagnetic intensity above a certain threshold suggesting that reversals may be associated with internal fluctuations rather than changes in mantle thermal or magnetic boundary conditions

peak metamorphic temperatures in type 6 ordinary chondrites an evaluation of pyroxene and plagioclase geothermometry quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade type 6 but results are inconsistent and span hundreds of degrees because different geothermometers and calibration models were used with different meteorites it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations we addressed this problem by performing twopyroxene geothermometry using quilf95 on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer nakamuta and motomura 1999 we also calculated temperatures for published pyroxene analyses from other type 6 h l and ll chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites pyroxenes record a narrow overlapping range of temperatures in h6 865 926 c l6 812 934 c and ll6 874 945 c chondrites plagioclase temperature estimates are 96 179 c lower than pyroxenes in the same type 6 meteorites plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak the average temperature for h l and ll chondrites 900 c which agrees with previously published oxygen isotope geothermometry is at least 50 c lower than the peak temperatures used in current asteroid thermal evolution models this difference may require minor adjustments to thermal model calculations

metal veins in the kernouve h6 s1 chondrite evidence for pre or syn metamorphic shear deformation abstract kernouve is an h6 chondrite that experienced a very small degree of late stage shock loading s1 however kernouve contains fe ni metal vein like structures whose formation have been attributed to an early impact event to establish the formation conditions of metal veins in kernouve we examined the three dimensional 3d arrangement of metal vein like structures and typical metal grains in two samples of kernouve with x ray microtomography ct at resolutions of 11 m voxel we additionally investigated the 3d structure of the porosity present in kernouve using ct at two different resolutions 3 and 11 m voxel these data and optical microscopy support the hypothesis that kernouve has experienced little post metamorphic shock however the moderate 5 8 vol porosity of kernouve is in the form of intergranular voids rather than cracks which indicates any cracking that may have existed in the relatively brittle silicate grains was annealed we estimate that 70 80 of the primordial porosity in kernouve was removed by impact related compaction moreover we found no collective orientation of metal grains so high metamorphic temperatures following compaction erased any common orientation of metal grains due to compaction we propose that the metal vein structures can be explained as a pre or syn metamorphic shock induced process which we infer was primarily shear deformation with some uniaxial compaction also occurring the coarse metal veins probably formed by accumulation of ductile metal grains along shear zones a process that would have been facilitated by having an already warm h chondrite parent body when shock occurred i e syn metamorphic shock the complexity of shape including numerous tendrils expanding from the primary structure of the veins in kernouve is likely due to the coalescence of metal by metamorphic growth after the shear event the metal veins in kernouve thus appear to record evidence for early shock induced metal mobilization and the maximum shock pressure experienced by kernouve may have been 21 gpa our study suggests that collision induced segregation of metal occurred at an early stage in low gravity planetesimals consistent with the idea that this process could have been important for the differentiation of such objects

extensive impact melting on the h chondrite parent asteroid during the cataclysmic bombardment of the early solar system evidence from the achondritic meteorite dar al gani 896 dag 896 is an olivine rich microporphyritic rock of komatiitic composition both the olivine composition fa 17 5 2 1 mn mg 0 0061 and the bulk oxygen isotopic composition delta 17 o 2 55 delta 18 o 3 50 indicate that dag 896 is a sample of the h chondrite parent body the bulk chemistry shows an h chondritic distribution of lithophile elements whereas chalcophile and siderophile elements are strongly depleted indicating formation through whole rock melting or nearly so of h chondrite material nearly complete loss of the metal plus sulfide component and crystallization without significant igneous fractionation superheated severely shocked chondritic relics similar to10 vol typically in the form of corroded lithic fragments 100 mum in size intimately distributed within the igneous lithology indicate that melting was triggered by a highly energetic impact which possibly induced shock pressures of similar to80 100 gpa the relatively young 3 704 0 035 ga ar 40 ar 39 crystallization age is consistent with the impact melting origin as magmatism in the asteroid belt was active only in the first hundred million years of solar system history 1ased on textural data and thermodynamic crystallization modelling we infer that dag 896 crystallized from a liquidus temperature of similar to1630degreesc under relatively slow cooling rates similar to10degreesc h 1 to 1300degreesc before quenching the two stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor metal silicate fractionation may have been accomplished at least at the centimeter scale of the studied meteorite sample through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism alternatively dag 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid for metal particles 1 to 10 mm in size typically observed in partially differentiated impact melt rocks stokes law indicates a settling velocity 1 in h 1 during the first few hours of crystallization on asteroidal bodies of 25 kin radius the similar to3 7 ga age of dag 896 nearly overlaps with the slightly older resetting ages of h chondrites all impact melts available from the literature indicating that the h chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system such an age overlap may also indicate early disruption of the initial h chondrite parent asteroid the close similarity between the bulk composition and degassing age of dag 896 and silicate inclusions in he iron meteorites is further evidence in support of a common origin by impact melting and metal silicate segregation on the h chondrite parent asteroid our new high precision oxygen isotopic measurements of h chondrites delta 17 o 0 77 0 04 should be extended to iies to verify this possible petrogenetic link

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

postglacial left slip rate and past occurrence of m 8 earthquakes on the western haiyuan fault gansu china high resolution hr air photographs and a 1 m horizontal and 2 m vertical resolution digital elevation model derived from them by stereophotogrammetry provide new constraints on the behavior of the western stretch of the active haiyuan fault in gansu province china the photographs cover three swaths along the fault each about 2 km long and at least 500 m wide near the village of songshan at 103 5oe this high resolution data set is used to map and measure cumulative horizontal offsets of alluvial terraces and risers that range between 115 and la 135 m and 70 and 90 m at two sites dating these terraces with 14c yields minimum and maximum ages of 8400 and 7600 and 14 200 years b p respectively this leads to a postglacial slip rate of 12 4 mm yr with a most likely minimum value of 11 6 1 1 mm yr the smallest stream offsets observed on the hr photographs range between 8 and 16 m and are interpreted as coseismic displacements of the last few earthquakes with m 8 that ruptured the 220 km long tianzhu gap of the fault west of the yellow river earthquakes of that size within this gap which has been quiescent for at least 800 years would recur at intervals of 1050 450 years

long term polarity stability and strength of the geomagnetic dipole palaeomagnetically observed reversals so far provide the strongest evidence1 that reversals originate from processes of core magnetohydrodynamics mhd thus furnishing a crucial constraint for geomagnetic dynamo modelling the long term variations in reversal frequency favouring2 independent mechanisms for the dynamo and its reversals provide an added constraint here we present evidence for a correlation between polarity superchrons and palaeointensity data and also marine magnetic and mags at anomalies our results indicate that the dipole moment is correlated to the reversal frequency being appreciably greater during the fixed polarity superchrons this new information derived from the reversal record provides fresh insight into the workings of the geodynamo and also explains why the oceanic crust associated with the cretaceous quiet zone carries such a strong thermoremanence

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

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

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

akimotoite mg fe sio3 a new silicate mineral of the ilmenite group in the tenham chondrite akimotoite mg fe sio 3 a new silicate mineral of the ilmenite group was found in the shock metamorphosed tenham chondrite it occurs as aggregates adjacent to clinoenstatite in fragments within shock induced melt veins chemical analyses show the simplified formula to be mg sub 0 79 fe sub 0 21 sio 3 the same as for clinoenstatite selected area electron diffraction saed patterns correspond to the synthetic mg fe sio 3 ilmenite phase with space group r3 lattice parameters derived from saed patterns are a 0 478 5 nm c 1 36 1 nm and v 0 269 8 nm 3 in the hexagonal setting the calculated density is 4 0 1 g cm 3 akimotoite in this occurrence is thought to have been transformed from original orthoenstatite in a solid state reaction produced by a shock event peak pressure and temperature generated by shock events in tenham are estimated to be 22 gpa max max 2000 degrees c assuming that equilibrium crystallization of aluminous majorite occurred in the melt veins this new mineral was named after syun iti akimoto

compositions of three low feo ordinary chondrites indications of a common origin with the h chondrites abstract burnwell eet 96031 and lap 04575 are ordinary chondrites oc that possess lower than typical olivine fa content than has been established for the h chondrites

experimental partial melting of the st severin ll and lost city h chondrites we have performed melting experiments between 1170 1325 c and at an oxygen fugacity of iw 1 on the ordinary chondrites lost city h and st severin ll in order to investigate the origin of diogenites and their possible relationship to eucrites low ca pyroxene disappears from the lost city residuum just above 1220 c at a mg near 66 accordingly lost city does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites in contrast orthopyroxene in st severin persists up to about 1325 c at which point it has a mg similar to that of typical diogenites 75 this is only marginally compatible with diogenite origins as the most magnesian pyroxenes from diogenites mg 82 and howardites mg 85 are more mgo rich than any pyroxene formed from a st severin melt accordingly if diogenites formed from a source having the bulk composition of an ordinary chondrite than either the fo2 had to be lower than that inferred for eucrite formation or more likely the parent body had a lower fesi ratio than that of an ll chondrite also low temperature melts of st severin are depleted in elements compatible with pyroxene and so while broadly eucritic do not closely match the compositions of eucrites that are thought to be primary partial melts hence it doesn t appear possible to produce both diogenites and eucrites from the same source region composition if eucrites are primary partial melts reconnaiss mce experiments at higher oxygen fugacity iw 2 produce broadly angritic melts for lost city as was observed for murchison and allende in contrast our na2o poor st severin charge produced a broadly eucritic melt at this higher fo2 this difference in melting behavior is probably because ll chondrites have a much lower bulk fesi ratio than the h cm or cv chondrites

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

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

a shock produced mg fe sio3 glass in the suizhou meteorite ovoid grains consisting of glass of stoichiometric mg fe sio3 composition that is intimately associated with majorite were identified in the shock veins of the suizhou meteorite the glass is surrounded by a thick rim of polycrystalline majorite and is identical in composition to the parental low ca pyroxene and majorite these ovoid grains are surrounded by a fine grained matrix composed of majorite pyrope garnet ringwoodite magnesiowstite metal and troilite this study strongly suggests that some precursor pyroxene grains inside the shock veins were transformed to perovskite within the pyroxene due to a relatively low temperature while at the rim region pyroxene grains transformed to majorite due to a higher temperature after pressure release perovskite vitrified at post shock temperature the existence of vitrified perovskite indicates that the peak pressure in the shock veins exceeds 23 gpa the post shock temperature in the meteorite could have been above 477 c this study indicates that the occurrence of high pressure minerals in the shock veins could not be used as a ubiquitous criterion for evaluating the shock stage of meteorites

petrogenesis of miller range 07273 a new type of anomalous melt breccia implications for impact effects on the h chondrite asteroid miller range 07273 is a chondritic melt breccia that contains clasts of equilibrated ordinary chondrite set in a fine grained 3 gpa possibly to 15 19 gpa and temperature 1350 c possibly to 2000 c the higher pressures and temperatures would have involved back reaction of high pressure polymorphs to pyroxene and olivine upon cooling silicates outside of melt matrix have compositions that were relatively unchanged owing to brief heating duration

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

jadeite formation in shocked ordinary chondrites abstract albitic feldspar in shocked ordinary chondrites yamato 791384 l6 and yamato 75100 h6 and albite recovered from static high pressure and high temperature synthetic experiments kubo et al 2010 were investigated with a transmission electron microscope tem subsequent to a conventional micro raman spectroscopy analysis to clarify albite dissociation reaction under high pressure and high temperature condition when jadeite forms from albite sio 2 phase as a residual phase of albite dissociation reaction should accompany jadeite from the stoichiometry however albitic feldspar in and adjacent to shock melt veins of the shocked chondrites dissociates into jadeite residual amorphous or poorly crystallized material having varied chemical compositions between jadeite and sio 2 phase tem observations of albitic feldspar in the shocked chondrites and albite recovered from the static high pressure and high temperature synthetic experiments show that jadeite crystallization is initiated by grain refinement of albite or albitic feldspar nucleation occurs along grain boundaries or at triple junctions of the fine grained albite crystal assemblage jadeite crystal starts to grow from the nucleus through grain boundary diffusion considering pressure condition recorded in the shock melt veins of the shocked chondrites stishovite is the most likely as a residual sio 2 phase accompanying jadeite high pressure and high temperature condition induced by a dynamic event is very short stishovite would be hardy formed through a dynamic event due to sluggish nucleation rate of stishovite compared with that of jadeite thus leading to induce heterogeneous and incomplete albite dissociation reaction albite dissociates into jadeite residual amorphous material

asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the pacific and indian oceans abstract a record of relative paleointensity from marine sediments in the equatorial indian ocean spanning the last 4 ma completes a previous dataset from ocean drilling program odp leg 138 sites in the equatorial pacific 1 the timescale was defined with a precision better than 20 kyr with different and independent methods most of the features present in the record from the equatorial pacific are confirmed the overall saw tooth pattern is observed across every field reversal and the short term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites the global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo a synthetic curve of field intensity changes during the last 4 ma is proposed as a first paleointensity timescale for future stratigraphic studies

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

shock experiments with the h6 chondrite kernouve pressure calibration of microscopic shock effects shock recovery experiments were carried out on samples of the h6 chondrite kernouve at shock pressures of 10 15 20 25 30 35 45 and 60 gpa and preheating temperatures of 293 k low temperature experiments and 920 k high temperature experiments using a calculated equation of state of kernouve pressure pulse durations of 0 3 to 1 2 s were estimated the shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine orthopyroxene oligoclase and troilite the following pressure calibration is proposed for silicates 1 undulatory extinction of olivine gpa 2 weak mosaicism of olivine from 10 15 gpa to 20 25 gpa 3 onset of strong mosaicism of olivine at 20 25 gpa 4 transformation of oligoclase to diaplectic glass completed at 25 30 gpa low temperature experiments and at 20 25 gpa high temperature experiments 5 onset of weak mosaicism in orthopyroxene at 30 35 gpa low temperature experiments and at 25 30 gpa high temperature experiments and 6 recrystallization or melting of olivine starting at 45 60 gpa low temperature experiments and at 35 45 gpa high temperature experiments and completed above 45 60 gpa in the high temperature experiments troilite displays distinct differences between the samples shocked at low and high temperatures in the low temperature experiments the following effects can be observed in troilite 1 undulatory extinction up to 25 gpa 2 twinning up to 45 gpa 3 partial recrystallization from 30 to 60 gpa and 4 complete recrystallization 35 gpa whereas in the high temperature experiments troilite shows 1 complete recrystallization from 10 up to 45 gpa and 2 melting and crystallization above 45 gpa localized shock induced melting is observed in samples shocked to pressures 15 gpa in the high temperature experiments and 30 gpa for the low temperature experiments in the form of feni metal and troilite melt injections and intergrowths and as pockets and veins of whole rock melt obviously the onset and abundance of shock induced localized melting strongly depends on the initial temperature of the sample

history of earth s magnetic field and possible connections to core mantle boundary processes the core mantle boundary cmb sets boundary conditions for processes occurring within the core thus the history of the geomagnetic field is intimately connected with the history of the cmb information about one can often provide information about the other it is a simple matter to separate sources of external origin from those of internal origin but there is no unique way to specify the location of the internal sources indirect arguments suggest that sources within earth s outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees building on the pioneering studies of roberts and scott and of backus in the 1960s the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data however even with appropriate assumptions about location of the magnetic field source there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines additional assumptions about the flow are therefore required sometimes leading to quite different estimates in the pattern of fluid flow the resulting velocity field estimates appear to be sensitive to conditions e g small lateral variations in temperature at the cmb consideration of mantle dynamics suggests that large changes in the cmb conditions probably occur on a 10 to 100 m y timescale secular variations with periods shorter than a million years but longer than several years almost certainly originate from processes operating in the outer core unfortunately there is not yet consensus as to what those processes are longer period variations in the paleomagnetic record including nonstationarities in the rates of magnetic field reversals in paleointensities and in paleosecular variation may reflect changes in cmb conditions because these boundary conditions are controlled primarily by mantle dynamics there have been several speculations regarding causal links between changes in earth s lithosphere and changes in earth s magnetic field finally the evidence linking lateral variations at the cmb to perceived systematics in polarity transition data is found to be intriguing but insufficient

oxygen isotope studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

cat mountain a meteoritic sample of an impact melted asteroid regolith cat mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule bearing clasts of l5 material these clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years probably within a melt breccia lens in the bottom of a large 1 km diameter crater on an l chondrite asteroid noble gas isotopes indicate that the sample was involved in at least two different impact events approximately 880 and 20 myr ago following the 4 55 ga accretion of primitive chondritic material the 880 ma event is responsible for the impact breccia texture of the sample and the 20 ma event reduced the sample to a meter sized object we also infer that another impact occurred between 880 and 20 ma possibly the 500 ma event recorded in many other l chondrites to jettison the material from the asteroid belt into an orbit that evolved into an earth crossing trajectory the shock metamorphic processes that occurred at 880 ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases this reduced the reflectance of the l5 material and decreased the amplitude of its spectral absorption features these characteristics are consistent with the spectral characteristics of some c class asteroids and suggest that some dark asteroids that appear to belong to the c class could be covered with shocked ordinary chondrite material if one assumes that cat mountain came from the same asteroid as other l chondrites with the same cosmic ray exposure age then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m furthermore the structural integrity of cat mountain and other l chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere however in a regime where the asteroid is the target of impact fragmentation rather than the projectile the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy in this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material

relationships among physical properties as indicators of high temperature deformation or post shock thermal annealing in ordinary chondrites collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals including the parent bodies of chondrites but the conditions under which these occurred are not well constrained a simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric to test this model we studied sixteen ordinary chondrites of different groups h l ll using x ray computed microtomography ct to measure porosity and metal fabric ideal gas pycnometry and 3d laser scanning to determine porosity and optical microscopy om to determine shock stage these included a subsample of six chondrites previously studied using transmission electron microscopy tem to characterize microstructures in olivine combining with previous data results support the simple model in general but not for chondrites with low shock porosity foliation low spf chondrites these include kernouve h6 portales valley h6 7 butsura h6 park l6 gro 85209 l6 estacado h6 mil 99301 ll6 spade h6 and queen s mercy h6 among others the data for these meteorites are best explained by high ambient heat during or after shock low spf chondrites tend to have older 40ar 39ar ages 4435 4526 ma than other non low spf type 6 chondrites in this study we conclude that the h l and ll asteroids all were shock compacted at an early stage while warm with collisions occurring during metamorphic heating of the parent bodies results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile and on the nature of accretion and lithification processes for planetesimals

deformation and thermal histories of ordinary chondrites evidence for post deformation annealing and syn metamorphic shock abstract we show that olivine microstructures in seven metamorphosed ordinary chondrites of different groups studied with optical and transmission electron microscopy can be used to evaluate the post deformation cooling setting of the meteorites and to discriminate between collisions affecting cold and warm parent bodies the l6 chondrites park shock stage s1 bruderheim s4 leedey s4 and morrow county s5 were affected by variable shock deformation followed by relatively rapid cooling and probably cooled as fragments liberated by impact in near surface settings in contrast kernouve h6 s1 portales valley h6 7 s1 and mil 99301 ll6 s1 appear to have cooled slowly after shock probably by deep burial in warm materials in these chondrites post deformation annealing lowered apparent optical strain levels in olivine additionally kernouve morrow county park mil 99301 and possibly portales valley show evidence for having been deformed at an elevated temperature 800 1000 c the high temperatures for morrow county can be explained by dynamic heating during intense shock but kernouve park and mil 99301 were probably shocked while the h l and ll parent bodies were warm during early endogenically driven thermal metamorphism thus whereas the s4 and s5 chondrites experienced purely shock induced heating and cooling all the s1 chondrites examined show evidence for static heating consistent with either syn metamorphic shock kernouve mil 99301 park post deformation burial in warm materials kernouve mil 99301 portales valley or both the results show the pitfalls in relying on optical shock classification alone to infer an absence of shock and to construct cooling stratigraphy models for parent bodies moreover they provide support for the idea that secondary metamorphic and tertiary shock processes overlapped in time shortly after the accretion of chondritic planetesimals and that impacts into warm asteroidal bodies were common

comparison of the trace element composition of tagish lake with other primitive carbonaceous chondrites a meteorite fall on 18 january 2000 was detected by u s defense department satellites which established its pre impact orbit fresh samples were collected from frozen tagish lake in british columbia a week later and some properties of these samples reveal it to be a unique meteorite we characterized tagish lake and 8 other samples using icpms and rnaa data for 47 elements reveal that each of 9 carbonaceous chondrites of different type exhibit the orgueil normalized plateaus expected for members of such types trends evident in tagish lake differ from all other carbonaceous chondrites including ci and cm samples of tagish lake collected later show similar patterns affected by weathering

portales valley petrology of a metallic melt meteorite breccia portales valley pv is an unusual metal veined meteorite that has been classified as an h6 chondrite it has been regarded either as an annealed impact melt breccia as a primitive achondrite or as a meteorite with affinities to silicated iron meteorites we studied the petrology of pv using a variety of geochemical mineralogical techniques our results suggest that pv is the first well documented metallic melt meteorite breccia mineral chemical and other data suggest that the protolith to pv was an h chondrite the composition of feni metal in pv is somewhat fractionated compared to h chondrites and varies between coarse vein and silicate rich portions it is best modeled as having formed by partial melting at temperatures of 940 1150 c with incomplete separation of solid from liquid metal solid metal concentrated in the coarse vein areas and s bearing liquid metal concentrated in the silicate rich areas possibly as a result of a surface energy effect both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid graphite nodules formed by crystallization from this liquid whereas phosphate formed by reaction between p bearing metal and clinopyroxene components depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal silicate interfaces some phosphate probably crystallized from p bearing liquids but most probably formed by solid state reaction at 975 725 c phosphate forming and feo reduction reactions were widespread in pv and entailed a change in the mineralogy of the stony portion on a large scale portales valley experienced protracted annealing from supersolidus to subsolidus temperatures probably by cooling at depth within its parent body but the main differences between pv and h chondrites arose because maximum temperatures were higher in pv a combination of a relatively weak shock event and elevated pre shock temperatures probably produced the vein and breccia texture with endogenic heating being the main heat source for melting and with stress waves from an impact event being an essential trigger for mobilizing metal portales valley is best classified as an h7 metallic melt breccia of shock stage s1 the meteorite is transitional between more primitive chondritic and evolved achondrite iron meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies

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

aqueous alteration without a pronounced oxygen isotopic shift implications for the asteroidal processing of chondritic materials primitive meteorites exhibit certain features that are consistent with aqueous and thermal alteration on asteroids but o isotopic analyses show only a modest heavy isotope shift interpreted as indicating modification in the nebula to understand the isotopic effects of asteroidal alteration we take the l group ordinary chondrites weathered in antarctica as an analogue the data show that alteration is a two stage process with an initial phase producing only a negligible isotopic effect although surprising a possible explanation is found when we consider the alteration of terrestrial silicates numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates we observe a similar texture alteration involves a restructuring of clay minerals along these narrow channels in which access of water is restricted the clay shows a topotactic relationship to the primary grain which suggests either epitaxial growth of the clay using the silicate as a substrate or inheritance of the original o structure by the clay our data suggests the latter with extensive inheritance of structural polymers by the weathering product the bulk o isotopic composition is comparatively unaffected this offers an explanation for the lack of an isotopic effect in the weathering of the l chondrites if substantial modification of chondritic materials may occur without a pronounced isotopic effect it also reconciles existing o analyses of cv chondrites with an asteroidal model of aqueous alteration

shock and thermal history of northwest africa 4859 an annealed impact melt breccia of ll chondrite parentage containing unusual igneous features and pentlandite abstract northwest africa 4859 nwa 4859 is a meteorite of ll chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite the most obvious unusual feature is a high proportion of large 3 cm diameter igneous textured enclaves lites interpreted as shock melts that were intruded into an ll chondrite host one such lite appears to have been produced by whole rock melting of ll chondrite initial rapid partial crystallization and subsequent slow cooling of the residual melt in the host to produce a differentiated object other unusual features include mm sized overgrowth objects fine grained plagioclase rich bands and coarse troilite 7 mm across grains all these features are interpreted as having crystallized from melts produced by a single transient shock event followed by slow cooling a subsequent shock event of moderate s3 intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype pentlandite together with associated troilite in nwa 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature 230 c following the second shock event nwa 4859 is interpreted to be an unusual impact melt breccia that contains shock melt which crystallized in different forms at depth within the parent body

smyer h chondrite impact melt breccia and evidence for sulfur vaporization smyer is an h chondrite impact melt breccia containing 20 vol 0 5 to 13 mm thick silicate rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension at the interface between some of the melt veins and chondritic clasts there are troilite rich regions consisting of unmelted crushed 0 2 to 140 m size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins troilite fills every available fracture in the silicates including some as thin as 0 1 m little metallic fe ni is present in these regions the fes fe modal ratio ranges from 25 1 to 500 1 far higher than the eutectic weight ratio of 7 5 1 the texture of these regions indicates that the sulfide formed from a fluid of very low viscosity the moderately high viscosity 0 2 poise and large surface tension of liquid fes its inability to wet silicate grain surfaces at low oxygen fugacities and the supereutectic fes fe ratios in the troilite rich regions indicate that the fluid was a vapor it seems likely that during the shock event that melted smyer many silicates adjacent to the melt veins were crushed upon release of shock pressure some of the troilite evaporated and dissociated molecules of s2 were transported and condensed into fractures and around tiny silicate grains there they combined with fe from small adjacent metallic fe ni grains to form troilite the ni content at the edges of some of these metal grains increased significantly co from these ni rich grains diffused into nearby kamacite impact induced s volatilization may have played a major role in depleting the surface of 433 eros and other chondritic asteroids in s

back transformation of high pressure phases in a shock melt vein of an h chondrite during atmospheric passage implications for the survival of high pressure phases after decompression abstract we investigated the h6 chondrite yamato 75267 in which a fusion crust cuts a shock melt vein the shock vein region more than 280 m from the fusion crust contains high pressure phases such as ringwoodite majorite pyrope ss garnet and naalsi 3 o 8 hollandite however the shock vein close to the fusion crust entirely consists of the low pressure polymorphs olivine low ca pyroxene and plagioclase glass the boundary between low and high pressure phase regions is parallel to the fusion crust during the atmospheric passage the peripheral part of the chondrite was melted to form the fusion crust our microscopic laser micro raman electron microprobe investigations and calculations indicate an area up to 300 m from the fusion crust experienced a temperature of 1400 c after 3 s during the melting of the peripheral part the high pressure phases would at this conditions quickly transform back to their low pressure polymorphs the result obtained here indicates that post shock temperatures in the interior part of the veins were much lower than 1400 c thus leading to the survival of high pressure phases in heavily shocked chondrites

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

the burnwell kentucky low iron oxide chondrite fall description classification and origin the burnwell kentucky meteorite fell as a single stone on 1990 september 4 the burnwell meteorite has lower fa in olivine 15 8 mol fs in orthopyroxene 13 4 mol co in kamacite 0 36 wt feo from bulk chemical analysis 9 43 wt and a170 0 5 1 f 0 02 0 and higher fe ni co metal 19 75 wt from bulk wet chemical analysis than observed in h chondrites the burnwell meteorite plots on extensions of h l ll chondrite trends for each of these properties towards more reducing compositions than in h chondrites extensions of this trend have been previously suggested in the case of other low feo chondrites or silicate inclusions in the iie iron netschaevo but interpretation of the evidence in these mete orites is complicated by terrestrial weathering chemical disequilibrium or reduction in contrast the burn well meteorite is an equilibrated fall that exhibits no evidence for reduction as such it provides the first definitive evidence for extension of the h l ll ordinary chondrite trend beyond typical h values towards more reducing compositions

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

melting experiments on the enstatite pyrope join at 80 152 kbar melting temperatures of the assemblage pyroxene garnet on the enstatite pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split sphere anvil apparatus ussa 2000 the compositions of pyroxene garnet and melt coexisting along the solidus were also determined comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene it is proposed that the observed variation in the garnet composition is caused by disorder in garnet the obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80 110 kbar contributed to an improvement of the garnet peridotite thermobarometry an internally consistent set of thermodynamic parameters was derived which allows calculation of the temperature pressure phase diagram for the enstatite pyrope join at 20 260 kbar

vostok ice core a continuous isotope temperature record over the last climatic cycle 160 000 years a continuous deuterium profile along the 160 000 year vostok ice core antarctica is interpreted in terms of atmospheric temperature changes this climatic record is the awaited terrestrial complement of the deep sea records supporting the existence of a relation between the pleistocene climate and orbital forcing

time variations of the earth s magnetic field from daily to secular variations in time of the earth s natural magnetic field have a frightfully wide spectrum ranging over more than 20 orders of magnitude they extend from well over 103 hz where they merge into the radioelectric noise spectrum to probably more than 100 m y in rough summary the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere whereas the longer periods arise from internal sources primarily fluid motions in the core at depths in excess of 2900 km methods for separating external and internal signals have been available since the time of gauss 1 838 but the accuracy the space distribution and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations although periods shorter than 1 yr should clearly be assigned to external sources the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress currie 1966 1 968 proposed that there was little overlap and a cutoff of 3 7 yr a long debated l l yr contribution due to the solar cycle was clearly demonstrated by currie 1973a courtillot le mouei 1976a b and alldredge 1976 and the cutoff period was thus rejected to at least 20 yr the situation seemed to be confused again when courtillot et al 1978a provided evidence for sharp impulses of 1 yr duration i n the internal so called secular variation the debate that followed over the reality of brief internal events is one of the topics addressed in this review we first provide a short historical account of successive discoveries

northern hemisphere glaciation and the evolution of plio pleistocene climate noise 1 deterministic orbital controls on climate variability are commonly inferred to dominate across timescales of 104 106 years although some studies have suggested that stochastic processes may be of equal or greater importance here we explicitly quantify changes in deterministic orbital processes forcing and or pacing versus stochastic climate processes during the plio pleistocene via time frequency analysis of two prominent foraminifera oxygen isotopic stacks our results indicate that development of the northern hemisphere ice sheet is paralleled by an overall amplification of both deterministic and stochastic climate energy but their relative dominance is variable the progression from a more stochastic early pliocene to a strongly deterministic late pleistocene is primarily accommodated during two transitory phases of northern hemisphere ice sheet growth this long term trend is punctuated by stochastic events which we interpret as evidence for abrupt reorganization of the climate system at the initiation and termination of the mid pleistocene transition and at the onset of northern hemisphere glaciation in addition to highlighting a complex interplay between deterministic and stochastic climate change during the plio pleistocene our results support an early onset for northern hemisphere glaciation between 3 5 and 3 7 ma and reveal some new characteristics of the orbital signal response such as the puzzling emergence of 100 ka and 400 ka cyclic climate variability during theoretical eccentricity nodes

akimotoite in the tenham meteorite crystal chemistry and high pressure transformation mechanisms the transformation of pyroxene to its high pressure polymorph akimotoite mgsio3 ilmenite type structure is documented in the shock induced melt veins of the l6 tenham chondrite four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein in one of the entrained enstatite grains the transformation to akimotoite is partial one third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite akimotoite fe0 4mg1 24ca0 07na0 12al0 14 si1 963al0 037 o6 is slightly enriched in ca cao 1 74 al and na with respect to enstatite cao 0 71 narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the surrounding enstatite intersect the other part of the grain in a second grain entrained in the shear melt vein tablets of akimotoite are interwoven with a pyroxene glass with a lower na and fe content and a higher mg content compared to the adjacent akimotoite this pyroxene glass is probably the product of mg fe sio3 perovskite amorphization polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein in that case akimotoite is also enriched in ca cao 2 85 na nao 1 72 and al al2o3 4 14 all these observations suggest that akimotoite is mainly formed by solid state transformation of former pyroxenes with subsequent diffusion of calcium aluminum and sodium from the chondritic melt of the shear melt vein finally the first complete x ray diffraction pattern of natural akimotoite is presented

multiple impact events recorded in the nwa 7298 h chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid abstract the major geologic process that has shaped the asteroids and led to development of their regoliths is impact petrofabrics in ordinary chondrites are undoubtedly the result of impact events on their asteroidal parent bodies and the foliation present in a chondrite serves as an enduring record of the magnitude of the most intense compacting event experienced by the material an overwhelming majority of chondrites have an internally consistent petrofabric contained within the spatial dimensions of the entire rock including across clasts or different petrographic domains this indicates that the magnitude of the most recent impact to have affected the assembled chondrite was significant enough to impart a foliation across all lithologies information of any previous impacts is largely lost because of the consistent realigned foliations we present x ray microtomography derived 3d petrofabric intensity and orientation data for three lithologies in the nwa 7298 breccia the internally inconsistent petrofabrics among differing lithologies indicate that the magnitude of the final impact event was smaller than previous ones this latter case preserves fabric information recorded during previous impacts and allows a more complete interpretation of the impact history of a local region of the asteroidal parent we used our data to infer the sequence and intensity of distinct impact events affecting the nwa 7298 parent asteroid we suggest a near surface impact debris zone on the h chondrite parent asteroid as an origin for nwa 7298 these observations yield new opportunities for investigating and interpreting the dynamic collisional evolution of asteroids

three dimensional petrography of metal phases in equilibrated l chondrites effects of shock loading and dynamic compaction abstract to examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring we have performed a quantitative three dimensional 3d study of metal grains in a suite of increasingly shocked l chondrites with synchrotron x ray microtomography xmt our data allow rigorous quantification of size number distributions and collective morphology of fe ni metal phases in chondritic meteorites at the resolution of our xmt measurements 8 4 17 9 m voxel the number of metal particles increase with higher degrees of petrographically identified shock loading indicating a coalescing of fe ni metal at or below this scale our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact related compaction and shock loading ductile metal grains in l chondrites begin to show foliation at peak shock pressures

thermal history of the shock melted antarctic ll chondrites from the yamato 79 collection abstract the sr and rare gas isotopic compositions and abundances of lithophile trace elements k rb sr ba and rees were determined for a series of shock melted yamato 79 ll chondrites to investigate their late thermal history and the chemical features of shock processes all meteorites show similarities in shock ages 1 2 ga as confirmed by rb sr internal isochron and k ar dating rare gas compositions as well as cosmic ray exposure ages 28 ma petrographie textures and sampling sites in antarctica these results indicate that all of these meteorites are part of the same fall the 1 2 ga shock event caused a severe partial to total melting followed by recrystallization of olivine and clinopyroxene vesiculation shock induced alkali homogenization and local isotopic equilibration or perturbation of the rb sr system the degrees of shock effects are variable from specimen to specimen and from portion to portion even in a single specimen model calculations of fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body from these model calculations and the evidence provided by cosmogenic rare gas compositions it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth 2 m shielded from cosmic rays in an impact crater by the 1 2 ga event the parent body was fragmented to meter size stones by an additional collision at 28 ma resulting in the formation of the parent material of the yamato 79 shocked chondrites

resolving milankovitch consideration of signal and noise milankovitch climate theory provides a fundamental framework for the study of ancient climates although the identification and quantification of orbital rhythms are commonplace in paleoclimate research criticisms have been advanced that dispute the importance of an astronomical climate driver if these criticisms are valid major revisions in our understanding of the climate system and past climates are required resolution of this issue is hindered by numerous factors that challenge accurate quantification of orbital cyclicity in paleoclimate archives in this study we delineate sources of noise that distort the primary orbital signal in proxy climate records and utilize this template in tandem with advanced spectral methods to quantify milankovitch forced paced climate variability in a temperature proxy record from the vostok ice core vimeux and others 2002 our analysis indicates that vostok temperature variance is almost equally apportioned between three components the precession and obliquity periods 28 a periodic 100 000 year cycle 41 and the background continuum 31 a range of analyses accounting for various frequency bands of interest and potential bias introduced by the saw tooth shape of the glacial interglacial cycle establish that precession and obliquity periods account for between 25 percent to 41 percent of the variance in the 1 10 kyr 1 100 kyr band and between 39 percent to 66 percent of the variance in the 1 10 kyr 1 64 kyr band these results are approximately two to four times greater than those published by wunsch 2004 for the same vostok time series in all cases most of the remaining variance is accounted for by the 100 000 year cycle which is distinct from a background continuum that resembles autoregressive red noise our analysis high lights the importance of a comprehensive assessment of the climate signals in geologic records and underscores the significance of orbital forcing pacing as a primary agent of pleistocene climate change

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