collisional history of h chondrites the cosmic ray exposure histories of ordinary chondrites were recently reviewed marti and graf 1992 here we discuss in detail currently available cosmic ray exposure records and the adopted exposure ages of individual h chondrites we further present analyses of the distinct histograms of petrographic subgroups and the implications for orbital evolution and collisional records in addition to the stochastic events 7 to 8 m y ago a peak at 33 ma represents a statistically significant deviation from an exponential age distribution and signals a major collisional event at this time the h6 subgroup also recorded a similar event 24 m y ago these records present significant arguments against a continuous supply of h chondrites from the asteroid belt we show that the records of fall times for h5s are distinct from other subgroups and that a different orbit for the h5 parent is indicated in collisional events 7 to 8 m y ago the systematics of noble gas losses in the h5 subgroup are consistent with this interpretation the evidence for stochastic fragmentation and subsequent delivery to earth of h chondrites suggests that their present predominance among falls may represent a deviation from a representative population distribution

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

ar ar and i xe ages and thermal histories of three unusual metal rich meteorites abstract portales valley sombrerete and northwest africa nwa 176 are three unrelated meteorites which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies measured 39 ar 40 ar ages of these meteorites are 4477 11 ma and 4458 16 ma two samples of portales valley 4541 12 ma and 4524 13 ma respectively ma million years all one sigma errors the ar ar data for portales valley show no evidence of later open system behavior suggested by some other chronometers measured 129 i 129 xe ages of these three meteorites are 4559 9 0 5 ma 4561 9 1 0 ma and 4544 ma respectively relative to shallowater 4562 3 0 4 ma from stepwise temperature release data we determined the diffusion characteristics for ar and xe in our samples and calculated approximate closure temperatures for the k ar and i xe chronometers adopting results and interpretations about these meteorites from some previous workers we evaluated all these data against various thermal cooling models we conclude that portales valley formed 4560 ma ago cooled quickly to below the i xe closure temperature then cooled deep within the parent body at a rate of 4 c ma through k ar closure we conclude that sombrerete formed 4562 ma ago and cooled relatively quickly nwa 176 likely formed and cooled quickly 4544 ma ago or later than formation times of most meteorite parent bodies for all three meteorites the ar ar ages are in better agreement with i xe ages and preferred thermal models if we increase these ar ar ages by 20 ma such age corrections would be consistent with probable errors in 40 k decay parameters in current use as suggested by others the role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system

multiphase flow modeling of a crude oil spill site with a bimodal permeability distribution fluid saturation particle size distribution and porosity measurements were obtained from 269 core samples collected from six boreholes along a 90 m transect at a subregion of a crude oil spill site the north pool near bemidji minnesota the oil saturation data collected 11 years after the spill showed an irregularly shaped oil body that appeared to be affected by sediment spatial variability the particle size distribution data were used to estimate the permeability k and retention curves for each sample an additional 344 k estimates were obtained from samples previously collected at the north pool the 613 k estimates were distributed bimodal lognormally with the two population distributions corresponding to the two predominant lithologies a coarse glacial outwash deposit and fine grained interbedded lenses a two step geostatistical approach was used to generate a conditioned realization of k representing the bimodal heterogeneity a cross sectional multiphase flow model was used to simulate the flow of oil and water in the presence of air along the north pool transect for an 11 year period the inclusion of a representation of the bimodal aquifer heterogeneity was crucial for reproduction of general features of the observed oil body if the bimodal heterogeneity was characterized hysteresis did not have to be incorporated into the model because a hysteretic effect was produced by the sediment spatial variability by revising the relative permeability functional relation an improved reproduction of the observed oil saturation distribution was achieved the inclusion of water table fluctuations in the model did not significantly affect the simulated oil saturation distribution

39ar 40ar evidence for early impact events on the ll parent body abstract we determined 39ar 40ar ages of eight ll chondrites and one igneous inclusion from an ll chondrite with the object of understanding the thermal history of the ll chondrite parent body the meteorites in this study have a range of petrographic types from ll3 3 to ll6 and shock stages from s1 to s4 these meteorites reveal a range of k ar ages from 3 66 to 4 50 ga and peak ages from 3 74 to 4 55 ga significantly three of the eight chondrites ll4 5 6 have k ar ages of 4 27 ga one of these mil99301 preserves an 39ar 40ar age of 4 23 0 03 ga from low temperature extractions and an older age of 4 52 0 08 ga from the highest temperature extractions in addition an igneous textured impact melt dom85505 22 has a peak 39ar 40ar age of 4 27 ga we interpret these results as evidence for impact events that occurred at about 4 27 ga on the ll parent body that produced local impact melts reset the 39ar 40ar ages of some meteorites and exhumed or interred others resulting in a range of cooling ages the somewhat younger peak age of 3 74 ga from gro95658 ll3 3 suggests an additional impact event close to timing of impact reset ages of some other ordinary chondrites between 3 6 3 8 ga the results from mil99301 suggest that some apparently unshocked s1 chondrites may have substantially reset 39ar 40ar ages a previous petrographic investigation of mil99301 suggested that reheating to temperatures less than or equal to type 4 petrographic conditions 600 c caused fractures in olivine to anneal resulting in a low apparent shock stage of s1 unshocked the 39ar 40ar age spectrum of mil99301 is consistent with this interpretation older ages from high t extractions may date an earlier impact event at 4 52 0 08 ga whereas younger ages from lower t extractions date a later impact event at 4 23 0 03 ga that may have caused annealing of feldspar and olivine

the l3 6 chondritic regolith breccia northwest africa nwa 869 i petrology chemistry oxygen isotopes and ar ar age determinations northwest africa nwa 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons it is an l3 6 chondrite and probably represents the largest sample of the rare regolith breccias from the l chondrite asteroid it contains unequilibrated and equilibrated chondrite clasts some of which display shock darkening impact melt rocks imrs both clast free and clast poor are strongly depleted in fe ni metal and sulfides an unequilibrated microbreccia two different light inclusions and two different sio2 bearing objects were found although the matrix of this breccia appears partly clastic it is not a simple mixture of fine grained debris formed from the above lithologies but mainly represents an additional specific lithology of low petrologic type we speculate that this material stems from a region of the parent body that was only weakly consolidated one imr clast and one sio2 bearing object show 17o values similar to bulk nwa 869 suggesting that both are related to the host rock in contrast one light inclusion and one imr clast appear to be unrelated to nwa 869 suggesting that the imr clast is contaminated with impactor material 40ar 39ar analyses of a type 4 chondrite clast yield a plateau age of 4402 7 ma which is interpreted to be the result of impact heating other impact events are recorded by an imr clast at 1790 36 ma and a shock darkened clast at 2216 40 ma demonstrating that nwa 869 escaped major reset in the course of the event at approximately 470 ma that affected many l chondrites

a comparison of micro ct and thin section analysis of lateglacial glaciolacustrine varves from glen roy scotland abstract despite the prevalence of thin section analysis in studies of quaternary sediments there are limitations associated with the production of thin sections sediment modification and the inherently 2d view that a thin section affords non destructive and rapid scanning technologies such as x ray computed microtomography ct enable material samples to be visualised and analysed in 3d in a quaternary context however such techniques are in their infancy this paper assesses the optimum approach to ct analysis of quaternary sediments applying the method on lateglacial glaciolacustrine varves from glen roy scotland scan datasets are examined at each stage of the thin section process and comparisons are made between 2d ct images and thin sections for the recognition of 2d sediment features with further appraisal of 3d models to identify 3d sediment structures comparable sediment features are observed in 2d ct images and thin sections however the ct imaging resolution determines the precision of microfacies descriptions additional 3d structures are distinguished from volumetric models that are otherwise impossible to identify in thin section slides these 3d structures can locally alter sediment properties e g layer thickness as seen in 2d thin sections and or digital images although such variation cannot be detected with these media it has been demonstrated that clear benefits exist in understanding the 3d structure of quaternary sediments both prior to thin sectioning to avoid complicating e g deformation structures and after thin sectioning to establish the complex 3d context of 2d datasets it is recommended that ct and thin section techniques are applied in parallel in future studies which will profit from the integration of true 3d data it is also advised that samples are scanned soon after field sampling due to the significant modification of in situ sediment structures that can occur during thin section processing

high resolution hydrothermal structure of hansbreen spitsbergen mapped by ground penetrating radar detailed ground penetrating radar gpr surveys at 50 and 200 mhz on hansbreen a polythermal glacier in southern svalbard are presented and interpreted comparison of the variations in character of the radar reflections with borehole thermometry and water levels in moulins suggests that gpr can be used to study the hydrothermal properties of the glacier the high resolution of the gpr data shows that the hydrothermal structure of the glacier is highly variable both along the centre line and on transverse profiles water contents for many places and depths within the glacier were calculated by estimating radar wave velocities to point reflectors we find typical water contents of 1 2 for the temperate ice but wetter ice associated with surface crevassing and moulins typically 4 water content there is evidence that wet ice sometimes overlays drier ice the hydrothermal structure is thus shown to be very complex temperature gradients in the cold ice indicate freezing rates of temperate ice below cold ice of 0 1 0 5 m a while isolated point reflectors within the cold ice indicate large water filled bodies that are probably related to the regular drainage structure of the glacier

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

ice thickness internal structure and subglacial topography of bowles plateau ice cap and the main ice divides of livingston island antarctica by ground based radio echo sounding ground based radio echo sounding studies of livingston island ice cap antarctica were started in 1999 at johnsons and hurd glaciers in hurd peninsula close to the spanish antarctic station juan carlos i radar profiling continued in 2000 and 2003 along the main ice divides of the ice cap and in the upper parts of huron and another unnamed glacier both draining bowles ice plateau towards moon bay more detailed radar studies on bowles ice plateau were performed in december 2006 using an icepenetrating radar virl 6 with central frequency of 20 mhz and a ramac gpr radar with 200 mhz antennae intended for deep penetration to bedrock and shallow penetration to determine the firn layer thickness respectively the radar equipment was installed on two nansen sledges and two pulkas towed by snowmobiles georeferencing of radar data was accomplished by using a gps receiver working in stand alone mode the endpoints of the radar profiles were more accurately georeferenced using differential gps the data collected were used to construct the ice thickness glacier surface and bedrock elevation maps and to estimate the average ice thickness and total ice volume of the plateau as well as the spatial distribution of snow accumulation for time to depth conversion an average radio wave velocity determined from common midpoint measurements at several locations in hurd peninsula was used the thickest ice 550 m was found in the upper part of huron glacier this thickness is almost twice the maximum thickness found along the main ice divides 330 m and almost three times that found in hurd peninsula 200 m the bedrock in two large areas in the northern and southern parts of the plateau towards moon bay and huntress glacier lies below sea level at depths down to 200 m indicating that should the ice cap fully disappear several separated islands would likely appear even if isostacic rebound is taken into account

versatile direct push profiler for the investigation of volatile compounds near the water table 1 the collection of representative samples of dissolved volatile compounds requires particular care to ensure that degassing does not occur during sampling particularly for sites where the water table is deep furthermore the investigation of physical and chemical processes involving volatile components can be enhanced by collecting vadose zone gas samples above the water table and collecting water samples very close to the water table here we describe the design and operation of a direct push profiler that allows the collection of gas and water samples from one hole the profiler is capable of collecting water samples close to the water table and preserves the integrity of the samples collected from below deep water tables with respect to volatile compounds the versatility of the profiler is due to the incorporation of a small diameter bladder pump into the tip of a waterloo profiler laboratory testing shows that samples collected from 10 cm below the water table and at 9 m depth below ground surface show no signs of degassing field measurements made at a petroleum spill site near bemidji minnesota provide a comparison of profiler results to samples collected from an existing well cluster demonstrating the ability of the profiler to obtain representative samples under field conditions furthermore the field tests demonstrate the utility of the direct push profiler in terms of obtaining high resolution high quality data through the vadose zone and into the upper few meters of the aquifer

magnetite scavenging and the buoyancy of bubbles in magmas part 1 discovery of a pre eruptive bubble in bishop rhyolite little is known about the presence distribution and size of bubbles in rhyolitic magmas prior to eruption using x ray tomography to study pumice from early erupted bishop rhyolite we discovered a large vesicle with abundant magnetite crystals attached to its walls attachment of magnetite crystals to bubble walls under pre eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals alternatively bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it we argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression the inferred presence of pre eruptive bubbles in the bishop magma confirms the interpretation that the magma was volatile saturated prior to eruption the pre eruptive size of this bubble is estimated based on three independent approaches 1 the current size of the vesicle 2 the total cross sectional area of the magnetite crystals and 3 the bubble size required for the aggregate to be neutrally buoyant these approaches suggest a pre eruptive bubble 300 850 m in diameter with a preferred value of 600 750 m

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

computation of refraction static corrections using first break traveltime differences differences in first arrival traveltimes between adjacent records in multifold reflection surveys can be used to compute the depth and velocity structure of near surface layers the procedure uses the redundancy of first break data in multifold surveys to enable a statistically reliable refraction analysis to be undertaken for either end on or split spread recording geometries the traveltime differences as a function of source receiver offset provide a direct indication of the number of refractors present with each refractor being defined by an offset range with a constant time difference for each refractor the time difference value at a common receiver from two shotpoints is used to partition the intercept time into the delay time at each shotpoint this procedure is repeated until the delay times at all shotpoints and for all refractors have been computed refractor depths and velocities are evaluated from this suite of delay times a surface consistent static correction to a selected datum level is then calculated at each surface station using a replacement velocity equal to that of the deepest refractor in a case history from the canadian rocky mountain foothills short and intermediate wavelength weathering static anomalies were resolved successfully elevation and weathering static corrections of up to 40 ms were computed with an estimated error of less than or 3 ms

modifications to the theory of intraglacial waterways for the case of subglacial ones on defend l idee que dans les glaciers de montagne temperes les voies d eau sont sous glaciaires pour un conduit noye sous glaciaire il faut introduire deux processus importants l accroissement de la deformation plastique sur les cotes amont des bosses et la chaleur apportee par l eau des sources qui en hiver rend compte de la plus grande partie du debit la theorie explique pourquoi rothlisberger 1972 dut adopter une valeur non realiste pour le parametre rheologique de la glace elle conduit a la conclusion que dans les zones non surcreusees la plus grande partie de l annee les conduits sous glaciaires doivent rester non noyes et a la pression atmospherique cas des surcreusements l incidence de ce nouveau modele sur la theorie du glissement des glaciers

chronology and petrology of silicates from iie iron meteorites evidence of a complex parent body evolution iie iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of h chondrites however these silicates show a wide range of alteration ranging from netschaevo and techado the inclusions of which are little altered to highly differentiated silicates like those in kodaikanal weekeroo station and colomera which have lost metal and sulfur and are enriched in feldspar we find these inclusions to show varying degrees of shock alteration we made 39ar 40ar age determinations of watson techado miles colomera and sombrerete watson has an ar ar age of 3 677 0 007 gyr similar to previously reported ages for kodaikanal and netschaevo we suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event the space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other iies 39ar 40ar ages inferred for the other four meteorites analyzed are considerably older than watson and are techado 4 49 0 01 gyr miles 4 405 0 012 gyr colomera 4 470 0 010 gyr and sombrerete 4 541 0 0012 gyr these ages are in fair agreement with previously reported rb sr isochron ages for colomera and weekeroo station although several mechanisms to form iie meteorites have been suggested it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by 0 8 gyr we suggest that those iies with older isotopic ages are a product of partial melting and differentiation within the parent body followed by mixing of silicate and metal while both were relatively hot netschaevo and watson may have formed by this same process or by impact mixing 4 5 gyr ago but their isotopic ages may have been subsequently reset by shock heating kodaikanal apparently is required to have formed more recently in which case impact melting and differentiation seems the only viable process we see no compelling reasons to believe that iie silicate and metal derived from different parent bodies or that the parent body of iies was the same as that of h chondrites

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

ar ar ages and thermal histories of enstatite meteorites abstract compared with ordinary chondrites there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies in this study we report 39ar 40ar dating results for five el chondrites khairpur pillistfer hvittis blithfield and forrest five eh chondrites parsa saint marks indarch bethune and reckling peak 80259 three igneous textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies zaklodzie queen alexandra range 97348 and queen alexandra range 97289 and three aubrites norton county bishopville and cumberland falls several ar ar age spectra show unusual 39ar recoil effects possibly the result of some of the k residing in unusual sulfide minerals such as djerfisherite and rodderite and other age spectra show 40ar diffusion loss few additional ar ar ages for enstatite meteorites are available in the literature when all available ar ar data on enstatite meteorites are considered preferred ages of nine chondrites and one aubrite show a range of 4 50 4 54 ga whereas five other meteorites show only lower age limits over 4 35 4 46 ga ar ar ages of several enstatite chondrites are as old or older as the oldest ar ar ages of ordinary chondrites which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies or were metamorphosed to lower temperatures compared to other chondrite types many enstatite meteorites are brecciated and or shocked and some of the younger ar ar ages may record these impact events although impact heating of ordinary chondrites within the last 1 ga is relatively common for ordinary chondrites only bethune gives any significant evidence for such a young event

the origin of sulfide rimmed metal grains in ordinary chondrites abstract we report results from an experimental study of sulfide chemistry during metamorphism on ordinary chondrite parent bodies artificial ll chondrite material composed of silicate iron metal and sulfide grains was placed in sealed evacuated silica tubes and heated to either 500 c or 900 c these temperatures are representative peak metamorphic temperatures experienced by type 3 and type 6 ordinary chondrites respectively rapid sulfur mobilization occurs during heating and results in the formation of sulfide rims around the metal grains and sulfur loss from the original sulfide crystals the newly formed sulfide rims have two distinct layers that incorporate nearby silicate grains we also observe narrow sulfide trails that follow silicate grain boundaries and connect separate sulfide rimmed metal grains the morphologies of the sulfide rims suggest that vapor transport is the main mechanism for sulfur mobilization sulfur loss is observed along cracks and crystal boundaries of the initial sulfide grains after extensive reaction patches of iron metal appear at the outer edges of the sulfide crystals and large pore spaces form throughout the original sulfides the experimental metal sulfide assemblages resemble those found in low metamorphic grade ordinary chondrites this suggests metamorphism is mainly responsible for the observed metal sulfide textures in these chondrites

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

portales valley a meteoritic sample of the brecciated and metal veined floor of an impact crater on an h chondrite asteroid a shower of meteorite fragments fell at 0730 h local time on 1998 june 13 near the town of portales new mexico thus far 51 pieces of the portales valley h6 meteorite have been recovered this meteorite has an unusually large number of metallic veins some of these veins are also unusually thick having widths on the order of centimeters these wide veins have fine widmanstatten structure which is the first time it has been seen in an ordinary chondrite this structure indicates the metallic veins and the host chondrite cooled slowly these veins appear to have been produced by shock metamorphic processes which we infer produced a 20 km diameter impact crater on an h chondrite planetesimal

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

post crystallization reheating and partial melting of eucrite eet90020 by impact into the hot crust of asteroid 4vesta 4 50 ga ago we performed petrologic radiometric ar ar sm nd and mn cr ages and ion microprobe studies of the basaltic eucrite eet90020 this is one of the few rare basaltic eucrites whose 39ar 40ar age has not been reset during impact bombardment on the hed parent body 4 ga ago and thus should provide a unique opportunity to study the nature of the early thermal events on its parent body presumably asteroid 4vesta hand specimen inspection shows that the rock consists of a fine grained and a coarse grained lithology microscopy indicates that the fine grained lithology has a granulitic texture with a coarser grained area and a large opaque assemblage embedded in the granulitic matrix the coarse grained lithology has an igneous subophitic texture the rock has pyroxenes similar to those in type 5 eucrites type 5 pyroxene and experienced prolonged thermal metamorphism after rapid crystallization from a near surface melt however minor mineral assemblages are unusual and suggest a complex thermal history tridymite occurs as large laths irregular crystals 1 5 gpa after the reheating event eet90020 seems to have experienced the following thermal history 1 crystallization during rapid cooling near the surface 2 some brecciation by impact 3 thermal metamorphism that produced type 5 pyroxene and 4 short reheating that caused partial melting and rapid cooling 39ar 40ar measurements show a relatively flat pattern and an age of 4 49 0 01 ga which is consistent with rapid cooling from high temperature event 4 resetting of the sm nd ages at 4 51 0 04 ga appears to be closely related to the remelting of ca phosphates rb sr data suggest rb loss from tridymite during partial melting the resetting of the mn cr age may have been related to the formation of cr ulvospinels event 4 we suggest that all these ages were reset by partial melting event 4 we further suggest that the partial melting event event 4 that reset the ages 4 50 ga ago was caused by an impact into eet90020 which was part of the hot crust of 4vesta and resulted in an increase in the temperature from the ambient temperature of 870 c to above the subsolidus temperature of eucrites of 1060 c

vertical distribution of water within the polythermal storglaciaren sweden 1 knowledge of water content and its distribution in polythermal glaciers is required to model their flow and thermal state however observations of water content variations with depth in polythermal glaciers are scarce water content can be estimated from radio wave speed because they depend on one another we obtained continuous profiles of radio wave speed variations with depth from zero offset radar profiles collected in boreholes approximately 80 m deep in the upper ablation area of storglaciaren northern sweden these profiles show that the microcrystalline water system in the temperate ice is relatively homogeneous the overall hydrothermal structure at this location is composed of a 20 m thick upper layer of cold water free ice underlain by a temperate ice layer whose average water content is 0 6 0 3 these results are corroborated by surface radar and thermistor measurements which show that the depth of the cold temperate transition is 21 m and the calculated water content at that transition is 0 6 0 1 these findings imply that the whole temperate ice layer is from 3 to 4 times softer than the cold ice and consequently that realistic ice flow models of polythermal glaciers should include the effect of water content on viscosity

subglacial water pressure and ice speed variations at johnsons glacier livingston island antarctic peninsula to study subglacial hydrological condition and its influence on the glacier dynamics we drilled johnsons glacier on livingston island in the antarctic peninsula region subglacial water pressure was recorded in boreholes at two locations over 2 years accompanied by high frequency ice speed measurements during two summer melt seasons water pressure showed two different regimes namely high frequency and large amplitude variations during the melt season january april and small fluctuations near the overburden pressure the rest of the year speed up events were observed several times in each summer measurement period ice motion during these events substantially contributed to total glacier motion for example fast ice flow over 1 week accounted for 70 of the total displacement over a 25 day long measurement period we did not find a clear relationship between subglacial water pressure and ice speed this was probably because subglacial hydraulic conditions were spatially inhomogeneous and thus our borehole data did not always represent a large scale subglacial condition ice temperature measurements in the boreholes confirmed the existence of a cold ice layer near the glacier surface our data provide a basis to better understand the dynamic and hydrological conditions of relatively unstudied glaciers in the antarctic peninsula region

h l chondrite lapaz icefield 031047 a feather of icarus abstract antarctic meteorite lap 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments its petrography oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between h and l chondrites petrographic indicators suggest lap 031047 suffered a shock metamorphic overprint below 10 gpa which did not destroy its unusually high porosity of 27 vol metallographic textures in lap 031047 indicate heating above 700 c and subsequent cooling which caused massive transformation of taenite to kamacite the depletion of thermally labile trace elements the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition and the occurrence of coarsely crystallized chondrule fragments is further evidence for post metamorphic heating to 700 750 c however this heating event had a transient character because olivine and low ca pyroxene did not equilibrate nearly complete degassing up to very high temperatures is indicated by the thorough resetting of lap 031047 s ar ar reservoir 100 55 ma ago a noble gas cosmic ray exposure age indicates it was reduced to a meter size fragment at

formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid we propose that mesosiderites formed when a 200 400 km diameter asteroid with a molten core was disrupted by a 50 150 km diameter projectile to test whether impacts can excavate core iron and mix it with crustal material we used a low resolution smoothed particle hydrodynamics computer simulation for 50 300 km diameter differentiated targets we found that significant proportions of scrambled core material and hence potential mesosiderite metal material could be generated for near catastrophic impacts that reduce the target to 80 of its original diameter and about half of its original mass the proportion of scrambled core material would be about 5 vol equivalent to 10 vol of mesosiderite like material the paucity of olivine in mesosiderites and the lack of metal poor or troilite rich meteorites from the mesosiderite body probably reflect biased sampling mesosiderites may be olivine poor because mantle material was preferentially excluded from the metal rich regions of the reaccreted body molten metal globules probably crystallized around small cool fragments of crust hindering migration of metal to the core if mantle fragments were much hotter and larger than crustal fragments little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally the rapid cooling rates of mesosiderites above 850 c can be attributed to local thermal equilibration between hot and cold ejecta very slow cooling below 400 c probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris we infer that our model is more plausible than an earlier model that invoked an impact at 1 km s to mix projectile metal with target silicates if large impacts cannot effectively strip mantles from asteroidal cores as we infer we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material this may help to explain why relatively few olivine rich a type and metal rich asteroids m type are known some s type asteroids may be scrambled differentiated bodies

origin and history of impact melt rocks of enstatite chondrite parentage we have conducted petrologic chemical and isotopic studies of two impact produced rocks of enstatite chondrite parentage ilafegh 009 is a total impact melt rock with no residual lithic clasts formation on the el chondrite parent body is suggested by its mineralogy and mineral compositions cooling of the impact melt was rapid at melt temperatures and decreased at subsolidus temperatures in contrast to previous studies we show that happy canyon is not a new enstatite achondrite but an impactmelt breccia of enstatite chondrite and not aubrite parentage this rock formed by impact melting and incorporation into the melt of clastic material which resulted in relatively rapid cooling at all temperatures mineralogical and bulk compositional data probably biased by the heterogeneous nature of this rock do not allow unequivocal determination of its parent body i e el vs eh although some data such as bulk total fe content seem to favor el parentage both rocks were subjected to post solidification shock which was more severe for ilafegh 009 than for happy canyon it appears that both impact melt rocks could have formed by impact melting 4 57 ga ago as is indicated by the nearly identical ixe closure ages of 1 6 and 1 4 ma before bjurbole for ilafegh 009 and happy canyon respectively an apparently younger 39ar40ar age of 4 53 ga for happy canyon may be due to small biases in the intercalibration of the ixe and 39ar40ar chronometers whereas the much younger 4 34 4 44 ga age for ilafegh 009 reflects thermal resetting during shock metamorphism shallowater which was impact derived from a different enstatite achondrite parent body has an ixe closure age 0 4 ma younger than that for ilafegh 009 and an 39ar40ar age of 4 53 ga the ancient ages of these three rocks attest to the intense early bombardment in this region of the solar system

bunburra rockhole exploring the geology of a new differentiated asteroid bunburra rockhole is the first recovered meteorite of the desert fireball network we expanded a bulk chemical study of the bunburra rockhole meteorite to include major minor and trace element analyses as well as oxygen and chromium isotopes in several different pieces of the meteorite this was to determine the extent of chemical heterogeneity and constrain the origin of the meteorite minor and trace element analyses in all pieces are exactly on the basaltic eucrite trend major element analyses show a slight deviation from basaltic eucrite compositions but not in any systematic pattern new oxygen isotope analyses on 23 pieces of bunburra rockhole shows large variation in both 17o and 18o and both are well outside the hed parent body fractionation line we present the first cr isotope results of this rock which are also distinct from heds detailed computed tomographic scanning and back scattered electron mapping do not indicate the presence of any other meteoritic contaminant contamination is also unlikely based on trace element chemistry we therefore conclude that bunburra rockhole represents a sample of a new differentiated asteroid one that may have more variable oxygen isotopic compositions than 4 vesta the fact that bunburra rockhole chemistry falls on the eucrite trend perhaps suggests that multiple objects with basaltic crusts accreted in a similar region of the solar system

use of dissolved and vapor phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface 1 at many sites contaminated with petroleum hydrocarbons methanogenesis is a significant degradation pathway techniques to estimate ch4 production consumption and transport processes are needed to understand the geochemical system provide a complete carbon mass balance and quantify the hydrocarbon degradation rate dissolved and vapor phase gas data collected at a petroleum hydrocarbon contaminated site near bemidji minnesota demonstrate that naturally occurring nonreactive or relatively inert gases such as ar and n2 can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface in the vadose zone regions of ar and n2 depletion and enrichment are indicative of methanogenic and methanotrophic zones and concentration gradients between the regions suggest that reaction induced advection can be an important gas transport process in the saturated zone dissolved ar and n2 concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path into the downgradient aquifer is largely controlled by physical processes slight but discernable preferential depletion of n2 over ar in both the saturated and unsaturated zones near the free phase oil suggests reactivity of n2 and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

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

seismic wave attenuation in the uppermost glacier ice of storglaciaren sweden we conducted seismic refraction surveys in the upper ablation area of storglaciaren a small valley glacier located in swedish lapland we estimated seismic wave attenuation using the spectral ratio method on the energy travelling in the uppermost ice with an average temperature of approximately 1 c attenuation values were derived between 100 and 300 hz using the p wave quality factor q p the inverse of the internal friction by assuming constant attenuation along the seismic line we obtained mean q p 6 1 we also observed that q p varies from 8 1 to 5 1 from the near offset to the far offset region of the line respectively since the wave propagates deeper at far offsets this variation is interpreted by considering the temperature profile of the study area far offset arrivals sampled warmer and thus more attenuative ice our estimates are considerably lower than those reported for field studies in polar ice 500 1700 at 28 c and 50 160 at 10 c and hence are supportive of laboratory experiments that show attenuation increases with rising ice temperature our results provide new in situ estimates of q p for glacier ice and demonstrate a valuable method for future investigations in both alpine and polar ice

formation of vesicles in asteroidal basaltic meteorites rare asteroidal vesicular basalts have previously been thought to form in surficial lava flows with co as the vesicle forming gas however vesicular lava flows are unlikely on small airless bodies such as asteroids to unravel the origin of these rocks we analyzed vesicle sizes and abundances for two angrites and two eucrites using high resolution x ray computed tomography and conducted numerical modeling of bubble formation in a dike of ascending magma modeling results indicate that thin 30 cm wide dikes are trapped at 5 km depth where 75 ppm of co and co2 contribute equally to vesicle formation vesicular eucrites were metamorphosed in this deep seated environment the gas was lost and they were excavated by impacts

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

40ar 39ar age of northwest africa 091 more evidence for a link between l chondrites and fossil meteorites abstract most 40ar 39ar ages of l chondrites record an event at approximately 500 ma indicating a large collisional impact at that time however there is a spread in ages from 400 to 600 ma in these meteorites that is greater than the analytical uncertainty identification of and correction for trapped ar in a few l chondrites has given an age of 470 6 ma this age coincides with ordivician fossil meteorites that fell to earth at 467 2 ma as these fossil meteorites were originally l chondrites the apparent conclusion is that a large impact sent a flood of l chondrite material to earth while material that remained on the l chondrite parent body was strongly heated and reset we have reduced 40ar 39ar data for northwest africa 091 using various techniques that appear in the literature including identification and subtraction of trapped ar these techniques give a range of ages from 455 to 520 ma and show the importance of making accurate corrections by using the most straightforward technique to identify and remove a trapped ar component which is neither terrestrial nor primordial an 40ar 39ar age of 475 6 ma is found for northwest africa 091 showing a temporal link to fossil meteorites in addition high temperature releases of northwest africa 091 contain evidence for a second trapped component and subtraction of this component indicates a possible second collisional impact at approximately 800 ma this earlier age coincides with 40ar 39ar ages of some h and l chondrites and lunar samples

complex exposure histories for meteorites with short exposure ages we report measurements of 26al and 10be activities in nine ordinary chondrites and of the light noble gas concentrations and 36cl and 41ca activities in subsets of those meteorites all but murray have low 21ne concentrations 1 0 10 8cm3stp g and have previously been used to estimate 21ne production rates ladder creek murchison sena and timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21ne production rates that are consistent with the standard l chondrite value of 0 33 10 8cm3stp g ma in contrast cullison guenie shaw and tsarev experienced complex irradiation histories they and several other meteorites with low nominal exposure ages also have lower 3he 21ne ratios than expected based on their 22ne 21ne ratios a general association between low 21ne contents and 3he losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia however meteorites with low 21ne contents one stage exposure histories and losses of cosmogenic 3he are rare possible explanations for the scarcity are 1 statistical 2 that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision and 3 that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia

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

hurd peninsula glaciers livingston island antarctica as indicators of regional warming ice volume changes during the period 1956 2000 johnsons and hurd glaciers are the two main glacier units of hurd peninsula ice cap livingston island south shetland islands antarctica they presently cover an area of about 10 km 2 johnsons is a tidewater glacier while hurd glacier ends on emerged land in this paper we estimate the changes in ice volume during the period 1956 2000 and compare them with the regional meteorological records the volume change estimates are based on the comparison of digital terrain models for the glacier surface constructed from aerial photographs taken by the british antarctic survey in 1956 and from our geodetic measurements in 1999 2000 the total volume estimates are based on an ice thickness map constructed from radio echo sounding profiles 18 25 mhz done in 1999 2001 showing maximum ice thickness of about 200 m we estimate the changes in ice volume during the period 1956 2000 to be 0 108 0 048 km 3 which represents a 10 0 4 5 decrease from the 1956 total volume of 1 076 0 055 km 3 and is equivalent to an average annual mass balance of 0 23 0 10 m w e during 1956 2000 ice thickness changes range from 40 to 20 m averaging 5 5 4 4 m most areas show ice thinning the thickening is limited to a small area within johnsons glacier all glacier fronts except johnsons calving front show retreat these changes are consistent with the regional meteorological records for mean summer temperature which show a trend of 0 023 0 0058 ca 1 during the period 1956 2000

impact melt products of chondritic material a large variety of objects in chondritic meteorites formed as a result of impact melting shock veins metal troilite mixtures metal and sulfide nodules melt pockets vugs agglutinates glassy and crystalline spherules and numerous types of melt rock clasts the type of object produced in an impact event is related to the interaction of the shock waves with the particular target rock impact melting has also affected large portions of several chondrite breccias many of these contain vugs and patches of silicate glass some workers have suggested that various iron meteorites including groups iab iiicd and iie as well as several ungrouped irons were formed from individual impact melt pools in chondritic regoliths the eagle station trio pallasites and several ungrouped meteorites bencubbin weatherford enon acapulco and allan hills a77081 may have formed from impact melted chondritic material it is also possible that the parent magmas of ureilites were derived from impact melted metal rich carbonaceous chondrite material

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

velocity of radio waves in glaciers as an indicator of their hydrothermal state structure and regime the results of measurements of radio wave velocities rwv by wide angle reflection war methods in the temperate abramov glacier in the alai mountain ridge and the two layered sub polar fridtjovbreen and hansbreen on svalbard using a low frequency 2 13 mhz radar are considered and discussed the experimental data obtained and the data from the literature show that the values of rwv could be a good indicator of the hydrothermal state of glaciers as such these data enable the identification of cold temperate and transitional two layered glaciers and can be used for estimation of the water content in glaciers and changes in the hydrothermal state

a new framework for estimating englacial water content and pore geometry using combined radar and seismic wave velocities 1 ice mechanical properties and hence the response of glaciers to climate change depend strongly on the presence of liquid water at ice grain boundaries the propagation velocities of radar and seismic waves are also highly sensitive to this water mixing laws typically the looyenga and riznichenko formulae have traditionally been used to quantify liquid water content within glaciers from such velocity data however it has become apparent that these mixing laws are geometrically inconsistent we present an inclusion based effective medium approximation in which we model water inclusions within solid ice two types of inclusions are used spherical inclusions to represent water in the grain junction nodes and high aspect ratio spheroidal inclusions to represent water in the grain boundary veins we apply this model to radar and seismic data from a polythermal glacier in svalbard to quantify both inclusion geometry and the unfrozen water content within the warm ice

radioglaciological studies on hurd peninsula glaciers livingston island antarctica we present the results of several radio echo sounding surveys carried out on johnsons and hurd glaciers livingston island antarctica between the 1999 2000 and 2004 05 austral summer campaigns which included both radar profiling and common midpoint measurements with low 20 25 mhz and high 200 mhz frequency radars the latter have allowed us to estimate the radio wave velocity in ice and firn and the corresponding water contents in temperate ice which vary between 0 and 1 6 depending on the zone maximum ice thickness is 200 m with a mean value of 93 6 2 5 m total ice volume is 0 968 0 026 km 3 for an area of 10 34 0 03 km 2 the subglacial relief of johnsons glacier is quite smooth while that of hurd glacier shows numerous overdeepenings and peaks the radar records suggest that hurd glacier has a polythermal structure contrary to the usual assumption that glaciers in livingston island are temperate this is also supported by other dynamical and geomorphological evidence

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

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

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

mineralogy petrology chronology and exposure history of the chelyabinsk meteorite and parent body three masses of the chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the chelyabinsk parent body the samples exhibit little to no postentry oxidation via mossbauer and raman spectroscopy indicating their fresh character but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result detailed studies show three distinct lithologies indicative of a genomict breccia a light colored lithology is ll5 material that has experienced thermal metamorphism and subsequent shock at levels near s4 the second lithology is a shock darkened ll5 material in which the darkening is caused by melt and metal troilite veins along grain boundaries the third lithology is an impact melt breccia that formed at high temperatures 1600 c and it experienced rapid cooling and degassing of s2 gas portions of light and dark lithologies from chel 101 and the impact melt breccias chel 102 and chel 103 were prepared and analyzed for rb sr sm nd and ar ar dating when combined with results from other studies and chronometers at least eight impact events e g 4 53 ga 4 45 ga 3 73 ga 2 81 ga 1 46 ga 852 ma 312 ma and 27 ma are clearly identified for chelyabinsk indicating a complex history of impacts and heating events finally noble gases yield young cosmic ray exposure ages near 1 ma these young ages together with the absence of measurable cosmogenic derived sm and cr indicate that chelyabinsk may have been derived from a recent breakup event on an neo of ll chondrite composition

an introduction to the application of x ray microtomography to the three dimensional study of igneous rocks imaging rocks in three dimensions through x ray microtomography enables routine visualization of structures in samples which can be spatially resolved down to the sub micron scale although x ray tomography has been applied in biomedical research and clinical settings for decades it has only recently been applied to studies of rocks and few geoscientists realize its value and potential this contribution provides an introduction to the principles and techniques of x ray microtomography to the study of igneous rock textures as well as reviewing the current state of the art we hope that this short review will encourage more geoscientists to apply x ray microtomography in their research and that this will lead to new insights into the processes that occur in magmatic as well as other geological systems

ice volume changes 1936 1990 and structure of aldegondabreen spitsbergen aldegondabreen is a small valley glacier ending on land located in the gronfjorden area of spitsbergen svalbard airborne radio echo sounding in 1974 75 using a 440mhz radar revealed a polythermal two layered structure which has been confirmed by detailed ground based radio echo sounding done in 1999 using a 15mhz monopulse radar the 1999 radar data reveal an upper cold layer extending down to 90m depth in the southern part of the glacier where the thickest ice 216m was also found a repeated pattern of diffractions from the southern part of the glacier at depths of 50 80m and dipping down glacier has been interpreted as an englacial channel which originates in the temperate ice from joint analysis of the 1936 topographic map a digital elevation model constructed from 1990 aerial photographs and the subglacial topography determined from radar data a severe loss of mass during the period 1936 90 has been estimated a glacier tongue retreat of 930m a decrease in area from 8 9 to 7 6 km in average ice thickness from 101 to 73m and in ice volume from 0 950 to 0 558 km which are equivalent to an average annual balance of 0 7mw e this is comparable with the only available data of net mass balance for aldegondabreen 1 1 and 1 35mw e for the balance years 1976 77 and 2002 03 and consistent with the 0 278c increase in mean summer air temperature in this zone during 1936 90 as well as the warming in spitsbergen following the end of the little ice age lia and the general glacier recession trend observed in this region introduction aldegondabreen is a small 7 6 km in 1990 valley glacier located in the gronfjorden area of spitsbergen svalbard 77858 n 14805 e fig 1 the glacier terminates on land and presently spans a 30 450m altitude range radio echo sounding from a helicopter using a 440mhz radar was performed in 1974 75 along a longitudinal profile of the glacier revealing a polythermal two layered structure and maximum ice thickness along the profile of 150m macheret and zhuravlev 1980 radio echo sounding has often been used as a tool for investigating the two layered structure of svalbard polythermal glaciers e g glazovskiy and others 1992 bjornsson and others 1996 moore and others 1999 there have been few glaciological studies of aldegondabreen well documented mass balance data are only available for the balance year 1975 76 for which a net mass balance of 1 10mw e was estimated for the same balance year the average snow cover thickness on aldegondabreen was estimated from 367 measurements as 1 16m troitskiy and others 1985 in 2002 03 the winter balance was measured as 0 45mw e and the summer balance as 1 80mw e resulting in a net annual balance of 1 35mw e in 2003 04 the values were 0 56 2 27 and 1 71mw e for winter summer and annual balances respectively b r mavlyudov and i y solovyanova unpublished information no documented data on ice velocities are available for aldegondabreen the hydraulic system of aldegondabreen is one of its most studied aspects the glacier has a well developed system of englacial channels and caves with explored galleries reaching 500m in length eraso and pulina 2001 two main water streams emerge from the glacier front one of them close to the northern glacier wall and the other in the central part of the snout though another stream or set of streams is known to flow under the lateral terminal moraine next to the southern wall aldegondabreen has experienced significant retreat since the end of the little ice age lia especially in the second half of the 20th century this retreat is clearly manifested by the system of terminal moraines it is also evidenced by several eskers present in the glacier forefield which also shows some striations and fluted moraines revealing past sliding the main objective of this paper is to quantify this retreat the other aim is to improve our knowledge of the structure of aldegondabreen as a representative of many small sized valley glaciers in spitsbergen ending on land

exposure history of the st robert h5 fall the compositionally typical h5 chondrite st robert has an exposure age 7 8 ma indistinguishable from that of the main cluster of h chondrites small values of the cosmogenic 22ne 21ne ratio in interior samples imply a pre atmospheric radius on the order of 40 cm sample depths based on tracks and the production rates of bhattacharya et al 1973 range from 6 to 40 cm and are generally larger than depths estimated from published 60co activities perhaps because the track production rates adopted are too high depth profiles of the production rates of 14c 36cl 26al 10be and 21ne in stony material show increases with depth and reach levels 5 to 15 higher than expected from modeling calculations the maximum concentrations in st robert are however generally comparable to those measured for the l5 chondrite knyahinya whose pre atmospheric radius of 45 cm is thought to lead to the maximum possible production rates in chondrites we infer that the pre atmospheric radius of st robert was within 5 cm of the value that supports maximum production rates i e 45 5 cm this radius corresponds to a pre atmospheric mass of 1 3 0 4 103 kg the agreement of exposure ages for st robert obtained in several different ways and the similarity of the depth profiles for 14c 26al 10be and 21ne argue against a lengthy pre exposure of st robert on the parent body and against a two stage exposure after launch from the parent body following morbidelli and gladman 1998 we suggest that st robert was chipped from deep in its parent body spent the next 7 8 ma without undergoing a major collision was nudged gradually into an orbital resonance with jupiter and then traveled quickly to earth

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

englacial water distribution in a temperate glacier from surface and borehole radar velocity analysis we have obtained common offset common midpoint cmp and borehole vertical vrp ground penetrating radar profiles close to the margin of falljokull a small steep temperate valley glacier situated in southeast iceland velocity analysis of cmp and vrp surveys provided a four layered velocity model this model was verified by comparison between the depths of englacial reflectors and water channels seen in borehole video and from the depths of boreholes drilled to the bed in the absence of sediment within the glacier ice radar velocity is inversely proportional to water content using mixture models developed by paren and looyenga the variation of water content with depth was determined from the radar velocity profile at the glacier surface the calculated water content is 0 23 0 34 velocity 0 166 m ns 1 which rises sharply to 3 0 4 1 velocity 0 149 m ns 1 at 28 m depth interpreted to be the level of the piezometric surface below the piezometric surface the water content drops slowly to 2 4 3 3 velocity 0 152 m ns 1 until 102 m depth where it falls to 0 09 0 14 velocity 0 167 m ns 1 the water content of the ice then remains low to the glacier bed at about 112 m these results suggest storage of a substantial volume of water within the glacier ice which has significant implications for glacier hydrology ice rheology and interpretations of both radar and seismic surveys