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

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

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

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

melting runoff and the formation of frozen lakes in a mixed snow and blue ice field in dronning maud land antarctica large scale melting phenomena such as meltwater drainage channels and meltwater accumulation basins of frozen lakes were surveyed on the land ice mass in jutulgryta dronning maud land antarctica during the norwegian antarctic research expedition in 1989 90 nare 1989 90 the largest frozen lake that was observed was close to 1 km in width these melting features were also detected in a landsat thematic mapper image recorded on 12 february 1990 then during nare 1993 94 a 5year glaciological programme was started in this area in spite of negative air temperatures and the presence of a frozen ice surface sub surface melting and runoff were found within the uppermost metre in blue ice fields the sub surface melting is a consequence of solar radiative penetration and absorption within the ice i e the solid state greenhouse effect temperatures in blue ice were about 6 c higher than for snow internal melt and meltwater transport were observed throughout the 1 month of measurements the conditions for active melting in jutulgryta are probably marginal a slight increase of air temperatures can result in more classical surface melting whereas a cooling may disable sub surface melting studies of how the extent and characteristics of the melting features change with time can be particularly valuable as indicators of climate change this ongoing programme clearly identifies the importance of analyzing how these melting features originate of mapping their present areal distribution of determining how sensitive they are to climate change and of studying changes in the past and possible changes in the future

post shock annealing of miller range 99301 ll6 implications for impact heating of ordinary chondrites mil 99301 is an ll chondrite that has experienced successive episodes of thermal metamorphism shock metamorphism and annealing the first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 as indicated by homogeneous olivine compositions significant textural recrystallization and the presence of coarse grains of plagioclase metallic fe ni and troilite the source of heat for this thermal episode is not identified the rock also experienced shock metamorphism to shock stage s4 as indicated by extensive silicate darkening caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic fe ni and troilite blebs polycrystalline troilite myrmekitic plessite a relatively high occurrence abundance oa of metallic cu 3 6 the presence of numerous chromite plagioclase assemblages and coarse grains of low ca clinopyroxene with polysynthetic twinning the shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type 6 levels otherwise the polycrystallinity of the troilite would have disappeared and the low ca clinopyroxene would have transformed into orthopyroxene despite abundant evidence of strong shock olivine and plagioclase in mil 99301 exhibit sharp optical extinction consistent with shock stage s1 and characteristic of an unshocked rock this implies that an episode of post shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction the rock was probably annealed to metamorphic levels approximating petrologic type 4 more significant heating would have transformed the low ca clinopyroxene into orthopyroxene it is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26al because this isotope would have decayed away by that time impact heating is a more plausible source of post metamorphic annealing of rocks in the vicinity of impact craters on low density high porosity asteroids with rubble pile structures

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

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

penecontemporaneous metamorphism fragmentation and reassembly of ordinary chondrite parent bodies the occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three h l ll parent asteroids of 100 km radius e g miyamoto et al 1982 this conventional onion shell model predicts an inverse relation between petrologic type and metallographic cooling rate but none has been observed scott and rajan 1981 devised a metamorphosed planetesimal model to explain this discrepancy whereby metamorphism occurs in planetesimals a few kilometers in radius which then accrete to form r 100 km parent bodies metallographic cooling rates are then controlled by burial depth thermal and collisional constraints on the metamorphosed planetesimal model are examined here and the model is found to be applicable only to highly insulating 26al rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface an alternative model is presented here in which onion shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled if reassembly times are short days then metallographic cooling rates would be determined by burial depth in the reaccreted parent body this model unlike previous ones can explain both coherent and incoherent cooling rates of breccia clasts by collisions during or after metamorphism respectively

regional magnetic overprinting of witwatersrand supergroup sediments south africa shales from the late archean witwatersrand supergroup of south africa yield stable wellgrouped remanent magnetizations after thermal and alternating field demagnetization virtual geomagnetic poles from 17 sites located throughout the central and western parts of the basin indicate that rocks of the basin were remagnetized after folding a paleomagnetic fold test shows that the fisherian precision parameter k decreases from 22 to 3 after restoring the beds to paleohorizontal illite crystallinity indices show that the remagnetized samples were subjected to at least middle greenschist facies temperatures whereas magnetic blocking temperature estimates indicate upper greenschist facies temperatures a k ar age obtained from clays of 1945 40 ma 2 is assigned to this metamorphic event the new paleomagnetic pole obtained from the witwatersrand basin sites 19 n 46 e k 22 a95 8 is consistent with other 1950 2050 ma poles from southern africa confirming independently the k ar age the distinctive overprint direction seen in shale specimens may be useful as a directional marker for orienting deep bore cores

comment on 40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology by paul r renne

pyrite zoning as a record of mineralization in the ventersdorp contact reef witwatersrand basin south africa the archean metasedimentary succession of the witwatersrand basin south africa hosts the largest au deposit in the world gold mineralization is mostly concentrated in conglomerate horizons or reefs and is tightly associated with pyrite trace element zoning of pyrite from the ventersdorp contact reef studied by x ray elemental as ni co and pb maps electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry indicates successive stages of pyrite formation each characterized by different textures and trace element composition as 2 2 wt ni 1 37 wt co 1 98 wt four generations have been distinguished generation 1 is detrital and includes compact nonporous porous and laminated pyrite generations 2 to 4 are postsedimentary authigenic generation 4 pyrite formed at near peak metamorphic conditions t 270 350 c chlorite geothermometry porous and concentrically laminated pyrite grains generation 1 are particularly enriched in au average 6 4 ppm maximum 70 ppm in addition to sb tl pb mn mo cu and ag in comparison with compact pyrite types of all generations in these grains au occurring as invisible gold and other trace elements might be finely dispersed with the phyllosilicates filling the pyrite pores trace element composition of porous and concentrically laminated pyrite is reminiscent of pyrite known to form in suboxic to anoxic environments black shales the presence of au in detrital pyrite indicates an early introduction of au in the ventersdorp contact reef gold is also present as secondary inclusions of electrum associated with the last pyrite generation generation 4 together with sphalerite chalcopyrite galena and pyrrhotite

an analytical electron microscopy aem investigation of opaque inclusions in some type 6 ordinary chondrites abstract a large number of ordinary chondrites contains micron sized particles of metal and or troilite dispersed in their silicate grains such metallic phases are responsible for the so called darkening of the silicate grains and might be either precipitates which formed during reduction of the silicates or inclusions injected as a melt during a shock event we have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered metamorphosed ordinary chondrites saint severin ll6 tsarev l6 and kernouve h6 we also looked for remnant shock indices our tem observations suggest the following sequence of events in the three meteorites first a number of relatively strong shock events occurred on the parent body bodies producing an fe fes melt that was injected into silicate grains along a dense network of open fractures most of these shock defects were subsequently erased by high temperature 700 900 c thermal metamorphism some remnants of the shock events are the observed trails of tiny metal and or sulfide inclusions that formed as a result of fracture healing chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high temperature annealing event resulting in ni rich inclusions this effect was especially pronounced in the l and ll chondrites studied during subsequent cooling of the body bodies inclusions of chromite and phosphate precipitated nucleating preferentially on lattice defects dislocations subgrain boundaries and on the metal and sulfide inclusions a later shock event of moderate intensity probably corresponding to the separation of the meteorite from its parent body produced new shock features in the silicate grains of the saint severin meteorite including mechanical twins in diopside and straight free screw dislocations in olivine

below surface ice melt on the coastal antarctic ice sheet in the jutulgryta area of dronning maud land antarctica subsurface melting of the ice sheet has been observed the melting takes place during the summer months in blue ice areas under conditions of below freezing air and surface temperatures adjacent snow covered regions having the same meteorological and climatic conditions experience little or no subsurface melting to help explain and understand the observed melt rate differences in the blue ice and snow covered areas a physically based numerical model of the coupled atmosphere radiation snow and blue ice system has been developed the model comprises a heat transfer equation which includes a spectrally dependent solar radiation source term the penetration of radiation into the snow and blue ice depends on the solar radiation spectrum the surface albedo and the snow and blue ice grain sizes and densities in addition the model uses a complete surface energy balance to define the surface boundary conditions it is run over the full annual cycle simulating temperature profiles and melting and freezing quantities throughout the summer and winter seasons the model is driven and validated using field observations collected during the norwegian antarctic research expedition nare 1996 97 the simulations suggest that the observed differences between subsurface snow and blue ice melting can be explained largely by radiative and heat transfer interactions resulting from differences in albedo grain size and density between the two mediums

a multiple sulfur and organic carbon isotope record from non conglomeratic sedimentary rocks of the mesoarchean witwatersrand supergroup south africa abstract multiple sulfur isotope ratios 36 s 34 s 33 s 32 s and organic carbon isotope ratios 13 c 12 c were measured from 198 non conglomeratic sedimentary samples that were collected from five deep diamond drill cores that cover the majority of the ca 2 96 2 82 ga witwatersrand supergroup 13 c org 34 s py and 33 s py values of the sample set range from 44 3 to 21 9 3 7 to 16 5 and 4 0 to 1 9 respectively these geochemical data vary relative to depositional facies proximal marine distal marine and fluvial dominated and tectonic setting trailing margin and foreland basin in the trailing margin setting of the 2 96 ga hospital hill subgroup the proximal marine depofacies is characterized by relatively high organic carbon contents up to 0 9 wt and 13 c org values around 28 in contrast to the distal marine depofacies that yields low organic carbon contents 0 01 wt and high 13 c org values up to 22 both depofacies yield low sulfur contents 0 02 wt a narrow range of 34 s values 3 2 and positive 33 s values up to 1 9 this data is consistent with photoautotrophic carbon fixation in shallow marine environments and limited organic carbon production preservation in deeper water settings due to longer transport distances and effective biological degradation e g fe reducing microbes positive 33 s values imply that sulfur was largely derived from a photochemical elemental sulfur reservoir in the foreland basin tectonic setting of the 2 94 ga government and 2 92 ga jeppestown subgroups shelf deposits associated with fluvial braidplain depofacies are characterized by 13 c depleted organic carbon 44 to 38 relatively high sulfur contents 0 2 1 3 wt variable 34 s values 3 7 to 16 5 and small negative 33 s values 0 4 these data suggest that the microbial community in the fluvial dominated depofacies may have consisted of photoautotrophs methanogens anaerobic methanotrophs and sulfate reducers sulfate was derived from a variety of sources that include photochemical crustal and marine sulfur reservoirs the occurrence of three regionally persistent diamictite deposits in the government subgroup coincides with the onset of compressional tectonics and development of pyritic shales with highly 13 c depleted organic carbon suggesting that an increase in continental sulfur flux and methane oxidation may have triggered the mesoarchean glaciations drawdown of h 2 and ch 4 however the link between large 33 s anomalies 4 0 to 1 2 and diamictite suggests low levels of atmospheric oxygen and minimal dilution of photochemical signatures

studies of snow surface characteristics by landsat tm in dronning maud land antarctica large scale melting phenomena like meltwater drainage channels and meltwater accumulation basins or frozen lakes have been surveyed on the land ice mass in dronning maud land antarctica these melting features were also detected in a landsat thematic mapper tm image recorded on 12 february 1990 image processing techniques such as principal component pc analysis band ratioing and histogram equalizing are carried out to emphasize the melting phenomena interestingly a histogram equalized single tm band 5 image appears to be a good discriminator leaving the melting phenomena as bright areas in the tm scene the largest frozen lake is close to 1km wide while some of the drainage channels stretch more than 5 km there is also a comparison of satellite derived and in situ reflectance and temperature the atmospheric corrected satellite data underestimate surface snow albedo by about 16 while tm measurements of surface temperature are 2 to 6 c lower than that measured on the ground

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

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

shock effects and argon loss in samples of the leedey l6 chondrite experimentally shocked to 29 70 ga pressures abstract shock recovery experiments between 29 and 70 gpa were made on the leedey l6 chondrite to determine the possible loss of radiogenic ar caused directly by passage of shock waves as opposed to purely thermal loss during prolonged post shock cooling experiments were made in vacuum to prevent shock implantation of terrestrial air and following neutron irradiation the 39 ar 40 ar ratios were measured by stepwise temperature release petrographie examination of experimentally shocked leedey showed predominantly solid state effects in the form of increasing disaggregation of component minerals with increasing shock pressure but very little melting even at the highest shock levels the melting behavior of the minor feldspar phase was difficult to document but likely was complete by the higher shock levels the 39ar 40ar release patterns were complex for both shocked and reference leedey probably because of a natural shock history for the meteorite at ca 3 8 gy the average 39ar 40ar age for shocked leedey decreased with increasing shock levels from essentially no change at 29 gpa to a 20 decrease in age at 60 gpa the feldspar phases degas at lower extraction temperatures and show even greater lowering of 39ar 40ar age with increasing shock pressure the sample shocked at 69 gpa shows less mechanical disruption than samples shocked to lesser pressures and also shows a smaller loss of ar compared to samples shocked at 45 and 60 gpa we conclude that observed ar loss is caused by extensive grain fracturing and heating during shock and that greater ar loss at low extraction temperatures is enhanced by feldspar melting none of the experiments duplicated the magnitude of ar loss commonly seen in naturally shocked materials that lack evidence of pervasive melting we conclude that large scale gas loss in shocked but unmelted samples requires that the materials reside for prolonged periods of time at elevated temperatures in a relatively hot impact formation either inside or outside the crater cavity

blue ice areas in antarctica derived from noaa avhrr satellite data we have mapped antarctic blue ice areas using the u s national oceanic and atmospheric administration noaa advanced very high resolution radiometer avhrr antarctica cloud free image mosaic established by the united states geological survey the mosaic consists of 38 scenes acquired from 1980 to 1994 our results show that approximately 60 000 km2 of blue ice exist for each of the two main types of blue ice melt induced and wind induced normally the former type is located on slopes in coastal areas where climate conditions i e persistent winds and temperature together with favourable surface orientation sustain conditions for surface and near surface melt the latter blue ice category occurs near mountains or on outlet glaciers often at higher elevations where persistent winds erode snow away year round and combined with sublimation creates areas of net ablation furthermore we have identified an additional area of 121 000 km2 as having potential for blue ice however in these areas features such as mixed pixels glazed snow surfaces crevasses and or shadows make interpretation more uncertain in conclusion a conservative estimate of antarctic blue ice area coverage by this method is found to be 120 000 km2 0 8 of the antarctic continent with a potential maximum of 241 000 km2 1 6 of the antarctic continent

kaapvaal craton changing first and second order controls on sea level from c 3 0 ga to 2 0 ga abstract well preserved supracrustal basin fills including the oldest large depository witwatersrand known makes the kaapvaal craton an ideal example for studying controls on first and second order stratigraphic cyclicity the duration of the three basins studied witwatersrand ventersdorp and transvaal from c 3 0 to c 2 0 ga is almost twice that of the entire phanerozoic period from which the paradigms of classical sequence stratigraphy have been derived amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through earth history this does not appear to be borne out by the precambrian record studied here instead a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first and second orders is preferred first order cycles within the c 3 0 2 0 ga period on kaapvaal are synonymous with the entire evolution of a supracrustal basin fill defined by a specific basin type second order cycles define basin fill packages of unique character which reflect large scale shifts in the accommodation sedimentation balance early to middle archaean evolution of the kaapvaal craton encompassed amalgamation of a mosaic of subdomains to produce an early south southeastern nucleus by c 3 1 ga accretion of composite terranes from both north and west followed from c 3 0 2 7 ga this was contemporaneous with evolution of the c 3 0 2 8 ga witwatersrand basin the high grade limpopo terrane was amalgamated with the kaapvaal craton between 2 95 and 2 69 ga although a younger age for this orogensis is also possible the witwatersrand first order depositional cycle was controlled largely by subduction related tectonic loading and had a duration of c 190 my the c 5 my first order cycle of the c 2 7 ga ventersdorp basin was essentially controlled by thermal uplift induced extensional subsidence and that of the c 650 my transvaal basin by extensional and subsequent thermal subsidence it is noteworthy in the three basins examined that first order cycles controlled by plate tectonics witwatersrand and transvaal basins were about two orders of magnitude longer relative to the ventersdorp plume tectonic cycle second order cycles also varied greatly from c 100 my to c 1 my within the witwatersrand and transvaal basins subsidence allowed extensive seaways to develop within which classical sequence stratigraphic systems tracts can be recognized reflecting different stages of shoreline shifts within the short lived ventersdorp continental basin system sedimentological criteria can be used to discriminate high and low accommodation systems tracts the transvaal first order cycle was terminated at c 2 0 ga by the bushveld complex plume event

measuring snow and glacier ice properties from satellite satellite remote sensing is a convenient tool for studying snow and glacier ice allowing us to conduct research over large and otherwise inaccessible areas this paper reviews various methods for measuring snow and glacier ice properties with satellite remote sensing these methods have been improving with the use of new satellite sensors like the synthetic aperture radar sar during the last decade leading to the development of new and powerful methods such as sar interferometry for glacier velocity digital elevation model generation of ice sheets or snow cover mapping some methods still try to overcome the limitations of present sensors but future satellites will have much increased capability for example the ability to measure the whole optical spectrum or sar sensors with multiple polarization or frequencies among the methods presented are the satellite derived determination of surface albedo snow extent snow volume snow grain size surface temperature glacier facies glacier velocities glacier extent and ice sheet topography in this review emphasis is put on the principles and theory of each satellite remote sensing method an extensive list of references with an emphasis on studies from the 1990s allows the reader to delve into specific topics

alumina rich rocks from the early precambrian of the kaapvaal craton as indicators of paleosols and as products of other decompositional reactions within early archaean to early proterozoic pelites and volcanics of the kaapvaal craton rocks with above 20 al2o3 are widely distributed the alumina rich nature of these rocks is ascribed to decompositional reactions during which up to about 60 of the original material was leached out of the source rock and removed in solution many of these rocks also show diagenetic enrichment in k2o of up to 10 the decomposition reactions in many cases affected the source rocks without disturbing fine structures such as perlitic devitrification cracks in volcanic glass granules the origin of the majority of these rocks can be ascribed to decomposition during subaerial weathering they represent paleosols for other occurrences the stratigraphic context rules out an explanation as paleosols the origin of this type is ascribed to subaqueous decomposition of lavas or tuffs assisted by hydrothermal exhalations it is shown that alumina rich rocks in greenschist belts almost exclusively formed subaqueously whereas the younger occurrences mostly are of weathering origin geotectonic differences are invoked to explain this contrast rocks of similar chemical composition are described from the canadian shield zimbabwe western australia india and the baltic shield they illustrate that the preservation potential of these rocks is high only the study of the stratigraphic context will allow a conclusion on whether a particular occurrence is the product of weathering or subaqueous decomposition

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

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

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

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

antarctic surface and subsurface snow and ice melt fluxes abstract this paper presents modeled surface and subsurface melt fluxes across near coastal antarctica simulations were performed using a physical based energy balance model developed in conjunction with detailed field measurements in a mixed snow and blue ice area of dronning maud land antarctica the model was combined with a satellite derived map of antarctic snow and blue ice areas 10 yr 1991 2000 of antarctic meteorological station data and a high resolution meteorological distribution model to provide daily simulated melt values on a 1 km grid covering antarctica model simulations showed that 11 8 and 21 6 of the antarctic continent experienced surface and subsurface melt respectively in addition the simulations produced 10 yr averaged subsurface meltwater production fluxes of 316 5 and 57 4 km3 yr 1 for snow covered and blue ice areas respectively the corresponding figures for surface melt were 46 0 and 2 0 km3 yr 1 respectively thus demonstrating the dominant role of subsurface ov

characterization of mason gully h5 the second recovered fall from the desert fireball network mason gully the second meteorite recovered using the desert fireball network dfn is characterized using petrography mineralogy oxygen isotopes bulk chemistry and physical properties geochemical data are consistent with its classification as an h5 ordinary chondrite several properties distinguish it from most other h chondrites its 10 7 porosity is predominantly macroscopic present as intergranular void spaces rather than microscopic cracks modal mineralogy determined via ps xrd element mapping via energy dispersive spectroscopy eds and x ray tomography for sulfide metal and porosity volume fractions consistently gives an unusually low olivine orthopyroxene ratio 0 67 0 76 for mason gully versus 1 3 for typical h5 ordinary chondrites widespread silicate darkening is observed in addition it contains a bright green crystalline object at the surface of the recovered stone diameter 1 5 mm which has a tridymite core with minor quartz and a rim of both low and high ca pyroxene the mineralogy allows the calculation of the temperatures and o2 characterizing thermal metamorphism on the parent body using both the two pyroxene and the olivine chromite geo oxybarometers these indicate that mg experienced a peak metamorphic temperature of 900 c and had a similar o2 to kernouve h6 that was buffered by the reaction between olivine metal and pyroxene there is no evidence for shock consistent with the observed porosity structure thus while mason gully has some unique properties its geochemistry indicates a similar thermal evolution to other h chondrites the presence of tridymite while rare is seen in other ocs and likely exogenous however the green object itself may result from metamorphism

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

noble gas retention chronologies for the st severin meteorite abstract 40 ar 39 ar and 129 xe 128 xe analyses were performed on two lithologies light and dark of the st severin ll6 chondrite for the light and dark fractions respectively we obtained 40 ar retention ages of 4 38 and 4 42 ae and 129 xe retention ages of 8 4 and 15 2 myr after bjurbole the two methods give age differences of opposite sense and by both methods the differences are significant both the 40 ar and the 129 xe ages are interpreted as dating relaxation of metamorphic conditions these two chronometers are decoupled however and do not date the same events 40 ar 39 ar reflect chondritic metamorphisrn on a 10 8 yr time scale the 129 xe 128 xe ages reflect isotopic closure at higher temperatures and earlier times

antarctica measuring glacier velocity from satellite images many landsat images of antarctica show distinctive flow and crevasse features in the floating part of ice streams and outlet glaciers immediately below their grounding zones some of the features which move with the glacier or ice stream remain visible over many years and thus allow time lapse measurements of ice velocities measurements taken from landsat images of features on byrd glacier agree well with detailed ground and aerial observations the satellite image technique thus offers a rapid and cost effective method of obtaining average velocities to a first order of accuracy of many ice streams and outlet glaciers near their termini

origin of thick first cycle quartz arenite successions evidence from the 1 7 ga ortega group northern new mexico abstract the 1700 ma old ortega group in northern new mexico represents a 1000 m thick succession of metamorphosed quartz arenites and subordinate pelites published ages of detrital zircon grains from the ortega group indicate that the sediments were derived mainly from 1760 1700 ma old volcano plutonic basement rocks and not from the archean wyoming province to the north evidence for recycling within the ortega group also is lacking consequently the ortega group quartz arenites are considered to be first cycle sediments the compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering abnormally high al contents of pelites within the ortega group imply intense chemical weathering in the provenance similar to present day low relief tropical regions quartz arenites in the ortega group were deposited exclusively within a shallow marine shelf environment influenced by tide storm and wave processes prolonged sediment transport especially on the tide dominated inner shelf in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains although the maturity of sediments in the ortega group can be accounted for the mode of origin of 850 m of quartz arenites remains enigmatic thick quartz arenite sequences of global distribution in the phanerozoic coincide with first order eustatic sea level rises world wide occurrences of 1700 ma old thick quartz arenite shelf successions suggest that the ortega group may have accumulated in response to a similar major eustatic sea level rise

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

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

evidence of tidal processes from the lower part of the witwatersrand supergroup south africa abstract a 1600 m succession of quartz arenites and associated shaley deposits comprising the hospital hill subgroup at the base of the witwatersrand supergroup is considered to have been deposited largely under the influence of tidal processes facies analysis indicates that deposition occurred in the following environments 1 marine shalf 2 shallow subtidal to intertidal 3 intertidal flat and 4 tidal inlet the presence of strong tidal currents implies that the witwatersrand basin was open to an ocean basin at least during the early stages of its evolution palaeocurrent trends and isopach data suggest that this probably lay to the southwest an area now occupied by the high grade natal namaqua metamorphic belt the contrast between the supermature quartz arenites of the hospital hill subgroup and the overlying gold bearing immature subgreywackes feldspathic quartzites and conglomerates of fluvial origin is believed to be a function of tidal reworking of sediments

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

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

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

an impact origin for the foliation of chondrites abstract the origin of the foliation of ordinary chondrites is addressed through the study of the anisotropy of magnetic susceptibility ams of these meteorites based on a database of 295 different meteorites including 35 achondrites hed and snc ams is a reliable proxy for preferred orientation in ordinary chondrites moreover the intensity of magnetic anisotropy is constant over a given chondrite the intensity of l chondrite petrofabric evaluated by means of ams measurements and shock stage determined independently by microscopic observation of shock features in silicates appear to be positively correlated this suggests that the oblate petrofabric in l chondrites is shock induced the inverse correlation between porosity and ams intensity suggests that hypervelocity impacts compacted and lithified an originally loose material causing the deformation of metallic fe ni grains ll and h ordinary chondrites cannot be studied as easily as l chondrites tetrataenite in ll chondrites affects the ams in an unpredictable way and shape anisotropy due to the strong susceptibility of h chondrites imposes strong constraints on sample shape our limited dataset for carbonaceous chondrites is compatible with a shock induced foliation model the very low degrees of ams in rumuruti chondrites testify that surprisingly the easily deformable fe sulfides in these chondrites show almost no deformation suggesting that sulfidation is a secondary phenomenon that occurred after the major impacts on the rumuruti parent body the weak magnetic anisotropy of martian meteorites implies that no significant macroscopic deformation took place during the severe impacts suffered by these rocks which is explained by the weak initial porosity and high compressive strength of these igneous products

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

fall recovery and characterization of the novato l6 chondrite breccia the novato l6 chondrite fragmental breccia fell in california on 17 october 2012 and was recovered after the cameras for allsky meteor surveillance cams project determined the meteor s trajectory between 95 and 46 km altitude the final fragmentation from 42 to 22 km altitude was exceptionally well documented by digital photographs the first sample was recovered before rain hit the area first results from a consortium study of the meteorite s characterization cosmogenic and radiogenic nuclides origin and conditions of the fall are presented some meteorites did not retain fusion crust and show evidence of spallation before entry the meteoroid was 35 5 cm in diameter mass 80 35 kg with a cosmic ray exposure age of 9 1 ma if it had a one stage exposure history a two stage exposure history is more likely with lower shielding in the last few ma thermoluminescence data suggest a collision event within the last similar to 0 1 ma novato probably belonged to the class of shocked l chondrites that have a common shock age of 470 ma based on the u th he age of 420 220 ma the measured orbits of novato jesenice and innisfree are consistent with a proposed origin of these shocked l chondrites in the gefion asteroid family perhaps directly via the 5 2 mean motion resonance with jupiter novato experienced a stronger compaction than did other l6 chondrites of shock stage s4 despite this a freshly broken surface shows a wide range of organic compounds less

precambrian continental freeboard and geological evolution a time perspective abstract continental crustal growth rates crustal volumes thicknesses and continental freeboard are considered to be intimately related however none are easy to quantify least of all in the precambrian additionally crustal volumes thicknesses and isostasy are related to changing mantle heat and the concomitant variation in thickness of ocean crust another set of variables would be the interaction of plate tectonics and the onset of plate tectonics a contentious issue in itself and mantle plumes superplumes further complication is provided by the ranges of freeboard elevations and eustatic sea level changes overlapping in scale and sharing common genetic causes the constant freeboard model suggests that the net results of the interaction of this complex set of variables has been that continental elevation relative to sea level has remained essentially similar on a global average basis over much of geological time assuming an essentially constant ocean volume since c 3 8 ga application of this model to the precambrian geological record suggests that sigmoidal crustal growth rate models are probably the most reliable a conundrum of the constant freeboard model would be that freeboard would tend to become lowered towards mean sea level for any specific craton over time this freeboard equilibrium would be interrupted for a particular craton by active geodynamic processes resulting from the interplay of plate tectonics and mantle thermal instabilities in this paper we test the validity of this thesis of alternating approximate freeboard equilibrium and active geodynamics by examining the volcano sedimentary record of a number of cratons kaapvaal singhbhum and pilbara for the singhbhum craton and bearing in mind its poor chronological data base freeboard equilibrium appears to have prevailed until c 2 1 ga but for kaapvaal and pilbara until c 1 8 ga the geodynamic processes largely outweighed the tendency towards equilibrium except for c 200 my from c 2 6 to 2 4 ga when global sea level appears to have remained high and transgression was the norm we thus tentatively add the plate tectonic cycle and mantle thermal processes to the already complex association of concepts controlling continental freeboard over time

the crustal architecture of the kaapvaal crustal block south africa between 3 5 and 2 0 ga following terrane amalgamation of early oceanic lithosphere the southern and central parts of the kaapvaal craton were a coherent unit by 3 1 ga juxta position of the northern and western granitoid greenstone terranes including the murchison island arc was the result of terrane accretion that started at 3 1 ga the culmination of these events was the collision of the kaapvaal craton the pre cratonic zimbabwe block and the central zone to generate the limpopo granulite gneiss terrane coeval with these orogenic events the central kaapvaal craton underwent extension to accommodate the development of the dominion witwatersrand pongola and ventersdorp basins the craton scale thabazimbi murchison lineament development during the 3 1 ga accretion event and continued to influence the tectonic evolution of the kaapvaal block throughout the period under review as indicated by the syn sedimentary tectonics of the 2 64 ga wolkberg group overlying black reef formation and the transvaal sequence the transvaal and griqualand west basins developed in the late archaean 2 55 ga with basin dynamics influenced by far field stresses related to the limpopo orogeny during this period the thabazimbi murchison lineament lay close to the northern margin of the depository reactivation of the lineament between 2 4 and 2 2 ga resulted in inversion of the transvaal basin and formation of the northward verging mhlapitsi fold and thrust belt the half graben setting envisaged for the deposition of the pretoria group was influenced by the thabazimbi murchison lineament as was the emplacement and subsequent deformation of the bushveld complex

spade an h chondrite impact melt breccia that experienced post shock annealing the low modal abundances of relict chondrules 1 8 vol and of coarse i e 200 m size isolated mafic silicate grains 1 8 vol in spade relative to mean h6 chondrites 11 4 and 9 8 vol respectively show spade to be a rock that has experienced a significant degree of melting various petrographic features e g chromite plagioclase assemblages chromite veinlets silicate darkening indicate that melting was caused by shock plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains during crystallization plagioclase developed igneous zoning low ca pyroxene that crystallized from the melt or equilibrated with the melt at high temperatures acquired relatively high amounts of cao metallic fe ni cooled rapidly below the fe ni solvus and transformed into martensite subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite ambiguities exist in the shock stage assignment of spade the extensive silicate darkening the occurrence of chromite plagioclase assemblages and the impact melted characteristics of spade are consistent with shock stage s6 low shock stage s2 is indicated by the undulose extinction and lack of planar fractures in olivine this suggests that spade reached a maximum prior shock level equivalent to stage s6 and then experienced post shock annealing probably to stage s1 these events were followed by a less intense impact that produced the undulose extinction in the olivine characteristic of shock stage s2 annealing could have occurred if spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket spade firmly establishes the case for post shock annealing this may have been a common process on oc asteroids

alternative model for the derivation of gold in the witwatersrand supergroup present models of the derivation of the gold in the witwatersrand conglomerates of south africa as detrital grains directly from primary deposits in a source area consisting mainly of archaean schist belts meets with a volume problem witwatersrand deposits have yielded about 923 kg km 2 au compared to about 65 kg km 2 au in the richest known schist belts differences in the extent of mining activities cannot account for these differences in yield gold fineness grain size and morphology of the gold particles are also difficult to reconcile with a purely detrital derivation data on the mobility of gold in the hydrosphere and in the weathering cycle are used to erect a model which predicts that most of the gold in the witwatersrand conglomerates was derived from a source area in trace amounts in a 9dissolved9 form and precipitated at or close to the edge of the depositional basin possibly under organic influences subsequent sedimentary reworking and metamorphic diagenetic remobilization led to the formation of the complex association of minerals observed now in the conglomerates

structural deformation of the leoville chondrite foliations defined by alignment of elongated chondrules have been noted previously in chondrites but none displays this effect so well as leoville cv3 the shapes of leoville chondrules were produced by deformation in situ as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone 1 9 and 2 0 by several methods and for the whole meteorite 2 0 indicate either that leoville deformed homogeneously or that it deformed as a framework of touching chondrules this amount of strain corresponds to approximately 33 uniaxial shortening assuming constant volume because the strain ellipse was measured in only one orientation this strain value is a minimum estimate for the meteorite lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause estimates of maximum deviatoric stresses in the interiors of asteroid sized bodies and constraints on maximum temperatures for cv3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules diffusional flow is also suggested by the scarcity of observed lattice dislocations deformation of leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite 600 c however differences in olivine grain size the presence of water or a more complex deformation history might explain this discrepancy

a naive zero dimensional sea ice model a single observation of the microwave brightness temperature over sea ice cannot determine the composition of the ice surface in terms of the three constituents open water first year ice and multiyear ice the ambiguity can be removed by interpreting the time series of brightness temperatures at a fixed location in the context of a model of how the composition evolves in time a model is discussed which includes the effects of ice growth melting deformation and aging the aging process accounts for the change of first year ice into multiyear ice at the beginning of the cold season the model reproduces the annual cycle of brightness temperature a procedure based on kalman filtering is presented for estimating the fractional areas of the three surface types the procedure combines the model of ice evolution with the time series of brightness temperatures the estimation errors are in the range 2 12 these methods can be used with existing passive microwave data to construct a climatology of sea ice based on the three type classification it is also possible to use the observations to improve model parameters such as the rate of melting and the rate of creation of open water by mechanical processes

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

a terrestrial shallow marine transition in the archean opemisca group east of chapais quebec abstract the stella and hauy formations of the archean opemisca group in the eastern chapais syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence during the first depositional phase stella formation relatively high gradient alluvial fans prograde into a shallow marine environment the coarse clastic sediments reflect a scott type depositional environment the initial depositional environment is envisaged to be a high energy wave and storm flood dominated coastline bordered by coastal alluvial fans shallow marine deposits consist of coarse to very coarse grained sandstones and intraclast and pebble to cobble conglomerates the prevalent shallow marine sediments are related to storm and or flood deposition terrigenous sediments of the donjek type depositional environment prevail during the second depositional phase hauy formation the shallow marine sediments although still wave and storm flood dominated are finergrained and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline furthermore contemporaneous volcanism of andesitic shoshonitic to basaltic composition occurs during this evolutionary stage the sedimentary basin gradually evolved during the two depositional phases the repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence coarsening and fining sequences in the conglomerates and the alluvial fan setting strongly suggest a fault bounded basin margin tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf whereas quiescent periods are documented by marine transgression the clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts the synvolcanic chibougamau pluton and the lake dore complex acted as a stable basement during deposition of the opemisca group the small landmass served as a stable platform for shallow marine deposition even when tectonically influenced a late phase intraplutonic cratonic development of an initial backarc basin system internal zone of the abitibi greenstone belt is proposed as the overall depositional setting of the chapais basin

density magnetic susceptibility and the characterization of ordinary chondrite falls and showers bulk and grain densities of 132 ordinary chondrites from the vatican observatory collection were measured and compared with their magnetic susceptibility for the most part using previously measured values ten new susceptibility measures were taken for this study grain density and magnetic susceptibility combined provide a reliable method of classifying un weathered ordinary chondrites unlike traditional chemical tests this method is fast nondestructive and characterizes the whole rock making it especially appropriate for surveying large collections the system is less viable for finds extensive weathering of metallic iron in an h chondrite can cause it to plot among l chondrites while heavily weathered l chondrites plot among the ll group this system has revealed outlier stones that may be misclassified meteorites or mislabeled samples in every case where the magnetic susceptibility of a meteorite does not fit its putative classification the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification an analysis of stones from five showers shows that excluding outliers these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility

shocked meteorites argon 40 argon 39 evidence for multiple impacts to contribute to the understanding of the impact history of asteroids we performed a high resolution 40ar 39ar study of ten moderately to highly shocked chondrites which we selected according to the shock classification given by stoffler et al 1991 two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite when possible we separated impact melt from host rock for separate analysis in total we studied 28 samples from 14 meteorites in some cases atmospheric ar that we associate with terrestrial weathering was identified and corrected for the ages we obtained range between 100 ma and 4 1 ga and are clearly distinct from primordial ages that correspond to solar system formation we reproduced the previously reported cluster of l chondrite ages 500 ma the most prominent result of our study is that in the case of chondrites melts generally are older than host rocks or melt embedded unmolten rocks to solve this apparent paradox we propose that the melt forming event which was the most severe shock episode in the history of these meteorites has not been the only occasion affecting their k ar systems at least one later impact metamorphism must have occured the response of the k ar clock to this second event was more severe in the host rock than in the previously in the first event generated melt veins and pockets because of different ar retention rates hence impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years

spectral signature of alpine snow cover from the landsat thematic mapper mapping of snow and estimation of snow characteristics from satellite remote sensing data require that we distinguish snow from other surface cover and from clouds and compensate for the effects of the atmosphere and rugged terrain the spectral signature of the snowpack is calculated from a radiative transfer model accounting for scattering and absorption by the ice grains water inclusions and particulates in interpreting the spectral reflectance measured from the thematic mapper or from any other satellite we need to account for topographic effects without requiring that satellite data be precisely registered to digital elevation data because the poor quality of most digital elevation data introduces considerable noise into calculations of slope and azimuth for a range of snow types atmospheric profiles and topographic illumination conditions i estimate typical spectral signatures for the landsat thematic mapper tm images of the southern sierra nevada are analyzed to distinguish several classes of snow from other surface covers snow can be reliably distinguished from other surfaces and from clouds at all sun angles encountered in the mid latitudes in tm band 1 or 2 snow is brighter than other natural surfaces and in band 5 clouds are usually much more reflective than snow therefore the intersection of the following criteria maps the snow cover planetary reflectance rp in tm band 1 is greater than about 0 16 rp tm5 is less than about 0 2 the normalized band 2 5 difference rp tm2 rp tm5 rp tm2 rp tm5 is greater than about 0 4 large surface grain sizes can be distinguished from areas where the grain size is finer at the snow surface using tm bands 2 4 and 5 because of saturation in tm band 1 estimation of the degree of contamination by absorbing aerosols is not feasible rp tm2 rp tm5 rp tm2 rp tm5 is

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

fluvial coastal nearshore and shelf deposition in the upper proterozoic to lower cambrian addy quartzite northeastern washington abstract quartzose and lesser argillaceous strata of the approximately 1300 m thick upper proterozoic to lower cambrian addy quartzite of northeastern washington record shelf coastal and continental sedimentation on an actively subsiding relatively young passive margin the lower 400 to 500 m of the addy quartzite was deposited immediately following the initiation of subsidence in a wave dominated coastal and eventually storm dominated shelf environment this sequence records relative sea level rise that culminated in high stand conditions these sediments grade upwards into approximately 150 m of fluvial strata that record progradation of terrestrial environments during a period of relative sea level fall and a regional low stand the upper 700 m of the addy quartzite fines upwards from nearshore to shallow marine dominated sediments into shelf dominated deposits this sequence marks the reestablishment of net sea level rise the overall fining upwards trend displayed by the addy quartzite indicates that regional subsidence of the newly formed passive margin was one of the most important factors controlling addy sedimentation however variation seen in this trend throughout the addy quartzite indicates that rates of sea level rise associated with subsidence fluctuated and occasionally even reversed causes for the variation are inferred to be related to variations in subsidence rates and sediment supply textural and compositional data suggest addy detritus was derived from relatively proximal recycled sedimentary source terranes and more distal continental crystalline terranes

palaeoproterozoic sedimentation on the singhbhum craton global context and comparison with kaapvaal the singhbhum craton has a limited palaeoproterozoic supracrustal record which suggests a three part history comprising a long period of high freeboard and palaeosol formation on granitoids subsequent rift related mafic ultramafic volcanism and subordinate sedimentation c 2 25 2 1 ga dhanjori and jagannathpur basin fills possibly also simlipal malangtoli and ongarbira basin fills which overlapped locally with mafic soil formation and a major regression at around2 0 ga followinga longhiatus theapproximately 1 6 gadhalbhum dalmasuccession was laid down probably under continental conditions this rather truncated record stands in con trast to the chronologically and geographically much more widespread supracrustal basin fills of the kaapvaal craton and there appears to be an overall poor comparison between these two early precambrian crustal blocks however on kaapvaal three analogous events are identified widespread approximately 2 2 ga mafic volcanism followed by a well developed palaeosol and a major transgression prior to 2 05 ga the three shared events between the two cratons are com patible with the postulate of a global approximate 2 45 2 2 ga shutdown of magmatic and tec tonic geodynamics with the origin of the triumvirate directly reflecting its resumption again after about 2 2 ga we recognize here that a large diversity of views on singhbhum s geodynamic history exists predicated on a lack of precise geochronology and commonly poor outcrops and the current hypotheses are presented with these factors in mind

snow dunes and glazed surfaces in antarctica new field and remote sensing data as part of the international trans antarctic scientific expedition project the italian antarctic programme undertook two traverses from the terra nova station to talos dome and to dome c along the traverses the party carried out several tasks drilling glaciological and geophysical exploration the difference in spectral response between glazed surfaces and snow makes it simple to identify these areas on visible near infrared satellite images integration of field observation and remotely sensed data allows the description of different mega morphologic features wide glazed surfaces sastrugi glazed surface fields transverse dunes and megadunes topography global positioning system ground penetrating radar and detailed snow surface surveys have been carried out providing new information about the formation and evolution of mega morphologic features the extensive presence up to 30 of glazed surface caused by a long hiatus in accumulation with an accumulation rate of nil or slightly negative has a significant impact on the surface mass balance of a wide area of the interior part of east antarctica the aeolian processes creating these features have important implications for the selection of optimum sites for ice coring because orographic variations of even a few metres per kilometre have a significant impact on the snow accumulation process remote sensing surveys of aeolian macro morphology provide a proven high quality method for detailed mapping of the interior of the ice sheet s prevalent wind direction and could provide a relative indication of wind intensity

simulation experiments for shocked primitive materials in the solar system abstract shock recovery experiments were conducted on porous mixtures initial porosity 35 5 in silicate metal and silicate metal sulfide systems in an attempt to simulate impact phenomena of unconsolidated porous materials similar to ordinary chondrites in a wide range of shock pressures from 20 to 70 gpa the textures and chemical compositions of shocked samples were investigated in detail features such as grain deformation fracture density decrease and heterogeneous meltings are found at

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

reflectances of glaciers as calculated using landsat 5 thematic mapper data landsat 5 thematic mapper tm data have been used in previous work in order to locate zones of similar spectral reflectance on glaciers in austria and alaska and to relate these zones to the glacier facies which are indicative of glacier mass balance in this paper landsat 5 digital numbers dns have been used to compute the at satellite planetary reflectances p on spectrally similar zones on the meares and schwanda glaciers in alaska and the grossglockner glacier group in austria the patterns of tm derived reflectances in the ablation areas of the grossglockner glacier group and the meares glacier compare favorably with published reflectance curves measured on the surface of glacier ice though the surface reflectance of snow covered ice is higher than the landsat derived reflectance for the glaciers studied the accuracy of the at satellite planetary reflectances is shown to be affected by topographic and atmospheric effects and by the anisotropic nature of snow reflectance use of reflectance values for analysis of snow and ice facilitates scene to scene comparisons of glaciers

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

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

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

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

gold accumulation in the archaean witwatersrand basin south africa evidence from concentrically laminated pyrite abstract concentrically laminated pyrite is a relatively common although volumetrically minor component of auriferous conglomerates in the archaean ca 3 0 2 7 ga witwatersrand basin of south africa this type of pyrite contains high amounts several tens of ppm of au but the origin of the pyrite is debated and the timing of au deposition in these grains is not known in order to constrain the formation of pyrite we have studied concentrically laminated pyrite and other coexisting types of pyrite inclusion rich massive pyrite by analysing the contents and distribution of au and other trace elements by laser ablation icp ms the s and fe isotope composition by sims and the mineral inclusions by scanning electron microscope and laser raman spectroscopy trace element maps indicate that concentrically laminated pyrite is enriched in sb mn au ag tl cu mo mn and contains two types of gold finely dispersed au invisible gold with au ag 0 1 and likely of primary origin and au inclusions with au ag 10 of secondary origin the study of mineral inclusions revealed the presence of muscovite chlorite fine grained carbonaceous matter monazite ti oxides and quartz iron and multiple s isotopes suggest that concentrically laminated pyrite and inclusion rich pyrite were formed from two separate pools of s and fe with different isotope characteristics sulfur was derived from atmospheric s that had undergone mass independent isotope fractionation to form so 4 2 with negative 33 s that constituted concentrically laminated pyrite and elemental s with positive 33 s that formed inclusion rich pyrite iron pools were derived from partial oxidation of fe 2 so that concentrically laminated pyrite formed from a low 56 fe residual fe 2 average 0 2 and inclusion rich pyrite formed from a high 56 fe fe 3 pool average 2 7 biological activity may have been involved in the reduction of so 4 2 causing a wide spread of 34 s values 25 s reducing microorganisms as well as in the partial oxidation of fe 2 anaerobic photosynthetic fe reducers or photosynthetic o 2 producers and in the formation of pyrite from fe 3 dissimilatory fe reducers we propose that concurrent biogenically mediated pyrite formation and au trapping suggest that microbial activity was responsible for the accumulation of au and other trace elements e g sb mn ag tl cu mo mn which are commonly enriched in organic matter rich sediments

spatial and temporal distribution of microbially induced sedimentary structures a case study from siliciclastic storm deposits of the 2 9 ga witwatersrand supergroup south africa abstract microbially induced sedimentary structures miss arise from the interaction of microbial mats with the physical sedimentary dynamics of shallow marine settings the structures occur in siliciclastic deposits of paleo archean age to modern where they record biostabilization and baffling trapping and binding of photoautotrophic microbenthos in the brixton formation witwatersrand supergroup south africa erosional remnants and pockets wrinkle structures and oscillation cracks include carbon and pyrite rich filament like textures that resemble modern trichomes of cyanobacteria c isotope ratios of 22 0 1 indicate potential photoautotrophy the miss occur at the turning points of regression transgression and are restricted to a specific sedimentary facies characterized by 2 20 cm thick fine sandstone beds predominantly composed of quartz the sandstone beds display ripple marks which record moderate hydraulic reworking this spatial and temporal distribution of miss in the brixton formation is consistent with occurrences of those structures in comparable environments throughout earth history