• I-Xe measurements of CAIs and chondrules from the CV3 chondrites Mokoia and Vigarano

      Whitby, J. A.; Russell, S. S.; Turner, G.; Gilmour, J. D. (The Meteoritical Society, 2004-01-01)
      I-Xe analyses were carried out for chondrules and refractory inclusions from the two CV3 carbonaceous chondrites Mokoia and Vigarano (representing the oxidized and reduced subgroups, respectively). Although some degree of disturbance to the I-Xe system is evident in all of the samples, evidence is preserved of aqueous alteration of CAIs in Mokoia 1 Myr later than the I-Xe age of the Shallowater standard and of the alteration of a chondrule (V3) from Vigarano ~0.7 Myr later than Shallowater. Other chondrules in Mokoia and Vigarano experienced disturbance of the I-Xe system millions of years later and, in the case of one Vigarano chondrule (VS1), complete resetting of the I-Xe system after decay of essentially all 129I, corresponding to an age more than 80 Myr after Shallowater. Our interpretation is that accretion and processing to form the Mokoia and Vigarano parent bodies must have continued for at least 4 Myr and 80 Myr, respectively. The late age of a chondrule that shows no evidence for any aqueous alteration or significant thermal processing after its formation leads us to postulate the existence of an energetic chondrule-forming mechanism at a time when nebular processes are not expected to be important.
    • Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission

      Mittlefehldt, D. W. (The Meteoritical Society, 2005-01-01)
      I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 +/- 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.
    • Ice dynamics of the Allan Hills meteorite concentration sites revealed by satellite aperture radar interferometry

      Coren, F.; Delisle, G.; Sterzai, P. (The Meteoritical Society, 2003-01-01)
      The ice flow conditions of a 100 x 100 km area of Victoria Land, Antarctica were analyzed with the synthetic aperture radar (SAR) technique. The area includes a number of meteorite concentration sites, in particular the Allan Hills ice fields. Regional ice flow velocities around the Mid- western and Near-western ice fields and the Allan Hills main ice field are shown to be less than or equal to 2.5 m yr-1. These sites are located on a horseshoe-shaped area that bounds an area characterized by higher ice flow velocities of up to 5 m yr-1. Meteorite find locations on the Elephant Moraine are located in this "high ice flow" area. The SAR derived digital elevation model (DEM) shows atypical low surface slopes for Antarctic conditions, which are the cause for the slow ice movements. Numerous ice rises in the area are interpreted to cap sub-ice obstacles, which were formed by tectonic processes in the past. The ice rises are considered to represent temporary features, which develop only during warm stages when the regional ice stand is lowered. Ice depressions, which develop in warm stages on the lee side of ice rises, may act as the sites of temporary build-up of meteorite concentrations, which turn inoperative during cold stages when the regional ice level rises and the ice rises disappear. Based on a simplified ice flow model, we argue that the regional ice flow in cold stages is reduced by a factor of at least 3.
    • Identification of mercurian volcanism: Resolution effects and implications for MESSENGER

      Milkovich, S. M.; Head, J. W.; Wilson, L. (The Meteoritical Society, 2002-01-01)
      The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high resolution, low sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER will take images with viewing geometries and resolutions appropriate for the identification of such features.
    • Identification of mineral impactors in hypervelocity impact craters in aluminum by Raman spectroscopy of residues

      Burchell, M. J.; Foster, N. J.; Kearsley, A. T.; Creighton, J. A. (The Meteoritical Society, 2008-01-01)
      Here we demonstrate the use of Raman spectroscopy techniques to identify mineral particle fragments after their impact into aluminum foil at ~6 km s^(-1). Samples of six minerals (olivine, rhodonite, enstatite, diopside, wollastonite, and lizardite) were fired into aluminum foil and the resulting impact craters were studied with a HeNe laser connected to a Raman spectrometer. Raman spectra similar to those of the raw mineral grains were obtained from the craters for impacts by olivine, rhodonite, enstatite, wollastonite, and diopside, but no Raman signals were found from lizardite after impact. In general, the impactors do not survive completely intact, but are fragmented into smaller fractions that retain the structure of the original body. Combined with evidence for SEM and FIB studies, this suggests that in most cases the fragments are relatively unaltered during impact. The survival of identifiable projectile fragments after impact at ~6 km s^(-1) is thus established in general, but may not apply to all minerals. Where survival has occurred, the use of Raman spectroscopic techniques for identifying minerals after hypervelocity impacts into a metallic target is also demonstrated.
    • Identification of minerals and meteoritic materials via Raman techniques after capture in hypervelocity impacts on aerogel

      Burchell, M. J.; Mann, J.; Creighton, J. A.; Kearsley, A. T.; Graham, G.; Franchi, I. A. (The Meteoritical Society, 2006-01-01)
      Mineral particles analogous to components of cosmic dust were tested to determine if their Raman signatures can be recognized after hypervelocity capture in aerogel. The mineral particles were accelerated onto the silica aerogel by light-gas-gun shots. It was found that all the individual minerals captured in aerogel could be identified using Raman (or fluorescence) spectra. The laser beam spot size was 5 micrometers, and in some cases the captured particles were of a similar small size. In some samples fired into aerogel, a broadening and a shift in the wave numbers of some of the Raman bands was observed, a result of the trapped particles being at elevated temperatures due to laser heating. Temperatures of samples were also estimated from the relative intensities of Stokes and anti-Stokes Raman bands, or, in the case of corundum particles, from the wave number of fluorescence bands excited by the laser. The temperature varied greatly, dependent upon laser power and the nature of the particle. Most of the mineral particles examined had temperatures below 200 degrees C at a laser power of about 3 mW at them sample. This temperature is sufficiently low enough not to damage most materials expected to be found captured in aerogel in space. In the worst case, some particles were shown to have temperatures of 500-700 degrees C. In addition, selected meteorite samples were examined to obtain Raman signatures of their constituent minerals and were then shot into aerogel. It was possible to find Raman signatures after capture in aerogel and obtain a Raman map of a whole grain in situ in the aerogel. It is concluded that Raman analysis is indeed well suited for an in situ analysis of micrometer-sized materials captured in aerogel.
    • Identification of the Bloody Creek structure, a possible impact crater in southwestern Nova Scotia, Canada

      Spooner, I.; Stevens, G.; Morrow, J.; Pufahl, P.; Grieve, R.; Raeside, R.; Pilon, J.; Stanley, C.; Barr, S.; McMullin, D. (The Meteoritical Society, 2009-01-01)
      An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near-continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments. Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink-banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock-metamorphic pressures less than or equal to 25 GPa. The age of the BCS is not determined. The low diameter-to-depth ratio of the crater, coupled with anomalously high shock-metamorphic pressures recorded at its exposed rim, may be a result of significant post-impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS.
    • Impact crater formation in icy layered terrains on Mars

      Senft, L. E.; Stewart, S. T. (The Meteoritical Society, 2008-01-01)
      We present numerical simulations of crater formation under Martian conditions with a single near-surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well-preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near-surface icy layers are widespread on Mars.
    • Impact cratering: Bridging the gap between modeling and observations, Houston, Texas, USA 2003 February 7-9

      The Meteoritical Society, 2002-01-01
      Announcement: Impact cratering: Bridging the gap between modeling and observations.
    • Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

      Barlow, Nadine G. (The Meteoritical Society, 2006-01-01)
      Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single-layer ejecta (SLE) morphology is most consistent with impact into an ice-rich target. Double-layer ejecta (DLE) and multiple-layer ejecta (MLE) craters also likely form in volatile-rich materials, but the interaction of the ejecta curtain and target-produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double-layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an ice-rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice-soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.
    • Impact ejecta in upper Eocene deposits at Massignano, Italy

      Glass, B. P.; Liu, S.; Montanari, A. (The Meteoritical Society, 2004-01-01)
      Previous workers have shown that an impact ejecta layer at Massignano, Italy contains a positive Ir anomaly, flattened spheroids (pancake spherules), Ni-rich spinel crystals, and shocked quartz with multiple sets of planar deformation features. Because of sample sizes and work by different investigators, it was not clear if the shocked quartz is associated with the Ir anomaly and pancake spherules or if it belongs to a separate impact event. To address this problem, we carried out a high-resolution stratigraphic study of this ejecta layer. The ejecta layer was sampled continuously at 1 cm intervals in two adjacent columns. The carbonate was removed with dilute HCl, and the noncarbonate fraction was gently sieved. Pancake spherules were recovered from the 250-500 micrometers size fraction and counted. At the peak abundance, the number of pancake spherules in the 250-500 micrometers size fraction is about 67/g of sample. The pancake spherules removed from the 250-500 micrometers size fraction are mostly translucent to opaque pale green, but some have a grey color or dark opaque patches due to a coating of Ni- and Cr-rich spinel crystals. Energy-dispersive X-ray analysis and Xray diffraction data indicate that the green spherules are composed of iron-rich smectite, probably nontronite. Black opaque spinel stringers (dark spinel-rich pancake spherules), usually 200 micrometers across, can be seen in a polished section of a block that includes the ejecta layer. None of the dark spinel-rich pancake spherules were recovered from the sieved non-carbonate fraction due to their fragile nature, but we believe that they are from the same impact event as the green pancake spherules. The <250 micrometers size fractions from both columns were disaggregated using ultrasonics and re-sieved. The 63-125 micrometers size fractions were then searched for shocked quartz using a petrographic microscope. At the peak-abundance level, the number of shocked quartz grains in the 63-125 micrometers size fraction is about 7/g of sample. Some of the shocked quartz grains have a toasted appearance. These grains have a brownish color and contain a patchy distribution of faint, densely spaced planar deformation features (PDFs). Polymineralic fragments containing one or two shocked quartz grains with one or two sets of PDFs were observed. They appear to have an organic matrix and are probably fragments of agglutinated foraminiferal tests. We searched for, but did not find, coesite or shocked zircons. We found that the peak abundance of the shocked quartz is within a centimeter of the peak abundance of the green pancake spherules. We conclude that the pancake spherules are diagenetically altered clinopyroxene-bearing spherules and that the shocked quartz, green (and presumably the dark spinel-rich) pancake spherules, and Ir anomaly all belong to the same impact event. This conclusion is consistent with previous suggestions that the cpx spherule layer may be from the 100 km-diameter Popigai impact crater in northern Siberia.
    • Impact glasses in fallout suevites from the Ries impact structure, Germany: An analytical SEM study

      Osinski, Gordon R. (The Meteoritical Society, 2003-01-01)
      Impact-generated glasses from fallout suevite deposits at the Ries impact structure have been investigated using analytical scanning electron microscopy. Approximately 320 analyses of glass clasts were obtained. Four glass types are distinguished on the basis of composition and microtextures. Type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers. Major oxide concentrations indicate the involvement of granitic rocks, amphibolites, and minor Al-rich gneisses during melting. Type 2 glasses are chemically heterogeneous, even within individual clasts, with variations of several wt% in most of the major oxides (e.g., 57-70 wt% SiO2). This suggests incomplete mixing of: 1) mineral-derived melts or 2) whole rock melts from a wide range of lithologies. Aluminium-rich clinopyroxene and Fe-Mg-rich plagioclase quench crystals are present in type 1 and 2 glasses, respectively. Type 3 glasses contain substantial amounts of H2O (~12-17 wt%), low SiO2 (50-53 wt%), high Al2O3 (17-21 wt%), and high CaO (57 wt%) contents. This suggests an origin due to shock melting of part of the sedimentary cover. Type 4 glasses form a ubiquitous component of the suevites. Based on their high SiO2 content (~85-100 wt%), the only possible protolith are sandstones in the lowermost part of the sedimentary succession. Calcite forms globules within type 1 glasses, with which it develops microtextures indicative of liquid immiscibility. Unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1, 2, and 4 glasses, indicating that montmorillonite was originally an impact melt glass. Clearly, the melt zone at the Ries must have incorporated a substantial fraction of the sedimentary cover, as well as the underlying crystalline basement rocks. Impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases (type 2, 3, and 4 glasses and carbonate melts).
    • Impact lithologies and their emplacement in the Chixulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico

      Kring, D. A.; Hörz, F.; Zurcher, L.; Urrutia Fucugauchi, J. (The Meteoritical Society, 2004-01-01)
      The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán-6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax-1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km- thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.
    • Impact melt rocks from the Paasselkä impact structure (SE Finland): Petrography and geochemistry

      Schmieder, M.; Moilanen, J.; Buchner, E. (The Meteoritical Society, 2008-01-01)
      Recently, samples of allochthonous melt rocks from the v10 km and less than or equal to 1.9 Ga Paasselkä impact structure, SE Finland, were obtained. In this study, we present a first detailed petrographic and geochemical description of clast-rich Paasselkä impact melt rocks. Shock metamorphic features comprise shocked feldspar grains, intensely shocked and toasted quartz, marginally molten and recrystallized clasts thought to have been diaplectic quartz glass, largely fresh and recrystallized feldspar glasses, decomposed biotite flakes, recrystallized fluidal silica glass (originally probably lechatelierite) in partially molten sandstone clasts, all set into a glassy to cryptocrystalline melt matrix. The degree of shock metamorphism of clasts suggests initial whole-rock melting at peak shock pressures of greater than or equal to 35 GPa and post-shock temperatures of up to ~1500 degrees C. Glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock (i.e., representing different monomineralic and mixed-mineral melts). Feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the Paasselkä melt rocks are not intensely altered. The geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield.
    • Impact metamorphism of CaCO3-bearing sandstones at the Haughton structure, Canada

      Osinski, G. R. (The Meteoritical Society, 2007-01-01)
      Impact-metamorphosed CaCO3-bearing sandstones at the Haughton structure have been divided into 6 classes, based to a large extent on a previous classification developed for sandstones at Meteor Crater. Class 1a sandstones (<3 GPa) display crude shatter cones, but no other petrographic indications of shock. At pressures of 3 to 5.5 GPa (class 1b), porosity is destroyed and well-developed shatter cones occur. Class 2 rocks display planar deformation features (PDFs) and are characterized by a "jigsaw" texture produced by rotation and shear at quartz grain boundaries. Calcite shows an increase in the density of mechanical twins and undergoes micro-brecciation in class 1 and 2 sandstones. Class 3 samples display multiple sets of PDFs and widespread development of diaplectic glass, toasted quartz, and symplectic intergrowths of quartz, diaplectic glass, coesite. Textural evidence, such as the intermingling of silicate glasses and calcite and the presence of flow textures, indicates that calcite in class 3 sandstones has undergone melting. This constrains the onset of melting of calcite in the Haughton sandstones to >10 <20 GPa. At higher pressures, the original texture of the sandstone is lost, which is associated with major development of vesicular SiO2 glass or lechatelierite. Class 5 rocks (>30 GPa) consist almost entirely of lechatelierite. A new class of shocked sandstones (class 6) consists of SiO2-rich melt that recrystallized to microcrystalline quartz. Calcite within class 4 to 6 sandstones also underwent melting and is preserved as globules and euhedral crystals within SiO2 phases, demonstrating the importance of impact melting, and not decomposition, in these CaCO3-bearing sandstones.
    • Impact microcrater morphology on Australasian microtektites

      Prasad, M. S.; Khedekar, V. D. (The Meteoritical Society, 2003-01-01)
      Scanning electron microscopy of 137 Australasian microtektites and fragments from 4 sediment cores in the Central Indian Ocean reveals more than 2000 impact-generated features in the size range of 0.3 to 600 micrometers. Three distinct impact types are recognized: destructive, erosive, and accretionery. A large variation in impact energy is seen in terms of catastrophic destruction demonstrated by fragmented microtektites through erosive impacts comprising glass-lined pit craters, stylus pit craters, pitless craters, and a small number of accretionery features as well. The size range of observed microtektites is from 180 to 2320 micrometers, and not only are the smaller microtektites seen to have the largest number of impacts, but most of these impacts are also of the erosive category, indicating that target temperature is an important factor for retaining impact-generated features. Further, microcratering due to collisions in impact-generated plumes seems to exist on a larger and more violent scale than previously known. Although the microcraters are produced in a terrestrially generated impact plume, they resemble lunar microcraters in many ways: 1) the size range of impacts and crater morphology variation with increasing size; 2) dominant crater number densities in micrometer and sub-micrometer sizes. Therefore, tektite-producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials, and for the lunar rocks to qualify as reliable cosmic dust flux detectors, their tumbling histories and lunar surface orientations have to be known precisely.
    • Impact penetration of Europa's ice crust as a mechanism for formation of chaos terrain

      Cox, R.; Ong, L. C. F.; Arakawa, M.; Scheider, K. C. (The Meteoritical Society, 2008-01-01)
      Ice thickness estimates and impactor dynamics indicate that some impacts must breach Europas ice crust; and outcomes of impact experiments using ice-over-water targets range from simple craters to chaos-like destroyed zones, depending on impact energy and ice competence. Firstorder impacts--into thick ice or at low impact energy--produce craters. Second-order impacts punch through the ice, making holes that resemble raft-free chaos areas. Third-order impacts--into thinnest ice or at highest energy--produce large irregular raft-filled zones similar to platy chaos. Other evidence for an impact origin for chaos areas comes from the size-frequency distribution of chaos+craters on Europa, which matches the impact production functions of Ganymede and Callisto; and from small craters around the large chaos area Thera Macula, which decrease in average size and density per unit area as a function of distance from Theras center. There are no tiny chaos areas and no craters 50 km diameter. This suggests that small impactors never penetrate, whereas large ones (berPenetrators: 2.5 km diameter at average impact velocity) always do. Existence of both craters and chaos areas in the size range 2-40 km diameter points to spatial/temporal variation in crust thickness. But in this size range, craters are progressively outnumbered by chaos areas at larger diameters, suggesting that probability of penetration increases with increasing scale of impact. If chaos areas do represent impact sites, then Europas surface is older than previously thought. The recalculated resurfacing age is 480 (-302/+960) Ma: greater than prior estimates, but still very young by solar system standards.
    • Impact processing of chondritic planetesimals: Siderophile and volatile element fractionation in the Chico L chondrite

      Norman, Marc D.; Mittlefehldt, David W. (The Meteoritical Society, 2002-01-01)
      A large impact event 500 million years ago shocked and melted portions of the L chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine-grained, clast-poor impact melt cutting shocked host chondrite. Coarse (1-2 cm diameter) globules of FeNi metal+sulfide are concentrated along the axis of the dike from metal-poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near-total fusion of the L chondrite target by the impact and negligible crystal-liquid fractionation during emplacement and cooling of the dike. Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal-silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (~10^2) effective metal/silicate-melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.
    • Impact tectonics in the core of the Vredefort dome, South Africa: Implications for central uplift formation in very large impact structures

      Lana, C.; Gibson, R. L.; Reimold, W. U. (The Meteoritical Society, 2003-01-01)
      The 80 km wide Vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure. Exposure of progressively older strata in the collar of the dome and of progressively higher-grade metamorphic rocks toward its center is consistent with differential uplift; however, the deepest levels exposed correspond to pre-impact mid- crust, rather than lower crust, as has been suggested previously. Pre-impact Archean gneissic fabrics in the core of the dome are differentially rotated, with the angle of rotation increasing sharply at a distance of ~16-19 km from the center. The present asymmetric dips of the collar strata, with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector, and the eccentric distribution of the pre-impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely NW-dipping target sequence during central uplift formation. The rocks in the core of the dome lack distinctive megablocks or large-slip-magnitude faults such as have been described in other central uplifts. We suggest that the large-scale coherent response of these rocks to the central uplift formation could have been accommodated by small-scale shear and/or rotation along pervasive pseudotachylitic breccia vein-fractures.
    • Impact-induced frictional melting in ordinary chondrites: A mechanism for deformation, darkening, and vein formation

      Van der Bogert, C. H.; Schultz, P. H.; Spray, J. G. (The Meteoritical Society, 2003-01-01)
      High speed friction experiments have been performed on the ordinary chondrites El Hammami (H5, S2) and Sahara 97001 (L6, S3) using an axial friction-welding apparatus. Each sample was subjected to a strain rate of 103 to 104 s-1, which generated 250 to 500 micrometer-deep darkened zones on each sample cube. Thin section analyses reveal that the darkened areas are composed of silicate glass and mineral fragments intermingled with dispersed submicron-size FeNi and FeS blebs. Fracturing of mineral grains and the formation of tiny metallic veins define the extent of deformation beyond the darkened shear zone. These features are not present in the original meteorites. The shear zones and tiny veins are quite similar to certain vein systems seen in naturally deformed ordinary chondrites. The experiments show that shock deformation is not required for the formation of melt veins and darkening in ordinary chondrites. Therefore, the presence of melt veins and darkening does not imply that an ordinary chondrite has undergone severe shock deformation. In fact, high strain rate deformation and frictional melting are especially important for the formation of veins at low shock pressures.