• L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating

      Korochantseva, Ekaterina V.; Trieloff, Mario; Lorenz, Cyrill A.; Buykin, Alexey I.; Ivanova, Marina A.; Schwarz, Winfried H.; Hopp, Jens; Jessberger, Elmar K. (The Meteoritical Society, 2007-01-01)
      Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ~480 Myr ago. An association with the L-chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 +/- 30 Myr and 520 +/- 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40Ar-39Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 +/- 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 +/- 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L-chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.
    • Labile trace elements in basaltic achondrites: Can they distinguish between meteorites from the Moon, Mars, and V-type asteroids?

      Wolf, S. F.; Wang, M.-S.; Lipschutz, M. E. (The Meteoritical Society, 2009-01-01)
      We report data for 14 mainly labile trace elements (Ag, Au, Bi, Cd, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U, and Zn) in eight whole-rock lunar meteorites (Asuka [A-] 881757, Dar al Gani [DaG] 262, Elephant Moraine [EET] 87521, Queen Alexandra Range [QUE] 93069, QUE 94269, QUE 94281, Yamato [Y-] 793169, and Y-981031), and Martian meteorite (DaG 476) and incorporate these into a comparative study of basaltic meteorites from the Moon, Mars, and V-type asteroids. Multivariate cluster analysis of data for these elements in 14 lunar, 13 Martian, and 34 howardite, eucrite, and diogenite (HED) meteorites demonstrate that materials from these three parents are distinguishable using these markers of late, low-temperature episodes. This distinguishability is essentially as complete as that based on markers of high-temperature igneous processes. Concentrations of these elements in 14 lunar meteorites are essentially lognormally distributed and generally more homogeneous than in Martian and HED meteorites. Mean siderophile and labile element concentrations in the 14 lunar meteorites indicate the presence of a CI-equivalent micrometeorite admixture of 2.6%. When only feldspathic samples are considered, our data show a slightly higher value of 3.4% consistent with an increasing micrometeorite content in regolith samples of higher maturity. Concentrations of labile elements in the 8 feldspathic samples hint at the presence of a fractionated highly labile element component, possibly volcanic in origin, at a level comparable to the micrometeorite component. Apparently, the process(es) that contributed to establishing lunar meteorite siderophile and labile trace element contents occurred in a system open to highly labile element transport.
    • Laboratory experiments on the weathering of iron meteorites and carbonaceous chondrites by iron-oxidizing bacteria

      Gronstal, A.; Pearson, V.; Kappler, A.; Dooris, C.; Anand, M.; Poitrasson, F.; Kee, T. P.; Cockell, C. S. (The Meteoritical Society, 2009-01-01)
      Batch culture experiments were performed to investigate the weathering of meteoritic material by iron-oxidizing bacteria. The aerobic, acidophilic iron oxidizer (A. ferrooxidans) was capable of oxidizing iron from both carbonaceous chondrites (Murchison and Cold Bokkeveld) and iron meteorites (York and Casas Grandes). Preliminary iron isotope results clearly show contrasted iron pathways during oxidation with and without bacteria suggesting that a biological role in meteorite weathering could be distinguished isotopically. Anaerobic iron-oxidizers growing under pH-neutral conditions oxidized iron from iron meteorites. These results show that rapid biologicallymediated alteration of extraterrestrial materials can occur in both aerobic and anaerobic environments. These results also demonstrate that iron can act as a source of energy for microorganisms from both iron and carbonaceous chondrites in aerobic and anaerobic conditions with implications for life on the early Earth and the possible use of microorganisms to extract minerals from asteroidal material.
    • Laboratory hydration of condensed magnesiosilica smokes with implications for hydrated silicates in IDPs and comets

      Rietmeijer, F. J. M.; Nuth, J. A.; Nelson, R. N. (The Meteoritical Society, 2004-01-01)
      Samples of silica-rich and MgO-rich condensed, amorphous magnesiosilica smokes were hydrated to monitor systematic mineralogical and chemical changes as a function of time and temperature controlled by their unique metastable eutectic compositions, their porous texture, and the ultrafine, nanometer grain size of all entities. At water supersaturated conditions, proto-phyllosilicates formed by spinodal-type homogeneous nucleation. Their formation and subsequent growth was entirely determined by the availability of water via pore spaces inherited from the original smokes and the textural continuity of magnesiosilica material with a mostly smectite-dehydroxylate composition. The results may have implications for the hydration of proto-CI material, the presence of rare periclase and brucite in primitive solar system bodies, and the pervasiveness of hydrated amorphous magnesiosilica dust and saponite proto-phyllosilicates in icy-protoplanets, such as comet nuclei.
    • Laboratory impacts into dry and wet sandstone with and without an overlying water layer: Implications for scaling laws and projectile survivability

      Baldwin, E. C.; Milner, D. J.; Burchell, M. J.; Crawford, I. A. (The Meteoritical Society, 2007-01-01)
      Scaling laws describing crater dimensions are defined in terms of projectile velocity and mass, densities of the material involved,strength of the target, and the local gravity. Here, the additional importance of target porosity and saturation, and an overlying water layer, are considered through 15 laboratory impacts of 1 mm diameter stainless steel projectiles at 5 km s^(-1) into a) an initially uncharacterized sandstone (porosity ~17%) and b) Coconino Sandstone (porosity ~23%). The higher-porosity dry sandstone allows a crater to form with a larger diameter but smaller depth than in the lower-porosity dry sandstone. Furthermore, for both porosities, a greater volume of material is excavated from a wet target than a dry target (by 27-30%). Comparison of our results with Pi-scaling (dimensionless ratios of key parameters characterizing cratering data over a range of scales) suggests that porosity is important for scaling laws given that the new data lie significantly beneath the current fit for ice and rock targets on a pi-v versus pi-3 plot (pi-v gives cratering efficiency and pi-3 the influence of target strength). An overlying water layer results in a reduction of crater dimensions, with larger craters produced in the saturated targets compared to unsaturated targets. A water depth of approximately 12 times the projectile diameter is required before craters are no longer observed in the targets. Previous experimental studies have shown that this ratio varies between 10 and 20 (Gault and Sonett 1982). In our experiments ~25% of the original projectile mass survives the impact.
    • Laboratory investigations of marine impact events: Factors influencing crater formation and projectile survivability

      Milner, D. J.; Baldwin, E. C.; Burchell, M. J. (The Meteoritical Society, 2008-01-01)
      Given that the Earths surface is covered in around two-thirds water, the majority of impact events should have occurred in marine environments. However, with the presence of a water layer, crater formation may be prohibited. Indeed, formation is greatly controlled by the water depth to projectile diameter ratio, as discussed in this paper. Previous work has shown that the underlying target material also influences crater formation (e.g., Gault and Sonett 1982; Baldwin et al. 2007). In addition to the above parameters we also show the influence of impact angle, impact velocity and projectile density for a variety of water depths on crater formation and projectile survivability. The limiting ratio of water depth to projectile diameter on cratering represents the point at which the projectile is significantly slowed by transit through the water layer to reduce the impact energy to that which prohibits cratering. We therefore study the velocity decay produced by a water layer using laboratory, analytical and numerical modelling techniques, and determine the peak pressures endured by the projectile. For an impact into a water depth five times the projectile diameter, the velocity of the projectile is found to be reduced to 26-32% its original value. For deep water impacts we find that up to 60% of the original mass of the projectile survives in an oblique impact, where survivability is defined as the solid or melted mass fraction of the projectile that could be collected after impact.
    • Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild-2

      Kearsley, A. T.; Burchell, M. J.; Hörz, F.; Cole, M. J.; Schwandt, C. S. (The Meteoritical Society, 2006-01-01)
      Metallic aluminum alloy foils exposed on the forward, comet-facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl-shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild-2, with a known and constant spacecraft-particle relative velocity and effective surface-perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well-documented particle size range from 10 micrometers to nearly 100 micrometers. Light gas gun buckshot firings of these particles at approximately 6 km s^(-1) onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild-2, independent of the active impact detector instruments aboard the Stardust spacecraft.
    • Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite

      Lee, M. R.; Smith, C. L.; Gordon, S. H.; Hodson, M. E. (The Meteoritical Society, 2006-01-01)
      Laboratory dissolution experiments using the LL6 ordinary chondrite Bensour demonstrate that meteoritic minerals readily react with distilled water at low temperatures, liberating ions into solution and forming reaction products. Three experiments were performed, all for 68 days and at atmospheric fO2 but using a range of water/rock ratios and different temperatures. Experiments 1 and 2 were batch experiments and undertaken at room temperature, whereas in experiment 3, condensed boiling water was dripped onto meteorite subsamples within a Soxhlet extractor. Solutions from experiment 1 were chemically analyzed at the end of the experiment, whereas aliquots were extracted from experiments 2 and 3 for analysis at regular intervals. In all three experiments, a very significant proportion of the Na, Cl, and K within the Bensour subsamples entered solution, demonstrating that chlorapatite and feldspar were especially susceptible to dissolution. Concentrations of Mg, Al, Si, Ca, and Fe in solution were strongly affected by the precipitation of reaction products and Mg and Ca may also have been removed by sorption. Calculations predict saturation of experimental solutions with respect to Al hydroxides, Fe oxides, and Fe (oxy)hydroxides, which would have frequently been accompanied by hydrous aluminosilicates. Some reaction products were identified and include silica, a Mg-rich silicate, Fe oxides, and Fe (oxy)hydroxides. The implications of these results are that even very short periods of subaerial exposure of ordinary chondrites will lead to dissolution of primary minerals and crystallization of weathering products that are likely to include aluminosilicates and silicates, Mg-Ca carbonates, and sulfates in addition to the ubiquitous Fe oxides and (oxy)hydroxides.
    • Large-ion lithophile element fractionation during the early differentiation of Mars and the composition of the martian primitive mantle

      McLennan, Scott M. (The Meteoritical Society, 2003-01-01)
      Basaltic shergottites display a systematic decrease in K/Th, K/U, and K/La ratios with increasing K content. These trends are interpreted as mixing lines between relatively young martian magmas derived from highly depleted mantle sources and an ancient large-ion lithophile (LIL) element-enriched crustal component. One implication of this is that a substantial fractionation of these ratios occurs during the early crustal differentiation on Mars. Isotopic evidence from SNC meteorites and compositional data from Pathfinder and orbital gamma ray spectroscopy suggest that in excess of 50% of the LIL element complement of Mars resides in the crustal reservoir. If so, the primitive mantle of Mars is significantly more volatile-depleted (i.e., lower K/Th, K/U, K/La) than previously thought but probably (though not necessarily) still less volatile-depleted than the primitive mantle of the Earth. The La/Th ratios of virtually all SNC meteorites are subchondritic, including those with the most severe LREE-depletion. Extrapolation of the basaltic shergottite trend suggests that both the depleted mantle end member and the enriched crustal end member have subchondritic La/Th ratios. This is in contrast with the Earth where basalts from LIL element-depleted sources such as MORB have superchondritic La/Th ratios, complementary to the subchondritic ratios of the continental crust. Accordingly, assuming that the refractory elements are in chondritic proportions for the Mars primitive mantle, an additional major geochemical reservoir must exist on Mars that may not yet have been sampled.
    • Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events

      Reimold, W. U.; Kelley, S. P.; Sherlock, S. C.; Henkel, H.; Koeberl, C. (The Meteoritical Society, 2005-01-01)
      In earlier studies, the 65-75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by KAr and Ar-Ar chronology at 342-368 Ma, with the commonly quoted age being 362.7 +/- 2.2 Ma (2 sigma, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 +/- 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 +/- 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% 39Ar release of 377.5 +/- 2.4 Ma (2 sigma). Our conservative estimate for the age of Siljan is 377 +/- 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 +/- 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earths biostratigraphic record.
    • Laser-fusion 40Ar/39Ar Ages of Darwin Impact Glass

      Lo, Ching-Hua; Howard, Kieren T.; Chung, Sun-Lin; Meffre, Sebastien (The Meteoritical Society, 2002-01-01)
      Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single-grain laser fusion technique, yielding isochron ages of 796-815 ka with an overall weighted mean of 816 +/- 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761-816 ka; with a mean weighted age of 803 +/- 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time.
    • Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias

      Sokol, A. K.; Bischoff, A.; Marhas, K. K.; Mezger, K.; Zinner, E. (The Meteoritical Society, 2007-01-01)
      Recent results of isotopic dating studies (182Hf-182W, 26Al-26Mg) and the increasing number of observed igneous and metamorphosed fragments in (primitive) chondrites provide strong evidence that accretion of differentiated planetesimals predates that of primitive chondrite parent bodies. The primitive chondrites Adrar 003 and Acfer 094 contain some unusual fragments that seem to have undergone recrystallization. Magnesium isotope analyses reveal no detectable radiogenic 26Mg in any of the studied fragments. The possibility that evidence for 26Al was destroyed by parent body metamorphism after formation is not likely because several other constituents of these chondrites do not show any metamorphic features. Since final accretion of a planetesimal must have occurred after formation of its youngest components, formation of these parent bodies must thus have been relatively late (i.e., after most 26Al had decayed). Al-Mg isotope data for some igneous-textured clasts (granitoids and andesitic fragments) within the two chondrite regolith breccias Adzhi-Bogdo and Study Butte reveal also no evidence for radiogenic 26Mg. As calculated from the upper limits, the formation of these igneous clasts, the incorporation into the parent body regolith, and the lithification must have occurred at least 3.8 Myr (andesite in Study Butte) and 4.7 Myr (granitoids in Adzhi-Bogdo) after calciumaluminum-rich inclusions (CAI) formation. The absence of 26Mg excess in the igneous inclusions does not exclude 26Al from being a heat source for planetary melting. In large, early formed planetesimals, cooling below the closure temperature of the Al-Mg system may be too late for any evidence for live 26Al (in the form of 26Mg excess) to be preserved. Thus, growing evidence exists that chondritic meteorites represent the products of a complex, multi-stage history of accretion, parent body modification, disruption and re-accretion.
    • Layered ejecta craters and the early water/ice aquifer on Mars

      Oberbeck, V. R. (The Meteoritical Society, 2009-01-01)
      A model for emplacement of deposits of impact craters is presented that explains the size range of Martian layered ejecta craters between 5 km and 60 km in diameter in the low and middle latitudes. The impact model provides estimates of the water content of crater deposits relative to volatile content in the aquifer of Mars. These estimates together with the amount of water required to initiate fluid flow in terrestrial debris flows provide an estimate of 21% by volume (7.6 x 10^7 km^3) of water/ice that was stored between 0.27 and 2.5 km depth in the crust of Mars during Hesperian and Amazonian time. This would have been sufficient to supply the water for an ocean in the northern lowlands of Mars. The existence of fluidized craters smaller than 5 km diameter in some places on Mars suggests that volatiles were present locally at depths less than 0.27 km. Deposits of Martian craters may be ideal sites for searches for fossils of early organisms that may have existed in the water table if life originated on Mars.
    • Letter to the Editor and the President of the Meteoritical Society

      Heymann, Deiter (The Meteoritical Society, 2003-01-01)
    • Light element geochemistry of the Tagish Lake CI2 chondrite: Comparison with CI1 and CM2 meteorites

      Grady, Monica M.; Verchovsky, A. B.; Franchi, I. A.; Wright, I. P.; Pillinger, C. T. (The Meteoritical Society, 2002-01-01)
      We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high resolution stepped combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), EET 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (~1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole rock carbon isotopic composition (delta-13C is approximately equal to +24.4 ppm) of the chip was dominated by 13C-enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; delta-13C is approximately equal to +67 ppm and delta-18O is approximately equal to +35 ppm, in the proportions ~4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (~2.6 wt%; delta-13C is approximately equal to -9 ppm; 1033 ppm N, delta-15N is approximately equal to +77 ppm), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ~18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (~3650-4330 ppm) than other carbonaceous chondrites, along with ~8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.
    • Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars

      Nekvasil, H.; McCubbin, F. M.; Harrington, A.; Elardo, S.; Lindsley, D. H. (The Meteoritical Society, 2009-01-01)
      In order to use igneous surface lithologies to constrain Martian mantle characteristics, secondary processes that lead to compositional modification of primary mantle melts must be considered. Crystal fractionation of a mantle-derived magma at the base of the crust followed by separation and ascent of residual liquids to the surface is common in continental hotspot regions on Earth. The possibility that this process also takes place on Mars was investigated by experimentally determining whether a surface rock, specifically the hawaiite Backstay analyzed by the MER Spirit could produce a known cumulate lithology with a deep origin (namely the assemblages of the Chassigny meteorite) if trapped at the base of the Martian crust. Both the major cumulus and melt inclusion mineral assemblages of the Chassigny meteorite were produced experimentally by a liquid of Backstay composition within the pressure range 9.3 to 6.8 kbar with bulk water contents between 1.5 and 2.6 wt%. Experiments at 4.3 and 2.8 kbar did not produce the requisite assemblages. This agreement suggests that just as on Earth, Martian mantle-derived melts may rise to the surface or remain trapped at the base of the crust, fractionate, and lose their residual liquids. Efficient removal of these residual liquids at depth would yield a deep low-silica cumulate layer for higher magmatic water content; at lower magmatic water content this cumulate layer would be basaltic with shergottitic affinity.
    • Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

      Hunze, S.; Wonik, T. (The Meteoritical Society, 2007-01-01)
      Bosumtwi is a very well-preserved 1.07 Myr old, complex terrestrial impact crater locatedin south-central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and wasformed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures. We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p-wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta-graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70 degrees) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north-south.
    • Lithostratigraphic and petrographic analysis of ICDP drill core LB-07A, Bosumtwi impact structure, Ghana

      Coney, L.; Gibson, R. L.; Reimold, W. U.; Koeberl, C. (The Meteoritical Society, 2007-01-01)
      Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB-07A in the Bosumtwi impact structure revealed two sequences of impactites below the post-impact crater sediments and above coherent basement rock. The upper impactites (333.38-415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67-470.55 m depth) consists essentially of monomict impact breccia formed from meta-graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55-545.08 m depth) is composed of lower greenschist-facies metapelites (shale, schist and minor phyllite), meta-graywacke, and minor meta-sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric-textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre-impact granitoid complexes. Impact melt fragments are not as prevalent in LB-07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within-crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB-07A appears to have been derived from the <35 GPa shock zone of the transient crater.
    • Local topographic effects on photometry and reflectance spectra of planetary surfaces: An example based on lunar photometry

      Domingue, D.; Vilas, F. (The Meteoritical Society, 2007-01-01)
      The photometric properties of the average lunar surface are characterized using Hapkes equations and whole disk observations ranging from 0.36 to 1.06 micrometers. Synthetic spectra across a crater topographic profile arecreated using the modeling results. The synthetic spectra are examined for spectral variations created by changes in lighting conditions induced by the topography. Changes above the modeling uncertainties are seen in both spectral slope and band depths, though the most pronounced change is in band depth. The data have insufficient spectral resolution to determine if there are any changes in band center due to photometric effects. No additional absorption features are introduced by the photometry. These results have serious implications on the interpretation of spectral observations in terms of abundance estimates and alteration processes as a function of location and association with geologic features.
    • Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

      Walton, E. L.; Herd, C. D. K. (The Meteoritical Society, 2007-01-01)
      The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low-Ca pyroxene poikilically enclosing cumulate olivine (Fo70-75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low- and high-Ca pyroxene, cumulate olivine (Fo68-71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large-scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35-45 GPa. Large (millimeter-size) melt pockets have crystallized magnesian olivine (Fo78-87) and chromite, embedded in an Fe-rich, Al-poor basaltic to picro-basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 degrees C/micrometer) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 x 10^3 crystals/mm^2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 x 10^5 crystals/mm^2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45-55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.