• Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

      Glass, Billy P.; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95-17957-2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock-metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine-grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm^2) of any known Australasian microtektite-bearing site and may be closer to the source crater than any previously identified Australasian microtektite-bearing site. A source crater in the vicinity of 22 degrees N and 10 degrees E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 +/- 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ~800 ka ago and that it is spread over an area of at least 3.7 x 10^7 km^2.
    • Focused ion beam recovery of hypervelocity impact residue in experimental craters on metallic foils

      Graham, G. A.; Teslich, N.; Dai, Z. R.; Bradley, J. P.; Kearsley, A. T.; Hörz, F. (The Meteoritical Society, 2006-01-01)
      The Stardust sample return capsule returned to Earth in January 2006 with primitive debris collected from Comet 81P/Wild-2 during the fly-by encounter in 2004. In addition to the cometary particles embedded in low-density silica aerogel, there are microcraters preserved in the aluminum foils (1100 series; 100 micrometers thick) that are wrapped around the sample tray assembly. Soda lime spheres (~49 micrometers in diameter) have been accelerated with a light gas gun into flight-grade aluminum foils at 6.35 km s^(-1) to simulate the capture of cometary debris. The experimental craters have been analyzed using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX) to locate and characterize remants of the projectile material remaining within the craters. In addition, ion beam-induced secondary electron imaging has proven particularly useful in identifying areas within the craters that contain residue material. Finally, high-precision focused ion beam (FIB) milling has been used to isolate and then extract an individual melt residue droplet from the interior wall of an impact. This has enabled further detailed elemental characterization that is free from the background contamination of the aluminum foil substrate. The ability to recover "pure" melt residues using FIB will significantly extend the interpretations of the residue chemistry preserved in the aluminum foils returned by Stardust.
    • 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.
    • Investigation of Shuttle Radar Topography Mission data of the possible impact structure at Serra da Cangalha, Brazil

      Reimold, Wolf U.; Cooper, Gordon R. J.; Romano, Rafael; Cowan, Duncan R.; Koeberl, Christian (The Meteoritical Society, 2006-01-01)
      The Serra da Cangalha crater structure in northeast Brazil, ~13 km in diameter, has long been widely considered to be a confirmed impact structure, based on reports of shatter cone findings. Only very limited field work has been carried out at this crater structure. Landsat Thematic Mapper (TM) and Shuttle Radar Topography Mission (SRTM) data sets for the region around this crater structure are compared here with regard to their suitability to determine first-order structural detail of impact crater structures. The SRTM data provide very detailed information regarding drainage patterns and topography. A pronounced central ring of up to 300 m elevation above the surrounding area, two comparatively subdued intermediate rings of 6 and 10.5 km diameter, respectively, and the broad, complex crater rim of up to >100 m elevation can be distinguished in the Serra da Cangalha data. The maximum cratering-related regional deformation (radial and concentric features) seems to be limited to a radial distance of 16-18 km from the center of the structure. A first comparison of macrostructural information from several impact structures with that from Serra da Cangalha does not yield firm trends, but the database is still very small at this stage. The varied nature of the target geology strongly influences the development of structural features in any impact event.
    • 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.
    • Macroscopic subdivision of silica aerogel collectors for sample return missions

      Ishii, H. A.; Bradley, J. P. (The Meteoritical Society, 2006-01-01)
      Silica aerogel collector tiles have been employed for the collection of particles in low Earth orbit and, more recently, for the capture of cometary particles by NASA's Stardust mission. Reliable, reproducible methods for cutting these and future collector tiles from sample return missions are necessary to maximize the science output from the extremely valuable embedded particles. We present a means of macroscopic subdivision of collector tiles by generating large-scale cuts over several centimeters in silica aerogel with almost no material loss. The cut surfaces are smooth and optically clear allowing visual location of particles for analysis and extraction. This capability is complementary to the smaller-scale cutting capabilities previously described (Westphal 2004; Ishii 2005a, 2005b) for removing individual impacts and particulate debris in tiny aerogel extractions. Macroscopic cuts enable division and storage or distribution of portions of aerogel tiles for immediate analysis of samples by certain techniques in situ or further extraction of samples suited for other methods of analysis. The capability has been implemented in the Stardust Laboratory at NASA's Johnson Space Center as one of a suite of aerogel cutting methods to be used in Stardust sample curation.
    • Microstructural study of micron-sized craters simulating Stardust impacts in aluminum 1100 targets

      Leroux, Hugues; Borg, Janet; Troadec, David; Djouadi, Zahia; Hörz, Friedrich (The Meteoritical Society, 2006-01-01)
      Various microscopic techniques were used to characterize experimental microcraters in aluminum foils to prepare for the comprehensive analysis of the cometary and interstellar particle impacts in aluminum foils to be returned by the Stardust mission. First, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to study the morphology of the impact craters and the bulk composition of the residues left by soda-lime glass impactors. A more detailed structural and compositional study of impactor remnants was then performed using transmission electron microscopy (TEM), EDS, and electron diffraction methods. The TEM samples were prepared by focused ion beam (FIB) methods. This technique proved to be especially valuable in studying impact crater residues and impact crater morphology. Finally, we also showed that infrared microscopy (IR) can be a quick and reliable tool for such investigations. The combination of all of these tools enables a complete microscopic characterization of the craters.
    • Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post-basin deposition of basaltic material at the site

      Zeigler, Ryan A.; Korotev, Randy L.; Haskin, Larry A.; Jolliff, Bradley L.; Gillis, Jeffrey J. (The Meteoritical Society, 2006-01-01)
      We present the petrography and geochemistry of five 2-4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high-Ti vitrophyric basalt compositionally similar to Apollo 17 high-Ti mare basalts, 2) a very high-Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red-black pyroclastic glass, 3) a coarsely crystalline high-Al basalt compositionally similar to group 5 Apollo 14 high-Al mare basalts, 4) a very low-Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High-Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small- to medium-sized post-basin impacts. On the basis of TiO2 concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (~300 km north) is the most likely source for the high-Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (~220 km east) is the most likely source for the low-Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).
    • Shock re-equilibration of fluid inclusions in crystalline basement rocks from the Ries crater, Germany

      Elwood Madden, Megan E.; Kring, David A.; Bodnar, Robert J. (The Meteoritical Society, 2006-01-01)
      This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two-phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single-phase inclusions increase, likely as a result of re-equilibration. In shock stages 2 and 3, both two-phase and single-phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single-phase and two-phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re-equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals.
    • SIMS studies of Allende projectiles fired into Stardust-type aluminum foils at 6 km/sec

      Hoppe, Peter; Stadermann, Frank J.; Stephan, Thomas; Floss, Christine; Leitner, Jan; Marhas, Kuljeet K.; Hörz, Friedrich (The Meteoritical Society, 2006-01-01)
      We have explored the feasibility of degrees C, N, and O isotopic measurements by NanoSIMS and of elemental abundance determinations by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on residues of Allende projectiles that impacted Stardust-type aluminum foils in the laboratory at 6 km/sec. These investigations are part of a consortium study aimed at providing the foundation for the characterization of matter associated with microcraters that were produced during the encounter of the Stardust space probe with comet 81P/Wild-2. Eleven experimental impact craters were studied by NanoSIMS and eighteen by TOF-SIMS. Crater sizes were between 3 and 190 micrometers. The NanoSIMS measurements have shown that the crater morphology has only a minor effect on spatial resolution and on instrumental mass fractionation. The achievable spatial resolution is always better than 200 nm, and degrees C and O isotopic ratios can be measured with a precision of several percent at a scale of several 100 nm, which is the typical size of presolar grains. This clearly demonstrates that presolar matter, provided it survives the impact into the aluminum foil partly intact, is recognizable even if embedded in material of solar system origin. TOF-SIMS studies are restricted to materials from the crater rim. The element ratios of the major rock-forming elements in the Allende projectiles are well-characterized by the TOF-SIMS measurements, indicating that fractionation of those elements during impact can be expected to be negligible. This permits chemical information on the type of impactor material to be obtained. For any more detailed assignments to specific chondrite groups, however, information on the abundances of the light elements, especially degrees C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.
    • Stratigraphy and composition of lava flows in Mare Nubium and Mare Cognitum

      Bugiolacchi, Roberto; Spudis, Paul D.; Guest, John E. (The Meteoritical Society, 2006-01-01)
      Three major periods of basaltic activity characterize the infill of the basins. Each of these periods was itself punctuated by discrete phases of widespread magma eruptions: three during both the Late Imbrian Epoch and the early Eratosthenian Period and then two in the late Eratosthenian Period. We found the youngest lavas off the eastern border of the Fra Mauro peninsula and, mantling a much larger area, over most of the central western Nubium basin.Our results place the Nubium/Cognitum basalts in the low-Ti category (1-5 wt% TiO2).The data indicate that the majority (~90%) of the mare terrain has iron content between 18 and 22 wt%. In particular, FeO contents tend to concentrate toward two compositional poles, each of ~20 wt%, and a much smaller one of ~15 wt%. These values are typical of nearside lunar maria.To complement our compositional data, we present a census of craters larger than 500 m using Orbiter IV images. The result was a crater count average with frequency 5.6 x 10^(-2)km^(-2), translating into an inferred mean age of 3300 Ma for the exposed lava flows.By combining lava chemistry with age, we find a possible correlation between the ages of the most prominent flow units and their estimated titanium content, with younger basalts becoming progressively Ti-richer with time (from 2-3 to 4-5 wt% TiO2).
    • TOF-SIMS analysis of Allende projectiles shot into silica aerogel

      Stephan, Thomas; Butterworth, Anna L.; Hörz, Friedrich; Snead, Christopher J.; Westphal, Andrew J. (The Meteoritical Society, 2006-01-01)
      Powdered Allende projectiles were fired into silica aerogel at 6.1 km/sec in order to evaluate particle retrieval and analysis techniques for samples from the Stardust mission. Since particles may disintegrate and ablate along the penetration paths in a high-porosity aerogel, TOF-SIMS analysis may be a suitable method to determine the distribution of such materials along the tracks as well as potential compositional modifications. Therefore, two 350 micrometer-sized tracks, residing at the surface of a keystone specimen that was flattened between two silicon chips, were analyzed. TOF-SIMS allows for a detailed study of the chemical composition of particles that survived the impact mostly intact and of fine-grained material from disintegrated projectiles. In the investigated keystone, material from light gas gun debris dominated. Besides the two tracks, a continuous, 40-micrometer-thick surface layer of implanted material - probably gun residue - was found. One of the two analyzed tracks is compositionally distinct from this surface layer and is likely to contain residual material of an Allende projectile. The analyses clearly demonstrate that tracks, resulting from impactors in the 5-10 micrometer size range, can be successfully analyzed with TOF-SIMS.