• Fabric analysis of Allende matrix using EBSD

      Watt, Lauren E.; Bland, Phil A.; Prior, Dave J.; Russell, Sara S. (The Meteoritical Society, 2006-01-01)
      Fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution. Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (~25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events.
    • Facies distribution of post-impact sediments in the Ordovician Lockne and Tvaren impact craters: Indications for unique impact-generated environments

      Frisk, Å. M.; Ormö, J. (The Meteoritical Society, 2007-01-01)
      The Lockne and Tvären craters formed in the Late Ordovician Baltoscandian epicontinental sea. Both craters demonstrate similarities concerning near-synchronous age, target seabed, and succeeding resurge deposits; however, the water depths at the impact sites and the sizes of the craters were not alike. The post-impact sedimentary succession of carbonates, i.e., the Dalby Limestone, deposited on top of the resurge sediments in the two craters, is nevertheless similar. At least three main facies of the Dalby Limestone were established in the Lockne crater, depending on sea-floor topography, location with respect to the crater, and local water currents. The dominating nodular argillaceous facies, showing low values of inorganic carbon (IC), was distributed foremost in the deeper and quiet areas of the crater floor and depressions. At the crater rim, consisting of crushed crystalline basement ejecta, a rim facies with a reef-like fauna was established, most certainly due to topographical highs and substrate-derived nutrients. Between these facies are occurrences of a relatively thick-bedded calcilutite rich in cephalopods (cephalopod facies). In Tvären, the lower part of the succession consists of an analogous argillaceous facies, also showing similar low IC values as in Lockne, followed by calcareous mudstones with an increase of IC. Occasionally biocalcarenites with a distinctive fauna occur in the Tvären succession, probably originating as detritus from a facies developed on the rim. They are evident as peaks in IC and lows in organic carbon (Corg). The fauna in these biocalcarenites corresponds very well with those of erratic boulders derived from Tvären; moreover, they correspond to the rim facies of Lockne except for the inclusion of photosynthesizing algae, indicating shallower water at Tvären than Lockne. Consequently, we suggest equivalent distribution patterns for the carbonates of the Dalby Limestone in Lockne and Tvären.
    • Fall, classification and cosmogenic records of the Sabrum (LL6) chondrite

      Ghosh, S.; Murty, S. V. S.; Shukla, P. N.; Shukla, A. D.; Mahajan, R. R.; Bhandari, N.; Pant, N. C.; Ghosh, J. B.; Shome, S. (The Meteoritical Society, 2002-01-01)
      The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described. Its mean mineral composition is defined by olivine (Fa31.4), orthopyroxene (Fs25.1,Wo2.0), clinopyroxene (Wo45En45.6Fs9.4) and plagioclase (An10.6Ab83.6Or5.8). The meteorite shows moderate shock features, which indicate that it, belong to the S4 category. Based on mineralogical and chemical criteria the meteorite is classified as LL6 brecciated veined chondrite. Several cosmogenic radioisotopes (46Sc, 7Be, 54Mn, 22Na and 26Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes and particle tracks density have been measured. Concentrations of cosmogenic 21Ne and 38Ar indicate that its cosmic ray exposure is 24.8 Ma. Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present. The track density in olivines is found to be (1.3 +/- 0.3 x 10^6/cm^2. Activities of most of the short lived isotopes are lower than those expected from solar cycle variation. 22Na/26Al (1.12 +/- 0.02) is found to be significantly anomalous, being approximately 25% lower than expected from the climax neutron monitor data. These results indicate that the cosmic ray flux during the terminal segment of the meteoroid orbit was low. The activities of 26Al and 60Co and the track density indicate small meteoroid size with radius ~15 cm.
    • Fassaites in compact type A Ca-Al-rich inclusions in the Ningqiang carbonaceous chondrite: Evidence for partial melting in the nebula

      Lin, Y.; Kimura, M.; Wang, D. (The Meteoritical Society, 2003-01-01)
      Fassaite is a major component of Ca-Al-rich inclusions (CAIs) of Types B and degrees C that crystallized from liquids. In contrast, this mineral is rarely reported in Type A inclusions and has been much less studied. In this paper, we report highly Ti-, Al-enriched fassaite that occurs as rims on perovskite in two compact Type A inclusions from the Ningqiang meteorite. In addition, one of the inclusions contains an euhedral grain of Sc-fassaite (16.4 wt% Sc2O3) isolated in melilite. The occurrence and mineral chemistry of the fassaite rims can be explained by a reaction of pre-existing perovskite with CAI melts. Hence, such rims may serve as an indicator for partial melting of Type A inclusions. The Sc-fassaite is probably a relict grain. A third spherical CAI contains several euhedral grains of V-fassaite (4.8-5.4 wt% V2O3) enclosed in a melilite fragment. The high V content of fassaite cannot be related to any Fremdlinge, magnetite, or metallic Fe-Ni, because these phases are absent in the inclusion. In the same CAI, other fassaites intergrow with spinel and minor perovskite, filling voids inside of the melilite and space adjacent to the Wark-Lovering rim. The fassaite intergrown with spinel is almost V-free. The coexistence of two types of fassaite suggests that this CAI has not been completely melted.
    • Fe-Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

      Kimura, M.; Grossman, J. N.; Weisberg, M. K. (The Meteoritical Society, 2008-01-01)
      We report the results of our petrological and mineralogical study of Fe-Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe-Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni-rich metal in chondrites of petrologic type lower than type 3.10 is enriched in CO relative to the kamacite in chondrules. However, Ni-rich metal in type 3.15-3.9 chondrites always contains less CO than does kamacite. Fe-Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni-rich regions. Metal in other type 3 chondrites is composed of fine- to coarse-grained aggregates of kamacite and Ni-rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni-rich grains in metal (number of Ni-rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe-Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe-Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01.
    • FeO-rich silicates in the Sahara 97159 (EH3) enstatite chondrite: Mineralogy, oxygen isotopic compositions, and origin

      Kimura, M.; Hiyagon, H.; Lin, Y.; Weisberg, M. K. (The Meteoritical Society, 2003-01-01)
      We report the mineralogy and oxygen isotopic compositions of FeO-rich silicates in the Sahara 97159 EH3 chondrite. This component is referred to as FeO-rich because it contains substantially more FeO than the characteristic FeO-poor silicates in the highly reduced enstatite meteorites. These FeO-rich silicates are mostly low-Ca pyroxene (Fs535) and their compositions suggest an origin under more oxidizing conditions, like those for the ordinary chondrites. However, the mafic silicates in ordinary and carbonaceous chondrites are dominantly olivine, and the FeO-rich silicates in the E chondrites are less commonly olivine. The oxygen isotopic compositions of the FeO- rich silicates are indistinguishable from those of FeO-poor silicates in Sahara 97159. These observations suggest that both the FeO-rich silicates and the FeO-poor silicates in EH chondrites formed from the same oxygen reservoir where redox conditions varied widely.
    • Ferric iron in SNC meteorites as determined by Mössbauer spectroscopy: Implications for martian landers and martian oxygen fugacity

      Dyar, M. Darby (The Meteoritical Society, 2003-01-01)
      Mössbauer spectra of martian meteorites are currently of great interest due to the Mössbauer spectrometers on the Athena mission MER rovers as well as the European Space Agency Mars Express mission, with its Beagle 2 payload. Also, considerable current effort is being made to understand the oxygen fugacity of martian magmas because of the effect of fO2 on mineral chemistry and crystallization processes. For these 2 reasons, the present study was conceived to acquire room temperature Mössbauer spectra of mineral separates and whole rock samples of 10 SNC meteorites. The results suggest that mineral identification using remote application of this technique will be most useful when the phases present have distinctive parameters arising from Fe in very different coordination polyhedra; for example, pyroxene coexisting with olivine can be discriminated easily, but opx versus cpx cannot. The MER goal of using Mössbauer spectroscopy to quantify the relative amounts of individual mineral species present will be difficult to satisfy if silicates are present because the lack of constraints on wt% FeO contents of individual silicate phases present will make modal calculations impossible. The remote Mössbauer spectroscopy will be most advantageous if the rocks analyzed are predominantly oxides with known stoichiometries, though these phases are not present in the SNCs. As for the detection of martian oxygen fugacity, no evidence exists in the SNC samples studied of a relationship between Fe3+ content and fO2 as calculated by independent methods. Possibly, all of the Fe3+ observed in olivine is the result of dehydrogenation rather than oxidation, and this process may also be the source of all the Fe3+ observed in pyroxene. The observed Fe3+ in pyroxene also likely records an equilibrium between pyroxene and melt at such low fO2 that little or no Fe3+ would be expected.
    • Fine-grained dust rims in the Tagish Lake carbonaceous chondrite: Evidence for parent body alteration

      Greshake, A.; Krot, A. N.; Flynn, G. J.; Keil, K. (The Meteoritical Society, 2005-01-01)
      The Tagish Lake carbonaceous chondrite consists of heavily aqueously altered chondrules, CAIs, and larger mineral fragments in a fine-grained, phyllosilicate-dominated matrix. The vast majority of the coarse-grained components in this meteorite are surrounded by continuous, 1.5 to 200 m wide, fine-grained, accretionary rims, which are well known from meteorites belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate. Texturally, the fine-grained rims in Tagish Lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object. They typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix. Compared to the matrix, the rims are much more finegrained and characterized by a significantly lower porosity. The rims consist of an unequilibrated assemblage of phyllosilicates, Fe,Ni sulfides, magnetites, low-Ca pyroxenes, and forsteritic olivines, and are, except for a much lower abundance of carbonates, very similar to the Tagish Lake matrix. Electron microprobe and synchrotron X-ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in Ca. X-ray elemental mapping and mineralogical observations indicate that Ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix. The analyses also show that Ca is strongly fractionated from Al in the rims, whereas there is no fractionation of the Ti/Al-ratios. Our data suggest that the fine-grained rims in Tagish Lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body. This pervasive alteration removed any potential evidence for preaccretionary alteration but did not change the overall texture of the Tagish Lake meteorite.
    • Fine-grained rims in the Allan Hills 81002 and Lewis Cliff 90500 CM2 meteorites: Their origin and modification

      Hua, X.; Wang, J.; Buseck, P. R. (The Meteoritical Society, 2002-01-01)
      Antarctic CM meteorites Allan Hills (ALH) 81002 and Lewis Cliff (LEW) 90500 contain abundant fine-grained rims (FGRs) that surround a variety of coarse-grained objects. FGRs from both meteorites have similar compositions and petrographic features, independent of their enclosed objects. The FGRs are chemically homogeneous at the 10 micrometer scale for major and minor elements and at the 25 micrometer scale for trace elements. They display accretionary features and contain large amounts of volatiles, presumably water. They are depleted in Ca, Mn, and S but enriched in P. All FGRs show a slightly fractionated rare earth element (REE) pattern, with enrichments of Gd and Yb and depletion of Er. Gd is twice as abundant as Er. Our results indicate that those FGRs are not genetically related to their enclosed cores. They were sampled from a reservoir of homogeneously mixed dust, prior to accretion to their parent body. The rim materials subsequently experienced aqueous alteration under identical conditions. Based on their mineral, textural, and especially chemical similarities, we conclude that ALH 81002 and LEW 90500 likely have a similar or identical source.
    • Fine-grained, spinel-rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions

      Aléon, J.; Krot, A. N.; McKeegan, K. D.; MacPherson, G. J.; Ulyanov, A. A. (The Meteoritical Society, 2005-01-01)
      Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al- Ti-diopside, melilite, and anorthite) within four relatively unaltered, fine-grained, spinel-rich Ca-Alrich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly 16O-rich (Delta-17O is less than or equal to -20 ppm) in all four CAIs; Al-Ti-diopside is similarly 16O-rich in all but one CAI, where it has smaller 16O excesses (-15 ppm is less than or equal to Delta-17O, which is less than or equal to -10 ppm). Anorthite and melilite vary widely in composition from 16O-rich to 16O-poor (-22 ppm is less than or equal to Delta-17O, which is less than or equal to -5 ppm). Two of the CAIs are known to have group II volatilityfractionated rare-earth-element patterns, which is typical of this variety of CAI and which suggests formation by condensation. The association of such trace element patterns with 16O-enrichment in these CAIs suggests that they formed by gas-solid condensation from an 16O-rich gas. They subsequently experienced thermal processing in an 16O-poor reservoir, resulting in partial oxygen isotope exchange. Within each inclusion, oxygen isotope variations from mineral to mineral are consistent with solid-state oxygen self-diffusion at the grain-to-grain scale, but such a model is not consistent with isotopic variations at a larger scale in two of the CAIs. The spatial association of 16O depletions with both elevated Fe contents in spinel and the presence of nepheline suggests that latestage iron-alkali metasomatism played some role in modifying the isotopic patterns in some CAIs. One of the CAIs is a compound object consisting of a coarse-grained, melilite-rich (type A) lithology joined to a fine-grained, spinel-rich one. Melilite and anorthite in the fine-grained portion are mainly 16O-rich, whereas melilite in the type A portion ranges from 16O-rich to 16O-poor, suggesting that oxygen isotope exchange predated the joining together of the two parts and that both 16O-rich and 16O-poor gaseous reservoirs existed simultaneously in the early solar nebula.
    • Fine-grained, spinel-rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy, petrology, and bulk chemistry

      Krot, A. N.; MacPherson, G. J.; Ulyanov, A. A.; Petaev, M. I. (The Meteoritical Society, 2004-01-01)
      Fine-grained, spinel-rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al-diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite-rich accretionary rims. In contrast to heavily altered fine-grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine-grained CAIs are 16O-enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine-grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel +/- hibonite +/- perovskite cores surrounded by layers of melilite and Al-diopside. Other fine-grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite-free and consist of tiny spinel +/- hibonite +/- perovskite grains surrounded by layers of anorthite and Al-diopside. The mantles are calcium-enriched, magnesium-depleted and coarser-grained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine-grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type degrees C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel-perovskite-melilite +/- hibonite gas-solid condensates from a reservoir that was 16O-enriched but depleted in the most refractory REEs. These aggregates later experienced low-temperature gas-solid nebular reactions with gaseous SiO and Mg to form Al-diopside and anorthite. The zoned structures of many of the fine-grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi-stage formation history of fine-grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse-grained CAIs in CV chondrites.
    • Finite difference time domain simulation of radar wave propagation through comet nuclei dielectric models

      Carley, Ruth A.; Heggy, Essam (The Meteoritical Society, 2008-01-01)
      The 90 MHz radar-wave experiment, Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), on board the Rosetta mission (ESA, 2004) is expected to probe the nucleus of the comet 67P/Churyumov-Gerasimenko (67P/C-G) to reveal information on its physical properties, chemical composition, and internal structure. This investigation assesses the potential to recognize lithological structure in the comet nucleus with a radar experiment such as CONSERT. Radar simulations at 90 MHz were performed with a finite difference time domain (FDTD) method. The amplitude and losses of the transmitted and reflected electric field components of an incident radar pulse were evaluated as a function of time. Seven different dielectric models of sections of a hypothetical comet nucleus were used, representative of existing theories of comet nuclei. Values of dielectric constant assigned to these models are based on mixing laws for a porous mixture of ice and meteoritic dust, employing laboratory measured values of relative permittivity for mainly chondritic meteorites. Our results confirm that structural differences such as layers or inclusions are discernable from transmitted and reflected radar signals at 90 MHz, the central frequency of the CONSERT instrument. Such simulations help to constrain the ambiguities that might exist in future radar data associated with the nature of the comet nuclei, whether conglomerate or layered in nature.
    • First discovery of stishovite in an iron meteorite

      Holtstam, D.; Broman, C.; Söderhielm, J.; Zetterqvist, A. (The Meteoritical Society, 2003-01-01)
      The first occurrence of stishovite in an iron meteorite, Muonionalusta (group IVA), is reported. The mineral occurs intimately mixed with amorphous silica, forming tabular grains up to ~3 mm wide, with a hexagonal outline. It was identified using X-ray diffraction and Raman microspectroscopy. The unit-cell parameters of stishovite are a = 4.165(3) Å and c = 2.661(6) Å, and its chemical composition is nearly pure SiO2. Raman spectra show relatively sharp bands at 231 and 754 cm-1 and a broad band with an asymmetric shape and a maximum around 500 cm-1. The rare grains are found within troilite nodules together with chromite, daubreelite, and schreibersite. From their composition and morphology, and by comparisons with silica inclusions in, e.g., the Gibeon IVA iron, we conclude that these rare grains represent pseudomorphs after tridymite. The presence of stishovite in Muonionalusta is suggested to reflect shock metamorphic conditions in the IVA parent asteroid during a cosmic impact event.
    • First petrographic results on impactites from the Yaxcopoil-1 borehole, Chicxulub structure, Mexico

      Tuchscherer, M. G.; Reimold, W. U.; Koeberl, C.; Gibson, R. L.; de Bruin, D. (The Meteoritical Society, 2004-01-01)
      The ICDP Yaxcopoil-1 (Yax-1) borehole located 60 km south-southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795-895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795-922 m) comprises an apparently reworked, poorly sorted and graded, fine-grained, clast-supported, melt fragment-bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823-846 m) and 3 (846-861 m) are groundmass-supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine-grained calcite, altered alkali element-, Ca-, and Si-rich cement, as well as occasional lithic fragments. Unit 4 (861-885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885-895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine-grained, carbonate-dominated groundmass. This groundmass could represent either a secondary ydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well-preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K-metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground-surge deposit.
    • Fluid inclusion evidence for impact-related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP-Chicxulub drill core Yax-1

      Lüders, V.; Rickers, K. (The Meteoritical Society, 2004-01-01)
      Fluid inclusions studies in quartz and calcite in samples from the ICDP-Chicxulub drill core Yaxcopoil-1 (Yax-1) have revealed compelling evidence for impact-induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 degrees C), highly saline, metal-rich, hydrocarbon-bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre-impact organic matter. The center of the crater is assumed to be the source of the hot quartz-forming brines. Fluid inclusions in abundant newly-formed calcite indicate lower cyrstallization temperatures (75100 degrees C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite-forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact-induced conductive heating of the sediments.
    • 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.
    • Foraminiferal biostratigraphy and paleoenvironmental reconstruction at Yaxcopil-1 drill hole, Chicxulub crater, Yucatán Peninsula

      Arz, J. A.; Alegret, L.; Arenillas, I. (The Meteoritical Society, 2004-01-01)
      The Yaxcopoil-1 (Yax-1) drill hole comprises Cretaceous limestones and calcarenites, the K/P boundary cocktail unit (including impact breccia), and a Danian marly clay layer overlain by calcareous marls. The biostratigraphy, paleobathymetry, and environmental turnover across the K/P interval were inferred after analyzing the planktic and benthic foraminiferal assemblages. The Cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle Campanian to Maastrichtian age, while low-diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic, occasionally stressed environment. The impact breccia and the redeposited suevite are overlain by a 46 cm-thick dolomitic calcareous sandstone unit that contains scarce, reworked planktic foraminiferal specimens. This unit probably represents the uppermost part of the initial infill of the crater. The uppermost centimeters of this unit are bioturbated, and its top represents a hiatus that spans at least the G. cretacea, Pv. eugubina, and part of the P. pseudobulloides biozones. This unit is overlain by a 3-4 cm-thick marly clay layer that represents a condensed layer. Benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer. The increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the G. compressa biozone toward the A. uncinata (P2) biozone. The Danian planktic and benthic foraminiferal assemblages indicate a deeper, probably bathyal environment.
    • Formation age of the lunar crater Giordano Bruno

      Morota, T.; Haruyama, J.; Miyamoto, H.; Honda, C.; Ohtake, M.; Yokota, Y.; Matsunaga, T.; Hirata, N.; Demura, H.; Takeda, H.; et al. (The Meteoritical Society, 2009-01-01)
      Using the Terrain Camera onboard the Japanese lunar explorer, SELENE (Kaguya), we obtained new high-resolution images of the 22-kilometer-diameter lunar crater Giordano Bruno. Based on crater size-frequency measurements of small craters (<200 m in diameter) superposed on its continuous ejecta, the formation age of Giordano Bruno is estimated to be 1 to 10 Ma. This is constructive evidence against the crater's medieval age formation hypothesis.
    • Formation of accretionary dust mantles in the solar nebula: Evidence from preirradiated olivines in CM chondrites

      Metzler, Knut (The Meteoritical Society, 2004-01-01)
      CM chondrites are regolith breccias consisting of lithic clasts embedded in a fine-grained clastic matrix. The majority of these lithic clasts belongs to a texturally well-defined rock type (primary rock) that can be described as an agglomerate of chondrules and other coarse-grained components, most of which are surrounded by fine-grained rims (dust mantles). Metzler et al. (1992) explain these textures as the result of accretionary processes in the solar nebula, while an alternative model explains them to be the result of regolith processes on the parent body (Sears et al. 1993). The main intention of the present study is to discern between both models by investigating the occurrence, frequency, spatial distribution, and textural setting of preirradiated (track-rich) olivines in CM chondrites. Track-rich olivines were studied in situ in six polished thin sections from 4 different CM chondrites (Cold Bokkeveld, Mighei, Murchison, Nogoya) by optical and scanning electron microscopy (SEM). It was found that their occurrence is restricted to the clastic matrix of these meteorites. The primary rock seems to have formed in an environment shielded from cosmic radiation, since fragments of this rock are free of track-rich grains and solar noble gases. This finding supports the solar nebula model for the formation of dust mantles around chondrules and other coarse-grained components, and points against a regolith origin. In Cold Bokkeveld, a small breccia-in-breccia clast was found, which has been irradiated as an entity within the uppermost millimeters to meters of its parent body for at least about 3 Ma. This clast seems to represent a compacted subsurface layer that was later excavated by impact and admixed to the host breccia. Furthermore, the results of this study may affect the interpretation of compaction ages obtained by fission track methods, since these ages may be mixtures of different contact ages between finegrained, U-rich dust and U-poor olivines. In some cases, they may date the formation of dust mantles in the solar nebula, while in other cases the lithification of the host breccias may be dated.
    • Formation of the binary near-Earth object 1996 FG3: Can binary NEOs be the source of short-CRE meteorites?

      Morbidelli, A.; Levison, H. F.; Bottke, W. F. (The Meteoritical Society, 2006-01-01)
      1996 FG3 is a binary near-Earth object (NEO) that was likely formed during a tidal disruption event. Our results indicate that the formation of this binary object was unlikely to have occurred when the progenitor had a encounter velocity with the Earth significantly smaller than its current value (10.7 km/s); The formation of the binary object on an orbit similar to the present one is possible, and the survival of the satellite constrains this to have happened less than 1.6 Ma ago. However, the binary object could also have been formed when the progenitor's encounter velocity with Earth was >12 km/s, and in this case we cannot constrain its formation age. Our results indicate that tidal disruptions occurring among NEOs with low velocity encounters with Earth are unlikely to produce long-lasting NEO binaries. Thus, tidal disruption may not be able to completely re-supply the observed population. This would imply that a significant fraction of the observed NEO binaries evolved out of the main asteroid belt. Overall, our results suggest to us that the CM2 meteorites having cosmic ray exposure (CRE) ages of ~200,000 yr were likely liberated by the tidal disruption of a primitive NEO with a relative velocity with the Earth significantly smaller than that of 1996 FG3. We propose a list of such objects, although as far as we know, none of the candidates is a binary for the reasons described above.