bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock and thermal history of Martian meteorite Allan Hills 84001 from transmission electron microscopy
Authors: Barber David J. , Scott Edward R. D.
Published: January 2010

Abstract— Microstructures in the Allan Hills 84001 meteorite were studied using optical and electron microscopy, putting emphasis on shock effects, which are widespread. Some orthopyroxene exhibits only (100) slip, but more typical grains suffered extensive slip, microfracturing, and frequently contain (100) clino‐inversion lamellae. In fracture zones, shock deformation of orthopyroxene has produced all three effects in profusion, together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae, which were apparently created by shearing on low index planes, usually (100) or {110}. Both types of plane are loci that pseudo‐planar fractures tend to follow. Thus, the glass lamellae, which have not been observed in other meteorites, probably formed by frictional heating during the sliding of microscale corrugated surfaces, one over another, leading to local melting. We infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock. Ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation. Nanocrystals of eskolaite (Cr2O3) coating silica glass grains are probably also a result of shock‐induced thermal decomposition of chromite. Iron sulfides (pyrite and pyrrhotite were identified) tended to be associated with plagioclase glass. A carbonate disk showing no evidence for shock deformation had a substructure of elongated, slightly misoriented subcells in the exterior; interior regions had more eqiaxed subcells. Both microstructures probably formed during growth, but the conditions are undetermined. Chemical composition varied on a micron scale, but the rim of the disk was more ferroan; oxide precipitates and voids were widely distributed as in fracture‐filling carbonates. If the fracture zones and opx glass are the result of strong shock, as we deduce, it is very unlikely that pores could have filled by carbonate long after the fracture zones formed. We infer that the carbonate, like the phosphate, olivine, pyrrhotite, eskolaite, and many euhedral, submicrometer chromites, crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase, silica, and orthopyroxene.
Shişr 043 (IIIAB medium octahedrite): The first iron meteorite from the Oman desert
Authors: AL‐KATHIRI A. , HOFMANN B. A. , GNOS E. , EUGSTER O. , WELTEN K. C. , KRÄHENBÜHL U.
Published: January 2010

Abstract— The iron meteorite Shişr 043 is a single mass of 8267 g found in the south Oman desert 42 km NE of the Shişr village. It is the first iron identified among the >1400 individual meteorites reported from Oman. The meteorite is a slightly elongated mass showing only minor rusting, a partially smooth and partially rough surface with octahedral cleavage, and a partially preserved metallic fusion crust typically 0.75 mm thick.

The undeformed Widmanstätten pattern with a mean kamacite bandwidth of 1.0 ± 0.1 mm (n = 97) indicates structural classification as a medium octahedrite. From the bulk composition, Ni = 8.06 wt%, Ga = 18.8 ppm, Ge = 37.25 ppm, and Ir = 3.92 ppm, the meteorite is classified as IIIAB, the most common group of iron meteorites. The cosmic‐ray exposure (CRE) age based on 3He, 21Ne, 38Ar concentrations and 10Be‐21Ne, 26Al‐21Ne, and 36Cl‐36Ar ratios is 290 ± 20 Ma. This age falls within the range observed for type IIIAB iron meteorites, but does not coincide with the main cluster. The cosmogenic noble gas and radionuclide data indicate that Shişr 043 had a relatively small preatmospheric mass. The low degree of weathering is consistent with a young terrestrial age of <10,000 years based on the saturated 41Ca concentration. Shişr 043 is not paired with any of the other eight known iron meteorites from the Arabian Peninsula.
Seismic analysis for the Lanzhou fireball
Authors: DAILU Rong , YARONG Li
Published: January 2010

Abstract— In this paper, we convert the Lanzhou fireball's trajectory using seismic data according to the analytical method presented in Pujol et al. (2005, 2006). Taking the same assumptions as Pujol et al., the position of the fireball burst at its terminal has been converted using a relative simple independent method. Both the trajectory and the position of burst are roughly coincident.
Magnetometer survey of the proposed Sirente meteorite crater field, central Italy: Evidence for uplifted crater rims and buried meteorites
Authors: ORMÖ Jens , Gomez‐Ortiz David , McGuire Patrick C. , Henkel Herbert , Komatsu Goro , Rossi Angelo Pio
Published: January 2010

Abstract— The Sirente crater field consists of a 120 m wide, rimmed main depression flanked to the northwest by about 30 smaller depressions. It has been dated to the first centuries A.D. An impact origin is suggested, but not confirmed. The small size combined with the properties of the target material (carbonate mud) would neither allow shock features diagnostic of impact, nor projectile vaporization. Consequently, a meteoritic component in the sediments would be very localized. At impacts of this size the projectile most likely is an iron meteorite. Any iron meteorites on the ground surface would, in Iron Age Europe, have been removed shortly after the event. However, if the depressions are of impact origin they should contain meteorites at great depth in analogy with known craters. The magnetic properties of iron meteorites differ distinctly from the very low magnetic sediments and sedimentary rocks of the Sirente area. We have used a proton precession magnetometer/gradiometer to produce magnetic anomaly maps over four of the smaller depressions (∼8 m diameter), as well as two crossing profiles over a fifth depression (∼22 m diameter). All show distinct magnetic anomalies of about 20 nT, the larger depression up to 100 nT. Magnetic modeling shows a best fit for structures with upturned strata below their rims, excluding a karstic origin but supporting an explosive formation. The 100 nT anomaly can only be explained by highly‐magnetic objects at a few meters depth. All together, the magnetic data provides a strong indication for an impact origin of the crater field.
Nitrogen components in IAB/IIICD iron meteorites
Authors: PONGANIS K. V. , MARTI K.
Published: January 2010

Abstract— Isotopic variations have been reported for many elements in iron meteorites, with distinct N signatures found in the metal and graphite of IAB irons. In this study, a dozen IAB/IIICD iron meteorites (see Table 1 for new classifications) were analyzed by stepwise pyrolysis to resolve nitrogen components. Although isotopic heterogeneity has been presumed to be lost in thermally processed parent objects, the high‐resolution nitrogen isotopic data indicate otherwise. At least one reservoir has a light nitrogen signature, δ15N = −(74 ± 2)‰, at 900 °C to 1000 °C, with a possible second, even lighter, reservoir in Copiapo (δ15N ≤ −82‰). These releases are consistent with metal nitride decomposition or low‐temperature metal phase changes. Heavier nitrogen reservoirs are observed in steps ≤700 °C and at 1200 °C to 1400 °C. The latter release has a δ15N signature with a limit of ≥−16‰. Xenon isotopic signatures are sensitive indicators for the presence of inclusions because of the very low abundances of Xe in metal. The combined high‐temperature release shows 131Xe and 129Xe excesses to be consistent with shifts expected for Te(n,γ) reaction in troilite by epithermal neutrons, but there are also possible alterations in the isotopic ratios likely due to extinct 129I and cosmic‐ray spallation.

The IAB/IIICD iron data imply that at least one light N component survived the formation processes of iron parent objects which only partially exchanged nitrogen between phases. Preservation of separate N reservoirs conflicts with neither the model of impact‐heating effects for these meteorites nor reported age differences between metal and silicates.
Noble gases in mineral separates from three shergottites: Shergotty, Zagami, and EETA79001
Authors: Schwenzer Susanne P. , Herrmann Siegfried , Mohapatra Ratan K. , Ott Ulrich
Published: January 2010

Abstract— This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic‐ray exposure ages from 3He, 21Ne, and 38Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic‐ray effects. While a contribution of this low‐energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic‐ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near‐surface siting. We derive an estimate for the 40Ar/36Ar ratio of the Martian interior component by subtracting from measured Ar in the (K‐poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129Xe, * excesses. Unless compromised by the presence of additional components, a high ratio of ∼2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40Ar/36Ar in the Martian mantle.
Thermal history of ALH 84001 meteorite by Fe2+‐Mg ordering in orthopyroxene
Authors: Domeneghetti M. Chiara , Fioretti Anna M. , Cámara Fernando , Molin Gianmario , Tazzoli Vittorio
Published: January 2010

Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe2+‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient kD. The 529 ± 30°C closure temperature (Tc) of the Fe2+‐Mg ordering process of ALH 84001 orthopyroxene (Fs28) was calculated using Stimpfl (2005a, 2005b) ln kD versus 1/T equation obtained for intermediate iron sample. At this Tc, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the Tc of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3.
Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende
Authors: Fagan T. J. , Guan Y. , MacPherson G. J.
Published: January 2010

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence.

The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of 26Mg, whereas the primary CAI phases mostly yield correlated excesses of 26Mg with increasing Al/Mg corresponding to “canonical” initial 26Al/27Al ∼ 4.5–5 × 10−5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in 26Mg/24Mg with increasing Al/Mg, yielding (26Al/27Al)0 = (4.9 ± 2.8) × 10−6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of 26Mg consistent with initial 26Al/27Al ∼ 4.5 × 10−5. Especially in the compact type A CAI, where 26Mg/24Mg in grossular correlates with increasing Al/Mg, these 26Mg excesses are almost certainly due to in situ decay of 26Al. They indicate a nebular setting for formation of the grossular.

The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.
Late accretion and lithification of chondritic parent bodies: Mg isotope studies on fragments from primitive chondrites and chondritic breccias
Authors: Sokol Anna K. , Bischoff Addi , Marhas Kuljeet K. , Mezger Klaus , Zinner Emst
Published: January 2010

Abstract— 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 calcium‐aluminum‐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.
Constraints on the cooling history of the H‐chondrite parent body from phosphate and chondrule Pb‐isotopic dates from Estacado
Authors: Blinova Alexandra , Amelin Yuri , Samson Claire
Published: January 2010

Abstract— To constrain the metamorphic history of the H‐chondrite parent body, we dated phosphates and chondrules from four H6 chondritic meteorites using U‐Pb systematics. Reconnaissance analyses revealed that only Estacado had a sufficiently high 206Pb/204Pb ratio suitable for our purposes. The Pb‐Pb isochron date for Estacado phosphates is measured to be 4492 ± 15 Ma. The internal residue‐second leachate isochron for Estacado chondrules yielded the chondrule date of 4546 ± 18 Ma. An alternative age estimate for Estacado chondrules of 4527.6 ± 6.3 Ma is obtained from an isochron including two chondrules, two magnetically separated fractions, and four bulk chondrite analyses. This isochron date might represent the age of termination of Pb diffusion from the chondrules to the matrix. From these dates and previously established closure temperatures for Pb diffusion in phosphates and chondrules, we estimate an average cooling rate for Estacado between 5.5 ± 3.2 Myr/°C and 8.3 ± 5.0 Myr/°C. Using previously published results for Ste. Marguerite (H4) and Richardton (H5), our data reveal that the cooling rates of H chondrites decrease markedly with increasing metamorphic grade, in agreement with the predictions of the “onion‐shell” asteroid model. Several issues, however, need to be addressed before confirming this model for the H‐chondrite parent body: the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved, diffusion and other mechanisms of element migration in polycrystalline solids must be better understood, and dating techniques should be further improved.
Non‐mass‐dependent oxygen isotopic fractionation in smokes produced in an electrical discharge
Authors: Kimura Yuki , Nuth Joseph A. , Chakraborty Subrata , Thiemens Mark H.
Published: January 2010

Abstract— We report the first production of non‐mass‐dependently fractionated silicate smokes from the gas phase at room temperature from a stream of silane and/or pentacarbonyl iron in a molecular hydrogen (or helium) flow mixed with molecular oxygen (or nitrous oxide). The smokes were formed at the Goddard Space Flight Center (GSFC) at total pressures of just under 100 Torr in an electrical discharge powered by a Tesla coil, were collected from the surfaces of the copper electrodes after each experiment and sent to the University of California at San Diego (UCSD) for oxygen isotopic analysis. Transmission electron microscopy studies of the smokes show that they grew in the gas phase rather than on the surfaces of the electrodes. We hypothesize at least two types of fractionation processes occurred during formation of the solids: a mass‐dependent process that made isotopically lighter oxides compared to our initial oxygen gas composition followed by a mass‐independent process that produced oxides enriched in 17O and 18O. The maximum Δ17O observed is + 4.7‰ for an iron oxide produced in flowing hydrogen, using O2 as the oxidant. More typical displacements are 1–2‰ above the equilibrium fractionation line. The chemical reaction mechanisms that yield these smokes are still under investigation.
Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources
Authors: Sahijpal S. , Soni P. , Gupta G.
Published: January 2010

Abstract— Numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26Al and 60Fe as the heat sources. Planetesimal accretion was started at chosen times up to 3 Ma after Ca‐Al‐rich inclusions (CAIs) were formed, and was continued for periods of 0.001–1 Ma. The planetesimals were initially porous, unconsolidated bodies at 250 K, but became sintered at around 700 K, ending up as compact bodies whose final radii were 20, 50, 100, or 270 km. With further heating, the planetesimals underwent melting and igneous differentiation. Two approaches to core segregation were tried. In the first, labelled A, the core grew gradually before silicate began to melt, and in the second, labelled B, the core segregated once the silicate had become 40% molten. In A, when the silicate had become 20% molten, the basaltic melt fraction began migrating upward to the surface, carrying 26Al with it. The 60Fe partitioned between core and mantle. The results show that the rate and timing of core and crust formation depend mainly on the time after CAIs when planetesimal accretion started. They imply significant melting where accretion was complete before 2 Ma, and a little melting in the deep interiors of planetesimals that accreted as late as 3 Ma. The latest melting would have occurred at <10 Ma. The effect on core and crust formation of the planetesimal's final size, the duration of accretion, and the choice of (60Fe/56Fe)initial were also found to be important, particularly where accretion was late. The results are consistent with the isotopic ages of differentiated meteorites, and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 Ma after CAIs.
Characterization of the log lithology of cores LB‐07A and LB‐08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility
Authors: SCHELL Christina , SCHLEIFER Norbert , ELBRA Tiiu
Published: January 2010

Abstract— Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock‐forming and rock‐changing processes, e.g., deformation or sedimentation.

To characterize the rocks in drill cores LB‐07A and LB‐08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite.

The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta‐graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB‐07A and LB‐08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta‐graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB‐07A towards core LB‐08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.
Microstructure and thermal history of metal particles in CH chondrites
Authors: GOLDSTEIN J. I. , JONES R. H. , KOTULA P. G. , MICHAEL J. R.
Published: January 2010

Abstract We have studied metal microstructures in four CH chondrites, Patuxent Range (PAT) 91546, Allan Hills (ALH) 85085, Acfer 214, and Northwest Africa (NWA) 739, to examine details of the thermal histories of individual particles. Four types of metal particles are common in all of these chondrites. Zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime, as has been shown previously. We have demonstrated that these particles were cooled rapidly to temperatures below 500 K after they formed, and that condensation effectively closed around 700 K. Zoned and unzoned particles with exsolution precipitates, predominantly high‐Ni taenite, have considerably more complex thermal histories. Precipitates grew in reheating episodes, but the details of the heating events vary among individual grains. Reheating temperatures are typically in the range 800–1000 K. Reheating could have been the result of impact events on the CH parent body. Some particles with precipitates may have been incorporated into chondrules, with further brief heating episodes taking place during chondrule formation. In addition to the four dominant types of metal particles, rare Ni‐rich metal particles and Si‐rich metal particles indicate that the metal assemblage in CH chondrites was a mixture of material that formed at different redox conditions. Metal in CH chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites.
Gain and loss of uranium by meteorites and rocks, and implications for the redistribution of uranium on Mars
Authors: DREIBUS Gerlind , HAUBOLD Reinhard , HUISL Wolfgang , SPETTEL Bernhard
Published: January 2010

Abstract Terrestrial alteration of meteorites results in the redistribution, gain, or loss of uranium and other elements. We have measured the maximum U adsorption capacity of a meteorite and two geochemical reference materials under conditions resembling terrestrial ones (pH 5.8). The basaltic eucrite Sioux County adsorbs 7 ppm of U. The result for the terrestrial granite AC‐E is similar (5 ppm), while the basalt BE‐N adsorbs 34 ppm of U.

We have also investigated U adsorption in the presence of phosphate (0.01 M or less) in imitation of conditions that probably occurred in the earlier history of Mars. Such a process would have alterated Martian surface material and would be noticeable in Martian meteorites from the affected surface. The experiments demonstrated the counteracting effects of phosphate, which increases U adsorption, but decreases the quantity of dissolved U that is available for adsorption. U adsorption by AC‐E increases to 7 ppm. The lowered value for BE‐N of 8 ppm results from the low quantity of dissolved U in the volume of solution used.

The results from the adsorption experiments and from leaching the Martian meteorite Zagami and a terrestrial basalt imply that the aqueous redistribution of U on Mars was moderate. Acidic liquids mobilized uranium and other metals, but present phosphate impeded the dissolution of U compounds. Some mobilized U may have reached the global sinks, while most of it probably was transported in the form of suspended particles over a limited distance and then settled.
Comment on: “New” lunar meteorites: Impact melt and regolith breccias and large‐scale heterogeneities of the upper lunar crust, by P. H. Warren, F. Ulff‐Møller, and G. W. Kallemeyn
Authors: JAMES Odette B. , COHEN Barbara A. , TAYLOR Lawrence A. , NAZAROV Mikhail A.
Published: January 2010

Abstract We described lunar meteorite Dhofar 026 (Cohen et al. 2004) and interpreted this rock as a strongly shocked granulitic breccia (or fragmental breccia consisting almost entirely of granulitic‐breccia clasts) that was partially melted by post‐shock heating. Warren et al. (2005) objected to many aspects of our interpretation: they were uncertain whether or not the bulk rock had been shocked; they disputed our identification of the precursor as granulitic breccia; and they suggested that mafic, igneous‐textured globules within the breccia, which we proposed were melted by post‐shock heating, are clasts with relict textures. The major evidence for shock of the bulk rock is the fact that the plagioclase in the lithologic domains that make up 80–90% of the rock is devitrified maskelynite. The major evidence for a granulitic‐breccia precursor is the texture of the olivine‐plagioclase domain that constitutes 40—45% of the rock; Warren et al. apparently overlooked or ignored this lithology. Textures of the mafic, igneous‐textured globules, and especially of the vesicles they contain, demonstrate that these bodies were melted and crystallized in situ. Warren et al. suggested that the rock might have originally been a regolith breccia, but the textural homogeneity of the rock and the absence of solar wind—derived noble gases preclude a regolith‐breccia precursor. Warren et al. classified the rock as an impact‐melt breccia, but they did not identify any fraction that was impact melt.
Bulk mineralogy and three‐dimensional structures of individual Stardust particles deduced from synchrotron X‐ray diffraction and microtomography analysis
Authors: Nakamura Tomoki , Tsuchiyama Akira , Akaki Takeshi , Uesugi Kentaro , Nakano Tsukasa , Takeuchi Akihisa , Suzuki Yoshio , Noguchi Takaaki
Published: January 2010

Abstract— During preliminary examination of particles released from 81P/Wild 2 short‐period comet, we analyzed 28 particles by nondestructive means, high‐sensitive X‐ray diffraction and high‐resolution X‐ray tomography, in order to characterize bulk mineralogy and three‐dimensional structures of individual particles. The analyses were performed at synchrotron facilities, KEK and SPring‐8 in Japan. Twenty‐eight particles from 5 to 25 μm in size, including 25 particles from Track 35 and 3 particles from Track 44, were first analyzed by X‐ray diffraction and then 4 out of 28 particles were analyzed by X‐ray tomography.

All particles are classified into two groups based on silicate crystallinity: crystalline type and amorphous‐rich type. The abundance of the former is approximately 10% of the particles investigated. Crystalline type shows very sharp reflections of olivine and low‐Ca pyroxene, while amorphous‐rich type shows no or very weak silicate reflections, suggesting that silicates are mostly amorphous. Broad reflections of Fe sulfides and Fe silicides are detected from most of amorphous‐rich type particles. Subsequent tomography analysis revealed that the crystalline type is non‐porous material consisting of coarse silicate crystals larger than 1 μm in size, while the amorphous‐rich type is very porous aggregates with amorphous silicates and small Fe sulfide and Fe metallic grains. All characteristics of amorphous‐rich type particles indicate that most of them are melted and rapidly solidified during capture in the silica aerogel. On the other hand, the crystalline type is indigenous cometary particle formed through high‐temperature heating episodes that have taken place prior to formation of comet Wild 2. One of the crystalline‐type particles (C2054,0,35,6,0) consists of Mg‐rich olivine, pyroxene, and kamacite and exhibits porphyritic or poikilitic texture very similar to chondrules.
Combined micro‐Raman, micro‐infrared, and field emission scanning electron microscope analyses of comet 81P/Wild 2 particles collected by Stardust
Authors: Rotundi A. , Baratta G. A. , Borg J. , Brucato J. R. , Busemann H. , Colangeli L. , D'Hendecourt L. , Djouadi Z. , Ferrini G. , Franchi I. A. , Fries M. , Grossemy F. , Keller L. P. , Mennella V. , Nakamura K. , Nittler L. R. , Palumbo M. E. , Sandford S. A. , Steele A. , Wopenka B.
Published: January 2010

Abstract— We report combined micro‐infrared, micro‐Raman, and field emission scanning electron microscope (FESEM) analyses of particles collected by the Stardust spacecraft during its flyby of comet 81P/Wild 2 on 2 January 2004 and successfully returned back to Earth on 15 January 2006. We present mid‐infrared (IR) spectra of six of these particles. The CH2/CH3 ratios inferred from the infrared data are greater than those seen in organics in the diffuse interstellar medium, possibly indicating the presence of longer or less branched aliphatic chains. The micro‐Raman data offer insights into the state of the order of the carbonaceous component present in the particles. Raman parameters for most of the particles span a similar range to that observed in interplanetary dust particles (IDPs) and the most primitive meteorites. Both the IR and Raman data imply the presence of a very labile carbonaceous component. Hydrated silicates may be present in two particles of Track 35, one of which may also contain carbonates, but further investigations with other techniques need to be performed to confirm these findings. In some cases, the analyses are difficult to interpret because of the presence of compressed aerogel mixed with the grains.
Evidence for a meteoritic origin of the September 15, 2007, Carancas crater
Authors: PICHON A. Le , ANTIER K. , CANSI Y. , HERNANDEZ B. , MINAYA E. , BURGOA B. , DROB D. , EVERS L. G. , VAUBAILLON J.
Published: January 2010

Abstract— On September 15th, 2007, around 11:45 local time in Peru, near the Bolivian border, the atmospheric entry of a meteoroid produced bright lights in the sky and intense detonations. Soon after, a crater was discovered south of Lake Titicaca. These events have been detected by the Bolivian seismic network and two infrasound arrays operating for the Comprehensive Nuclear‐Test‐Ban Treaty Organization, situated at about 80 and 1620 km from the crater. The localization and origin time computed with the seismic records are consistent with the reported impact. The entry elevation and azimuthal angles of the trajectory are estimated from the observed signal time sequences and back‐azimuths. From the crater diameter and the airwave amplitudes, the kinetic energy, mass and explosive energy are calculated. Using the estimated velocity of the meteoroid and similarity criteria between orbital elements, an association with possible parent asteroids is attempted. The favorable setting of this event provides a unique opportunity to evaluate physical and kinematic parameters of the object that generated the first actual terrestrial meteorite impact seismically recorded.
Post‐impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?
Authors: MUTTIK Nele , KIRSIMÄE Kalle , Somelar Peeter , Osinski Gordon R.
Published: January 2010

Abstract— Alteration of surficial suevites at Ries crater, Germany was studied by means of X‐ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post‐impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al‐Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post‐impact cooling, then the alteration was of limited character and is completely masked by later weathering.
The effect of target lithology on the products of impact melting
Authors: OSINSKI G. R. , GRIEVE R. A. F. , COLLINS G. S. , MARION C. , SYLVESTER P.
Published: January 2010

Abstract— Impact cratering is an important geological process on the terrestrial planets and rocky and icy moons of the outer solar system. Impact events generate pressures and temperatures that can melt a substantial volume of the target; however, there remains considerable discussion as to the effect of target lithology on the generation of impact melts. Early studies showed that for impacts into crystalline targets, coherent impact melt rocks or “sheets” are formed with these rocks often displaying classic igneous structures (e.g., columnar jointing) and textures. For impact structures containing some amount of sedimentary rocks in the target sequence, a wide range of impact‐generated lithologies have been described, although it has generally been suggested that impact melt is either lacking or is volumetrically minor. This is surprising given theoretical constraints, which show that as much melt should be produced during impacts into sedimentary targets. The question then arises: where has all the melt gone? The goal of this synthesis is to explore the effect of target lithology on the products of impact melting. A comparative study of the similarly sized Haughton, Mistastin, and Ries impact structures, suggests that the fundamental processes of impact melting are basically the same in sedimentary and crystalline targets, regardless of target properties. Furthermore, using advanced microbeam analytical techniques, it is apparent that, for the structures under consideration here, a large proportion of the melt is retained within the crater (as crater‐fill impactites) for impacts into sedimentary‐bearing target rocks. Thus, it is suggested that the basic products are genetically equivalent but they just appear different. That is, it is the textural, chemical and physical properties of the products that vary.
Comment on: Constraints on the depth and variability of the lunar regolith, by B. B. Wilcox, M. S. Robinson, P. C. Thomas, and B. R. Hawke
Authors: OBERBECK Verne R.
Published: January 2010

Abstract— Any permanent presence on the Moon will require use of materials from the lunar regolith, the surface soil layer on the Moon. Thus, knowledge of the thickness of the lunar regolith is essential. It has been proposed that crater counts obtained from high Sun angle photography give larger estimates of impact crater equilibrium diameters than for low Sun angle photography, and thus deeper estimates of lunar surface regolith than were previously made using crater morphology, size of blocky rimmed craters, and equilibrium diameters determined on low Sun angle images. The purpose of this comment is to evaluate this result as a means of resolving this important question before planning for future lunar missions is undertaken
Rotational excitation and damping as probes of interior structures of asteroids and comets
Authors: SAMARASINHA Nalin H.
Published: January 2010

Abstract— Rotational excitation and damping are discussed in the context of inferring structural properties of asteroids and comets. Opportunities for carrying out deterministic experiments are outlined and basic concepts involving space missions are discussed. Spacecraft carrying an impactor or explosives together with an orbiter are suggested as effective probes of the interiors of asteroid and comets. The feasibility of such missions, especially to near‐Earth objects (NEOs), is highlighted as NEOs provide an appropriate cost‐effective path to explore interiors of asteroids and comets.
Characterization of micron‐sized Fe,Ni metal grains in fine‐grained rims in the Y‐791198 CM2 carbonaceous chondrite: Implications for asteroidal and preaccretionary models for aqueous alteration
Authors: CHIZMADIA Lysa J. , XU Youren , SCHWAPPACH Chris , BREARLEY Adrian J.
Published: January 2010

Abstract— –The presence of apparently unaltered, micron‐sized Fe,Ni metal grains, juxtaposed against hydrated fine‐grained rim materials in the CM2 chondrite Yamato (Y‐) 791198 has been cited as unequivocal evidence of preaccretionary alteration. We have examined the occurrence, composition, and textural characteristics of 60 Fe,Ni metal grains located in fine‐grained rims in Y‐791198 using scanning electron microscopy (SEM) and electron microprobe analysis. In addition, three metal grains, prepared by focused ion beam (FIB) sample preparation techniques were studied by transmission electron microscopy (TEM). The metal grains are heterogeneously distributed within the rims. Electron microprobe analyses show that all the metal grains are kamacite with minor element contents (P, Cr, and Co) that lie either within or close to the range for other CM2 metal grains. X‐ray maps obtained by electron microprobe show S, P, and/or Ca enrichments on the outermost parts of many of the metal grains. Z‐contrast STEM imaging of FIB‐prepared Fe,Ni metal grains show the presence of a small amount of a lower Z secondary phase on the surface of the grains and within indentations on the grain surfaces. Energy‐filtered TEM (EFTEM) compositional mapping shows that these pits are enriched in oxygen and depleted in Fe relative to the metal. These observations are consistent with pitting corrosion of the metal on the edges of the grains and we suggest may be the result of the formation of Fe(OH)2, a common oxidation product of Fe metal. The presence of such a layer could have inhibited further alteration of the metal grains. These findings are consistent with alteration by an alkaline fluid as suggested by Zolensky et al. (1989), but the location of this alteration remains unconstrained, because Y‐791198 was recovered from Antarctica and therefore may have experienced incipient terrestrial alteration. However, we infer that the extremely low degree of oxidation of the metal is inconsistent with weathering in Antarctica and that alteration in an extraterrestrial environment is more probable. Although the presence of unaltered or incipiently altered metal grains in these fine‐grained rims could be interpreted as evidence for preaccretionary alteration, we suggest an alternative model in which metal alteration was inhibited by alkaline fluids on the asteroidal parent body.
Explicating the behavior of Mn‐bearing phases during shock melting and crystallization of the Abee EH‐chondrite impact‐melt breccia
Authors: RUBIN Alan E.
Published: January 2010

Abstract— –Literature data show that, among EH chondrites, the Abee impact‐melt breccia exhibits unusual mineralogical characteristics. These include very low MnO in enstatite (<0.04 wt%), higher Mn in troilite (0.24 wt%) and oldhamite (0.36 wt%) than in EH4 Indarch and EH3 Kota‐Kota (which are not impact‐melt breccias), low Mn in keilite (3.6–4.3 wt%), high modal abundances of keilite (11.2 wt%) and silica (∼7 wt%, but ranging up to 16 wt% in some regions), low modal abundances of total silicates (58.8 wt%) and troilite (5.8 wt%), and the presence of acicular grains of the amphibole, fluor‐richterite. These features result from Abee's complex history of shock melting and crystallization. Impact heating was responsible for the loss of MnO from enstatite and the concomitant sulfidation of Mn. Troilite and oldhamite grains that crystallized from the impact melt acquired relatively high Mn contents. Abundant keilite and silica also crystallized from the melt; these phases (along with metallic Fe) were produced at the expense of enstatite, niningerite and troilite. Melting of the latter two phases produced a S‐rich liquid with higher Fe/Mg and Fe/Mn ratios than in the original niningerite, allowing the crystallization of keilite. Prior to impact melting, F was distributed throughout Abee, perhaps in part adsorbed onto grain surfaces; after impact melting, most of the F that was not volatilized was incorporated into crystallizing grains of fluor‐richterite. Other EH‐chondrite impact‐melt breccias and impact‐melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories.
Northwest Africa 2526: A partial melt residue of enstatite chondrite parentage
Authors: KEIL Klaus , BISCHOFF Addi
Published: January 2010

Abstract— NWA 2526 is a coarse‐grained, achondritic rock dominated by equigranular grains of polysynthetically twinned enstatite (˜85 vol%) with frequent 120° triple junctions and ˜10–15 vol% of kamacite + terrestrial weathering products. All other phases including troilite, daubreelite, schreibersite, and silica‐normative melt areas make up <˜1 vol% of the rock Oxygen isotopic analyses are well within the range of those for enstatite chondrites and aubrites. We show that the “enstatite achondrite” (Russell et al. 2005) Northwest Africa (NWA) 2526 is actually a partial melt residue of an enstatite chondrite‐like lithology that experienced ˜20 vol% partial melting. We suggest that the heat source was internal to the parent body. The FeS‐Fe,Ni and plagioclase‐enstatite partial melts were removed from the parent lithology, leaving NWA 2526 as a residue highly depleted in troilite and lacking plagioclase. Sub‐solidus slow cooling and annealing is responsible for the coarsegrained, recrystallized texture of the rock. We also suggest that the parent lithology of NWA 2526, prior to partial melting, experienced a shock event which formed the curvilinear trails of blebs of minor troilite and rare metal that are enclosed in enstatite crystals; thus, these represent relicts. After partial melting and annealing, NWA 2526 experienced a second, relatively mild impact event (<˜20 GPa) that caused formation of the polysynthetic twinning in the enstatite.

We suggest that the meteorite Zaklodzie, which has been referred to as a “primitive enstatite achondrite” (Przylibski et al. 2005), did not form from a magma of internal origin, but that it is an impact‐melt breccia of enstatite chondrite‐like parentage, as previously discussed by Burbine et al. (2000) and Keil (2007).

Finally, the “metal‐rich enstatite meteorite with achondritic texture” Itqiy (Patzer et al. 2001) formed by processes very similar to those responsible for formation of NWA 2526 and is also the residue of ˜20 vol% partial melting of an enstatite chondrite‐like parent lithology, with the FeS‐Fe,Ni and plagioclase‐enstatite partial melts having been removed from the residue. It also experienced an impact event after partial melting that was responsible for the formation of the mixed Mg‐Mn‐Fe‐sulfides and the shock stage S3 features of the enstatite. These similarities indicate that NWA 2526 and Itqiy may have formed on the same parent body. This body was different from the EH, EL, Shallowater and aubrite parent bodies, and NWA 2526 and Itqiy may represent samples from yet another, fifth enstatite meteorite parent body.
Understanding the textures and origin of shock melt pockets in Martian meteorites from petrographic studies, comparisons with terrestrial mantle xenoliths, and experimental studies
Authors: WALTON Erin L. , SHAW Cliff S. J.
Published: January 2010

Abstract— We present a textural comparison of localized shock melt pockets in Martian meteorites and glass pockets in terrestrial, mantle‐derived peridotites. Specific textures such as the development of sieve texture on spinel and pyroxene, and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets. Based on petrographic and compositional observations it is concluded that void collapse/variable shock impedance is able to account for the occurrence of pre‐terrestrial sulfate‐bearing secondary minerals in the melts, high gas emplacement efficiencies, and S, Al, Ca, and Na enrichments and Fe and Mg depletion of shock melt compositions compared to the host rock; previously used as arguments against such a formation mechanism. Recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in Martian meteorites.