bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluid inclusion evidence for impact‐related hydrothermal fluid and hydrocarbon migration in Creataceous sediments of the ICDP‐Chicxulub drill core Yax‐1
Authors: Lüders Volker , Rickers Karen
Published: January 2010

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 °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 (75–100 °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.
Cooling of the Kärdla impact crater: II. Impact and geothermal modeling
Authors: JÕELEHT Argo , KIRSIMÄE Kalle , PLADO Jüri , VERSH Evelin , IVANOV Boris
Published: January 2010

Abstract— Impact and geothermal modeling was performed to explain hydrothermal alteration in a 4 km marine complex crater at Kärdla, Estonia. The impact modeling was used to simulate the formation of the crater and the post‐impact temperature distribution in crater environment. The geothermal modeling accounted for coupled heat transfer and multi‐phase fluid flow in a variably saturated medium. The modeling results suggest that strong convective fluid flow was initiated. During the first stage, the cooling was rapid due to the effect of the latent heat of vaporization, which efficiently decreased the temperature to the boiling point. The modeling results are consistent with geological observations.
A new hematite formation mechanism for Mars
Authors: MINITTI Michelle E. , LANE Melissa D. , BISHOP Janice L.
Published: January 2010

Abstract— The origin of hematite detected in Martian surface materials is commonly attributed to weathering processes or aqueous precipitation. Here, we present a new hematite formation mechanism that requires neither water nor weathering. Glass‐rich basalts with Martian meteorite‐like chemistry (high FeO, low Al2O3) oxidized at high (700 and 900 °C) temperatures in air and CO2, respectively, form thin (<1 μm) hematite coatings on their outermost surfaces. Hematite is manifested macroscopically by development of magnetism and a gray, metallic sheen on the glass surface and microscopically by Fe enrichment at the glass surface observed in element maps. Visible and near‐infrared, thermal infrared, and Raman spectroscopy confirm that the Fe enrichment at the oxidized glass surfaces corresponds to hematite mineralization. Hematite formation on basaltic glass is enabled by a mechanism that induces migration of Fe2+ to the surface of an oxidizing glass and subsequent oxidation to form hematite. A natural example of the hematite formation mechanism is provided by a Hawaiian basalt hosting a gray, metallic sheen that corresponds to a thin hematite coating. Hematite coating development on the Hawaiian basalt demonstrates that Martian meteorite‐like FeO contents are not required for hematite coating formation on basalt glass and that such coatings form during initial extrusion of the glassy basalt flows. If gray hematite originating as coatings on glassy basalt flows is an important source of Martian hematite, which is feasible given the predominance of igneous features on Mars, then the requirement of water as an agent of hematite formation is eliminated.
The onset of metamorphism in ordinary and carbonaceous chondrites
Authors: GROSSMAN Jeffrey N. , BREARLEY Adrian J.
Published: January 2010

Abstract— Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X‐ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO‐rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr‐rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr‐rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr3+. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re‐enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.
A unique type B inclusion from Allende with evidence for multiple stages of melting
Authors: Simon S. B. , Grossman L. , Davis A. M.
Published: January 2010

Abstract— A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (˜1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 μm long that occur at the inclusion rim. The core consists of blocky melilite grains 20–50 μm across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ˜1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine‐grained (1–10 μm) spinel. The rim laths are normally zoned (Åk30–70) inward from the rim of the inclusion with reverse zoning over the last ˜200 μm to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk5–12) occur. These grains are enclosed in strongly zoned (Åk15–70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na2O‐free, but the outer parts of the overgrowths are not. In the laths at the rim, Na2O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 2–9 wt% TiO2 + Ti2O3. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ˜1400 °C and melts except for gehlenitic (Åk5–12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na2O‐free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ˜1260 °C. Melilite more gehlenitic than Åk40 does not melt. During cooling, Na2O‐bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co‐crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na‐free and Na‐bearing melilite, plus a positive correlation between Na2O and åkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass‐fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse‐grained inclusions also have features consistent with a sequence of early, high‐temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.
Shock waves—Phenomenology, experimental, and numerical simulation
Authors: Thoma Klaus , Hornemann Ulrich , Sauer Martin , Schneider Eberhard
Published: January 2010

Abstract— The purpose of this paper is to review the results of long‐term cooperation between Dieter Stöffler and the authors in the field of shock wave deformation of minerals and rocks. First, the principal phenomena of shock wave generation and propagation, predominantly in solid media, are presented, and then analytical and numerical mathematical treatment of shock wave processes on the basis of mass, momentum, and energy conservation laws will be described and discussed. Experimental methods of shock wave investigations by means of impact and explosive techniques are summarized, including hypervelocity acceleration facilities and high‐pressure explosive devices. Shock pressure barometry by means of mineralogical evidence of distinct material phase transitions and characteristic shock structures is also discussed.
Shock melting of ordinary chondrite powders and implications for asteroidal regoliths
Authors: Hörz Friedrich , Cintala Mark J. , See Thomas H. , Le Loan
Published: January 2010

Abstract— A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse‐grained “regolith.” We then subjected the 125–250 μm fines from this sample to reverberation shock stresses of 14.5–67 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact.

The initial pore space (40–50%) was completely closed at 14.5 GPa and a dense aggregate of interlocking grains resulted. Grain‐boundary melting commenced at <27 GPa and ˜50% of the total charge was molten at 67 GPa; this stress corresponds to typical asteroid impacts at ˜5 km/sec. Melting of the entire sample most likely mandates >80 GPa, which is associated with impact velocities >8 km/sec. The Fe‐Ni and troilite clasts of the original meteorite melted with particular ease, forming immiscible melts that are finely disseminated throughout the silicate glass. These metal droplets are highly variable in size, extending to <100 nm and most likely to superparamagnetic domains; such opaques are also observed in the natural melt veins of ordinary chondrites. It follows that melting and dissemination of pre‐existing, Fe‐rich phases may substantially affect the optical properties of asteroidal surfaces. It seems unnecessary to invoke reduction of Fe2+ (or Fe3+) by sputtering or impact‐processes—in analogy to the lunar surface—to produce “space weathering” effects on S‐type asteroids. We note that HED meteorites contain ample FeO (comparable to that in lunar basalts) for reduction processes to take place, yet their probable parent object(s), Vesta and its collisional fragments, display substantially unweathered surfaces. Howardites, eucrites, and diogenites (HEDs), however, contain little native metal (typically <0.5%), in contrast to ordinary chondrites (commonly 10–15%) and their S‐type parent objects. These considerations suggest that the modal content of native metal and sulfides is more important for space weathering on asteroids than total FeO.
Bosumtwi impact structure, Ghana: Geochemistry of impactites and target rocks, and search for a meteoritic component
Authors: Dai Xiongxin , Boamah Daniel , Koeberl Christian , Reimold Wolf Uwe , Irvine Gordon , McDonald Iain
Published: January 2010

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.
Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole, Chicxulub impact structure, Mexico: Implications for target composition
Authors: Tuchscherer Martin G. , Reimold Wolf Uwe , Koeberl Christian , Gibson Roger L.
Published: January 2010

Abstract—We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO2content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.
Structural analysis of the collar of the Vredefort Dome, South Africa—Significance for impact‐related deformation and central uplift formation
Authors: Wieland Frank , Gibson Roger L. , Reimold Wolf Uwe
Published: January 2010

Abstract— Landsat TM, aerial photograph image analysis, and field mapping of Witwatersrand supergroup meta‐sedimentary strata in the collar of the Vredefort Dome reveals a highly heterogeneous internal structure involving folds, faults, fractures, and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort impact event. Broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike‐slip displacements of, typically, tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from ˜17% at a radius from the dome center of 21 km to <5% at a radius of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults or earlier, during the shock pulse. In addition to shatter cones, quartzite units show two fracture types—a cmspaced rhomboidal to orthogonal type that may be the product of shock‐induced deformation and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints, and the presence of radial and tangential dikes of impact melt rock, confirm the impact timing of these features and are suggestive of late‐stage collapse of the central uplift.
Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites
Authors: Guan Yunbin , Huss Gary R. , Leshin Laurie A. , MacPherson Glenn J. , McKeegan Kevin D.
Published: January 2010

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26Al‐26Mg systematics, only one aluminum‐rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 ± 2.4) × 10−6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites.

The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ˜0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O‐rich compositions with a range in δ18O of about ˜12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs).

Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite‐forming region.
Petrographic comparison of refractory inclusions from different chemical groups of chondrites
Authors: LIN Y. , KIMURA M. , MIAO B. , DAI D. , MONOI A.
Published: January 2010

Abstract— Twenty‐four refractory inclusions (40–230 μm, with average of 86 ± 40 μm) were found by X‐ray mapping of 18 ordinary chondrites. All inclusions are heavily altered, consisting of finegrained feldspathoids, spinel, and Ca‐pyroxene with minor ilmenite. The presence of feldspathoids and lack of melilite are due to alteration that took place under oxidizing conditions as indicated by FeO‐ZnO‐rich spinel and ilmenite. The pre‐altered mineral assemblages are dominated by two types: one rich in melilite, referred to as type A‐like, and the other rich in spinel, referred to as spinelpyroxene inclusions. This study and previous data show similar type and size distributions of refractory inclusions in ordinary and enstatite chondrites.

A survey of refractory inclusions was also conducted on Allende and Murchison in order to make unbiased comparison with their counterparts in other chondrites. The predominant inclusions are type A and spinel‐pyroxene, with average sizes of 170 ± 130 μm (except for two mm‐sized inclusions) in Allende and 150 ± 100 μm in Murchison. The relatively larger sizes are partially due to common conglomerating of smaller nodules in both chondrites. The survey reveals closely similar type and size distributions of refractory inclusions in various chondrites, consistent with our previous data of other carbonaceous chondrites. The petrographic observations suggest that refractory inclusions in various groups of chondrites had primarily formed under similar processes and conditions, and were transported to different chondrite‐accreting regions. Heterogeneous abundance and distinct alteration assemblages of refractory inclusions from various chondrites could be contributed to transporting processes and secondary reactions under different conditions.
Variation of chemical composition in Australasian tektites from different localities in Vietnam
Authors: Amare Kassa , Koeberl Christian
Published: January 2010

Abstract— One hundred and thirteen Australasian tektites from Vietnam (Hanoi, Vinh, Dalat, and Saigon areas) were analyzed for their major and trace element contents. The tektites are either of splash form or Muong Nong‐type. The splash‐form tektites have SiO2 contents ranging from 69.7 to 76.8 wt%, whereas Muong Nong‐type tektites, which are considerably larger than splash‐form tektites and have a blocky and chunky appearance, have slightly higher silica contents in the range of 74–81 wt%. Major‐element relationships, such as FeO versus major oxides, Na2O versus K2O, and oxide ratio plots, were used to distinguish the different groups of the tektites. In addition, correlation coefficients have been calculated for each tektite group of this study. Many chemical similarities are noted between Hanoi and Vinh tektites from the north of Vietnam, except that the Hanoi tektites contain higher contents of CaO than Vinh; the higher content of CaO might be due to some carbonate parent material. Both Dalat and Saigon tektites have nearly similar composition, whereas the bulk chemistries of the tektites from Hanoi and Vinh appear different from those of Saigon and Dalat. There are differences, especially in the lower CaO and Na2O and higher MgO, FeO, for the tektites of Dalat and Saigon in comparison to that of Hanoi tektites. Furthermore, the Dalat and Saigon tektites show enrichments by factors of 3 and 2 for the Ni and Cr contents, respectively, compared to those of Hanoi and Vinh. The difference in chemistry between the North Vietnam tektites (Hanoi, Vinh) to that of South Vietnam tektites (Saigon, Dalat) of this study indicate that the parent material was heterogeneous and possibly mixing between different source rocks took place.

Muong Nong‐type tektites are enriched in the volatile elements such as Br, Zn, As, and Sb compared to the average splash‐form tektites of this study. The chemical compositions of the average splash‐form and Muong Nong‐type tektites of this study closely resemble published data for average splash‐form and Muong Nong‐type indochinites, indicating that they have the same source. The trace element ratios Ba/Rb (2.7), Th/U (5.2), Th/Sc (1.3), Th/Sm (2.2), and the rare earth element (REE) abundances of this study show close similarities to those of average upper continental crust.
Shock, post‐shock annealing, and post‐annealing shock in ureilites
Authors: Rubin Alan E.
Published: January 2010

Abstract— The thermal and shock histories of ureilites can be divided into four periods: 1) formation, 2) initial shock, 3) post‐shock annealing, and 4) post‐annealing shock. Period 1 occurred ˜4.55 Ga ago when ureilites formed by melting chondritic material. Impact events during period 2 caused silicate darkening, undulose to mosaic extinction in olivines, and the formation of diamond, lonsdaleite, and chaoite from indigenous carbonaceous material. Alkali‐rich fine‐grained silicates may have been introduced by impact injection into ureilites during this period. About 57% of the ureilites were unchanged after period 2. During period 3 events, impact‐induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals. Some ureilites experienced reduction as FeO at the edges of olivine grains reacted with C from the matrix. Annealing may also be responsible for coarsening of graphite in a few ureilites, forming euhedral‐appearing, idioblastic crystals. Orthopyroxene in Meteorite Hills (MET) 78008 may have formed from pigeonite by annealing during this period. The Rb‐Sr internal isochron age of ˜4.0 Ga for MET 78008 probably dates the annealing event. At this late date, impacts are the only viable heat source. About 36% of ureilites experienced period 3 events, but remained unchanged afterwards. During period 4, ˜7% of the ureilites were shocked again, as is evident in the polymict breccia, Elephant Moraine (EET) 83309. This rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction.

Ureilites may have formed by impact‐melting chondritic material on a primitive body with heterogeneous O isotopes. Plagioclase was preferentially lost from the system due to its low impedance to shock compression. Brief melting and rapid burial minimized the escape of planetary‐type noble gases from the ureilitic melts. Incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements.
Focused ion beam recovery of hypervelocity impact residue in experimental craters on metallic foils
Authors: Graham G. A. , Teslich N. , Dai Z. R. , Bradley J. P. , Kearsley A. T. , Hörz F.
Published: January 2010

Abstract— The Stardust sample return capsule returned to Earth in January 2006 with primitive debris collected from comet 81P/Wild‐2 during the flyby 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 μm thick) that are wrapped around the sample tray assembly. Soda lime spheres (˜49 μm 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.
Microstructural study of micron‐sized craters simulating Stardust impacts in aluminum 1100 targets
Authors: Leroux Hugues , Borg Janet , Troadec David , Djouadi Zahia , Hörz Friedrich
Published: January 2010

Abstract— 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.
SIMS studies of Allende projectiles fired into Stardust‐type aluminum foils at 6 km/sec
Authors: Hoppe Peter , Stadermann Frank J. , Stephan Thomas , Floss Christine , Leitner Jan , Marhas Kuljeet K. , Hörz Friedrich
Published: January 2010

Abstract— We have explored the feasibility of 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 μm. 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 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 C, is crucial. This information could not be obtained in the present study due to unavoidable contamination during impact experiments.
Macroscopic subdivision of silica aerogel collectors for sample return missions
Authors: Ishii H. A. , Bradley J. P.
Published: January 2010

Abstract— 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.
Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics
Authors: Barlow Nadine G.
Published: January 2010

Abstract— Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single‐layer ejecta (SLE) morphology is most consistent with impact into an ice‐rich target. Double‐layer ejecta (DLE) and multiple‐layer ejecta (MLE) craters also likely form in volatile‐rich materials, but the interaction of the ejecta curtain and target‐produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double‐layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an icerich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice‐soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.
Nature of the Martian uplands: Effect on Martian meteorite age distribution and secondary cratering
Authors: Hartmann William K. , Barlow Nadine G.
Published: January 2010

Abstract— Martian meteorites (MMs) have been launched from an estimated 5–9 sites on Mars within the last 20 Myr. Some 80–89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near‐surface coherent rock (≤102 m deep), whereas Noachian surfaces generally should have ≥102 m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1–2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial‐derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic‐ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ˜45 km, ˜19 km, and ˜10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.
Morphology and geometry of the distal ramparts of Martian impact craters
Authors: Mouginis‐Mark Peter J. , Baloga Stephen M.
Published: January 2010

Abstract— We used Mars Orbiter Laser Altimeter (MOLA), Thermal Emission Imaging System visible light (THEMIS VIS), and Mars Orbiter Camera (MOC) data to identify and characterize the morphology and geometry of the distal ramparts surrounding Martian craters. Such information is valuable for investigating the ejecta emplacement process, as well as searching for spatial variations in ejecta characteristics that may be due to target material properties and/or latitude, altitude, or temporal variations in the climate. We find no systematic trend in rampart height that would indicate regional variations in target properties for 54 ramparts at 37 different craters 5.7–35.9 km in diameter between 52.3°S to 47.6°N. Rampart heights for multi‐lobe and single‐lobe ejecta are each normally distributed with a common standard deviation, but statistically distinct mean values. Ramparts range in height from 20–180 m, are not symmetric, are typically steeper on their distal sides, and may be as much as ˜4 km wide. The ejecta blanket proximal to parent crater from the rampart may be very thin (<5 m). A detailed analysis of two craters, Toconao crater (21°S, 285°E) (28 measurements), and an unnamed crater within Chryse Planitia (28.4°N, 319.6°E) (20 measurements), reveals that ejecta runout distance increases with an increase in height between the crater rim and the rampart, but that rampart height is not correlated with ejecta runout distance or the thickness of the ejecta blanket.
The planforms of low‐angle impact craters in the northern hemisphere of Mars
Authors: Herrick Robert R. , Hessen Katie K.
Published: January 2010

Abstract— We have surveyed Martian impact craters greater than 5 km in diameter using Viking and thermal emission imaging system (THEMIS) imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle. We infer the impact angles at which the changes occur by assuming a sin2θ dependence for the cumulative fraction of craters forming below angle θ. At impact angles less than ˜40° from horizontal, the ejecta become offset downrange relative to the crater rim. As the impact angle decreases to less than ˜20°, the ejecta begin to concentrate in the cross‐range direction and a “forbidden zone” that is void of ejecta develops in the uprange direction. At angles less than ˜10°, a “butterfly” ejecta pattern is generated by the presence of downrange and uprange forbidden zones, and the rim planform becomes elliptical with the major axis oriented along the projectile's direction of travel. The uprange forbidden zone appears as a “V” curving outward from the rim, but the downrange forbidden zone is a straight‐edged wedge. Although fresh Martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow, the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum. The planforms are different from those for Venusian craters and experimental impacts in a dense atmosphere. We interpret our results to indicate that Martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow.
Spatial heterogeneity of 26Al/27Al and stable oxygen isotopes in the solar nebula
Authors: Boss Alan P.
Published: January 2010

Abstract— The degree of isotopic spatial heterogeneity in the solar nebula has long been a puzzle, with different isotopic systems implying either large‐scale initial spatial homogeneity (e.g., 26Al chronometry) or a significant amount of preserved heterogeneity (e.g., ratios of the three stable oxygen isotopes, 16O, 17O, and 18O). We show here that in a marginally gravitationally unstable (MGU) solar nebula, the efficiency of large‐scale mixing and transport is sufficient to spatially homogenize an initially highly spatially heterogeneous nebula to dispersions of ˜10% about the mean value of 26Al/27Al on time scales of thousands of years. A similar dispersion would be expected for 17O/16O and 18O/16O ratios produced by ultraviolet photolysis of self‐shielded molecular CO gas at the surface of the outer solar nebula. In addition to preserving a chronological interpretation of initial 26Al/27Al ratios and the self‐shielding explanation for the oxygen isotope ratios, these solar nebula models offer a self‐consistent environment for achieving large‐scale mixing and transport of thermally annealed dust grains, shock‐wave processing of chondrules and refractory inclusions, and giant planet formation.
Thermodynamic constraints on fayalite formation on parent bodies of chondrites
Authors: Zolotov Mikhail Yu. , Mironenko Mikhail V. , Shock Everett L.
Published: January 2010

Abstract— Thermochemical equilibria are calculated in the multicomponent gas‐solution‐rock system in order to evaluate the formation conditions of fayalite, (Fe0.88–1.0Mg0.12–0)2SiO4, Fa88–100, in unequilibrated chondrites. Effects of temperature, pressure, water/rock ratio, rock composition, and progress of alteration are evaluated. The modeling shows that fayalite can form as a minor secondary and transient phase with and without aqueous solution. Fayalite can form at temperatures below ˜350 °C, but only in a narrow range of water/rock ratios that designates a transition between aqueous and metamorphic conditions. Pure fayalite forms at lower temperatures, higher water/rock ratios, and elevated pressures that correspond to higher H2/H2O ratios. Lower pressure and water/rock ratios and higher temperatures favor higher Mg content in olivine. In equilibrium assemblages, fayalite usually coexists with troilite, kamacite, magnetite, chromite, Ca‐Fe pyroxene, and phyllosilicates. Formation of fayalite can be driven by changes in temperature, pressure, H2/H2O, and water/rock ratios. However, in fayalite‐bearing ordinary and CV3 carbonaceous chondrites, the mineral could have formed during the aqueous‐to‐metamorphic transition. Dissolution of amorphous silicates in matrices and/or silica grains, as well as low activities of Mg solutes, favored aqueous precipitation of fayalite. During subsequent metamorphism, fayalite could have formed through the reduction of magnetite and/or dehydration of ferrous serpentine. Further metamorphism should have caused reductive transformation of fayalite to Ca‐Fe pyroxene and secondary metal, which is consistent with observations in metamorphosed chondrites. Although bulk compositions of matrices/chondrites have only a minor effect on fayalite stability, specific alteration paths led to different occurrences, quantities, and compositions of fayalite in chondrites.
Density, magnetic susceptibility, and the characterization of ordinary chondrite falls and showers
Authors: Consolmagno G. J. , Macke R. J. , Rochette P. , Britt D. T. , Gattacceca J.
Published: January 2010

Abstract— Bulk and grain densities of 132 ordinary chondrites from the Vatican Observatory collection were measured and compared with their magnetic susceptibility (for the most part using previously measured values; ten new susceptibility measures were taken for this study). Grain density and magnetic susceptibility combined provide a reliable method of classifying unweathered ordinary chondrites. Unlike traditional chemical tests, this method is fast, nondestructive, and characterizes the whole rock, making it especially appropriate for surveying large collections. The system is less viable for finds; extensive weathering of metallic iron in an H chondrite can cause it to plot among L chondrites, while heavily weathered L chondrites plot among the LL group. This system has revealed outlier stones that may be misclassified meteorites or mislabeled samples; in every case where the magnetic susceptibility of a meteorite does not fit its putative classification, the grain density is also found to be in disagreement in a manner consistent with either severe weathering or misidentification. An analysis of stones from five showers shows that, excluding outliers, these samples tend to cluster tightly within their appropriate groups in a plot of grain versus magnetic susceptibility.
In situ identification, pairing, and classification of meteorites from Antarctica through magnetic susceptibility measurements
Authors: Folco Luigi , Rochette Pierre , Gattacceca Jérôme , Perchiazzi Natale
Published: January 2010

Abstract— We report on the effectiveness of using magnetic measurements in the search for meteorites on the Antarctic ice sheet, which is thus far the Earth's most productive terrain. Magnetic susceptibility measurements carried out with a pocket meter (SM30) during the 2003/04 PNRA meteorite collection expedition to northern Victoria Land (Antarctica) proved to be a rapid, sensitive, non‐destructive means for the in situ identification, pairing, and classification of meteorites. In blue ice fields characterized by the presence of moraines and glacial drifts (e.g., Miller Butte, Roberts Butte, and Frontier Mountain), magnetic susceptibility measurements allowed discrimination of meteorites from abundant terrestrial stones that look like meteorites thanks to the relatively high magnetic susceptibility of the former with respect to terrestrial rocks. Comparative measurements helped identify 16 paired fragments found at Johannessen Nunataks, thereby reducing unnecessary duplication of laboratory analyses and statistical bias. Following classifications schemes developed by us in this and previous works, magnetic susceptibility measurements also helped classify stony meteorites directly in the field, thereby providing a means for selecting samples with higher research priority. A magnetic gradiometer capable of detecting perturbations in the Earth's magnetic field induced by the presence of meteorites was an efficient tool for locating meteorites buried in snow along the downwind margin of the Frontier Mountain blue ice field. Based on these results, we believe that magnetic sensors should constitute an additional payload for robotic search for meteorites on the Antarctic ice sheet and, by extension, on the surface of Mars where meteorite accumulations are predicted by theoretical works. Lastly, magnetic susceptibility data was successfully used to crosscheck the later petrographic classification of the 123 recovered meteorites, allowing the detection of misclassified or peculiar specimens.