• Diffusion-driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

      Dauphas, N. (The Meteoritical Society, 2007-01-01)
      Iron meteorites show resolvable Fe and Ni isotopic fractionation between taenite and kamacite. For Toluca (IAB), the isotopic fractionations between the two phases are around +0.1 ppm/amu for Fe and -0.4 ppm/amu for Ni. These variations may be due to i) equilibrium fractionation, ii) differences in the diffusivities of the different isotopes, or iii) a combination of both processes. A computer algorithm was developed in order to follow the growth of kamacite out of taenite during the formation of the Widmanstätten pattern as well as calculate the fractionation of Fe and Ni isotopes for a set of cooling rates ranging from 25 to 500 degrees C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4 ppm/amu for Fe and Ni (Beta = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 degrees C/Myr. This value agrees with earlier cooling rate estimates based on Ni concentration profiles. This supports the idea that the fractionation measured for Fe and Ni in iron meteorites is driven by differences in diffusivities of isotopes. It also supports the validity of the value of 0.25 adopted for Beta for diffusion of Fe and Ni in Fe-Ni alloy in the temperature range of 400-700 degrees C.
    • The Meteoritical Bulletin, No. 92, 2007 September

      Connolly, H. C.; Smith, C.; Benedix, G.; Folco, L.; Righter, K.; Zipfel, J.; Yamaguchi, A.; Chennaoui Aoudjehane, H. (The Meteoritical Society, 2007-01-01)
      In this edition of The Meteoritical Bulletin, 1394 recognized meteorites are reported, 27 from specific locations within Africa, 133 from Northwest Africa, 1227 from Antarctica (from ANSMET, PNRA, and PRIC expeditions), and 7 from Asia. The Meteoritical Bulletin announces the approval of four new names series by the Nomenclature Committee of the Meteoritical Society, two from Africa and one from Asia, including Al Haggounia, from Al Haggounia, Morocco, which is projected to be on the order of 3 metric tons of material related to enstatite chondrites and aubrites. Approved are two falls from Africa, Bassikounou Mauritania) and Gashua (Nigeria). Approved from areas other than Antarctica are one lunar, two Martian, 32 other achondrites, three mesosiderites, two pallasites, one CM, two CK, one CR2, two CV3, one CR2, and four R chondrites. The Nomenclature Committee of the Meteoritical Society announces 48 newly approved relict meteorites from two new name series, sterplana and Gullhögen (both from Sweden).
    • Tsunami generation and propagation from the Mjølnir asteroid impact

      Glimsdal, S.; Pedersen, G. K.; Langtangen, H. P.; Shuvalov, V.; Dypvik, H. (The Meteoritical Society, 2007-01-01)
      In the late Jurassic period, about 142 million years ago, an asteroid hit the shallow paleo-Barents Sea, north of present-day Norway. The geological structure resulting from the impact is today known as the Mjølnir crater. The present work attempts to model the generation and the propagation of the tsunami from the Mjølnir impact. A multi-material hydrocode SOVA is used to model the impact and the early stages of tsunami generation, while models based on shallow-water theories are used to study the subsequent wave propagation in the paleo-Barents Sea. We apply several wave models of varying computational complexity. This includes both three-dimensional and radially symmetric weakly dispersive and nonlinear Boussinesq equations, as well as equations based on nonlinear ray theory. These tsunami models require a reconstruction of the bathymetry of the paleo- Barents Sea. The Mjølnir tsunami is characteristic of large bolides impacting in shallow sea; in this case the asteroid was about 1.6 km in diameter and the water depth was around 400 m. Contrary to earthquake- and slide-generated tsunamis, this tsunami featured crucial dispersive and nonlinear effects: a few minutes after the impact, the ocean surface was formed into an undular bore, which developed further into a train of solitary waves. Our simulations indicate wave amplitudes above 200 m, and during shoaling the waves break far from the coastlines in rather deep water. The tsunami induced strong bottom currents, in the range of 30-90 km/h, which presumably caused a strong reworking of bottom sediments with dramatic consequences for the marine environment.
    • Magnetic characterization of Cretaceous-Tertiary boundary sediments

      Villasante-Marcos, V.; Martínez-Ruiz, F.; Osete, M. L.; Urrutia-Fucugauchi, J. (The Meteoritical Society, 2007-01-01)
      Rock magnetic properties across several K-T boundary sections have been investigated to reveal any possible magnetic signature associated with the remains of the impact event at the end of the Cretaceous. Studied sections locations vary in distance to the Chicxulub structure from distal (Agost and Caravaca, Spain), through closer (ODP Hole 1049A, Blake Nose, North Atlantic), to proximal (El Mimbral and La Lajilla, Mexico). A clear magnetic signature is associated with the fireball layer in the most distal sections, consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization (SIRM), and a decrease in remanence coercivity. Magnetic properties in these sections point to a distinctive ferrimagnetic phase, probably corresponding to the reported Mg- and Ni-rich, highly oxidized spinels of meteoritic origin. At closer and proximal sections magnetic properties are different. Although there is an increase in susceptibility and SIRM associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections, and with a similar limonitic layer on top of the spherule bed that defines the boundary at Blake Nose, the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine-grained goethite. Based on previous geochemical studies at Blake Nose and new geochemical and PGE abundance measurements performed in this work at El Mimbral, this goethite-rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of Fe. There is not enough evidence to assert that this Fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material. Magnetic, geochemical, and iridium data reject it as a primary meteoritic phase.
    • Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources

      Sahijpal, S.; Soni, P.; Gupta, G. (The Meteoritical Society, 2007-01-01)
      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 planetesimals 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.
    • Comprehensive imaging and Raman spectroscopy of carbonate globules from Martian meteorite ALH 84001 and a terrestrial analogue from Svalbard

      Steele, A.; Fries, M. D.; Amundsen, H. E. F.; Mysen, B. O.; Fogel, M. L.; Schweizer, M.; Boctor, N. Z. (The Meteoritical Society, 2007-01-01)
      We report a comprehensive imaging study including confocal microRaman spectroscopy, scanning electron microscopy (SEM), and 3-D extended focal imaging light microscopy of carbonate globules throughout a depth profile of the Martian meteorite Allan Hills (ALH) 84001 and similar objects in mantle peridotite xenoliths from the Bockfjorden volcanic complex (BVC), Svalbard. Carbonate and iron oxide zoning in ALH 84001 is similar to that seen in BVC globules. Hematite appears to be present in all ALH 84001 carbonate-bearing assemblages except within a magnesite outer rim found in some globules. Macromolecular carbon (MMC) was found in intimate association with magnetite in both ALH 84001 and BVC carbonates. The MMC synthesis mechanism appears similar to established reactions within the Fe-C-O system. By inference to a terrestrial analogue of mantle origin (BVC), these results appear to represent the first measurements of the products of an abiotic MMC synthesis mechanism in Martian samples. Furthermore, the ubiquitous but heterogeneous distribution of hematite throughout carbonate globules in ALH 84001 may be partly responsible for some of the wide range in measured oxygen isotopes reported in previous studies. Using BVC carbonates as a suitable analogue, we postulate that a low temperature hydrothermal model of ALH 84001 globule formation is most likely, although alteration (decarbonation) of a subset of globules possibly occurred during a later impact event.
    • Meteorite 3-D synchrotron microtomography: Methods and applications

      Ebel, D. S.; Rivers, M. L. (The Meteoritical Society, 2007-01-01)
      Methods of synchrotron X-ray computed microtomography (XRCMT) are described, which allow nondestructive, high spatial and contrast resolution imaging of the density structures of meteorites and their components in three dimensions. Images of bulk chondrites (to one cubic centimeter in size) reveal compound chondrules, chondrule/matrix volumetric ratios, metal and sulfide distribution, petrofabrics, and 3-D chondrule and calcium-aluminum inclusion (CAI) sizes and shapes. Images of separated chondrules and CAIs reveal void spaces, mineral intergrowth textures, and the true locations of crystal rims and cores, at resolutions to <8 cubic micron/volume element. Images of achondrites reveal mineral fabrics and crystal zoning. Lunar glass spherules can be searched for phenocrysts bearing deeply sourced melt inclusions. A companion DVD and URL contain images for classroom and research use. Numerical techniques for quantification of X-ray computed microtomography (XRCMT) data and its potential applications are discussed. Three-dimensional X-ray images of meteorites provide a way to discover components of interest and to precisely slice samples to expose these components with minimal damage and loss of material. Three-dimensional studies of petrographic features (size, shape, texture, and modal abundance) of chondrites and their components, as well as other meteorites, have definite advantages over standard 2-D studies using randomly sliced thin sections.
    • Osmium, tungsten, and chromium isotopes in sediments and in Ni-rich spinel at the K-T boundary: Signature of a chondritic impactor

      Quitté, G.; Robin, E.; Levasseur, S.; Capmas, F.; Rocchia, R.; Birck, J.-L.; Allègre, C. J. (The Meteoritical Society, 2007-01-01)
      It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re-Os, Hf-W, and Mn-Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous-Tertiary (K-T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni-rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.
    • Crater geometry and ejecta thickness of the Martian impact crater Tooting

      Mouginis-Mark, P. J.; Garbeil, H. (The Meteoritical Society, 2007-01-01)
      We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ~29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/ diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ~8120 km^2 ejecta blanket. Because the pre-impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has 500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre-impact terrain) is ~380 km^3 and the volume of materials above the pre-impact terrain is ~425 km^3. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ~100 m of additional ejecta thickness being deposited down-range compared to the up-range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi-layered ejecta craters. Tooting crater serves as a fresh end-member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials.
    • Amino acid composition, petrology, geochemistry, 14C terrestrial age and oxygen isotopes of the Shişr 033 CR chondrite

      Martins, Z.; Hofmann, B. A.; Gnos, E.; Greenwood, R. C.; Verchovsky, A.; Franchi, I. A.; Jull, A. J. T.; Botta, O.; Glavin, D. P.; Dworkin, J. P.; et al. (The Meteoritical Society, 2007-01-01)
      We have analyzed Shişr 033, a CR chondrite from the Omani desert, using several different analytical techniques designed to study the degree of terrestrial alteration of this meteorite and also its petrologic classification. Bulk chemical analyses (including organic carbon and mean total H2O content) are consistent with a CR classification. Additionally, oxygen isotope analysis on a bulk sample indicates that Shişr 033 is of type CR2. Amino acid analysis using liquid chromatography with UV fluorescence detection (HPLC-FD) and liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS) show that the absolute and the relative amino acid content of Shişr 033 is distinct from other carbonaceous chondrites. Oxygen isotope analysis of a phyllosilicate-rich dark inclusion shows that this inclusion is closer to CV3 or CO3 chondrites. The effects of terrestrial weathering in Shişr 033 are evident from the dark inclusion carbon isotopic data, bulk chemistry (through the elevated concentrations of Sr and Ba), and amino acid data, which suggests extensive amino acid contamination of the meteorite from the fall site soil. Nevertheless, Shişr 033 contains a small fraction of indigenous components, as indicated by the presence of the extraterrestrial amino acid -aminoisobutyric acid (AIB) that was not detected in the Shişr soils. Finally, the terrestrial age of Shişr 033 was determined and is discussed in the context of high levels of contamination.
    • The GRO 95577 CR1 chondrite and hydration of the CR parent body

      Weisberg, M. K.; Huber, H. (The Meteoritical Society, 2007-01-01)
      We carried out a petrologic and instrumental neutron activation analysis (INAA) whole chondrite compositional study of Grosvenor Mountains (GRO) 95577. GRO 95577 has many petrological similarities to the CR chondrites. Although the INAA data show patterns indicative of terrestrial weathering, some of the elemental abundances are consistent with a relationship to CR chondrites. The oxygen isotopic composition of GRO 95577 plots close to the Renazzo CR chondrite on the three-isotope diagram. However, GRO 95577 is remarkable in that the chondrules are completely hydrated, consisting almost entirely of phyllosilicates, magnetite, and sulfides. Although GRO 95577 is completely hydrated, the initial chondrule textures are perfectly preserved. The chondrules are in sharp contact with the matrix, their fine-grained rims are clearly visible, and the boundaries of the dark inclusions can be easily discerned. Many chondrules in GRO 95577 have textures suggestive of type I chondrules, but the phenocrysts have undergone perfect pseudomorphic replacement by yellow to brownish serpentine-rich phyllosilicate, with sharp original crystal outlines preserved. The chondrule mesostasis is a green aluminous chlorite-rich material, most likely a hydration product of the feldspathic mesostasis commonly found in anhydrous type I chondrules. Some chondrules contain magnetite spheres, most likely formed by oxidation of metal. We propose that GRO 95577 be classified as a CR1 chondrite, making it the first known CR1 chondrite and expanding the range of alteration conditions on the CR parent body.