• Numerical simulations of the production of extinct short-lived nuclides by magnetic flaring in the early solar system

      Sahijpal, S.; Soni, P. (The Meteoritical Society, 2007-01-01)
      Using the X-ray flare observations of low-mass protostars, we developed numerical simulations of thermal processing and irradiation of protoCAIs in the magnetic reconnection ring within the X-wind formulation. Observed X-ray flare luminosities have been used to model various simulation flare characteristics. Several approximations have been made regarding the thermal evolution that involve condensation, evaporation, and coagulation of protoCAIs. Ensembles of refractory cores with ferromagnesian mantles were evolved for irradiation production of the short lived nuclides 7Be, 10Be, 41Ca, 36Cl, 26Al, and 53Mn. Three distinct grain-size distributions of protoCAIs with refractory cores in the ranges of 32 micrometer-20 mm, 125 micrometer-16 mm, and 500 micrometer-13 mm were thermally evolved for irradiation. The latter two size distributions were found to result in the accumulation of protoCAIs in the reconnection ring during an X-wind cycle, and hence can account for the total inventory of 26Al in the early solar system. The canonical value of ~5 x 10^(-5) for 26Al/27Al can be inferred from the impulsive flare simulations by a suitable choice of simulation parameters. However, in most of the remaining simulations, the irradiation of protoCAIs by superflare(s) with Lx > 10^32 ergs s^(-1) subsequent to their thermal processing in the reconnection ring would be required to explain the experimental abundances of the short-lived nuclides. These superflares have never been reliably observed in young stellar objects. If they are real, they would be extremely rare. The paucity of these superflares could impose stringent constraints on the validity of the X-wind irradiation scenario as the source of the short-lived nuclides.
    • Gain and loss of uranium by meteorites and rocks, and implications for the redistribution of uranium on Mars

      Dreibus, G.; Haubold, R.; Huisl, W.; Spettel, B. (The Meteoritical Society, 2007-01-01)
      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.
    • Book Review: Meteors and Meteorites: Origins and Observations, Martin Beech

      ReVelle, D. O. (The Meteoritical Society, 2007-01-01)
    • Book Review: Sulfide Mineralogy and Geochemistry, D. J. Vaughan (Ed.)

      Greenwood, J. P. (The Meteoritical Society, 2007-01-01)
    • Zoning patterns of Fe and V in spinel from a type B Ca-Al-rich inclusion: Constraints on subsolidus thermal history

      Paque, J. M.; Burnett, D. S.; Beckett, J. R. (The Meteoritical Society, 2007-01-01)
      We obtained two-dimensional concentration maps for the minor elements Fe and V in 21 spinel crystals in the Allende type B1 inclusion TS-34 with a 4-5 micrometer resolution. Locally high concentrations of Fe occur along at least one edge of the spinels and decrease toward the center of the grains. Enrichment in V can also occur along edges or at corners. In general, there is no overall correlation of the Fe and V distributions, but in local regions of two grains, the V and Fe distributions are correlated, strongly suggesting a local source for both elements. In these two grains, opaque assemblages are present that appear to locally control the V distributions. This, coupled with previous work, suggests that prior to alteration, TS-34 contained V-rich metal. Oxidation of this metal during alteration can account for the edge/corner V enrichments, but provide only minor FeO contributions, explaining the overall lack of correlation between Fe and V. Most of the FeO appears to have been externally introduced along spinel boundaries during alteration. These alteration phases served as sources for diffusion of FeO into spinel. FeO distributions in spinel lead to a mean attenuation length of ~8 micrometer and, using literature diffusion coefficients in isothermal and exponential cooling approximations for peak temperatures in the range 600-700 degrees C, this leads to a time scale for calciumaluminum- rich inclusion (CAI) alteration in the range of decades to centuries.
    • Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

      Bell, M. S. (The Meteoritical Society, 2007-01-01)
      Shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron-carbonate were initiated. Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten-alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe^+2 in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (~50-100 nm), compositions (100% magnetite to 80% magnetite-20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ~32 GPa pressure and a resultant thermal pulse of ~100-110 degrees C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica-rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to >470 degrees C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well.
    • Microstructure and thermal history of metal particles in CH chondrites

      Goldstein, J. I.; Jones, R. H.; Kotula, P. G.; Michael, J. R. (The Meteoritical Society, 2007-01-01)
      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.
    • Alkalic parental magmas for chassignites?

      Nekvasil, H.; Filiberto, J.; McCubbin, F. M.; Lindsley, D. H. (The Meteoritical Society, 2007-01-01)
      Detailed analysis of cumulate and melt inclusion assemblages in the chassignites provide important constraints on the nature of the melt trapped as inclusions in cumulus olivine (and, by extension, parental magma compositions), the pressures of crystallization, and magmatic volatile contents. These mineral assemblages show strong similarities to the experimental fractionation assemblages that produce the sodic silica-saturated alkalic lavas on Earth (e.g., Ascension Island, Azores, the Nandewar volcano of Australia). The experimental assemblages were produced from silica-saturated hawaiite at pressures above 4.3 kbar with dissolved water contents above 0.5 wt%. Such pressures are consistent with Ti:Al ratios of the melt-inclusion pyroxenes in the Chassigny meteorite. Pyroxene compositions suggest early high crystallization temperatures and thus relatively low initial water and F contents. Feldspars indicate that melt evolution proceeded to rhyolite compositions both within the interstices of the cumulate olivine and within the melt inclusions, even though rhyolitic glass is only found within olivine-hosted polyphase melt inclusions. The observed rhyolite glass is compositionally similar to the alkali-rich rhyolite of Ascension Island which is produced experimentally by crystallization of hawaiite. It is proposed that the melt trapped in cumulus olivine of the Chassigny dunite was similar to a terrestrial silica-saturated hawaiite, while that trapped in olivine of the Northwest Africa (NWA) 2727 dunite was less evolved, perhaps mildly alkalic basalt. Melts similar to terrestrial intra-plate tholeiite could be parental to the cumulus minerals and evolve upon crystallization at pressures above 4.3 kbar and water contents above ~0.4 wt% to mildly alkalic basalt, silica-saturated hawaiite, and alkali-rich rhyolite. The melt inclusion assemblages are inconsistent with either crystallization of a low-Al, high-Fe basalt, or low pressure crystallization of a terrestrial-like tholeiite.
    • An integrated approach to understanding Apollo 16 impact glasses: Chemistry, isotopes, and shape

      Delano, J. W.; Zellner, N. E. B.; Barra, F.; Olson, E.; Swindle, T. D.; Tibbetts, N. J.; Whittet, D. C. B. (The Meteoritical Society, 2007-01-01)
      The major- and minor-element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ~30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non-volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open-system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low-Mg high-K Fra Mauro; lmHKFM) were selected for 40Ar/39Ar dating. The individual fragments of lmHKFM glass all yielded ages of ~3750 +/- 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 +/- 40 Ma, close to the accepted age for Imbrium.
    • Complex brecciation and shock effects in the Buck Mountain Wash (H3–5) chondrite

      Hutson, M.; Ruzicka, A.; Pugh, R.; Sloan, L.; Thompson, E. (The Meteoritical Society, 2007-01-01)
      Buck Mountain Wash (BMW) is a new genomict breccia (H35) found in the Franconia (H5) strewn field in Arizona that shows complex brecciation and shock effects. It contains three distinct chondritic lithologies in sharp contact: a) a main lithology that consists primarily of petrographic type 5 material but which has finely intermixed type 3 and 4 material, b) a shockblackened (shock stage S5) type 3 lithology (lithology A), and degrees C) a shock-blackened type 3/4 lithology(lithology B). Buck Mountain Wash was lithified after impact-mixing and impact-melting of weakly and strongly metamorphosed materials, possibly at depth in the regolith of the parent body. Shock effects included brecciation on a fine scale, localized impact-melting of silicates, partial melting, and mobilization of metal-sulfide, and chemical fractionations that produced non-H-group composition kamacite by two disequilibrium mechanisms. Shock heating did not cause significant thermal metamorphism in the shock-blackened lithologies of BMW, except possibly in areas adjacent to whole-rock shock melt. During lithification, cooling must have been rapid at high temperatures to preserve glass and inhomogeneous silicate compositions, but not so fast at lower temperatures as to produce dendritic metal-sulfide globules or martensite.
    • Book Review: Impact Structures in Canada, Richard A. F. Grieve

      Kring, D. A. (The Meteoritical Society, 2007-01-01)
    • 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

      James, O. B.; Cohen, B. A.; Taylor, L. A.; Nazarov, M. A. (The Meteoritical Society, 2007-01-01)
      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 granuliticbreccia 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 windderived 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.