• Laboratory hydration of condensed magnesiosilica smokes with implications for hydrated silicates in IDPs and comets

      Rietmeijer, F. J. M.; Nuth, J. A.; Nelson, R. N. (The Meteoritical Society, 2004-01-01)
      Samples of silica-rich and MgO-rich condensed, amorphous magnesiosilica smokes were hydrated to monitor systematic mineralogical and chemical changes as a function of time and temperature controlled by their unique metastable eutectic compositions, their porous texture, and the ultrafine, nanometer grain size of all entities. At water supersaturated conditions, proto-phyllosilicates formed by spinodal-type homogeneous nucleation. Their formation and subsequent growth was entirely determined by the availability of water via pore spaces inherited from the original smokes and the textural continuity of magnesiosilica material with a mostly smectite-dehydroxylate composition. The results may have implications for the hydration of proto-CI material, the presence of rare periclase and brucite in primitive solar system bodies, and the pervasiveness of hydrated amorphous magnesiosilica dust and saponite proto-phyllosilicates in icy-protoplanets, such as comet nuclei.
    • Non-mass-dependent oxygen isotopic fractionation in smokes produced in an electrical discharge

      Kimura, Y.; Nuth, J. A.; Chakraborty, S.; Thiemens, M. H. (The Meteoritical Society, 2007-01-01)
      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 Delta-17O observed is +4.7 per mil for an iron oxide produced in flowing hydrogen, using O2 as the oxidant. More typical displacements are 1-2 per mil above the equilibrium fractionation line. The chemical reaction mechanisms that yield these smokes are still under investigation.
    • Planetary accretion, oxygen isotopes, and the central limit theorem

      Nuth, J. A.; Hill, H. G. M. (The Meteoritical Society, 2004-01-01)
      The accumulation of presolar dust into increasingly larger aggregates such as calciumaluminum- rich inclusions (CAIs) and chondrules, asteroids, and planets should result in a drastic reduction in the numerical spread in oxygen isotopic composition between bodies of similar size, in accord with the central limit theorem. Observed variations in oxygen isotopic composition are many orders of magnitude larger than would be predicted by a simple, random accumulation model that begins in a well-mixed nebula, no matter what size objects are used as the beginning or end points of the calculation. This discrepancy implies either that some as yet unspecified but relatively long-lived process acted on the solids in the solar nebula to increase the spread in oxygen isotopic composition during each and every stage of accumulation, or that the nebula was heterogeneous (at least in oxygen) and maintained this heterogeneity throughout most of its nebular history. Depending on its origin, large-scale nebular heterogeneity could have significant consequences for many areas of cosmochemistry, including the application of well-known isotopic systems to the dating of nebular events and the prediction of bulk compositions of planetary bodies on the basis of a uniform cosmic abundance. The evidence supports a scenario wherein the oxygen isotopic composition of nebular solids becomes progressively depleted in 16O with time due to chemical processing within the nebula, rather than a scenario where 16O-rich dust and other materials are injected into the nebula, possibly causing its initial collapse.