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NASA NTRS1999

Trapped Melt in IIIAB Irons: Solid/Liquid Elemental Partitioning During the Fractionation of the IIIAB Magma

Group IIIAB, the largest iron-meteorite group, shows compositional trends (including a three-order-of-magnitude It concentration range) indicating that it formed by fractional crystallization of a metallic magma. Because about 200 irons are available, and all degrees of crystallization are well represented, IIIAB offers an excellent set of samples for the study of crystallization at all depths of the asteroidal core. On log-log Ir-Au, and Ir-As diagrams IIIAB forms a broad band; the breadth represents real meteorite-to-meteorite variations, far outside experimental or sampling uncertainties. A successful model must explain the width of this band; I suggest that it mainly resulted from the trapping of parental magma within the crystallizing solid. Because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid. The trapped-melt model is supported by the observation that irons having higher S contents plot closer to the inferred composition of the magmatic parental liquid. The lowest S values are found in the irons occupying the left envelope of the IIIAB Ir-Au or Ir-As compositional fields, thus it is this set of irons that should be interpreted as the solid products of a fractionating magma. This simplifies the modeling of the crystallization process and allows inferences regarding the distribution ratios for other elements in the evolved IIIAB system. The large (multiton) Cape York irons show wide variations in their trapped-melt fractions; their compositions seem best understood in terms of a low initial S content of the IIIAB magma, about 20 mg/g. The inferred initial IIIAB distribution coefficient for Ir, 4.6, is much higher than published values based on laboratory studies of low-S systems; I suggest that low-S (and low-P) partition-ratio measurements tend to err in the direction of unity. In IIIAB distribution coefficients for Au, As, and Ni were still < 1 when the most evolved IIIAB irons formed, another indication of a low initial S content.

Wasson, John T.↗

NASA NTRS1986

Isotopic characterisation of prebiotic synthesis of organic material

Many primitive meteorites contain an insoluble organic material, much like terrestrial kerogen, whose mode of origin is currently unknown. When sujbected to stepwise decomposition, this material, unlike its terrestrial counterpart, reveals characteristic release patterns for the stable isotopes of carbon, hydrogen and nitrogen as a function of fractional release of each element. The purpose of this study is to try to match those release patterns using organic matter synthesised in the laboratory under controlled conditions. If successful, such a study would shed light on the origin of kerogen-like organic matter in the early solar system and, by extension, on prebiotic organic synthesis in general. The range of possible syntheses, starting materials and reaction conditions to be investigated is considerable. Samples analysed to date include: a heavy oil produced by Fischer-Tropsch-type catalysis of CO + H2; a solid residue generated by a plasma discharge in CO + H2 + N2; a solid deposited on the electrodes of a Miller-Urey synthesis operating on CH4 + H2O + N2; and a solid residue formed by polymerization of light hydrocarbons procured by a Miller-Urey discharge acting on CH4. Significant structure is observed in the release patterns for the carbon and hydrogen isotopes from the synthetic samples, though there is little evidence for isotopic fractionation during the analysis itself.

Kerridge, J. F.↗

NASA NTRS1994

Chemical investigation of Titan and Triton tholins

We report chromatographic and spectroscopic analyses of both Titan and Triton tholins, organic solids made from the plasma irradiation of 0.9:0.1 and 0.999:0.001 N2/CH4 gas mixtures, respectively. The lower CH4 mixing ratio leads to a nitrogen-richer tholin (N/C greater than 1), probably including nitrogen heterocyclic compounds. Unlike Titan tholin, bulk Triton tholin is poor in nitriles. From high-pressure liquid chromatography, ultraviolet and infrared spectroscopy, and molecular weight estimation by gel filtration chromatography, we conclude that (1) several H2O-soluble fractions, each with distinct UV and IR spectral signatures, are present, (2) these fractions are not identical in the two tholins, (3) the H2O-soluble fractions of Titan tholins do not contain significant amounts of nitriles, despite the major role of nitriles in bulk Titan tholin, and (4) the H2O-soluble fractions of both tholins are mainly molcules containing about 10 to 50 (C + N) atoms. We report yields of amino acids upon hydrolysis of Titan and Triton tholins. Titan tholin is largely insoluble in the putative hydrocarbon lakes or oceans on Titan, but can yield the H2O-soluble species investigated here upon contact with transient (e.g., impact-generated) liquid water.

Mcdonald, Gene D.↗