• Precursors of Mars: Constraints from nitrogen and oxygen isotopic compositions of martian meteorites

      Mohapatra, R. K.; Murty, S. V. S. (The Meteoritical Society, 2003-01-01)
      We present an approach to assess the nature of materials involved in the accretion of Mars by the planets nitrogen (delta-15N) and oxygen (Delta-17O) isotopic compositions as derived from data on martian meteorites. delta-15N for Mars has been derived from nitrogen and xenon systematics, while Delta-17O has been taken from the literature data. These signatures indicate that Mars has most probably accreted from enstatite and ordinary chondritic materials in a ratio of 74:26 and may not have a significant contribution from the carbonaceous (CI, CM, or CV) chondrites. This is consistent with the chromium isotopic (Eta-53Cr) signatures of martian meteorites and the bulk planet Fe/Si ratio for Mars as suggested by the moment of inertia factor (I/MR2) obtained from the Mars Pathfinder data. Further, a simple homogeneous accretion from the above two types of materials is found to be consistent with the planets moment of inertia factor and the bulk composition of the mantle. But, it requires a core with 6.7 wt% Si, which is consistent with the new results from the high pressure and temperature melting experiments and chemical data on the opaque minerals in enstatite chondrites.
    • The petrogenesis of type B1 Ca-Al rich inclusions: The spinel perspective

      Connolly, H. C.; Burnett, D. S.; McKeegan, K. D. (The Meteoritical Society, 2003-01-01)
      Minor element variations in MgAl2O4 spinel from the type B1 calcium-aluminum-rich inclusion (CAI) Allende TS-34 confirm earlier studies in showing correlations between the minor element chemistry of spinels with their location within the inclusion and with the chemistry of host silicate phases. These correlations result from a combination of crystallization of a liquid produced by re-melting event(s) and local re-equilibration during subsolidus reheating. The correlation of the Ti and V in spinel inclusions with the Ti and V in the adjacent host clinopyroxene can be qualitatively explained by spinel and clinopyroxene crystallization prior to melilite, following a partial melting event. There are, however, difficulties in quantitative modeling of the observed trends, and it is easier to explain the Ti correlation in terms of complete re-equilibration. The correlation of V in spinel inclusions with that in the adjacent host clinopyroxene also cannot be quantitatively modeled by fractional crystallization of the liquid produced by re-melting, but it can be explained by partial re-equilibration. The distinct V and Ti concentrations in spinel inclusions in melilite from the edge regions of the CAI are best explained as being affected by only a minor degree of re-equilibration. The center melilites and included spinels formed during crystallization of the liquid produced by re-melting, while the edge melilites and included spinels are primary. The oxygen isotope compositions of TS-34 spinels are uniformly 16O-rich, regardless of the host silicate phase or its location within the inclusion. Similar to other type B1 CAIs, clinopyroxene is 16O-rich, but melilite is relatively 16O-poor. These data require that the oxygen isotope exchange in TS-34 melilite occurred subsequent to the last re-melting event.
    • Acid-susceptive material as a host phase of argon-rich noble gas in the carbonaceous chondrite Ningqiang

      Nakamura, T.; Zolensky, M.; Sekiya, M.; Okazaki, R.; Nagao, K. (The Meteoritical Society, 2003-01-01)
      A fine-grained dark inclusion in the Ningqiang carbonaceous chondrite consists of relatively pristine solar nebular materials and has high concentrations of heavy primordial rare gases. Trapped 36Ar concentration amounts to 6 x 10^(-6) cc STP/g, which is higher than that of Ningqiang host by a factor of three. Light HF-HCl etching of the dark inclusion removed 86, 73, and 64% of the primordial 36Ar, 84Kr, and 132Xe, respectively. Thus, the majority of the noble gases in this inclusion are located in very acid-susceptive material. Based on the elemental composition, the noble gases lost from the dark inclusion during the acid-treatments are Ar-rich, and the noble gases remaining in the inclusion are Q and HL gases. Transmission electron microscopy showed that the acid treatments removed thin Si, Mg, and Fe-rich amorphous rims present around small olivine and pyroxene grains in the dark inclusion, suggesting that the Ar-rich gases reside in the amorphous layers. A possible origin of the Ar-rich gases is the acquisition of noble-gas ions with a composition fractionated relative to solar abundance favoring the heavy elements by the effect of incomplete ionization under plasma conditions at 8000 K electron temperature.
    • Magnetic classification of stony meteorites: 1. Ordinary chondrites

      Rochette, P.; Sagnotti, L.; Bourot-Denise, M.; Consolmagno, G.; Folco, L.; Gattacceca, J.; Osete, M. L.; Pesonen, L. (The Meteoritical Society, 2003-01-01)
      We present a database of magnetic susceptibility measurements on 971 ordinary chondrites. It demonstrates that this parameter can besuccessfully used to characterize and classify ordinary chondrite meteorites. In ordinary chondrites, this rapid and non-destructive measurement essentially determines the amount of metal in the sample, which occurs in a very narrow range for each chondrite class (though terrestrial weathering can result in a variable decrease in susceptibility, especially in finds). This technique is particularly useful not only for a rapid classification of new meteorites, but also as a check against curation errors in large collections (i.e., unweathered meteorites, the measured susceptibility of which lies outside the expected range, may well be misclassified or misidentified samples). Magnetic remanence, related to magnetic field measurements around asteroids, is also discussed.
    • Yamato 86029: Aqueously altered and thermally metamorphosed CI-like chondrite with unusual textures

      Tonui, E. K.; Zolensky, M. E.; Lipschutz, M. E.; Wang, M.-S.; Nakamura, T. (The Meteoritical Society, 2003-01-01)
      We describe the petrologic and trace element characteristics of the Yamato 86029 (Y-86029) meteorite. Y-86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse- and fine-grained phyllosilicates, Fe-Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine-rich grains are present within a very fine-grained phyllosilicate-rich matrix. Analyses of 14 thermally mobile trace elements suggest that Y-86029 experienced moderate, open-system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500-600 degrees C for Y-86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re-crystallized olivine. This transformation appears to proceed through `intermediate' highly-disordered `poorly crystalline' phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg-rich carbonate precursors. Y-86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe-rich clasts (300-500 micrometers). Fine-grained carbonates with uniform texture are also present as small (10-15 micrometers in diameter), rounded to sub-rounded `shells' of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y-86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock-induced melt re- crystallization. Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y- 86029 are quite similar to those of Y-82162, the only other heated CI-like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.
    • Numerical modeling of impact heating and cooling of the Vredefort impact structure

      Turtle, E. P.; Pierazzo, E.; O'Brien, D. P. (The Meteoritical Society, 2003-01-01)
      Large meteorite impacts, such as the one that created the Vredefort structure in South Africa ~2 Ga ago, result in significant heating of the target. The temperatures achieved in these events have important implications for post-impact metamorphism as well as for the development of hydrothermal systems. To investigate the post-impact thermal evolution and the size of the Vredefort structure, we have analyzed impact-induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the Vredefort structure. When compared with the extent of estimated thermal shock metamorphism observed at different locations around Vredefort, our model results support our earlier estimates that the original crater was 120-160 km in diameter, based on comparison of predicted to observed locations of shock features. The simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0.3 Myr but no more than 30 Myr.
    • Book Review: Disturbing the Solar System, Alan E. Rubin

      Lorenz, R. (The Meteoritical Society, 2003-01-01)
    • An experimental test of Henry's Law in solid metal-liquid metal systems with implications for iron meteorites

      Chabot, N. L.; Campbell, A. J.; Jones, J. H.; Humayun, M.; Agee, C. B. (The Meteoritical Society, 2003-01-01)
      Experimental solid metal-liquid metal partition coefficients have been used to model the crystallization of magmatic iron meteorites and understand the evolution of asteroid cores. However, the majority of the partitioning experiments have been conducted with trace elements doped at levels that are orders of magnitude higher than measured in iron meteorites. Concern about Henrys Law and the unnatural doping levels have been cited as one reason that two recent iron meteorite studies have dismissed the experimental partition coefficients in their modeling. Using laser ablation ICP-MS analysis, this study reports experimentally determined solid metal-liquid metal trace element partition coefficients from runs doped down to the levels occurring in iron meteorites. The analyses for 12 trace elements (As, Co, Cr, Cu, Ga, Ge, Ir, Os, Pd, Pt, Re, and W) show no deviations from Henrys Law, and these results support decades of experimental work in which the partition coefficients were assumed to be independent of trace element concentration. Further, since our experiments are doped with natural levels of trace elements, the partitioning results are directly applicable to iron meteorites and should be used when modeling their crystallization. In contrast, our new Ag data are inconsistent with previous studies, suggesting the high Ag-content in previous studies may have influenced the measured Ag partitioning behavior.
    • From the Editor

      Jull, A. J. T. (The Meteoritical Society, 2003-01-01)
    • A primitive dark inclusion with radiation-damaged silicates in the Ningqiang carbonaceous chondrite

      Zolensky, M.; Nakamura, K.; Weisberg, M. K.; Prinz, M.; Nakamura, T.; Ohsumi, K.; Saitow, A.; Mukai, M.; Gounelle, M. (The Meteoritical Society, 2003-01-01)
      A petrologic and TEM study of a remarkable dark inclusion (DI) in the Ningqiang CV3 chondrite reveals that it is a mixture of highly primitive solar nebula materials. The DI contains two lithologies. The first, lithology A, contains micron-sized olivine and pyroxene grains rimmed by amorphous materials with compositions similar to the underlying crystalline grains. The second, lithology B, appears to preserve the mineralogy of lithology A before formation of the amorphous rims. Overall, the Ningqiang DI appears to record the following processes: 1) formation (condensation and Fe-enrichment) of olivine crystals in the nebula with compositions of Fo42­62; 2) irradiation, resulting in amorphitization of the olivine and pyroxene to varying degrees; 3) partial annealing, resulting in formation of fairly large, euhedral olivine and pyroxene grains with remnant amorphous sharply-bounded rims; 4) in some cases, prolonged annealing, resulting in the formation of microcrystalline olivine or pyroxene rims. The latter annealing would have been a natural consequence of irradiation near the critical temperature for olivine; and 5) mixture of the above materials (lithology A) with nebular condensate high-Ca pyroxene and olivine, which escaped nebular processing, to become lithology B. We suggest that the amorphous rims in lithology A formed in an energetic solar event such as a bi-polar outflow or FU-orionis flare.