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FIB-TEM Investigations of Fe-NI-Sulfides in the CI Chondrites Alais and Orgueil
| Content Provider | Semantic Scholar |
|---|---|
| Author | Berger, Eve L. Lauretta, Dante S. Zega, Thomas J. Keller, Lindsay P. |
| Copyright Year | 2013 |
| Abstract | Introduction: The CI chondrites are primitive meteorites with bulk compositions matching the solar photosphere for all but the lightest elements [1]. They have been extensively aqueously altered, and are composed primarily of fine-grained phyllosilicate matrix material which is host to carbonates, sulfates, sulfides, and minor amounts of olivine and pyroxene [2, and ref. therein]. The alteration, while extensive, is heterogeneous. For example, CI-chondrite cubanite [3] and carbonate grains [4] differ on mm to sub-mm scales, demonstrating multiple aqueous episodes. CI-chondrite variability is also evidenced by degree of brecciation, abundance and size of coarse-grained phyllosilicates, olivine and pyroxene abundance [4], as well as Nicontent and size of sulfide grains [4,5]. Our previous work [6] revealed Orgueil sulfide grains with variable Ni-contents, metal:S ratios, crystal structures and textures. We continue to explore the variability of CI-chondrite pyrrhotite (Po, (FeNi)1-xS) and pentlandite (Pn, (Fe,Ni)9S8) grains. We investigate the microstructure of sulfides within and among CIchondrite meteorites in order to place constraints on the conditions under which they formed. Samples & Techniques: Samples of Orgueil and Alais were provided by the Vatican Observatory and the Smithsonian Institution, respectively. EMPA was done on the Cameca SX-100 (JSC). Electron transparent thin sections were prepared and analyzed on the Quanta 3D FIB-SEM and JEOL 2500 SE STEM (JSC). Results: The bulk of the investigated Fe-Ni sulfide grains (n=107) in Orgueil have <1.0 at.% Ni and metal:S ratios consistent with 4C monoclinic Po, (FeNi)7S8. However, EDX mapping of 64 grains reveals that ~50% have areas of increased Ni-content, which trends with an increase in the metal:S ratio (see table 1, figs. 1-6). TEM analyses of 9 nine grains show that the textures of the Ni-enriched regions coarsen with increasing Ni-content and metal:S ratios. Low-Ni grains are structurally consistent with 4C monoclinic Po. The microstructures of those with higher Nicontents vary and include lenses of Ni-rich 4C Po, regions of Ni-rich 4C Po, intergrowths of 4C Po and 6C monoclinic Po/Pn, and discrete Pn and Po crystals. Seven of 15 EDX-mapped Alais sulfide grains contain areas of enriched Ni-content. To date, FIB-TEM analyses have been done on 2 of the grains, one of which showed minor Ni-enrichment in thin section. The first grain is 4C monoclinic Po, the second is an assemblage of discrete Pn and 4C Po crystals. Discussion: Plotting the compositional data on the 100-135°C Fe-Ni-S ternary [7] reveals that the Ni-rich material in Orgueil is distinct from the Ni-rich material in Alais [5]. While the Alais compositional data fits the 100-135°C ternary [5], the Orgueil Fe-Ni-sulfide compositions show a better correspondence to phase relations in the 25°C Fe-Ni-S ternary [8]. We propose that higher metal:S ratios and Ni-contents in Orgueil Po grains result from a low-T process in which Ni fills vacancies (☐) in the crystal structure and preferentially replaces Fe, as ferric ions are more easily disrupted from the crystal structure relative to ferrous ones [9]. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130011092.pdf |
| Alternate Webpage(s) | https://www.lpi.usra.edu/meetings/lpsc2013/pdf/1615.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
