Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.200b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 200
Other
6
Scientific paper
Two hundred twenty-three micrometeorites collected in Greenland (157) and Antarctica (66) were studied by REM and electron microprobe (Maurette et al., 1987, 1991). From the Greenland particles (size: 100-200 micrometers) about 82% are spherical particles, 6% porous particles, and 12% others (monomineralic or objects that could not be classified). The smaller Antarctic particles (size: 50-100 micrometers) can be subdivided into porous particles (62%), spheres (24%) and others (14%). Spheres can be characterized either as porphyritic, barred or glassy particles. Based on the study of 129 Greenland spheres a relationship between texture and chemical composition is obvious. Particles with barred texture have on average the highest FeO and lowest SiO2 contents and glassy spheres the highest SiO2 and the lowest FeO contents (Table 1). The spherical particles have approximately cosmic Ca/Al-ratios (Fig. 1). This relationship between texture and composition is less clear for the spheres from Antarctica; however, so far only 16 spheres were studied. Spheres from Greenland and Antarctica are depleted in Na, K, S, and Ni. Several objects of the 41 porous particles have textural similarities to CI and CM meteorites (e.g. magnetite framboids, sulfides). Minerals identified within these particles include olivine, pyroxene, sulfides, Fe,Ni-metal, magnetite, perovskite and ilmenite. Their chemistry is significantly different from that of the spheres. As noted by others (e.g. Klock et al., 1992; Kurat et al., 1992) they are depleted in CaO, Ni, and S. Some contain significant contents of Na2O and K2O (Table 1). Most particles plot far below the cosmic Ca/Al-ratio. Conclusions: The chemical composition appears to have an influence on the texture of the spherical particles. High SiO2 contents may reduce crystal nucleation upon rapid cooling. Based on differences in Ca/Al-ratios between spheres and porous particles we rule out that the porous materials are precursors of spheres prior to melting in the atmosphere. Low Ca-content of the porous particles is most probably preterrestrial (Kurat et al., 1992), since small primitive carbonaceous chondrites from Antarctica like Y- 82162, Y-82042 or ALH 85085 did not suffer significant Ca- loss in terrestrial environment. References: Klock W., Beckerling W., Spettel B., Flynn G., and Sutton S. (1992) Lunar Planet. Sci. (abstract) 23, 697. Kurat G., Presper T., and Brandstatter F. (1992) Lunar Planet. Sci. (abstract) 23, 747. Maurette M., Jehanno C., Robin E., and Hammer C. (1987) Nature 328, 699-702. Maurette M., Olinger C., Christophe Michel-Levy M., Kurat G., Pourchet M., Brandstatter F., and Bourot-Denise M. (1991) Nature 351, 44-47. TABLE 1: Average compositions of spherical particles with barred (ba), porphyritic (por), and glassy texture (gl) from Greenland and porous micrometeorites (p) from Antarctica. Numbers in parentheses = number of particles analyzed; all data in wt%; total Fe as FeO. ba(62) por(41) gl(26) p(41) SiO2 38.6 40.3 46.0 31.4 TiO2 0.13 0.12 0.15 0.14 Al2O3 2.64 2.80 2.76 2.75 Cr2O3 0.44 0.55 0.29 0.57 FeO 27.3 21.5 14.6 24.5 MnO 0.35 0.37 0.39 0.22 MgO 25.1 28.0 29.9 16.7 CaO 2.30 1.88 2.65 0.66 Na2O 0.15 0.25 0.16 0.47 K2O <0.02 0.05 0.05 0.19 Ni 0.35 0.24 0.07 0.18 S 0.05 0.21 0.08 0.56 Total 97.43 96.27 97.10 78.34 Figure 1, which in the hard copy appears here, shows CaO and Al2O3 concentrations of spherical barred, porphyritic and glassy micrometeorites from Greenland (averages), and of individual porous particles from Antarctica (41) and Greenland (9).
Beckerling W.
Bischoff Addi
Klöck W.
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