Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.221f&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 221
Mathematics
Logic
3
Scientific paper
While most small micrometeorites show no morphological evidence of extreme heating, a few spherical micrometeorites have been collected from the stratosphere. Although heated particles may be altered morphologically, mineralogically, and chemically, they are important because particles <20 micrometers must have high geocentric velocities (consistent with cometary parent bodies) to melt on atmospheric entry (Flynn, 1989). Trace element abundances were measured by Synchrotron X-Ray Fluorescence on 4 spherical particles having major element abundances, except S, consistent with chondritic. Each sphere is substantially depleted in Zn, an effect linked to atmospheric entry heating (Flynn and Sutton, 1992; Flynn et al., 1992). Unlike the low-Zn particles previously described (Flynn and Sutton, 1992), which had approximately chondritic abundances of other volatiles, these chondritic spheres exhibit substantial depletions in Cu, Ga, Ge, and Se as well as Zn. This suggests these spheres were heated longer or to higher temperatures than the irregularly shaped low-Zn particles. Cr and Mn are also depleted, suggesting metal loss. Transmission Electron Microscope examinations of ultramicrotome thin-sections showed L2005Y5, L2005J5, and L2005J20 having well-developed magnetite rims >50 nm thick, a feature linked to entry heating (Keller et al., 1992). These magnetite rims consist of polycrystalline plates of magnetite. L2005J20 also contains grains of FeNi metal. FeNi metal and Fe-bearing olivine are the dominant phases in L2005J13. No magnetites were observed in L2005J13. The Ni content of each sphere is between 1/3xCI and 3xCI, in contrast to the order-of-magnitude Ni depletions in almost all large micrometeorites recovered from polar ices (Maurette et al., l991). Even irregularly shaped micrometeorites from Greenland that have ~CI abundances of Zn and the other volatile trace elements (Flynn et al., 1991) have low Ni contents. Separation of a Ni-rich, metallic nugget during atmospheric entry was suggested to explain the low Ni content of spherical micrometeorites from the ocean (Brownlee, 1985). The low Ni content of large polar micrometeorites is unlikely to have resulted from atmospheric entry heating or these chondritic spheres would also be Ni-depleted (unless there is a lower size limit >11 micrometers for the nugget separation mechanism). Thus the low Ni content of micrometeorites from polar ices either reflects their pre-atmospheric composition or Ni was removed during their residence in or extraction from the ices. The recovery of 100- micrometer chondritic spheres with Ni/Fe ~CI from Canadian ice (Cresswell and Herd, 1992) suggests Ni loss occurs in Greenland and Antarctic ices or the Ni concentration in large micrometeorites varies significantly with time. REFERENCES: Brownlee D.E. (1985) Ann. Rev. Earth Planet. Sci, 13, 147-173. Cresswell R.G. and Herd R.K. (1992) Meteoritics, 27, 81-85. Flynn G.J. (1989) Icarus, 77, 287-310. Flynn G.J. and Sutton S.R. (1992) Proc. Lunar Planet. Sci. Conf., Vol. 22, 171-184. Flynn G.J. et al. (1991) Meteoritics, 26, 334-335. Flynn G.J. et al. (1992) Lunar Planet. Sci. XXIII, 375-376. Keller L.P. et al. (1992) Lunar Planet. Sci. XXIII, 675-676. Maurette M. et al. (1991) Nature, 351, 44-47. Table 1. Fe and CI normalized abundances in chondritic spheres. Particle Diam. Cr Mn Fe Ni Cu Zn Ga Ge Se Br L2005J5 10 um <0.006 <0.13 1 1.1 <0.04 <0.013 <0.1 <0.03 0.2 0.8 L2005J13 10 um 0.3 <0.1 1 3.3 <1.1 <0.02 <0.1 <0.03 <0.1 <0.6 L2005J20 11 um 0.03 <0.1 1 0.4 <0.2 <0.01 <0.2 <0.03 <0.3 0.8 L2005Y5 8 um 0.47 0.2 1 1.0 2.6 0 04 <1 <0.3 <0.6 8 Avg. Cosmic Dust 1.05 1.4 1.2 1.0 2.2 2.7 1.6 1.4 3.1 26
Bajt Saa
Flynn George James
Keller Lindsay P.
Sutton Richard S.
Thomas Kathie L.
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