Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.250l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 250
Physics
3
Scientific paper
Ultra-high sensitivity instrumental Neutron Activation Analysis (INAA) techniques developed for the analysis of individual stratospheric dust particles (Lindstrom, 1990) have been applied to 24 "unmelted" Antarctic micrometeorites (AMM) in the size range 50-100 micrometers. These weigh about 0.05-1.7 micrograms, or about 10-100x more than the Interplanetary Dust Particles (IDPs) previously analyzed. Samples were collected at Cap Prudhomme (Maurette et al., 1991). Four of the samples broke during handling and were analyzed separately. In all cases, the splits were very similar in composition, showing that sampling is not a serious problem. Two samples (B-5-24 and 91-19-11) had clearly terrestrial signatures, including low Ir contents (<0.07 and <0.014 ppm, respectively). One sample (B3-3-31) had the unmistakable compositional characteristics of a CAI: high CaO, REE, Hf, Th, and Sc, and low Fe, Cr, Co, and Ni. This is a fine-grained particle with a very flat (Group I) REE pattern at about 23x chondrites. A two sigma upper limit for Eu corresponds to 20x CI, so there is no positive Eu anomaly, suggesting that it is a Type B inclusion (e.g., Grossman, 1980). The remainder of the samples have approximately chondritic compositions, but with occasional outliers and some systematic differences that may provide valuable information on the processes that these particles have undergone, including atmospheric entry heating and weathering in the Antarctic ice. For example, seven of the 22 micrometeorite particles contain apparent U abundances of more than one ppm (>100 x CI). These seven include smaller particles and three of the four (porous?) particles that broke during handling, suggesting that the U contents are surface correlated. Most likely these small amounts of U (about 10^9 atoms) are leached from small amounts of terrestrial volcanic ash in the melted ice (in the relatively stable form of uranyl ion, UO(sub)2^2+) and adsorbed on the surfaces of the particles. These U-rich samples include 7 of the 10 samples significantly enriched in K relative to CIs. Note that our U analyses are for ^238U, which is not abundant in the ice like the products of its radioactive decay (Fireman, 1986). The few previous numbers for U in larger AMM (Koeberl and Hagen 1989; Koeberl et al., 1992) also suggest the presence of terrestrial U on some particles. Of 22 particles, 15 have Fe levels (determined using energy dispersive X-ray analyses on a scanning electron microscope) significantly below the CI chondrite value of 18.2%, suggesting that normalization of trace element abundances to CI values for Fe (Klock et al., 1992; Sutton et al., 1992) is not a good idea. Abundances of lithophile trace elements such as Sc and Cr, which are less likely to have been altered by atmospheric heating or terrestrial weathering, are in agreement with the idea that Fe is lost, since Cr/Fe and Sc/Fe ratios are well correlated and range from one to four times CI values, suggesting that as much as 3/4 of the original iron might have been lost. In agreement with previous work we observe sizable depletions in Ni. Co abundances are also low, and in fact, both Ni and Co are usually depleted relative to Fe. Zn is depleted in all of our samples by at least 2-10x. Br enrichments, when observed, correlate with K or Na enrichments, and probably are terrestrial. References: Fireman E. (1986) Proc. Lunar Planet Sci. 16th, D539-D544. Grossman L. (1980) Ann. Rev. Earth Planet. Sci. 8, 559-608. Klock W., Beckering W., Spettel B., Flynn G., and Sutton S. (1992) Lunar Planet Sci. (abstract) 23, 697. Koeberl C. and Hagen E. (1989) Geochim. Cosmochim. Acta 53, 937-944. Koeberl C., Kurat G., Presper T., Brandstatter F., and Maurette M. (1992) Lunar Planet. Sci. (abstract) 23, 709. Lindstrom D. (1990) Nucl. Instr. Meth. Phys. Res. A299, 584-588. Maurette M., Olinger C., Christophe Michel-Levy M., Kurat G., Pourchet M., Brandstatter F., and Bourot-Denishe M. (1991) Nature 351, 44-47. Maurette M., Kurat G., Presper T., Brandstatter F., and Perreau M. (1992) Lunar Planet. Sci. (abstract) 23, 861. Sutton S., Prinz M., Maurette M., Nehru C., Weisberg M., and Bajt S. (1992) Lunar Planet. Sci. (abstract) 23, 1391.
Klöck W.
Lindstrom David J.
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