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Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.220f&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 220
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2
Scientific paper
Synchrotron X-Ray Fluorescence (SXRF) allows high-sensitivity element abundance measurements from Cr to Br. Although abundances in individual chondritic cosmic dust particles generally scatter from 1/3xCI to 3xCI abundances (Flynn and Sutton, 1992a), enrichments are more common than depletions. We previously reported average compositions of 18 (Flynn and Sutton, 1991) and 24 (Flynn and Sutton, 1992b) chondritic cosmic dust particles showing volatile enrichments. Jessberger et al. (1991) reported similar results for 8 particles. Their average composition of 42 chondritic particles follows the previous trend. Each element from Cr to Br having a nebula condensation temperature lower than 1277 K is enriched over its CI meteorite concentration, generally by about the same factor as the depletion from CI exhibited by CII or CIII meteorites. Although many particles are volatile rich, two types have lower volatile contents. Low-Zn particles probably lost much of their initial Zn during atmospheric entry heating (Flynn and Sutton, 1992a; Flynn et al., 1992). Some low-Zn particles, presumably the most extremely heated survivors, are also depleted in other volatile elements (Cu, Ga, Ge, and Se). Since 10 low-Zn particles are included in the 42, the calculated average abundances may be lower than pre-atmospheric values, especially for very volatile elements. We have noted that Cu/Fe versus Se/Fe in almost all chondritic cosmic dust particles plots on the volatile-rich extension of the line defined by these ratios in the CI, CII, and CIII meteorites (Flynn and Sutton, 1992b). The eight particles that fall within the chondritic range (Cu/Fe and Se/Fe both <1.2 x CI) show much smaller deviations from CI for other elements. Their average volatile trace element content falls between CI and CII, except for a large Br enrichment. Schramm et al. (1989) indicate the porous (generally anhydrous) and smooth (generally hydrated) cosmic dust particles form two chemical groups, with Ca depleted from CI in the smooth group. We separated the particles into normal-Ca and low-Ca (Ca<1/3xCI) groups using SXRF Ca abundance or JSC Catalog EDX spectra. Although this does not provide a perfect separation into hydrated and anhydrous particles, the average compositions of the low-Ca and normal-Ca groups show distinct differences. The normal-Ca group is enriched in Mn, Cu, Ga, Ge, and Se by ~2x over the low-Ca group. Br and Zn are each enriched by approximately the same factor in both groups, but the average Zn content is dominated by one Zn-rich particle (probably containing a large ZnS) in each group. If Ca abundance is a general indicator of mineralogy, the anhydrous particles are more volatile rich than the hydrated particles. This is opposite the trend in meteorites, with hydrated CI meteorites being the most volatile rich. REFERENCES: Flynn G.J. and Sutton S.R. (1991) Meteoritics, 26, 334. Flynn G.J. and Sutton S.R. (1992a) Proc. Lunar Planet. Sci. Conf., Vol. 22, 171-184. Flynn G.J. and Sutton S.R. (1992b) Lunar Planet. Sci. XXIII, 373-374. Flynn G.J. et al. (1992) Lunar Planet. Sci. XXIII, 375-376. Jessberger E.K. et al. (1991) Meteoritics, 26, 352. Schramm L.S. et al. (1989) Meteoritics, 24, 99-112. Figure 1, which in the hard copy appears here, shows the average trace element contents of chondritic cosmic dust.
Flynn George James
Sutton Richard S.
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