Chondritic IDPs that are collected in the lower stratosphere are enriched in bromine: up to 40 x CI in an individual IDP . The average enrichment is 19 x CI . Volatile element enrichments in chondritic IDPs show a general increase with increased element volatility [1,2] which is consistent with solar nebula condensation models. However, the bromine enrichment is markedly up from the general volatile element enrichment trend. Stratospheric bromine derives from anthropogenic and volcanic activities and micrometeoroid ablation and evaporation . It is possible that a portion or all bromine in IDPs is stratospheric surface contamination. In this case there should be an inverse correlation between bromine content and IDP size [1,4]. This correlation is not obvious because it may be complicated by the different ability of exposed IDP surfaces to adsorb volatile elements . To evaluate this model it is important to know whether bromine occurs in a distinct mineral phase  or in a surface layer that might not survive the curatorial rinsing procedure. Another factor is the IDP stratospheric residence time. In my continued AEM analyses of ultrathin CP IDP sections, I recently observed round Br-containing grains associated with CP IDP W7029E5. These grains (11.6-744 nm in diameter) have a volatile matrix with abundant nanocrystals. Their bulk composition shows the presence of Na, K, Br, Cl, and S. Polycrystalline rings in their diffraction patterns are consistent with KBrO(sub)3, KCl, Na(sub)2SO(sub)3 and Na(sub)2S(sub)2 [Gail Fraundorf, written comm., 1991]. These round grains resemble sulfuric acid droplets associated with silica grains in other CP IDPs . The sulfuric acid was washed off the silica grain surface during curatorial hexane rinsing of IDPs. The AEM data confirm a Br-bearing layer on W7029E5. This study is the first, and so far only, observation of Br-bearing material associated with chondritic IDPs in support of a stratospheric bromine surface contamination layer. I calculate the atmospheric residence time of individual particles using a new two-stage model for settling of atmospheric dust that emphasizes a function between individual particle diameter and the gas mean free path of the atmosphere . The result in Figure 1 shows a linear correlation [corr. coeff. = 0.93] instead of the scattergram when using particle dimension [cf. 1]. The particles U2022B2 and W7013H17 are omitted; both have an atypical IDP morphology with considerable smooth surface area. CONCLUSION. The AEM data together with a new model for the atmospheric residence time of individual IDPs argue in favor of a bromine layer on chondritic IDPs due to stratospheric contamination. The excellent correlation also suggests that curatorial rinsing may not strongly affect the bromine content of most chondritic IDPs. Figure 1, which in the hard copy appears here, shows mass- normalized bromine content and stratospheric residence time of IDPs; cf. ref. 1 for bromine data. References. 1. Flynn G.J. & Sutton S.R. (1990) Proc. 20th LPSC, 335-342; 2. Flynn G.J. & Sutton S.R. (1992) Meteoritics 26, 334; 3. Sutton S.R. & Flynn G.J. (1990) Proc. 20th LPSC, 357-361; 4. Jessberger E.K. et al. (1992) Intern Rpt. Max Planck Inst. Heidelberg, 13 pp.; 5. Rietmeijer F.J.M. (1989) Meteoritics 24, 319-320; 6. Rietmeijer F.J.M. (1988) JVGR 34, 173184; 7. Rietmeijer F.J.M. (1992) JVGR, in press. This work is supported by NASA Grant NAG 9-160.
- Pub Date:
- July 1992