Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.303w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 303-303
Other
1
Scientific paper
Numerous studies since 1987 demonstrate that, on average, Antarctic populations of specific meteorite groups differ from non-Antarctic falls. Some differences could conceivably reflect alteration during the meteorites' residence in Antarctica while others clearly are preterrestrial origin, predating fall on Earth. Concentrations of certain trace elements (Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, Zn) determined by RNAA in 45 H4-6 chondrites first provided evidence for Antarctic/non-Antarctic meteorite population differences [1]. Most of these elements are thermally labile (easily lost during extended chondritic heating chondrites) so that their concentrations give important information on the thermal history of meteoritic material. Refractory elements cannot give such information. Factors possibly complicating establishment of compositional differences as preterrestrial--meteorite pairing, population reproducibility, analyst bias, and statistical modeling--are of demonstrated in consequence [1-4]. Indeed, compositional differences exist [3,4] between Antarctic meteorite populations (Victoria Land vs. Queen Maud Land) and among observed falls (Cluster 1 vs. other falls). Possibilities for meteorite compositional alteration during Antarctic weathering must be re-assessed as new data are obtained: here, we summarize the current status of this problem. 1. Highly weathered meteorites: Ten of our suite of trace elements have significantly lower mean concentrations (presumably because of leaching) in H5 chondrites of weathering types B/C and C, than in types A, A/B, and B [1]. Meteorites of types A to B-- whether exhibiting efflorescence or not--seem uncompromised [5]. 2. Antarctic meteorites of high weathering susceptibility: Carbonaceous chondrites and lunar meteorites are essentially unaltered by weathering. For example, data for LEW 90500 C(1?) chondrite reported here demonstrate that the 8 most volatile elements (Se, Cs, Te, Zn, Cd, Bi, Tl, In) have a mean Cl- normalized weight ratio of 0.585+-0.069. Other elements--even Rb, which should be easily transported in a phyllosilicate exposed to water--show no evidence for gain or loss in Antarctica. This is true also for 39 other Antarctic C2-6 chondrites [6]; 3 additional Cl-2 chondrites thermally metamorphosed in their parent bodies [7]; and lunar meteorites studied by us and others. Hydration effects are absent in these meteorites. 3. Eucrites exhibiting evidence for Ce transport: A pair of eucrite clast samples (EET 87503,23 interior and exterior), was previously studied by INAA yield REE data suggesting addition of LREE (except Ce) to the interior during Antarctic residence (Mittlefehldt, personal communication). The exterior/interior ratio for Ce, 1.1, is the same as the mean value for our RNAA trace element suite, 1.1+-0.5. Despite the large uncertainty of this ratio (reflecting the normally heterogeneous distribution of labile elements in eucrites--including falls [8]), results for EET 87503,23 are consistent with the interpretation that our suite of labile trace elements is unaffected by the process that affected REE other than Ce. Our elements are probably dispersed among many host sites, rather than being sited in a single host, like whitlockite. More RNAA measurements of additional eucrite pairs should be done to confirm this result. Further, a putative C3 clast exhibits no evidence for terrestrial alteration of RNAA elements, saponitic matrix, etc. even though REE have apparently been leached from basalts in its host eucrite, LEW 85300 [5]. After five years, numerous investigations confirm meteorite population differences consistent with the RNAA results. While Antarctic processes may have affected REE contents in some eucrites, at present no evidence exists for labile trace element transport into/out of interiors of meteorites of weathering types A to B. The absence of evidence is not evidence of absence, so continued vigilance remains necessary. Research supported by NASA grant NAG 9-48, aided by DOE grant DE-FG07-80ER10725J and NATO grant 0252/89. References: 1. Dennison J. E. and Lipschutz M. E. (1987) Geochim. Cosmochim. Acta 51, 741-754. 2. Lipschutz M. E. and Samuels S. M. (1991) Geochim. Cosmochim. Acta 55, 19-34. 3. Wolf S. F. and Lipschutz M. E. (1992) Lunar Planet. Sci. (abstract) XXIII, 1545-1546. 4. Wolf S. F. and Lipschutz M. E. (abstract), this conference. 5. Zolensky M. E., Hewins R. H., Mittlefehldt D. W., Lindstrom M. M., Xiao X., and Lipschutz M. E. (1992) Meteoritics, submitted. 6. Xiao X. and Lipschutz M. E. (1992) J. Geophys. Res. Planets, in press. 7. Paul R. L. and Lipschutz M. E. (1989) Z. Naturf. 44a, 978-987. 8. Paul R. L. and Lipschutz M. E. (1990) Geochim. Cosmochim. Acta 54, 3185-3195.
Lipschutz Michael E.
Wang Shao-Min
Xiao Xianbo
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