Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.573s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 573
Other
2
Activation, Neutron, Cosmochemistry, Craters, Impact, Impacts, Metals, Noble
Scientific paper
Lunar highland rocks contain an excess of siderophile elements, which has been attributed to meteoritic influx after the formation of the lunar crust [1-4]. Siderophile element enrichment has subsequently become a standard method for the identification of terrestrial impact craters. Janssens et al. [5], Grieve [6] and Palme et al. [7] have shown the dominant role of impact melt as the main carrier of meteoritic material at large terrestrial impact craters. This has been demonstrated at Clearwater East [8], Lappajarvi [9-11], Saaksjarvi [12], Brent [6] and Rochechouart [5]. The amount of projectile material incorporated in impact melt sheets is generally low (<1%). The highest recorded is 8% at East Clearwater, where the siderophiles are carried in a sulphide phase. In other cases, searches for siderophile anomalies at some impact structure have been largely unsuccessful. Melt bearing mixed breccias (suevitic melt) and fall-back sediments have been found to be free of meteoritic components in Brent, Lappajarvi and Ries samples [6,9,12-14]. However, from approximately 130 craters which are currently known on Earth only four clearly identified chondrites have been found as projectiles of large craters [15,16]. In this study we analyzed twenty-two impact melt samples (10 g) from Saaksjarvi (Finland), Mien and Dellen (Sweden) impact craters for Os, Re, Ir, Ru, Rh, Pd and Au by a slightly modified version of the fire assay neutron activation method using nickel sulphide as the collector [13,14]. All samples were obtained from the collection of the University of Munster. Only fresh, nearly fragment-free, fine grained samples without any sign of alteration were selected for chemical studies. All samples have been described previously [17]. The INAA procedure involved two irradiations: a short irradiation for Rh and a long irradiation for the other elements. Impact melts from Saaksjarvi are highly enriched in PGEs. The flat siderophile pattern suggests that the meteoritic component (PGE/CI = 3x10^-3 to 9x10^-3 relative to CI) in the Saaksjarvi impact melt is relatively unfractionated. Stony-iron meteorites (pallasites) as proposed earlier [7] can therefore be excluded as possible contaminants because Pd and Ir are highly fractionated in pallasites [18]. Impact melts from Mien and Dellen are moderately enriched in PGE. The concentrations are similar (PGE/CI = 3x10^-4 to 1x10^-3 relative to CI). The flat siderophile pattern of the Mien and Dellen impact samples are compatible with carbonaceous chondrite type of material, but a clear geochemical association of any of the known meteorite types is not possible because of the low signal-to-background ratio for Rh, Ru, Pd, and Au. Samples from all impact craters have low Os/PGE ratios (Os/Ir(sub)melt =0.24) compared to chondritic ratios (Os/Ir(sub)CI=1.06). It seems that the oxygen fugacity at the time of impact melting, vaporization and crystallization of the melt body may play the dominant role in the fractionation of Os and PGEs. If Os have been partially lost by volatilization of OsO4 under oxidizing conditions, then Ir is the only element to confirm meteoritic enrichments down to a level of 2x10^-4 CI chondrite. None of the other elements determined are sufficiently sensitive indicators to confirm small meteoritic enrichments, equivalent to <10^-3 CI chondrite, because of low CI/Earth crust-ratios. Acknowledgements. This work was supported by DFG. References: [1] Wasson J. T. et al. (1975) Moon, 13, 121-141. [2] Gros J. et al. (1976) Proc. LSC 7th, 2403-2425. [3] Hertogen J. et al. (1977) Proc. LSC 8th, 17-45. [4] Palme H. (1980) Proc. LPSC 11th, 481-506. [5] Janssens M.-J. et al. (1977) JGR, 82, 750-758. [6] Grieve R. A. F. (1978) Proc. LPSC 9th, 2579-2608. [7] Palme H. et al. (1980) LPSC XI, 848-850. [8] Palme H. et al. (1978) GCA, 42, 313-323. [9] Reimold W. U. and Stoffler D. (1980) Meteoritics, 14, 526-528. [10] Reimold W. U. (1980) Ph. D. thesis, Univ. of Munster, 172 pp. [11] Gobel E. et al. (1980) Z. Naturforsch., 35a, 197-203. [12] Morgan J. W. et al. (1979) GCA, 43, 803-815. [13] Schmidt G. and Pernicka E. (1991) Meteoritics, 26, 392. [14] Schmidt G. and Pernicka E. (1994) GCA, 58, 5083-5090. [15] Palme H. et al. (1981) GCA, 45, 2417-2424. [16] Grieve R. A. F. (1991) Meteoritics, 26, 174-194. [17] Maerz U. (1979) Diploma thesis, Univ. of Munster, 115 pp. [18] Davis A. M. (1977) Ph. D. thesis, Yale University, 285 pp.
Kratz Karl Ludwig
Palme Herbert
Schmidt Georg
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