Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..314s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 314
Mathematics
Logic
1
Scientific paper
Several differences between Antarctic and non-Antarctic meteorite populations have been recognized that are attributed to two main causes, namely a general difference in their parent meteoroid populations and/or secondary effects like weathering or pairing (see e.g.[1]). In the last few years several hundred new meteorites have become available from hot desert areas (Nullabor Plain, Roosevelt County, Sahara Desert). Compared to Antarctic meteorites these stones are stored on Earth in very different climatic environments. Thus, both groups should show different patterns of weathering. In this paper we report petrographic observations and noble gas measurements concerning terrestrial alterations of a suite of ordinary chondrites from the Sahara desert and Antarctica. Forty thin sections of desert meteorites (Acfer, El Djouf, Illafegh, Daraj, and Roosevelt County) and 60 thin sections of Antarctic ordinary chondrites (Allan Hills and Frontier Mountain) were investigated microscopically. In addition, in some of these meteorites the concentration and isotopic composition of all noble gases were determined. In this report we will discuss the trapped Kr and Xe only. As already noted by Jull et al. [2] many desert meteorites are heavily weathered. Compared to Antarctic meteorites their proportion of metal and troilite is smaller, while iron oxides and iron hydroxides are more abundant. In Antarctic meteorites most silicates are not severely altered. H- and L-group chondri- tes from the Acfer region and Roosevelt County, however, show a remarkable weathering feature of some silicates: In many of these samples nest-like structures are found that consist of silicate fragments embedded in iron oxide or hydroxide. Different stages of their development can be observed. It starts with the filling by these iron compounds of fine cracks of the original grains. With the process of weathering proceeding these cracks grow and finally destroy larger crystals completely to smaller fragments. Together with the oxides and hydroxides these mineral fragments then form distinct clusters sometimes without direct contact to FeNi or FeS. In Antarctic chondrites, oxides and hydroxides are mostly observed around metal. The concentrations of trapped noble gases in ordinary chondrites are correlated with the chemical-petrological classification [3]. Generally, for types 5 and 6 of modern falls or Antarctic finds, concentrations of ^84Kr and ^132Xe are less than 1 10^-10 and 2 10^-10 cm^3STP/g, respectively. In chondrites from hot deserts, however, especially the concentration of Kr is higher. This can be explained by adsorption and incorporation of atmospheric Kr and Xe ((^84Kr/^132Xe)(sub)atm = 27.8) into weathering products, resulting in higher Kr/Xe-ratios. The presence of atmospheric Xe also shows up in its isotopic composition. Several authors have reported on adsorbed noble gases with isotope ratios similar to those in the terrestrial atmosphere that are tightly bound to the minerals of stony meteorites or lunar rocks, i.e., these gases are not removed by a preheating of the sample at several hundred degrees C [4,5]. This has been attributed to an "irreversible adsorption process" [5]. However, as demonstrated here, weathering on Earth can also influence the concentration of Kr and Xe. Stepwise heating experiments and mineral separations are in progress to determine the siting and retentivity of this terrestrial contamination. References: [1] Koeberl C. and Cassidy W.A. (eds.) (1990) LPI Tech. Rep. 90-01, 102 pp. [2] Jull A.J.T., Wlotzka F., Palme H. and Donahue D.J. (1990) Geochim. Cosmochim. Acta 54, 2895. [3] Weber H.W., Schultz L. and Begemann F. (1990) Meteoritics 25, 405. [4] Schelhaas N., Ott U. and Begemann F. (1990) Geochim Cosmochim. Acta 54, 2869. [5] Niedermann S. and Eugster 0. (1992) Geochim. Cosmochim. Acta 56, 493.
Loeken Th.
Scherer Peter
Schultz Ludolf
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