Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.371i&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 371
Mathematics
Logic
1
Argon, Breccia, Clasts, Isotopes, Krypton, Petrology, Xenon
Scientific paper
The Dengli meteorite was found in the Karakum desert in July 1976. Previous studies [1] have shown that the meteorite is a complex chondritic breccia containing unusual components of achondritic affinity. In order to understand the origin of the breccia, we studied its mineralogy, petrology, and isotopic composition of noble gases. In addition, in cooperation with Dr. J. N. Goswami, we are now studying minerals in achondritic clasts using an ion microprobe. The Dengli meteorite has a distinct chondritic texture and consists of chondrules and chondrule fragments embedded into a fine-grained matrix. Equilibrated and unequilibrated populations were found among the chondrules, the latter of which are characterized by normal or inverse Mg-Fe zoning. Olivine (Fa19.6, N = 52, C.V. = 19.3) and pyroxene (Fs18.2, N = 27, C.V. = 17.0) displays large compositional variations. Based on textural and chemical characteristics, Dengli can be classified as an H3.8 chondrite. Two objects with unique compositions were identified. One has a round shape and consists of silica, orthopyroxene (Fs18.1 Wo6.2), clinopyroxene (Fs17.6 Wo27.6), and feldspar (An76.2 Ab23.2). The other object is an angular clast, which is composed of olivine (Fa17.2), Al-rich clinopyroxene (Fs6.3 Wo46.8), and feldspar (An44.7 Ab54.7). The presence of clinopyroxene and Ca-rich feldspar in these objects indicates their similarity with an achondritic material. The Ar, Kr, and Xe contents measured in Dengli are very close to those in other ordinary chondrites of petrological types 3 through 5 [2]. The Xe and Kr isotopic compositions at any temperature step differ from those of solar Xe and Kr. High ^38Ar concentrations were found. The exposure age was estimated to be 7.6 Ma, which is close to the average (6.3 +/- 0.2 Ma) exposure age of 350 H chondrites. The Dengli K/Ar age, 3.73 +/- 0.10 Ga, is close to K/Ar ages of the majority of H chondrites. Thus, Dengli is depleted in noble gases as compared to regolith breccias, and its noble gas isotopic compositions differ from those of the solar wind. This means that Dengli cannot be a regolith breccia. Preliminary ion-microprobe measurements of Mg isotopes in feldspar grains in the clasts described above did not reveal a clear excess of ^26Mg, which could result from an ^26Al decay. This suggests that the clasts were formed when ^26Al became extinct. In summary, we have to conclude that Dengli is a complex breccia that was not formed on a surface of a parent body. It should be of a nebular (accretional) origin. Meteorites with a possibly similar origin are Severnyi Kolchim [3], Study Butte, Beddgelert [4], and Barwell [5]. These meteorites also contain clasts of achondritic affinity as well as silica-bearing and other unusual objects that, in turn, should be considered as products of nebular processes. References: [1] Ivanova M. A. et al. (1992) Meteoritics, 27, 463-465. [2] Ivanova M. A. et al. (1993) LPSC XXIV, 695-696. [3] Nazarov M. A. et al. (1993) LPSC XXIV, 1055-1056. [4] Fredriksson K. et al. (1989) Z. Naturforsch., 44a, 945-962. [5] Hutchison R. et al. (1988) EPSL, 90, 105-118.
Assonov Sergey S.
Ivanova Marina A.
Kononkova Natal'ia N.
Shukolyukov Yu. A.
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