Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.367h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 367
Computer Science
Chondrules, Evaporation, Kamacite, Recondensation, Semarkona, Sulfides, Unequilibrated Chondrites
Scientific paper
The fact that many chondrules in UOCs contain metal associated with sulfide has been attributed to either low temperature of formation (<680 K) and lack of subsequent heating sufficient to cause evaporation [1] or metamorphism after chondrule formation [2]. We have examined the metal and sulfide in group A and B chondrule interiors and rims in the most primitive ordinary chondrite, Semarkona, in order to further explore these options. Most group A1 chondrules contain abundant metal(3-4 wt%), which is mainly as rounded grains of kamacite (< 1 micrometer~60 micrometers) usually situated in the mesostasis near chondrule edge. For the 37 group A1 chondrules investigated, only five contain sulfide and in only one was it abundant. Usually the sulfides were found associated with metal near the chondrule surfaces, and in a few cases, the metal grains were enclosed in sulfides. The "dusty metal" [3] is common in group A1 chondrules, but is not found in group B chondrules, and the host olivine is often embayed by metal-free pyroxene, which has lower Fe/Fe+Mg ratio than the coexisting olivine. In contrast, metal in group B1 chondrules is much less abundant (generally less than 1 wt%) and occurs as both kamacite and taenite. It is often associated with sulfide, with the sulfide being more abundant than metal. Metal in group A1 chondrules is generally poorer in Ni than the metal in group B1 chondrules (Fig. 1). A similar observation was made for type IA and II chondrules [4,5], which are subsets of group A and B respectively. Additionally, metal in chondrules with Fe-poor olivine contains lower abundance of Ni and Co than metal in chondrules with Fe-rich olivine (Fig. 1) [6]. Group A1 chondrules are more frequently rimmed than group B1 chondrules (~70% by number, compared with ~30% ) and seem to have higher ratios of rim thickness to chondrule diameter (Fig. 2). Most group A1 chondrule rims contain ultra-fine-grained metal- and sulfide-rich materials, which are not observed in chondrites of higher petrographic grades. In contrast, group B1 chondrule rims, when present, contain fine-grained matrix-like materials with dispersed or massive sulfide and metal, which, in contrast to the ultra-fine sulfides/metal-rich rims in group A chondrules, are also observed in higher petrographic types [7]. These results can best be explained by reduction of ferrous olivine and loss of FeS by evaporation during group A1 chondrule formation with the recondensation of FeS and/or reactions between recondensed metal and H2S in the nebular gas at lower temperatures. Thermoluminescence, cathodoluminescence, and compositional zoning in several Semarkona group A1 chondrules has also been interpreted in terms of recondensation of major volatile elements like Na and Mn [8,9]. References: [1] Grossman J. N and Wasson J. T. (1983) In Chondrules and their Origins (E. K. King, ed.), 88-121. [2] Wood J. A. (1993) personal communication; see also Grossman J. N. (1988) In Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), 680-696. [3] Rambaldi E. R. and Wasson J. T. (1982) GCA, 46, 929-939. [4] Jones R. H. and Scott E. R. D. (1989) LPS XIX, 523-536. [5] Jones R. H. (1990) GCA, 54, 1785- 1802. [6] Snellenburg J. (1978) Ph.D. Thesis, State University of New York at Stony Brook. [7] Allen J. S. et al. (1980) GCA, 44, 1161-1176. [8] DeHart J. M. (1989) Ph.D. Thesis, University of Arkansas. [9] Matsunami S. et al (1992) GCA (in press). Fig. 1, which appears here in the hard copy, shows chondrule melt compositions (data from [4,6,8]). Fig. 2, which appears here in the hard copy, shows rim thickness against chondrule diameter with regression lines.
Benoit Paul H.
Huang Shizhen
Sears Derek W. G.
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