Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.288s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 288
Mathematics
Logic
1
Scientific paper
The fine-grained rims on ordinary and carbonaceous chondrite chondrules are often regarded as accretionary, mainly because they are often volatile-rich (King and King, 1981). However, Sears et al. (1991) recently argued that some chondrule rims in CM chondrites may be the result of aqueous alteration. The rims, like many features in primitive chondrites, stand out especially clearly in cathodoluminescence (CL) imagery due to the distinctive bright red CL of their abundant fine-grained forsterite. Chondrules similarly stand out well in CL images. Group A chondrules show the strong red CL of Fe-free olivine, sometimes associated with the bright yellow CL of anorthite- normative mesostases, while group B chondrules containing Fe-rich olivines and quartz-normative mesostases do not produce CL (Sears et al., 1992). Using a CL mosaic, we measured chondrule and rim diameters for every chondrule in a 17x10 mm section of Murchison. 38% of the chondrules were group A, compared with 46% in the Semarkona (LL3.0) and 61% in the Dhajala (H3.8) ordinary chondrites. While both group A and group B chondrules have rims, those on group A chondrules are significantly thicker than those on group B chondrules, the rim-to-diameter ratios being 0.2-0.5 for group A chondrules and 0.1-0.2 for group B chondrules (Fig. 1). There are two reasonable explanations for the relationship between rim thickness and chondrule group. Either (1) the composition of chondrule mainly determines the thickness of the rim, e.g., the rims were produced by the aqueous alteration of the host chondrule, or (2) the two chondrule groups were formed in different environments, say a very dusty locale favoring thick rims versus a less favorable relatively dust-free location. We note that (a) both types of chondrule coexist in the same rock, (b) CL textures at the rim/matrix are sharp while at the rim/chondrule interface they are irregular (see Fig. 7d,e in Sears et al., 1991), (c) all faces on the objects in Murchison have rims of some sort, including the fracture faces of chondrule fragments, (d) the redistribution of volatiles will have accompanied aqueous alteration, and (e) mesostases of calcic plagioclase composition are more susceptible to hydrolysis than the more SiO2-rich glasses. We therefore suggest that the evidence favors the idea that these rims formed by in situ aqueous alteration. We suspect that most of this alteration predated the complex multistage, multienvironment brecciation process (Metzler et al., 1992). We speculate that some of the coarse-grained rims observed in higher petrologic type meteorites (e.g., Rubin, 1984) were produced by metamorphism of these fine- grained rims. King E. A. and King E. A. (1982) Icarus 48, 460-472. Metzler K. Bischoff A. and Stoffler D. (1992) Geochim. Cosmochim. Acta (in press). Rubin A. E. (1984) Coarse-grained chondrule rims in type 3 chondrites. Geochim. Cosmochim. Acta 48, 1779-1789. Sears D. W. G., Batchelor D. J., Lu J., Keck B. D. (1991) Proc. NIPR Symp. Antarct. Meteor., No. 4, 319-343. Sears D. W. G., Lu J., Benoit P. H., DeHart J. M. and Lofgren G. E. (1992) Nature (in press). Figure 1, which in the hard copy appears here, shows chondrule rim thickness to diameter ratio for 70 chondrules in Murchison. "Group A" and "Group B" refers to the compositional chondrule groups (Sears et al., 1992).
Benoit Paul H.
Jie Lu
Sears Derek W. G.
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