Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.252l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 252
Computer Science
Scientific paper
Several rare chondrules with an unusual combination of phenocryst minerals and mesostases have been found in the Semarkona chondrite. Chondrules SC-28-1, SC-16-2, and SC-16-6 are all POP in texture with unusual CL properties. Their mesostases are greenish yellow in CL, and enclose mineral grains with no CL. The extremely high CaO contents (>17.2 wt%) of the mesostases resemble those of group A1 chondrules. However, the FeO-rich silicates (Fa(sub)>17.0%, FS(sub)>17.0%) are typical of group B1 chondrules (Lu et al., 1991; Sears et al., 1992). Such an unusual combination of melt and phenocrysts is apparent in Fig. 1, in which the CaO/MgO molar ratios in the olivines are plotted against the same ratios for the coexisting mesostasis. Most of the chondrules cluster in a well-defined region that shows a positive correlation between olivine and mesostasis CaO/MgO, suggesting that the phenocrysts were formed in situ. The three unusual chondrules, however, plot outside of this region and have unusually high CaO/MgO ratios for their mesostases. A close examination of the largest chondrule (SC-28-1) reveals that the olivines are either poikilitically enclosed in pyroxenes or form discrete clusters. All the olivines have resorbed and rounded edges. The textures are those expected from incomplete melting of pre-existing mineral clusters. Unlike the strongly zoned olivines in group B1 chondrules, olivines in this chondrule are rather uniform in composition. The pyroxenes are highly FeO-rich (Fs(sub)17.4Wo(sub)1.9) and have high-Ca pyroxene rims (Fs(sub)12.4Wo(sub)38.9) when they are in contact with the mesostasis. The mesostases of these chondrules are highly anorthitic (normative An > 80 wt%). More importantly, a sightly MgO-rich relic pyroxene core (Fs(sub)13.8Wo(sub)0.4) was found inside the FeO-rich pyroxene, which also encloses some poikilitic olivines (Fig. 2). The olivines and pyroxenes are closely associated and have Fs/Fa ratios close to one. However, the pyroxenes are not at equilibrium with the coexisting mesostases. Although the olivine compositions and textures are similar to the relic grains described by Nagahara (1983), the association of "incompatible" phenocrysts and melts in these unusual chondrules suggests that they were formed through cementation of unmelted mineral clusters by non-cogenetic melts. Considering the high CaO contents of group A1 and A2 chondrules, the viscosity of the melts must be much lower than the SiO(sub)2-rich melts of group B1 chondrules. It is therefore possible that some of these low viscosity melts may spin off from their host droplets and cement the FeO-rich mineral clusters in the vicinity. The presence of these unusual chondrules suggests that chondrule-forming processes were very dynamic and that while most chondrules were formed through in situ melting of pre-existing solid material, possibly accompanied by reduction and volatilization, some chondrules could have been formed through cementation of unmelted mineral clusters by feldspathic melts. Lu J., Sears D.W.G., Benoit P.H., Prinz M. and Weisberg M.K. (1991) Meteoritics 26, 367. Sears D.W.G., Lu J., Benoit P.H., DeHart J.M. and Lofgren G.E. (1992) Nature, in press. Nagahara H. (1983) Chondrules and Their Origins (ed. King E.A.), Lunar and Planet. Inst., Houston, 211-222. Figure 1, which in the hard copy appears here, shows CaO/MgO (molar) in olivine vs. CaO/MgO (molar) in mesostasis for Semarkona chondrules. Figure 2, which in the hard copy appears here, shows a BSE image of a fragment of chondrule SC-28-1 chiseled from Semarkona. ol = olivine, py = pyroxene (gray), rpy = relic pyroxene (dark gray), m = mesostasis.
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