Sodium‐metasomatism in chondrules in CO3 chondrites: Relationship to parent body thermal metamorphism

Abstract

Sodium-metasomatism in chondrules in CO3 chondrites:Relationship to parent body thermal metamorphism Abstract-We have studied the mineralogy and petrology of mesostases of 783 type I chondrules in seven CO3 chondrites that range in petrologic subtype from 3.0 to 3.7. Chondrule mesostases in the CO chondrite of subtype 3.0 consist mainly of primary glass and plagioclase, while chondrule mesostases in the CO chondrites of higher subtypes (3.2-3.7) contain various amounts of nepheline in addition to glass and plagioclase. Nepheline has replaced glass and plagioclase, forming finegrained aggregates and thin parallel lamellar intergrowths with plagioclase. The nephelinization has proceeded preferentially from the outer margins of chondrules toward the inside. Although the degree of nephelinization differs widely among chondrules in each of the metamorphosed chondrites, our modal analyses and bulk chemical analyses of individual mesostases indicate that the amounts of nepheline in chondrules systematically increase with the increasing petrologic subtype of the host chondrites. Nepheline also has a tendency to increase in grain size with increasing petrologic subtype. We conclude that nepheline in chondrules in the CO3 chondrites has formed largely as a result of effects related to heating on the meteorite parent body. We suggest that nepheline initially formed as hydrous nepheline under the presence of aqueous fluids and subsequently was dehydrated after exhaustion of aqueous fluids. The degree of hydrothermal activity must have increased with increasing degree of heating, and thus, chondrules in more thermally metamorphosed chondrites produced larger amounts of nepheline. The results imply that CO3 chondrites have gone through lowgrade aqueous alteration and subsequent dehydration at the early stage of heating on the meteorite parent body.

Northwest Africa 1232—a CO3 carbonaceous chondrite with two lithologies

“…Glass has been partially replaced by nepheline. These characteristics are similar to those of the CO3.2 -3.5 chondrites (Tomeoka and Itoh, 2004). CAIs and AOIs.…”

“…The mineralogy of the chondrules, CAIs, AOIs, and matrix are also consistent with this classification (e.g., Scott and Jones, 1990;Kojima et al, 1995;Rubin, 1998;Tomeoka and Itoh, 2004). However, lithology A and lithology B show minor, but significant differences in mineralogy.…”

“…These differences in Fe distribution can be explained by the deduction that lithology B has gone through a higher degree of thermal metamorphism than lithology A such that Fe atoms in the Ferich matrix diffused into Fe -poor chondrules (and Mg atoms diffused out from chondrules to the matrix) during thermal metamorphism, and the overall Fe distribution became more homogenized in lithology B than in lithology A. Chondrules and CAIs in lithology B show a higher degree of nephelinization than those in lithology A. Previous studies showed that the amount of nepheline in chondrules and CAIs in CO3 chondrites systematically increases with an increase in the petrologic subtype of the host chondrites, and thus suggested that nephelinization in these objects is correlated with thermal metamorphism of their host chondrites (Kojima et al, 1995;Russell et al, 1998;Tomeoka and Itoh, 2004). We therefore conclude that the differences in Fe/Mg distribution and degree of nephelinization between lithology A and lithology B are related to the difference in degree of thermal metamorphism.…”

Ion microprobe Al–Mg dating of single plagioclase grains in an Efremovka chondrule

“…The black and white lines are probably attributable, respectively, to nepheline and FeS, even though measuring pure chemical compositions of these lines is difficult because the excitation volume by the electron beam was expanded to about 2-3 µm even though the electron beam diameter was confined to less than 0.1 µm. Similar black lines were found in a chondrule of a CO3 chondrite by Tomeoka and Itoh (2004). They suggested that the lines were generated by replacement of plagioclase by nepheline through the sodiummetasomatism because of parent body thermal metamorphism.…”

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

“…There is also no reason to assume that chondrites classified as type 3 have escaped aqueous alteration. Hydrous minerals are well known in type 3 chondrites like Semarkona (e.g., Hutchison et al 1987), and there is evidence for aqueous alteration in CO chondrites (e.g., Rubin 1998;Greenwood and Franchi 2004;Tomeoka and Itoh 2004).…”