Anhydrous alteration of Allende chondrules in the solar nebura II: Alkali-Ca exchange reactions and formation of nepheline,sodalite and Ca-rich phases in chondrules

Abstract

Fifty-eight Allende chondrules were studied to investigate secondary anhydrous alteration of chondrules. Nepheline and sodalite replace anorthite-normative cryptocrystalline groundmass and plagioclase. These reactions presumably took place by replacement of Ca of the anorthite component by 2 (Na, K) introduced from chondrules, and are called alkali-Ca exchange reactions. The degree of the reaction is variable among chondrules; a few chondrules display the reaction in the peripheral parts only. In contrast, the primary groundmass in some other chondrules was totally replaced by nepheline and sodalite. Most of the Allende chondrules were subjected to the alkali-Ca exchange reaction. Some Ca-rich phases such as hedenbergite, andradite, grossular, kirschsteinite and wollastonite, rarely occur intimately with nepheline and sodalite in groundmass. These Ca-rich phases are also secondary products in chondrules. During the alkali-Ca exchange reaction, part of the CaO released from anorthite was used to form the Ca-rich phases in chondrules. Since the abundance of the Ca-rich phases is too low, CaO was mostly lost from chondrules during the alkali-Ca exchange reaction. CAIs and chondrules in oxidized CV chondrites include similar secondary minerals, suggesting that both of them were subjected to the alkali-Ca exchange reaction under similar conditions.