Anhydrous alteration of Allende chondrules in the solar nebula III: Alkali-zoned chondrules and heating experiments for anhydrous alteration

The groundmass of some chondrules in Allende shows chemical zonation from alkali-poor and CaO-rich cores to alkali-rich and CaO-poor rims, and the zonation was defined to be original groundmass (zone I), continuous zoning region (zone II), and discontinuously high-Na and low-Ca zone (zone III). The zonation was formed after chondrule solidification and prior to accretion on the Allende parent body, and was probably produced by reactions with an oxidized nebular gas in equilibrium with nepheline, sodalite, high-Ca pyroxene, and ferroan olivine. Heating experiments using synthetic glass beads were conducted, and zonation similar to the alkali-zoned chondrules was formed. The experimental results suggests that the reaction temperature and duration for the Allende alkali-zoned chondrules were about 600°-400℃ and 1-10^4 years, respectively