Chemical and physical studies of chondrites: X. Cathodoluminescence and phase composition studies of metamorphism and nebular processes in chondrules of type 3 ordinary chondrites

Abstract

The cathodoluminescence (CL) properties of eight type 3 ordinary chondrites and one L5 chondrite have been determined, and phenocryst and mesostasis compositions have been analyzed in the chondrules of four of them (Semarkona, type 3.0; Krymka, 3.1; Allan Hills A77214, 3.5; and Dhajala, 3.8) in order to investigate their origins and metamorphic history. Two major classes of chondrule with eight subdivisions have been identified mainly on the basis of CL properties, and >95% of the chondrules can be assigned to these groups on the basis of phenocryst and mesostasis composition. Class A chondrules, consisting of those with plagioclase-normative mesostasis with bright CL, are subdivided into groups A1, A2, A3, A4, and A5. Class B chondrules, with little or no CL and having quartz-normative mesostases in the least metamorphosed chondrites which becomes feldspathic with metamorphism, are subdivided into groups B1, B2, and B3. Relationships between the eight chondrule groups can be deduced from their relative abundance in each of the nine chondrites. Groups A1, A2, B1, and A5 are present in Semarkona (15, 20, 60, 5% by number, respectively), group A5 chondrules in Semarkona being more heterogeneous than A5 chondrules in chondrites of higher petrologic type. Chondrule group A1 evolves into A3 then A4 and then A5 during metamorphism while A2 evolves into A4 and then A5. Chondrule group B1 evolves into B2, B3, and then A5 but higher levels of metamorphism are required to complete the series than for the A1–5 series. Conversion of a group to an adjacent group can be observed in Allan Hills A77214 (3.5), where several chondrules are group B3 in their central regions and group A5 in their outer regions. The present chondrule groups are essentially independent of texture. Since group A and group B chondrules differ in bulk composition, redox state, and possibly oxygen isotope systematics, their relative abundance might be a factor in the creation of the nine chondrite classes.