Abstract
CAIs in CO3 chondrites contain various amounts of nepheline. Nepheline were formed by secondary alteration (nephelinization). However, the location where the alteration occurred has been controversial. We here present the results of mineralogical and petrographic study of CAIs in 4 CO3 chondrites, NWA1232 consisting of lithology A (subtype 3.4) and B (3.7), Y81020 (3.0), Y82050 (3.2), and Y790992 (3.5). Our goal was to reveal the relationship between the amounts of nepheline in CAIs (the degree of nephelinization) and the metamorphic grade (subtype) of the host chondrites and to determine where the alteration occurred.
Our results have revealed that the amounts of nepheline in CAIs tend to increase with increasing subtype of the host chondrites. So, we conclude that the nephelinization in CAIs in CO3 chondrites is the process related to thermal metamorphism in the host chondrites, and occurred on the meteorite parent body.
