4.4 Ga Alteration of Chondrules in Allende (cv3) Parent Body: Evidence from the Rb-sr System

Introduction: It has been revealed that both of the thermal and aqueous alteration had occurred in the CV parent body [1, 2, 3]. To give a constraint to the alteration processes, the Allende meteorite can be a best example, because the following results indicate that the Allende is the strongest altered meteorite among the CV chondrites, those are: (a) homogeneity of the olivine in matrix of the CV3 chondrites increase from Kaba, Mokoia, Vigarano, Grosnaja to Allende [4, 5]; (b) Allende is markedly depleted in noble gases [6] (c) the abundance of interstellar SiC content decreases significantly from Leoville, Vigarano and Efremovka to Allende [7, 8]. To examine the alteration processes of the Allende meteorite, Rb-Sr system is suitable, because the disturbance of the Rb-Sr system of the Allende chondrite has been reported [9, 10, 11]. Since the opaque assemblage in the CAIs, chondrules and matrix of the Allende chondrite had equilibrated at the similar temperature [12] and since the disturbance of the Rb-Sr system of chondrules is identical to that of the CAIs [9,10], the alteration processes affected the Allende inclusions identically. To avoid the differences in the chemical and thermal properties among the inclusions, it should be better to limit a target. Among the inclusions, the chondrules should be suitable, because they have wide variety of chemical compositions, especially in alkaline metal elements. Additionally, to reduce the effect of relict minerals, we focused upon the barred olivine chondrules. The petrographic type and the degree of alteration for the individual chondrule were also determined. In this study, we have improved our preliminary work [11] and give a new insight into the alteration process in the CV parent body. The major goal of this study is to elucidate of the alteration process in the CV parent body based on the Rb-Sr system of the Allende chondrules.