Significance of the System CaMgSi2O6-CaAl2SiO6-CaTiAl2O6 to Ca-Al- Rich Inclusions in Carbonaceous Chondrites

The system CaMgSi_2O_6-CaAl_2SiO_6-CaTiAl_2O_6 was found to be significant for elucidating the crystallization of Ca-Al-rich inclusions in carbonaceous chondrites. Two clinopyroxenes (ss)(ss : solid solution) were found in the present system : One is Ti-bearing diopside (ss) and the other is Ti-pyroxene (ss) containing TiO_2 up to 8.7wt.%. The chemical composition of this Ti-pyroxene (ss) is very close to that of clinopyroxene found in Ca-Al-rich inclusions of the Allende meteorite. An invariant point was confirmed at 1230℃ where the Ti-pyroxene coexists with spinel, perovskite, anorthite, melilite, and liquid. The assemblage of the crystalline phases is the same as that of Ca-Al-rich inclusions of the Allende meteorite. The bulk chemical compositions of the Type B inclusions of the Allende meteorite (GROSSMAN, 1975) are calculated in terms of CaMgSi_2O_6,CaAl_2SiO_6,CaTiAl_2O_6 and MgAl_2O_4 (spinel), and projected to the present system. The possible formation mechanism of the inclusions is discussed with reference to the crystallization of the system