Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XVIII, Nos.1-2


Study of the System CaMgSi2O6-CaFe3+AlSiO6-CaAl2SiO6-CaTiAl2O6 : II. The Join CaMgSi2O6-CaAl2SiO6-CaTiAl2O6 and its Bearing on Ca-Al-Rich Inclusions in Carbonaceous Chondrite

Onuma, Kosuke;Kimura, Makoto

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In the join CaMgSi2O6-CaAl2SiO6-CaTiAl2O6, forsterite, anorthite, spinel, perovskite, and clinopyroxene(ss) crystallize as the primary phases. Spinel has the widest liquidus field. Perovskite has a small liquidus field, but it is present at the subliquidus and sub solidus temperatures. Forsterite is consumed by the reaction with liquid at about 1250℃. Melilite does not have its own liquidus field, but crystallizes below the temperature of 1250°-1300℃ as a sub liquidus and subsolidus phase. Two clinopyroxenes(ss) were found in the join : one is Ti-bearing diopside(ss) and the other is Ti-pyroxene(ss) containing TiO2 up to 8.7%. The liquidus temperature of Ti-pyroxene(ss) ranges from l234℃ to 1250℃ and its liquidus field is bounded by that of forsterite, diopside(ss), perovskite, spinel and anorthite. Four univariant assemblages are encountered: spinel + anorthite + melilite + Ti-pyroxene(ss) + liquid, spinel + anorthite + melilite + perovskite + liquid, anorthite + melilite + perovskite + Ti-pyroxene(ss) + liquid, and spinel + anorthite + melilite + diopside(ss) + liquid. An invariant point spinel + anorthite + melilite + perovskite + Ti-pyroxene(ss) + liquid was confirmed at 1230℃. Two invariant points are assumed from the distribution of univariant lines: diopside(ss) + anorthite + Ti-pyroxene(ss) + perovskite + melilite + liquid, and spinel + anorthite + melilite + Ti-pyroxene(ss) + diopside(ss) + liquid. The chemical composition of the Ti-pyroxene(ss) is very close to that of clinopyroxene found in Ca-Al rich inclusions of the Allende meterorite. This clinopyroxene coexists with spinel, perovskite, melilite, and anorthite and this assemblage is the same as that of the invariant point confirmed in the present study. From the observation mentioned above, the join CaMgSi2O6–CaAl2SiO6–CaTiAl2O6 is related to the formation of the inclusions in the Allende meteorite. The bulk compositions of the Type B inclusions (Grossman, 1975) are calculated in terms of CaMgSi2O6, CaAl2SiO6, CaTiAl2O6, and MgAl2O4 (spinel), and projected to the join. The possible formation mechanism of the inclusions is discussed with reference of the crystallization of the join.