南极碳质球粒陨石中两个富尖晶石球粒状难熔包体的岩石学和矿物化学特征

Petrography and mineral chemistry of two spinel-rich spherules in carbonaceous chondrites from the Antarctica

富尖晶石球粒状CAI(富Ca-Al难熔包体)是球粒陨石中一种特殊类型的CAI,在南极格罗夫山碳质球粒陨石GRV020025和GRV021579中共发现两个富尖晶石球粒状CAI———GRV020025-3RI8和GRV021579-3RI5。GRV020025-3RI8具有占统治地位的尖晶石,在球粒的最外边存在严重蚀变的不规则边,钙钛矿主要分布在靠近边的位置。与GRV020025-3RI8比较,GRV021579-3RI5的尖晶石中的钙钛矿消失,深绿辉石出现,薄薄的蚀变层位于尖晶石核和富钙辉石边之间。两个富尖晶石球粒状CAI的尖晶石均具有低含量FeO和ZnO的特征,而且GRV021579-3RI5具有较GRV020025-3RI8更高的TiO2含量。岩石学和矿物化学特征表明,GRV020025-3RI8和GRV021579-3RI5都经历过熔融结晶过程,它们的蚀变均发生在非氧化的含水或无水的环境中。

Spinel-rich spherule is a special Ca-, Al-rich inclusions （CAIs） in chondrites. Two spinel-rich spherules （GRV 020025-3RI8 and GRV 021579-3RI5） have been found from GRV 020025 and GRV 021579 carbonaceous chondrites collected from the Grove Mountains （GRV）, Antarctica. GRV 020025-3RI8 has a predominant spinel with a thin and irregular rim that has been strongly altered. A few grains of perovskite are enclosed in spinel mainly close to the rim. Compared with GRV 020025-3RI8, perovskite disappears and Ti-Al-rich Ca-pyroxene （fassaite） appears with spinel in the core and a whole Ca-pyroxene rim, and a thin layer of alteration between the spinel core and Ca-pyroxene rim in GRV 021579-3RI5. Both FeO and ZnO of spinel are low in the two spinel-rich spherule, and there is more TiO2 in spinel from GRV 021579-3RI5 than that from GRV 020025-3RI8. The petrography and mineral chemistry characteristics of the two spinel-rich spherules show features of crystallization from liquids and their alteration taken place under non-oxidized surrounding.