Formation conditions of aluminum-rich chondrules

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• 作者：Elodie J. Tronche ; Roger H. Hewins ; Glenn J. MacPherson
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2007
• 出版时间：1 July 2007
• 年：2007
• 卷：71
• 期：13
• 页码：3361-3381
• 全文大小：6155 K

文摘

We have studied the formation conditions of Al-rich chondrules by doing isothermal and dynamic crystallization experiments at one atmosphere on four different chondrule analogue compositions within the pure CaO–MgO–Al₂O₃–SiO₂ system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (T_max), at cooling rates from 5–1000 °C/h. The starting compositions include two with anorthite and two with forsterite as the dominant liquidus phases, all at or near spinel-saturation. One of each pair evolves towards diopside crystallization, and the others cordierite or enstatite crystallization, giving a total of four completely different crystallization sequences analogous to the four basic varieties of Al-rich chondrule recently proposed. Bulk composition is the main controlling factor, both in terms of mineralogy and texture. The textures of the anorthite-rich compositions are more sensitive to T_max than they are to cooling rate, whereas the textures of the forsterite-rich compositions are more sensitive to cooling rate. Comparisons of natural Al-rich chondrules having similar compositions to our synthetic analogues indicate that the natural objects reflect a range of peak heating temperatures, 1400–1500 °C, and cooling rates of 10–500 °C/h for porphyritic chondrules and possibly higher (1000 °C/h) for barred chondrules. These conditions are consistent with the conditions inferred for ferromagnesian chondrules but differ from those inferred for some calcium–aluminum-rich inclusions.