Formation conditions of aluminum-rich chondrules

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-Al2O3SiO2 system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (T-max), at cooling rates from 5-1000 degrees 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, similar to 1400-1500 degrees C, and cooling rates of 10-500 degrees C/h for porphyritic chondrules and possibly higher (1000 degrees 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. (c) 2007 Elsevier Ltd. All rights reserved.