Other
Scientific paper
Jul 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.3361t&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 13, p. 3361-3381.
Other
7
Scientific paper
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 (Tmax), 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 Tmax 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.
Hewins Roger H.
MacPherson Glenn J.
Tronche Elodie J.
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