The correlation of chondrule texture and magnesium isotope abundance in Allende meteorite

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The Mg isotope abundance of the individual chondrules in the Allende meteorite was measured by ion microprobe mass analysis. The amounts of ^<24>Mg excess with respect to the terrestrial fractionation line were obtained for each chondrule. Barred olivine and glassy chondrules tend to have relatively large ^<24>Mg excess, whereas porphyritic and radial pyroxene chondrules have relatively small ^<24>Mg excess. The following three factors were taken into consideration in connection with the formation process of these chondrules in the Allende meteorite; (1) the temperature conditions at the formation of the chondrules, (2) the relative abundance of each chondrule type, (3) the amount of ^<24>Mg excess in each chondrule type. It is shown that chondrules formed at high temperature or by rapid cooling have relatively large ^<24>Mg excess and their relative abundance is small, whereas chondrules formed at low temperature or by slow cooling tend to have small ^<24>Mg excess and their relative abundance is large. A model of chondrule formation process in the early solar nebula is proposed to explain the relations between factors (1)-(3).
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