Abstract

The crystal-rich, chemically zoned Toba Tuffs commonly contain accessory allanite. Electron-microprobe analyses of this LREE-rich phase exhibit significant variations that correlate generally with magmatic evolution and temperature. Allanite compositions from the different Toba Tuffs mostly overlap, although selected elements allow distinction among the units. Fractionation of allanite from the Toba magmas can be demonstrated from bulk-rock and glass inaa. Crystal/liquid distribution coefficients permit calculation of fractionated proportions and modal abundances. Toba Tuffs that erupted below 800°C contain allanite regardless of bulk-rock composition, whereas samples that erupted above 800°C contain none. Bishop Tuff allanites generally have higher concentrations of LREEs than Toba’s, even though their bulk rocks have lower LREE abundances. Apparently temperature is a critical factor that influences allanite crystallization, and magmatic LREE concentrations are of little importance.

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