Trace element partitioning between titanite and groundmass in silicic volcanic systems

Data for titanite and groundmass from seven Tertiary ignimbrites show a wide range of titanite trace-element concentrations and partitioning values. The range of partitioning values is due to lattice strain (with an ideal radius near Gd) and melt polymerization and is likely affected by the water content of the melt. The REEs, Y, Sc, Th and U are compatible in titanite whereas Rb, Sr, Ba and Pb are incompatible. Sector and growth zoning in titanite indicate disequilibrium crystal growth. Furthermore, differences in zoning, chemical composition and substitution mechanisms between volcanic and plutonic titanite suggest that volcanic titanite forms in systems characterized by down-T crystallization and high melt fractions whereas plutonic titanite forms at near-solidus conditions. Chemical similarities between titanite from the Fish Canyon magma system and plutonic titanite crystals provide evidence that the partitioning behavior of Fish Canyon titanite is the closest representation of titanite partitioning in plutonic systems