PARTITIONING OF RARE EARTH AND HIGH FIELD STRENGTH ELEMENTS BETWEEN TITANITE AND PHONOLITIC LIQUID

Accessory phases in igneous rocks are increasingly recognized as repositories of petrogenetic information. Titanite (CaTiSiO₅) is found as an accessory in phonolites and high-fO₂ silicic magmas. It exhibits high values of the partition coefficient D [=(concentration in solid phase)/(concentration in liquid), C_s/C_l] for REE and HFSE, a strong preferences for middle REE over light and heavy REE and for Ta over Nb. Fractionation of titanite during crystallization-differentiation therefore leaves a distinctive imprint of depletion in middle REE and increasing Nb/Ta in the residual liquid. There is nonetheless a shortfall of detailed information on titanite-melt partitioning of lithophile elements from natural samples, in particular from phonolites.

Here we present the results of a LA-ICPMS study of titanites and associated glasses from phonolitic pumice deposits of the 309 ± 6 ka Fasnia Member of the Diego Hernández Formation, Tenerife, Canary Islands. Fasnia titanites can be grouped into two types, and REE and HFSE contents are highly variable and systematically different between the two types. We employ a method of identifying equilibrium titanite-glass (mineral-melt) pairs in order to best constrain REE and HFSE partition coefficients. For 82 titanite analyses, apparent D_REE values were calculated using LA-ICPMS-determined concentrations of REE in the titanites and ~500 phonolite glasses. Equilibrium between the titanites and different glasses is evaluated using the lattice strain model equation for seven-coordinated REE³⁺ with known ionic radii. We show that the two titanite types most likely crystallised from different phonolitic liquids. Resulting partition coefficients are comparable to experimental data. We predict seven-coordinated ionic radii for REE that have no such reported values. Yttrium partitions less strongly into titanite than do REE with similar ionic radii, indicating that effects other than charge and radius play a minor but discernable role in trace element fractionation at magmatic temperatures.