Rare-earth element partitioning between sphene and coexisting silicate liquid at high pressure and temperature

Over 80 sets of partition coefficients (D) for 4 rare-earth elements (REE) (La, Sm, Ho, Lu) partitioning between sphene and coexisting silicate liquid at 7.5 to 30 kbar and 900° to 1120°C, demonstrate that D-values increase with: (a) increasing SiO₂ content of the coexisting liquid; (b) increasing pressure; and (c) decreasing temperature. The overall D pattern is convex upwards, with sphene strongly favouring the middle REE (Sm, Ho), and favouring Lu slightly more than La. Using Sr²⁺ as an analog for Eu²⁺, it is shown that sphene does not accept Eu²⁺, in contrast to its ready accommodation of the trivalent REE. This REE substitution appears to be coupled:. Ca²⁺+Ti⁴⁺⇌∑REE³⁺+(Al,Fe)³⁺. Sphenes with between 1 and 21 wt. % ∑RE₂O₃ have been grown experimentally, and it is shown that for this composition range D is independent of the REE content of the sphere. However, these D-values are less than most determined from phenocryst-groundmass pairs. Thus, since there is only a partial overlap of REE concentration between the natural and synthetic sphenes, caution is needed in the direct application of these results to natural rocks. Nevertheless, the data confirm the potential importance of sphene in controlling REE distribution and indicate the parameters affecting REE partitioning, which need to be considered when choosing D-values for sphene in geochemical modelling of magmatic processes.