The influence of pressure, mineral composition and water on trace element partitioning between clinopyroxene, amphibole and basanitic melts

Trace and minor elements in clinopyroxenes, amphiboles and basanitic melts produced in experiments at 0.5 to 2.0 GPa and 1000 to 1050°C were analysed by LAM-ICP-MS. The analyses reproduce the results of previous electron microprobe analyses by Adam & Green (1994) but also provide data for many new elements. The expanded data set was used to evaluate the trace element partitioning models of Blundy & Wood (1994) and Wood & Blundy (1997). Although the lattice-strain model of Blundy & Wood (1994) could be successfully fitted to our data, the thermodynamic model of Wood & Blundy (1997) does not predict the negative correlations that we found between D_REE and pressure. This is not explained by variations in the amounts of H₂O in our experiments. Instead, we attribute the variations in D_REE to volume-reducing substitutions in octahedral and tetrahedral sites that increase the activities of +3 and +4 ions, and decrease those of +1 ions, in sites of 8-fold coordination as pressure increases. We were able to quantitatively model some of these effects using a combination of simple electrostatics, Pauling bond strengths and bond-valence calculations. Notwithstanding this, it is apparent that pressure, temperature and the properties of the melt phase also influence partition coefficients independently of mineral composition and crystal-chemical effects.