Effect of pressure on rare earth element partition coefficients in common magmas

Abstract

Geochemical modelling of magma generation and crystal fractionation processes at pressures corresponding to those of the lower crust1 and upper mantle2 is critically dependent on the trace element partition coefficients used in the calculations. The effects of temperature and melt composition on partition coefficients are reasonably well established3,4, but the possible effect of pressure has not been closely evaluated, apart from some work on nickel partitioning5. Rare earth elements (REE) are important in geochemical modelling, and the need for data on the pressure effect on REE partitioning has been pointed out3. We have done a series of experiments in which the role of pressure was isolated from that of temperature and melt composition (or melt structure), and we are able to demonstrate a systematic increase in La, Sm, Ho and Lu partition coefficients for sphene/silicate liquid and clinopyroxene/silicate liquid as pressure increases from 7.5 to 30 kbar.