Trace element partitioning during the initial stages of melting beneath mid-ocean ridges

Abstract

We have measured partition coefficients for U, Th, REE, and high field strength element for orthopyroxene–liquid (3) and garnet–liquid (9) pairs from 2.4 to 2.8 GPa and for clinopyroxene–liquid (6) from 1.2 to 2.8 GPa. These are the first measurements of partition coefficients from experiments that are close to multiple saturation with an anhydrous lherzolite assemblage, similar to what is believed to be the source of primary melts formed beneath mid-ocean ridges. In determining the phase compositions of the anhydrous lherzolite we found that the composition of clinopyroxene on the anhydrous lherzolite solidus changes from high-Ca to low-Ca with increasing pressure. Garnet and clinopyroxene partition coefficients for the elements we studied (U, Th, Nb, Hf, Y, Zr, Ce, Nd, Sm, Er, Yb and Lu) are dependent on pressure, temperature, and composition; although crystal composition appears to be the dominant control on pyroxene coefficients. The garnet partition coefficients most applicable to lherzolite melting are significantly higher than those from previous studies, whereas the clinopyroxene partition coefficients are significantly lower. These new partition coefficients in combination with the new phase equilibria relax the constraints from uranium–thorium disequilibria on mantle porosities in the melting region and reconcile the implications of excess 230Th with melting models based on other geochemical and geophysical parameters [Kelemen et al., A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges, Philos. Trans. R. Soc. London Ser. A 355 (1997) 283-3182; Toomey et al., Tomographic constraints on upper mantle structure beneath the MELT region of the East pacific Rise, Trans. AGU 78 (1997) 705]. With these new partition coefficients significant 230Th excess can be created with residual porosities of up to 1%.