The effect of temperature on the equilibrium distribution of trace elements between clinopyroxene, orthopyroxene, olivine and spinel in upper mantle peridotite

Abstract

The abundance of 30 trace elements has been determined in the minerals of 16 well-equilibrated spinel lherzolite xenoliths by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS). Major elements were analysed by electron microprobe. The xenoliths span a range of equilibration temperatures from 1150 to 1500 K (calculated at an assumed pressure of 1.5 GPa from two-pyroxene geothermometry), allowing the trace-element partitioning relationships among the phases (olivine, orthopyroxene, clinopyroxene, spinel, and in some lower temperature xenoliths, amphibole) to be quantified as a function of temperature. Most elements show smooth partitioning trends among all phases that depend primarily on temperature but with some influences from bulk composition, particularly the amount of Na in clinopyroxene. Although most incompatible trace elements are concentrated into clinopyroxene, the effect of increasing temperature is to redistribute these elements into orthopyroxene and even olivine, such that these latter phases hold non-negligible proportions of many trace elements at the temperature at which peridotite would be in equilibrium with basaltic melts. The inter-crystalline trace-element partition coefficients reported in this study can be used to reconstruct the trace-element abundances in clinopyroxene at melting temperatures, and should also prove useful in elucidating the histories of more complex mantle peridotites with unequilibrated mineral compositions and textures.