THE EFFECT OF SODIUM AND TITANIUM ON CRYSTAL-MELT PARTITIONING OF TRACE ELEMENTS

Abstract

Eclogite of recycled slab origin has often been invoked in the source region of mid-ocean ridge and ocean-island basalts. Melts of this eclogitic material are expected to be enriched in incompatible elements including major elements such as Na and Ti. In order to investigate the controls on trace element chemistry of a melt from such a recycled component we have performed trace element partitioning experiments in the simple systems CMAS (CaO-MgO-Al2O3-SiO2), NCMAS (Na2O-CMAS) and Ti-CMAS (TiO2-CMAS) at 3 GPa and 1298–1500°C using analogue eclogitic compositions.We show that sodium has a profound effect on clinopyroxene-melt partition coefficients. NCMAS is characterized by elevated partition coefficients, relative to CMAS for all elements except Li. The increase is more pronounced for more highly charged cations, resulting in negative partitioning anomalies for Li, Sr, Ba and Pb. In contrast to sodium, titanium has very little effect on trace element partitioning for all elements except Nb and Ta, which are retained by the rutile that is saturated in this run.