Trace element partitioning between wollastonite and alkaline silicate magmas

Abstract

The partitioning of trace elements between wollastonite and melt provides a choice tool for understanding differentiation processes and trace element fractionation in alkaline-rich and silica-undersaturated magmatic systems at crustal conditions, but very few data are currently available. Here we provide the first partition coefficients and associated lattice strain parameters between wollastonite and silicate magmas of Oldoinyo Lengai (Tanzania). Trace element partitioning of isovalent cations shows a parabolic dependence between the partition coefficients and ionic radii explained by the lattice strain model with the site radius (r0) decreasing with increasing charge from r01+ = 1.2 Å to r05+ = 0.6 Å. Bivalent cations are moderately incompatible (DMg = 0.12 and DSr = 0.5) to compatible (DMn = 1). High field strength elements such as Zr and Nb are strongly incompatible in wollastonite (D < 0.01), and rare earth element (REE) partition coefficients increase with decreasing ionic radius from DLa = 0.19 to DLu = 2.8. The crystallization of wollastonite could eventually strongly influence REE fractionation (and more specifically the light-heavy REE ratios) during magmatic differentiation of alkali-rich foiditic melts and should therefore be considered to fully understand trace element evolution and partitioning in alkaline and silica-undersaturated magmas.