Compositional dependence of REE partitioning between diopside and melt at 1 atmosphere

Abstract

The effects of composition on pyroxene-melt partitioning of several REE (rare earth elements), Y, and Sr were experimentally evaluated. Using the synthetic model systems anorthite–diopside, diopside–titanite and anorthite–diopside–titanite different diopsides were grown at atmospheric conditions in a double-ellipsoid mirror furnace. The single samples were melted and crystallised in a Pt/Au crucible with compositions corresponding to the invariant points of these systems. Rotational motion with approximately 25 rpm around the longitudinal axis of the crucible increases the prevailing convection flows. By this means, the exclusively diffusional transport of assembly groups onto the growing crystals is avoided. Quenching is achieved by dropping the crucible into water. Crystals up to 2 mm were obtained and analysed by electron microprobe. No inhomogeneities or compositional zonation, either in the diopsides or in the coexisting melts, were observed within the analytical uncertainty of the electron microprobe. The crystallised diopsides occur as both euhedral single crystals and large symplectitic lamellar intergrowths with anorthite or titanite. The chemical homogeneity and the texture indicate near-equilibrium conditions. The analyses show strong positive correlations between D_REE and tetrahedrally coordinated Al in diopside but are not affected by octahedral Al or Ti-concentration. By means of correlations and mass balances the incorporation of REE can be described by 2 different coupled substitutions:

The Al-coupled incorporation of REE³⁺ (1) dominates the D-values. The Na-coupled substitution (2) is of minor importance. Depending on the compositions investigated the D-values vary by up to a factor of 10. This range overlaps most of the published pyroxene-melt partition coefficients. Because we conducted isothermal and isobaric experiments, this overlap indicates that a wide range of D-values is a function of composition. For the coupled substitutions (1) and (2) this indicates that the D_REE strongly depends on the amount of tetrahedrally coordinated Al³⁺ in clinopyroxenes.