Crystal vs. melt compositional effects on the partitioning of the first-row transition and high field strength elements between clinopyroxene and silicic, alkaline, aluminous melts

Abstract

The first-row transition element (FRTE) and high field strength element (HFSE) systematics are powerful tools for tracking the source and evolution of mantle-derived magmas. Clinopyroxene is generally considered a key fractionating mineral controlling the partitioning of trace elements between melt and residual solid during mantle melting.Although partitioning of FRTE and HFSE between clinopyroxene and basaltic melts has been well studied, experimental constraints on their partitioning behaviors in the presence of siliceous, aluminous and alkali-rich melts are still lacking. Here we present clinopyroxene-silicic melt (67-69 wt% SiO2) partitioning experiments at 1 bar pressure and 1070-1100 °C for Co, Mn, Ni, Cu, Zn, Fe, Sc, Cr, V, Ti, Zr, Hf, Nb, and Ta. Run products consist of diopsidic clinopyroxene coexisting with various melt compositions with non-bridging oxygen to tetrahedral cation ratio (NBO/T) ranging from 0.10 to 0.22.Using our new partition coefficients (Ds) and combining literature data we assess some of the effects of crystal chemistry and the melt composition on the partitioning of FRTE and HFSE in this simple system.We show that partitioning of FRTE varies from mildly incompatible (e.g., D = ~0.1−1 for V, Cu, and Zn) to highly compatible (e.g., D > 10 for Cr and Ni), with highest compatibilities observed for Ni (DNi = 13−34). The partitioning of HFSE varies from highly incompatible (D = 0.01−0.08) for Nb and Ta to mildly incompatible (D = 0.18−0.82) for Zr, Hf, and Ti. Our measured clinopyroxene-melt Ds are consistent with the theoretical predictions of the lattice strain model. Ds data for most tri-, tetra-, and pentavalent elements tend to increase with increasing tetrahedrally-coordinated Al This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.