Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism

Experiments in the systems diopside-albite (Di-Ab) and diposide-albite-dolo mite (Di-Ab-Dmt), doped with a wide range of trace elements, have been used to characterise the difference between clinopyroxene-silicate melt and cli nopyroxene-carbonate melt partitioning. Experiments in Di-Ab-Dmt yielded cl inopyroxene and olivine in equilibrium with CO2-saturated dolomitic carbona te melt at 3 GPa, 1375 degrees C. The experiments in Di-Ab were designed to bracket those conditions (3 GPa, 1640 degrees C and 0.8 GPa, 1375 degrees C), and so minimise the contribution of differential temperature and pressu re to partitioning. Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt systems. Notably, in the carbonate system clinopyroxene-melt partition coe fficients for Si, Al, Ga, heavy REE, Ti and Zr are higher by factors of 5 t o 200 than in the silicate system. Conversely, partition coefficients for N b, light REE, alkali metals and alkaline earths show much less fractionatio n (<3). The observed differences compare quantitatively with experimental d ata on partitioning between immiscible carbonate and silicate melts, indica ting that changes in melt chemistry provide the dominant control on variati on in partition coefficients in this case. The importance of melt chemistry in controlling several aspects of element partitioning is discussed in lig ht of the energetics of the partitioning process. The compositions of clino pyroxene and carbonate melt in our experiments closely match those of near- solidus melts and crystals in CMAS-CO2 at 3 GPa, suggesting that out partit ion coefficients have direct relevance to melting of carbonated mantle lher zolite. Melts so produced will be characterised by elevated incompatible tr ace element concentrations, due to the low degrees of melting involved, but marked depletions of Ti and Zr, and fractionated REE patterns. These are c ommon features of natural carbonatites. The different behaviour of trace el ements in carbonate and silicate systems will lead to contrasted styles of trace element metasomatism in the mantle.