Trace element partitioning during partial melting of carbonated eclogites

Crystal/melt partition coefficients for Sr, U, and three Rare Earth Elements (La, Gd and Yb) have been measured during melting of carbonated eclogitic composition in the 4-7 GPa range. The experiments were conducted in a multi-anvil apparatus. Trace element concentrations were determined using electron microprobe and laser ablation ICP-MS in some cases. The obtained clinopyroxene/melt partition coefficients are very low whether the melt is silicate or carbonate in nature. This feature is due to Na presence and high vacancy concentration in the clinopyroxene. On the other hand, garnet/melt partition coefficients fall at the lower limit of or are slightly lower than literature data dealing with carbonate melt-bearing systems. We note that clinopyroxene/carbonated melt partition coefficient values for the Rare Earth Elements cannot be modeled using the lattice strain model. The reasons for the discrepancy lie on assumptions about melt structure as well as clinopyroxene solution model. In the latter case, high Na combined with high vacancy concentration precludes ideal mixing on octahedral sites to be a valid assumption. Taking charge balance on clinopyroxene crystallographic sites does not solve the discrepancy, suggesting that melt structure can be important. Garnet/melt partition coefficients can be predicted but we note a possible influence of the melt structure. The newly obtained partition coefficients have been used to test some simple models of mantle metasomatism.