The melting relations of the CaCO3 MgCO3 system are investigated and trace element partition coefficient of Li, Na, K, Mn, Fe, Sr, Ba, Pb, Nb, Y and rare earth elements (REEs) between carbonates (Mg-calcite, Ca-magnesite) and dolomitic melt are established from high pressure (6 and 9 GPa) and temperature (1300 1800 ℃) experiments utilizing a rocking multi anvil press. We show that Ca Mg carbonates are stable within the subducting slab beyond ~300 km (9 GPa) but will (partial) melt beneath mid ocean ridges and in upwelling plumes. In contrast to previous studies, we report incongruent melting of carbonates producing a carbonate melt and periclase between 4 and 9 GPa. Partial melting of carbonates produces dolomitic melts whereby the trace element signature largely depends on the Ca/Mg-ratio of the bulk system. For instance, REE will be fractionated by two orders or magnitude between Ca magnesite and dolomitic melt. In contrast, melting of Ca rich carbonates will not lead to a significant REE fractionation.

The here published data set includes all chemical analysis (major and trace elment composition) of run products and starting materials. From this data set we obtained the melting relations and partition coefficients reported in Sieber et al. (2020); Sieber et al. (under review).