Melting of carbonated pelites at 2.5-5.0 GPa, silicate-carbonatite liquid immiscibility, and potassium-carbon metasomatism of the mantle

Melting experiments on a Fe-rich carbonate-saturated pelite were performed at 850-1300 °C and 2.5-5.0 GPa to define melting relations, melt compositions, and the conditions under which carbonates remain residual. In the selected fertile bulk composition, 30 wt.% potassic granite (2.5 GPa) or phonolite (5.0 GPa) melt is generated at the fluid-absent solidus. The temperature of the latter increases from 900 °C at 2.4 GPa to 1070 °C at 5.0 GPa. Phengite + quartz/coesite control initial silicate melting and melt productivity through the reaction phengite + quartz/coesite +clinopyroxene + calcite = silicate melt + kyanite + garnet, which leaves most of the Fe-Mg-calcite in the residue. Na remains compatible in clinopyroxene (D_Na^cpx/melt = 3.1 to 7.3 at the fluid-absent solidus), resulting in silicate melts with K₂O/Na₂O wt-ratios of 5.8-8.6. Such highly potassic carbonated silicate melts represent ideal metasomatic agents for the source mantle of group II kimberlites. From 3.7 to 5.0 GPa, Fe-Mg-calcite disappears only through the formation of Ca-carbonatite at 1100 °C. The experiments provide a possible source for Ca-carbonatites in combination with alkaline granitic to phonolitic melts at temperatures unlikely to be achieved during ongoing subduction. Large scale carbonate transfer to the subarc mantle can thus only be achieved when burying rates slow considerably down or subducted crust becomes incorporated into the mantle. Consequently, it is likely that carbonates will not be extensively mobilized in a typical subarc region, thus extending and confirming earlier results from subsolidus studies (Connolly, J.A.D., 2005. Computation of phase equilibria by linear programming: a tool for geodynamic modelling and its application to subduction zone decarbonation. Earth Planet. Sci. Lett. 236, 524-541.), that > 70-80% of the subducted carbonate will bypass the volcanic arc region and get buried to larger depths.