Quantification of intraplate volcanism magmatic processes: New partition coefficients between minerals & alkaline melts

International audiencePartition coefficients (D) are key tools to identify and quantify processes such as partial melting, fractional crystallization, and melt/rock interactions and depend strongly on thermodynamic parameters and on the melt composition. Intraplate volcanism, in particular in continental rift area, is characterized by the abundance of silica-undersaturated and alkali-rich magmas. Partition coefficients are choice tools to understand the evolution of intraplate volcanism, however, in the case of alkali-rich magmas, few D data are available. In order to provide the community with new quantitative tools, we have quantified D between alkali-rich silicate melt and clinopyroxene (Cpx), nepheline (Neph), garnet (Grt), apatite (Ap) and wollastonite (Woll) based on natural samples. All those phases are widespread in alkaline systems and their crystallization may control the differentiation of alkaline magma series. In alkaline magmas and carbonatites Cpx display sinusoidal REE pattern with unusual enrichments in heavy REE (HREE) and in Zr-Hf. We show that the specific DREE partitioning of Cpx in some alkaline systems is not consistent with the standard models assuming incorporation of all REE in the M2 site; rather HREE substitutes both in M1 and M2 sites; depending on the Cpx chemistry (Fe3+, Mn, Mg, AlIV). A new parametrized model based on Cpx major element composition is provided in Baudouin et al. (2020). New D values are also provided for the partitionning of trace elements between alkaline melts and Grt, Ap, and Neph. More specifically, we show that DREE varies from 0.2 (DLa), to 15 (DLu) for Grt (Ti-andradite), from 5 (DLa) to 1 (DLu) for Ap, and from 0.006 (DLa) to 0.001 (DLu) for Neph. Rb (0.7), Sr (0.12) and Nb (0.001) are also incompatible elements in Neph. The first D between Woll and silicate melts are presented in Baudouin & France (2019). Zr and Nb are strongly incompatible in Woll (D < 0.01), and DREE increase from DLa = 0.2 to DLu = 3. The crystallization of Woll eventually strongly influence REE fractionation during magmatic differentiation of alkali-rich melts and should therefore be considered if we are to fully understand trace element evolution and partitioning in alkaline magma series. Baudouin et al. (2020) Contrib Min Pet 175, 1-15. Baudouin & France (2019) Chem Geol 523, 88-94