Trace element partitioning in silica-undersaturated alkaline magmatic systems

Abstract

Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na2O + K2O = 4.15–14.97 wt%) and silica-undersaturated (SiO2 = 36.73–45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025–1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations.