Fractional crystallization of melilitite and Mg-nephelinite at 1 GPa: An experimental study on the petrogenesis of phonolite and related immiscible carbonatite magmas

Carbonatite deposits are rare magmas which represent the primary source of Rare Earth Elements (REE) on Earth; those are often associated with alkaline silicate magmas. A unique opportunity to investigate the natural carbonatite system exists through the study of the only active extrusive carbonatite volcano on Earth, the Oldoinyo Lengai (Tanzania). It has been proposed that carbonatites could be produced by low-degree partial melting of upper mantle domains associated with protracted differentiation that eventually form phonolite together with immiscible carbonatite melts. In such a model, the primitive magmas are proposed to be melilitites or Mg-nephelinite. However, a comprehensive study of the crystallization and immiscibility processes related to those parental melts is still missing.

In this study, we performed equilibrium and fractional crystallization experiments in order to understand the differentiation behaviour of carbonatites' parental magmas at high-pressure, and to decipher on the role of both melilitites or Mg-nephelinite in carbonatite genesis. Volatiles were introduced to both melilitite and Mg-nephelinite starting compositions, and the initial volatile contents (1.2 wt.% of H₂O and 0.6 wt.% of CO₂) were referred to the previous melt inclusion study by Mourey et al. (2023). The conditions of the first fractional crystallization experiments were established to be just below the liquidus temperature of each composition, i.e., for melilitite = 1250 °C and for Mg-nephelinite = 1200 °C and the subsequent experiments are performed by decreasing the temperature by 15 to 30 °C in each step. At each step, the melt composition in equilibrium with crystallized minerals is determined using an electron microprobe and, mass balance calculations for volatiles. All the experiments were performed at a nominal pressure of 1 GPa (relevant for lower crustal conditions in the Oldoinyo Lengai system), using a piston-cylinder apparatus. A double capsule setup made of AuPd was used to avoid iron and volatile losses during the experiments.

Equilibrium crystallization has not been able to produce phonolite magmas, the evolved term that is observed in the natural system in equilibrium with immiscible alkaline carbonatites. The fractional crystallization experiments of Mg-nephelinite composition highlight an evolution towards the alkaline-rich phonolites after 60% of fractionation, while the evolution of melilitite experiments seems to follow chemical trends that are not consistent with the natural liquid line of descent. The alkaline silicate melt remains abundant in all the fractional crystallization experiments (>65%), which facilitates an efficient equilibrium with silicate minerals and oxides, such as olivine, spinel, clinopyroxene, magnetite, perovskite, phlogopite, melilite and garnet. Mineral assemblages present along the liquid line of descent are consistent with the natural record that has been documented at Oldoinyo Lengai by cognate plutonic samples. Experiments are still ongoing to explore the potential occurrence of carbonatite-silicate melt immiscibility at later stages of the crystallization sequence.