Extreme peralkalinity in delhayelite- and andremeyerite-bearing nephelinite from Nyiragongo volcano, East African Rift

Highly peralkaline leucite nephelinite from the active volcano Nyiragongo in the Virunga province of the East African Rift contains globules of iron- and volatile-rich, highly peralkaline silicate glass with (Na+K)/Al up to 18 which has formed as a late differentiate of less peralkaline precursors, probably by fractional crystallization at a shallow level in the volcanic system. A number of uncommon minerals coexist with this glass (kalsilite, kirschsteinite, chlorbartonite, götzenite, delhayelite, umbrianite, zirconian cuspidine, andremeyerite (BaFe₂Si₂O₇), other Ba-Fe-Ti silicate minerals, and unnamed alkali-barium phosphate and Zr-Nb-Ti silicate minerals). These minerals are members of late magmatic assemblages that have survived sub-solidus recrystallization. Combeite occurs as a near-solidus mineral.

Low-variance mineral assemblages in Nyiragongo nephelinite define a cooling trend from eruptive temperatures ≥980 °C to the solidus of extremely peralkaline residual liquids at ca. 600 °C, followed by sub-solidus recrystallization and metasomatism down to ca. 500 °C. Oxygen fugacity well below the QFM buffer (QFM-2 to -3) persisted throughout the magmatic crystallization stage, but increased to above QFM during the final stage of postmagmatic recrystallization.

Highly alkaline, volatile-rich minerals such as delhayelite, götzenite and cuspidine were stabilized by a combination of high peralkalinity and elevated activity of chlorine and fluorine; these conditions persisted to sub-solidus temperatures.